Vehicle No.103
Design of a Competition ATV for the 2011 SAE Baja Series Monroe Community College, Rochester NY Ronald George Captain, Suspension Engineer
Carlo Inglese Treasurer, Controls Engineer
Daniel Sze Secretary, Powertrain Engineer Copyright © 2007 SAE International
ABSTRACT The design of MCC’s prototype baja vehicle is discussed in this report. The paper is written for a general audience and consists of an introduction summarizing the goals and design approach, followed by a description of vehicle systems present in the car. In places, relevant research done by students pertaining is also included to enhance subject matter. The appendix contains vehicle specifications, relevant images, plots and calculations.
INTRODUCTION In the summer of 2010, a group of new student leaders were elected to the board of officers for the 2011 MCC Baja Team. When the annual budget was made available, the team registered itself into the 2011 SAE Baja Illinois Competition as “Car No. 103 Tribunes Racing”. DESIGN PROBLEM – Every product must serve a purpose, otherwise no one would buy it. Team leaders sourced this main purpose from the SAE Rulebook.[1] MAN, MONEY, TIME – The project was essentially a systems engineering task. The automotive team was broken down into four major departments without overly dividing labor for a small group. The 12-person team was structured into sub teams (Fig. D4). Financial resources were divided among the various systems and subsystems (Fig. D5) and a Gantt chart was designed for the 9 month project (Fig. D6). BRAINSTORMING & RESEARCH – The project was started with questions in order to explore every possibility right away. Modest research was done to understand the trends and safety issues in the ATV industry. The team visited local ATV showrooms and studied commercial catalogues. In preliminary design
meetings, the team brainstormed ideas and benchmarked the competition by studying images and competition videos. An early concept sketch that resulted is shown in Fig. A1. FUNCTIONAL OBJECTIVES – The team sought to design with a focused purpose and develop the architecture with a logical process. The Rulebook requirements, combined with research material and ideas drawn from discussions among team patrons led to the formulation of a broad set of requirements. These requirements were graded for priority shown in Fig. A2. ERGONOMICS & PACKAGING – Ideation for the packaging was driven first and foremost by the safety and comfort needs of the driver. The geometry of the th 95 percentile human along with some of the team’s drivers helped provide seating and control hard points. After this, an envelope was created around the driver to set up environment dimensions (Fig. A3). PRODUCT ENGINEERING & FABRICATION – Prepared with a starting set of specifications (Fig. A4), engineering could proceed. In the following pages, each vehicle system is described and the choices behind designs justified on the basis of cost, time, manufacturing ease, and structural considerations.
ERGONOMICS OBJECTIVE – The SAE rulebook calls for a vehicle th design accommodating all individuals between 95 th percentile male to the 5 percentile female. The goal was to design a vehicle which would fit humans in this range so that the machine could achieve near universal operability. This had positive implications from a design and marketability standpoint. RESEARCH – Motor vehicle operation is a set of control tasks. Control movements are easiest when the limbs
are moderately flexed and extreme body positions avoided. Some clues as to what optimum to shoot for were discovered in equipment design research. [2] Some interesting information gleaned from the study was as follows:
design a practical structure that did not hinder an emergency escape. Finally, different side impact member heights were explored to locate the optimal position for egress and protection during a rock crawl event.
1. The best elbow angle for exerting force in the seated position is approximately 120°. 2. For heavy loads, the radius of hand operated wheels should not exceed 20” or fall below 7”. 3. For control pedal operation, the long axis of the feet and lower leg should form a 90° angle as this requires the least muscular effort to hold in place. 4. Where rapid, continued pedal movements are required, the pedal should be toe operated, with the fulcrum at the base of the heel. 5. The leg exerts maximum force with the knee angle at about 130-150°. 6. When maximum pedal pressures are sought, the fore-and-aft seat reference point to pedal distance should be about 47.5% of the height of the driver. When great force is not needed, the distance should be increased for comfort. 7. As pedals are moved laterally from the midline of the body in plan view, force exerted decreases. It falls to 90% when the pedal is placed 3” to either side; to 73% with a 6.7” shift; and to a 63% with a 10.2” shift. Hence, it should not be placed more than 3-5” from the midline. 8. For frequently but not continuously used leg operated pedals like the brake, a pressure of about 30% of maximum exertable is reasonable. For continuous usage pedals like the accelerator, resistance should not exceed 10 pounds. For toe operated pedals, the best resistance is from 6.5 to 9 pounds. 9. Pedals operated by ankle action should have a maximum travel of 2”, corresponding to an angle of 10-12°. When heavy footgear is anticipated, pedal travel should be increased. 10. For all foot controls, the direction of travel should be down, or away from the body, in line with the long axis of the lower leg and roughly parallel to the midsagittal plane of the body.
SEATING – A custom Kirky seat is used to increase driver comfort. It is made of MIG welded .125” thick 5052 grade aluminum. The seat is as low as practical in the vehicle to lower center of gravity (c.o.g). From practical experimentation, the previous car’s c.o.g was measured to be 22” +/- 2”. It was deemed desirable to decrease this to 20” to reduce possibility of roll-over. The seats from previous years led to distraction because the rib supports tightened unsuitably around the driver’s rib cage. Hence, the existing seat was modified by the fabrication group. In the process, 5 pounds of weight in metal was dropped. Students also decided that a tub-like seating envelope around the driver would be an optimal plan. This not only increased driver comfort by adding to the range of motion of leg movement but also modeled the exterior of the Baja car like a “hull” for future water competitions.
STATIC FIT – Armed with research knowledge, student designers began by obtaining physique data of the intended operators. The test subjects - 3 males and one female - were given standard racing gear to wear and were sat against a mock firewall oriented at 20° from the vertical. Important parameters like hip joint height, kneetorso angle, knee angle, thigh-seat angle, degree of ankle bend, head to roof clearance, elbow and wrist angles were explored. DYNAMIC FIT – Drivers were moved from their static positions until muscle tension, such as on wrist, ankle and the under part of the thigh were eased. The drivers were made to flex and abduct their elbows to explore the envelope of hand control. The range of visibility was also checked in order to design a non-obstructive front bracing structure. Driver egress was mocked in order to
CAD MODEING – The hard points obtained through the above exercise were then transferred to Solid works. CAD designers on the team used these points to create datum planes that the structural features of the vehicle would reference. ALTERNATIVES CONSIDERED: 1. In the CAD design stage, designers could have th employed a generic 95 male 3D model. However, this alone was deemed insufficient since the model wasn’t representative of a fully geared driver nor did it account for gender variation. 2. The population sample size for fit analysis could have been increased. However, from the team’s standpoint, this would entail an unprofitable expenditure of time and effort. 3. 3D laser scanners are used by some schools to model part geometry to custom fit a driver. [3] While this is certainly attractive, it would incur unnecessary costs.
POWERTRAIN OBJECTIVE - The design goal in this department was to produce a 40:1 overall transmission ratio power system that is reliable, serviceable, safe and easy to manufacture. Following is a description of the major systems comprising the powertrain. ENGINE – The air-cooled engine used on the car is a 4 3 stroke, 18.64 in (305cc) displacement Briggs & Stratton motor of “Over Head Valve” type (OHV). It is supplied to teams by Briggs and Stratton at sponsored cost and is to remain stock as per the Rulebook. [4] It has a .75” keyway PTO shaft as output and a compression ratio of 8.0 to 1. The engine weighs roughly 64 lbs. Engine idle RPM is set to 1750 RPM. At competition, the governor is usually set at max 3800 RPM, well below its RPM
capabilities. This is done for safety reasons so that the engine does not reach lethal speeds and destroy itself.
an low drive ratio of 4.5:1 and a high of 1:1 when the secondary is fully engaged.
Power and torque – Engine output has two components – power and torque. Power is important for the baja vehicle to perform under steady loads while torque gives the car the ability to cope with sudden loads. The engine has an advertised maximum torque of 19.65 N.m (14.5 ft.lbs) and a nominal power rating of 10 hp. However, data from an actual dyno test was in order. Due to lack of dyno facilities in the area that could supply us with printed graphs, test data was obtained from another team to help with tuning (Fig. C3).
Belts – Power and torque transmission is carried out via a rubber V-belt. Due to the adjustable engine plate, multiple center-center belts can be used in this system, from 8” c-c all the way to 9 ¼ “c-c
Fuel and oil – The engine has an oil capacity of 28 fl. oz (0.77 L) and a fuel capacity of 4 qts (3.78L). As recommended by Briggs, 5W-20 oil is used for lubrication. This oil behaves like a 5-weight oil during cold weather starts and gives the protection of 20-weight at high temperatures. It also has a float carburetor for consistent easy starting. A custom removable fuel tank is made out of .125” aluminum sheet for rapid and spill-free refueling. The fuel consumption in gallons per hour at a particular load level could be an important parameter for pit crew. This data was able to be obtained through good record keeping of past engineering data. See Fig. C1. Engine plate – A custom engine plate is designed to seat the engine. The plate allows the user to adjust the engine 1 inch fore and aft as it has parallel counter bored slots. This allows flexibility in use of multiple CVT center-center belts and allows for optimum CVT tuning. The engine is mounted to the plate using either 5/16-18 UNC or 5/16-24 UNF bolts, grade 5 or better. Noise, Vibration, Harshness (NVH) – It is the nature of single cylinder 4-stroke engines to vibrate. To dampen these vibrations, a thin layer of vibration resistant rubber is applied between the engine plate and the rollcage. Noise is one of the most common occupational health hazards and it is not uncommon for machine operators to complain because of its fatiguing nature. [5] To provide one solution in this area, the exhaust muffler can be changed to a less noisy one. The noise level at 4 meters away from the engine varies as a function of RPM and load for a given muffler (Fig.C2). A 2.5 to 3% reduction in db(A) levels can be attained by using Super Lo-Tone muffler, which is why it was chosen for the exhaust of this vehicle. For perspective, the standard for street-legal exhaust noise emissions in motorcycles is 80 dB (A). [6] CONTINUOUSLY VARIABLE TRANSMISSION – The torque transferring CVT used is a descendant of drag racing clutch, made especially for mini baja applications by Gaged Engineering in Nebraska. This system is primarily made out of aluminum and has one of the lightest overall weights at roughly 10 lbs +/- 1 lb. Transmission Ratios - The GX8 clutch system has a primary with pulleys of 5.5” diameter and a secondary with pulleys of 7.5” diameter pulleys. The clutch provides
Serviceability – A special inner post is used in the secondary CVT that allows the use of a splined 1” OD jackshaft. This system is easy to install and disassemble compared to Woodruff keys. Justification - A CVT was chosen because theoretically, a tuned system will shift out at the peak power output of the engine, thereby yielding an efficient system. [7] In discrete-gear transmissions, lesser time is spent at the power peak during shifts. Given equal power to weight ratios, a vehicle with a tuned CVT should theoretically outperform a stick shift system. CVT’s are not cheap, especially the Gaged model. However, to save on financial resources, last year’s model was re-used as it had no perceivable problems other than light scuff marks on the Primary pulleys. CVT Case - The CVT case is a 1/8 in. thick rectangular box made of 6061-T6 aluminum in a welded construction. It has two holes in it for the engine shaft input and the jack shaft output which interfaces with the Secondary. The box itself is made out of 1/8” thick 6061 aluminum sheet metal welded together to allow a 1” clearance on all sides for the CVT itself. A rectangular box is easier to manufacture than a curved box. PLANETARY GEARBOX – The design features a reduction gearbox after the CVT secondary stage to bring down RPM to usable levels. A 5:1 gearbox from MATEX Gears is featured on the car. This is the most compact inline planetary gearbox offered on the market. The 5:1 system offers high reduction, while keeping weight and moment of inertia balanced. Planetary Case - The gearbox is encased within a custom made case. It is designed in a way to allow CNC milling on a stock 6061 billet without any welding required at all. The case is robust and works for the vehicle needs. CHAIN DRIVE - With 4.5:1 in the CVT and 5:1 in planetary, the desired overall ratio was targeted by a 2:1 chain drive system. The system features a steel, hardened 17T sprocket and an aluminum 38T sprocket. The driver sprocket was held on the splined jackshaft with a retainer ring and a threaded nut. The driven sprocket was positioned on the splined axle using a custom made sprocket holder that was specially designed to withstand torsional stresses. Power is transmitted using a standard 428 motorcycle chain. The chain has a pitch of 0.50 in. and a tensile strength rating of 23.6 kN (5300 lbs). The theoretical chain load calculated at max power transmission is approximately 3.2KN (724 lbs) (Fig C3).
Chain tensioning & alignment – Through many past failures, the design team is appreciative of the damage a 50 inch long motorcycle chain can do. At full load, without proper tensioning, not only will power not be transmitted, but weak sprockets, their holders and connecting spline work can be torn apart. After many trials and errors with different designs, a simple delrin wheel tensioner is used to remove the slack in the chain. The drive system is tensioned to ensure a 1 in. sag height at the mid portion of the drive system. To ensure alignment in the chain drive system, a motorcycle chain aligner is used to inspect the installation. Chain case – The chain drive is encased within a 3/16” thick 6061 aluminum sheet metal case that is welded together. The case is thick to ensure a robust case that it impervious to damage from side impacts and rocks. Equivalency graph is displayed in Fig.C6. JACKSHAFTS - The CVT and planetary gearbox are connected by a 1 in. diameter primary jackshaft. The secondary jackshaft transfers power from the gearbox to the driving gear. At this end, it is supported by a double bearing housing that butts up against the collar of the shaft and a single bearing housing which the side end nut bolts to. The bearings are 2”x1”x0.5625” that are rated up to 1400 lbs. The jackshaft passes through the pivot of the swing arm. The concentric pivot system ensures that the chain drive follows the arc of the swing arm, rendering constant chain centers. This prevents excessive chain slackening and the need for elaborate tensioning schemes as seen on some motorcycles. ALTERNATIVES CONSIDERED: 1. An alternative solution was to forgo the planetary altogether and apply all the reduction into the final chain drive system. This would dictate larger sizes for the sprockets sizes, leading to the possibility of damage due to ground clearance issues. 2. Discussion was made about applying soundproofing materials such as polyether, urethanebased, thermo-acoustic foam on the sides of the CVT case. However, due to lack of time and complexity, it was not pursued. Definitely, this area has room for new ideas and maybe pursued by the design team in future. 3. Another alternative solution was to use lighter chains to decrease the weight of the final drive. However, with a load analysis, it was found that lighter chains cannot withstand the load and torque variations in the drive system reliably given their capacities. 4. An HDPE plastic chain case was proposed. This is certainly an attractive solution, however more study needs to be done in order to understand how the case would be manufactured and then fastened together. 5. Considerable time was spent on studying the feasibility of a belt drive. While it certainly has high tensile strength, the belt is not forgiving to dirt, and improper tensioning. It is also weak in the abrasion
resistance area. Hence, the benefit of the doubt was given to the chain this year.
SUSPENSION OBJECTIVE – The goal of suspension design is to help the wheels put traction onto the ground while keeping tire scrub to a minimum. It should reduce vibrations imparted to the sprung mass while providing adequate chassis to ground clearance. It should prevent uncomfortable pitching moments, and excessive squat and dive during transient conditions of acceleration and deceleration. It will be subjected to both low speed vibrations and high loading from jumps so linkages must be structurally sound. Another important goal was to keep linkages and joints in the system to a minimum so as to reduce complexity, friction and slop. REAR SUSPENSION – The rear suspension chosen is a classic twin-shock swing arm with solid axle that has proven very durable over the past 5 years. Though it is a non-independent suspension, it features less moving parts, is easy to manufacture and has high roll stiffness. Vehicle dynamics suggests that such a system will induce rear over-steer which has been validated through observations, at least on off-road terrain. Swingarm System – 1.00 x3.00 x.065” mild steel rectangular tubing was used for the trailing arms and diagonal support beams with a 3.00” OD x .065” thick mild steel tube for the cross tube. The drive axle was run through the cross tube, which was placed at the end of the side beams. The shape of the swing arm arrangement resembled the letter “U”. The swingarm is mounted to the firewall on a triangular sheet metal bracket and fixed in place with a delrin-metal sleeve combination for smooth movement. The weight of the system is around 30 lbs. Shocks – A pair of 7 inch. stroke coil-over shocks were selected to provide progressive vibration dampening. Shocks were supplied by AFCO. The 16 series small body gas shock comes with a slender 1 ¾” diameter aluminum mono-tube body and an external reservoir. It has a coil-over kit and 2 ¼” OD springs. Dampening adjustment is provided for both low speed compression and rebound. Dual springs were suitably selected to provide a progressive support and to provide a desired natural frequency of 1.4 Hz. Mounting brackets for these shocks were oriented along the axis of the shock body so as to reduce bending moments. Rear Axle – One of the goals of rear suspension design was to decrease wheel track. Therefore, a 51” center to center sprint car axle was selected from Hyper Racing. This is one of the lightest axles on the market. It is made out of heat treated aluminum and is 2” outer diameter. It features a swaged construction to save weight, shoulders for bearing support and involute splined ends for wheel mounting.
WHEELPATH - The swingarm was mounted on the RRH at a height of 10 in. from the bottom frame member of the car. The higher the swingarm is mounted, the more rearward is the wheelpath. Thus, the wheel can acts naturally in the direction of oncoming bumps. This is similar to how mountain bikes are designed. The final mounting point height led to a wheelpath tangency angle of 8°. TRAVEL – Ride height is determined by the height of the solid axle from the ground, which is 10 inches. Designed up travel is 5 inches and droop travel is 6.5 inches. ALTERNATIVES CONSIDERED: 1. Independent rear suspension was proposed several times among team members. However, the system has some strong cons. First, independent suspensions have more moving parts because it dictates the need for a custom gearbox design, CV/half-shafts, additional linkages, fasteners and custom wheel uprights. Secondly, due to the complexity of the semi-trailing and a-arm suspension systems, more design time is involved in getting the geometry right. Due to time constraints, it is possible for a designer to overlook the importance of roll center placement. Incorrect roll center placement in the rear suspension can lead to jacking and sluggish suspension behavior. To combat this problem, an often sought out solution is to restrain the system using anti-sway bars. However, by doing this, the independence of the suspension is reduced which defeats the initial design goal. Moreover, CV shafts have to be designed and oriented properly to deliver power. It is not uncommon to see these shafts breaking catastrophically on the field because of poor oversight. Thus, it was decided that independent suspension should be a future subject exploration. 2. KING remote reservoir shocks were considered for the rear with the thinking that they would be a sturdier system to support the rear end. After a unit was bought and analyzed, it was deemed too big for use and was returned. The customer service was not too accomodating either.
FRONT SUSPENSION – The front suspension chosen is a double wishbone design. The system comprises of steering knuckle, linkages, their geometry and the shocks. The following discussion describes these components. Suspension Wishbones - A short-long arm suspension was chosen for the front of the vehicle. These arms, called wishbones, are made out of 1 in. OD 4130 steel. The upper wishbone, which takes less stress, has a thickness of 0.049” and a length of 15.5” from pivot center to outboard mounting center. The lower one is 0.065” thick and 17.5” in length from pivot to outboard mounting center. The outboard ends of both arms have a threaded tube adaptor that is press fit and welded to the wishbone body. These adaptors receive the rod end.
The wishbones were designed to be asymmetrical in splay (Fig. B1). One leg is straight while the other leg is splayed. When installed on the car, the angle of the splayed leg gives enhanced clearance at the front as protection in a collision. Each wishbone has a base mounting width of 10”. This decreases tension and compression forces in the individual legs on loading and provides adequate clearance to fit a standard coilover shock. Each wishbone was made using one single pass on a tube bender. The manufacturing ease comes also from another factor. While setting up the arms for fabrication, the straight leg of the wishbone provides a good datum for reference. Bushings – For the inboard side of the wishbone, a delrin-metal bushing sleeve is used. The bushing is press-fit into a 4130 tube that is welded to the wishbone body. The entire assembly is held together by a hollow metal pin through which a ¼” bolt is run and fastened. The reasons for choosing bushings are twofold. A) Bushings are have a certain amount of compliancy and can be expected to absorb some of the vibrations, preventing them from being transferred to the unsprung chassis. B) Delrin is a load bearing plastic, is easy to machine and has good dimensional stability in manufacturing processes. [9] When many units of the same bushing are made, it is important that tolerances in plastic pieces do not vary considerably. Rod ends – For the outboard side of the wishbone, ½”20 TPI VCAM-8 male rod end bearings are used. These were sourced from Aurora Bearings. This rod end is a two piece rod end having 10 degrees range of motion per side. Using a 3/8” bolt through each rod end, with high misalignment spacers, afforded the vehicle a deflection 20 degrees per side. The rod ends are rated for an ultimate radial static load capacity of 13425 lbs. Since the front suspension application is a slowly oscillating scenario of less than 100 RPM, careful attention was put in ensuring that suspension loads would not exceed 15% of the radial static load capacity, or roughly 2000 lbs [8]. The steering knuckle design, described below, ensures that bending loads are not applied to the threads of the rod ends. Steering Knuckle – The steering knuckle design is an evolution of past year’s design. The knuckle is constructed out of 6061-T6 aluminum, a bold departure from steel construction. The bearing is positioned inside the knuckle, the splined body of which is positioned on a stub axle. The axle has a positive stop at one end to locate the wheel and a threaded end at the other to take a lock nut. This center lock nut is drilled across its body to accept a slide pin, which ensures that the nut doesn’t loosen in vibratory situations. The geometry of the knuckle went through 2 iterations and a round of FEA analysis. This was to ensure that the part topology was suitable for easy manufacture, and had the ability of withstanding combined static loading, emergency brake torque and fatigue (Fig. B3). Mounting brackets of the steering knuckle interface with the rod ends on the wishbone and steering rods to position them in double
shear. There is an offset in the upper and lower mounting brackets to yield a kingpin angle of 5.7 degrees to the suspension geometry. Due to this angle, scrub radius is reduced from the 2” that it was in the past year’s car to 1.5” - a 25% reduction. By reducing the scrub radius, steering feedback should theoretically decrease, aiding in comfort for both a driver and a crew member walking the vehicle. Mounting brackets – The mounting bracket design for the front shock, on the wishbone side, is robust because the bracket is positioned on both the shock tube and the curved end of the wishbone (Fig B1). This gives it more surface area for setup and welding. The mounting bracket on the rollcage is designed to orient it along the axis of the shock. All mounting brackets were made out of 0.125” thick sheet steel with a bending radius of 0.25”. They were laser cut and CNC bent by Canon Industries. Shocks – A pair of AFCO 7” stroke coil-over shocks were selected to provide progressive vibration dampening in the front. The body dimensions were described before. Dual springs were suitably selected to provide a progressive support and rendered a natural frequency of 1.2 Hz, which is about 16% lesser than the rear suspension frequency. This discrepancy was adopted to make the rear suspension quicker so that pitching would be avoided, given a wheelbase of 60” and an average speed of 28 mph. The shocks were mounted on lower wishbone, closer to the wheel end to reduce bending moments. TRACK AND TRAVEL – Static track width is set at 55”. This was determined from the need to fit on a customer’s pick-up truck bed, if the need came for the vehicle to be hauled. A narrow car is also agile in turns. Ride height is determined by the height of the front end chassis bottom to the ground, which is set at 12”. Designed up travel is 5.5” and droop travel is 6”. There is more droop to aid in wheel traction while negotiating pits. WHEELPATH – The in-board mounting orientation of the wishbones is 10 degrees from horizontal. Like the rear wheels, the front wheels are designed to recede on suspension compression in elevation view. CASTOR - The 10 degree steering axis angle, as noted above, gives a castor trail of 1.8”. Positive trail and tire side force due to slip angle combine to produce a selfrestoring torque to produce some amount of straight line stability. This was keyed in after experimentation and from experience. The designers believe that excessive castor angles beyond 10-12 degrees are not needed as the vehicle does not lean into a turn like a motorcycle to benefit from it. Moreover, it creates other issues. A) Excessive castor can make the steering feel heavy and less responsive. B) Excessive castor angles may transmit more vibration jar to the driver. C) Big angles can create structural complications when fabricating a roll cage.
SCRUB, CAMBER – The short-long arm front suspension induces camber variation and tire scrub. Careful attention was given to engineer a suitable camber profile that prevents tire wear while helping adequately with traction performance on cornering. From video studies of similar vehicles on rough terrain using Logger Pro, vehicle roll angles ranged from 0-10 degrees and depends a lot of on vehicle design and setup of the car suspension. Constraining roll to acceptable ranges ensures that wheel camber does not adversely affect the tires. The wishbone lengths and configurations were chosen to keep tire scrub to a minimum. In maximum parallel suspension bump, the static wheel track can be expected to increase by +2” while in maximum parallel droop, a change of -3.5” can be expected. The worst case scenario is a diagonal bump-droop configuration where the wheel track change is +4”. These numbers may be taken into account while scheduling a tire maintenance program. TOE – 1.24” of thread is available on the suspension rod ends make adjustments to wheel toe. Toe is adjusted by turning the rod ends to a suitable position. Hex nuts provide a positive lock to the assembly after adjustment. ALTERNATIVES CONSIDERED: 1. Most of the suspension analysis is done from a purely kinematic, rigid body type analysis. To check the effect of bushing compliances, high level physics software like MSC Adams will have to be used. The team looks forward to orienting itself to student version of this software in future. 2. Welded construction was suggested for the wishbones, however bending was attractive from both a manufacturing and cost perspective.
WHEELS OBJECTIVE - The goal of the wheel choice was to provide a durable interface between the vehicle, tire and terrain. Durability, traction and weight were the deciding factors in the choice of these tires. But ease of installation cannot be overlooked so this was also a central factor in their selection. Rims - The vehicle runs on 10”x5” rims on all four corners. The rims were provided by Hiper Technologies. Previous cars have used the .190” wall aluminum rims with mixed results. While they were lightweight, they lacked the ability to take the punishment from the terrain. Aluminum rims are deformable upon direct impact. 75% of the aluminum rims the team had used in the past have been bent or crushed under aggressive driving. To remedy this problem, the team switched to carbon reinforced rims with bead locks. This allows the rim toward slightly under hard impacts and return to its original form. The rims also include a bead lock ring to retain tire bead seating under hard lateral loads.
TIRES OBJECTIVE – Tires are an integral part of the suspension system. A change of 5-10psi of pressure can make perceivable effects on handling, rolling resistance and anti-bottoming capability. The contact patches deliver the tractive force needed to propel the vehicle. All lateral cornering loads are transmitted through the tires. As the tires perform very important functions, the objective of the selection process is to pick a set that offers optimum performance in both wet and dry conditions, while being lightweight, nimble in acceleration and durable throughout its demanded life. Selection Process - The selection of tires for the car was somewhat of an arduous process. There were several factors and variables that were considered before a final selection was made. Not only is there a myriad of tire sizes available, there also exists many different lug designs and tread patterns designed for use in a variety of track conditions. The ideal tire would be a good compromise between wet and dry traction, and also one that would perform well in the rock crawl environment at Illinois. Rear tires - Interco Swamp Lite ATV tire is chosen for the front, in a size of 22-8-10. The outside diameter of the tire is selected to provide the vehicle with adequate ground clearance at the compromise of some added rotational inertia. Having a wider tire helps envelope pits in a rock crawl event. Considering that the Peoria, IL competition takes place in the middle of June, it can be assumed that conditions will be mostly dry. This assumption came from assessing the city’s weather history. The Swamp Lite is a choice that offers a compromise between wet and dry performance, because it still maintains a somewhat chevron shaped tread, but it also has large sidewall tread blocks that will be beneficial in hard packed dirt and the rock crawl competition. Front Tires - At the front, Maxxis M934 RAZR ATV tires were used in the 21x7-10 size. These are similar to the previous year’s Titan AT 489 tires but with drastic improvements. These tires are 6 ply in nature. At 13 lbs per tire they are one of the lightest tires on the market. A pendulum type test was performed on the set of these tires in-house. By measuring the period of the swinging tire and doing a bit of math, it was found that its moment 2 of inertia is 1.8 lb.ft . In future, we hope to do more tests to compare its standing with other tires in our inventory. ALTERNATIVES CONSIDERED: 1. Tires intended for use in thick mud were considered. They have a chevron-shaped tread pattern, designed to sling mud out from underneath the tire as it spins at high speed. The scooping effect of this style of tire is very effective, but at the loss of traction in drier conditions. The contact patch of a mud tire is much smaller than one designed for on road, or hard packed dirt conditions.
2. Perfect hard pack tires were also considered. This tire is one that uses numerous small tread blocks, to increase the overall contact patch, and provide "bite" into the earth similar to that of a rugged work boot. These tires offer minimal performance in mud due to their inability to do the aforementioned slinging of mud, and they often end up caked with mud, drastically reducing their effectiveness to propel the vehicle. For the final consideration of rock crawl performance, where the track consists of large rocks and boulders, it was deemed wise to select a tire with as many large tread blocks as possible so they can grip and crawl over the rocks as best as possible. Obviously a smooth tire would not be preferred here.
BRAKES OBJECTIVE - The goal of the braking system was to provide good pedal feel/travel, effective braking distance, while reducing brake drag. MASTER CYCLINDERS - To conform to the rules set forth, an individual braking circuit is employed for each axle. A pair of Wilwood's 260-2636 Girling style master cylinders is used. The 5/8 Bore provides adequate hydraulic pressure to actuate the brake calipers with minimal pedal effort. The master cylinders are actuated using the Wilwood 340-5180 reverse swing mount brake pedal. Both master cylinders are mounted to the pedal bracket. To vary the amount of force applied between the two master cylinders, the balance bar on the pedal assembly can be adjusted, to suitably divide braking force between the front and rear braking circuits. To fine tune this calibration, an adjustable proportioning valve is also used in series with the brake lines. Though all wheels must be able to be fully locked upon heavy braking, it is possible to slow the vehicle with light pedal pressure by applying more of the pressure to the front brakes. The rears will be able to spin freely, and allow for a theoretically faster acceleration after braking. BRAKE ROTORS - Martin Custom Products manufactured the brake rotors seen in this vehicle. The rotors are .200" inches thick, with an outside diameter of 7”. The diameter was chosen for front wheel packaging reasons and the thickness was dictated by the caliper specifications. It was also used because it has no slots or holes cut into it. It was the experience of previous teams that dirt and mud serves as an abrasive to brake pads. In muddy environments it was found that the mud would occupy the slots and/or holes, and score the pads excessively. BRAKE CALIPERS - The final, crucial components of the braking system are the calipers. Since the SAE Baja application does not call for not a high speed, heavy duty brakes, we chose the Wilwood PS-1 caliper due to its compact size, light weight, and low cost. It is a single piston caliper, but it is more than adequate for the vehicle. Multiple piston calipers would likely be too powerful, and cause the wheels to lock with even the
slightest pedal pressure. The calipers are also compact, to provide enough clearance with the wheels that were selected. HARDWARE - All brake hardware was made to AN spec. This was chosen to reduce the confusion of using different types of fittings together. By using this system, the number of fittings needed to plumb the cars’ brake system together was reduced. Flexible brake lines were also used at the A-arms and the swing arm. They were made as short as possible to reduce pedal “sponginess”. ALTERNATIVES CONSIDERED - A single, dual circuit master cylinder was considered but physical size and available piston sizes were limited. The packaging of these items prevented them from working with the car’s current configuration. Hence, the idea was disqualified. It was also suggested that cutting brakes be used. While the Rulebook does not prohibit the use of cutting brakes, it was decided to postpone their use to future in order to study their effects on driver performance.
STEERING OBJECTIVE - The goal of the steering system was to provide a system with quick steer ratio, a balanced feedback response and limiting bump steer to a minimum in crucial areas of the suspension cycle. STEERING RACK AND STEERING WHEEL – A rack and pinion steering setup from Stiletto Products is chosen in the vehicle. Its quick steer ratio of 6.4:1 is ideal for quick steering in maneuverability and rock crawl conditions. The rack is placed in front of the wheel stub axle to work with the steering knuckle design. The rack is placed 2.5” above the center of the lower frame tube in the suspension compartment. This distance was arrived after geometrical treatments to reduce bump steer. A 13” diameter steering wheel serves as the control interface between the car and driver. TIE RODS – 13 in. tie rods made out of 7/8 in. OD 6061 drawn tubing is used to provide the push-pull steering force to the wheels. They are made by AFCO. The rods are heat treated and anodized for strength and aesthetics. The rod has a LH 5/8-18 thread at one end and a RH 5/8-18 thread at the other. This ensures that the length can be adjusted and locked in using 5/8”-18 jam nuts sourced from Fastenal. ROD ENDS – ALM-10 5/8-18 RH rod ends were used on the outboard side of the tie rods. These link up the rods to the steering knuckle and complete the steering system. A generic 5/8-18 LH rod end was used for the inboard side of the tie rod.
CHASSIS OBJECTIVE - The goal in designing the chassis was to produce a low weight, structurally sound space mounting bracket for all the major components listed above. The cage must be capable of safely and efficiently dispersing
loads throughout the entire vehicle. It must also be th accommodating of all anthropometrics from the 95 th percentile male to the 5 percentile female while they are in their full racing gear. ROLLCAGE – The rollcage features an all-metal construction. Design was carried out with ergonomics, ease of construction and cost in mind. Careful optimization of the tubing led to a 20 lb weight decrease compared to previous frames. Primary Members – Critical areas were constructed out of primary tubing. These areas are firewall hoop, front bracing members, shock supper tower, over head hoop, seat belt members, side impact members and rear engine envelope. The optimization analysis plots (Fig D2 & D3) suggests that the most lightweight tubing for primary members that would meet SAE strength requirements was 1.125” OD 4130 with 0.083” wall thickness. However, this size is not picked for rollcage construction mainly because of two reasons. A) This size was not offered by the supplier, Aircraft Spruce. B) The die needed to bend such tubing is not readily at the time. Thus, for time and cost reasons, a stock size of 1.25” OD tubing at 0.065” wall thickness was selected. This difference in weight/foot of this tube compared to the optimal solution is negligible. Mounting brackets – Generic mounting brackets were designed to mount the body panels and seat. They are made out of .125 in. thick sheet steel and were laser cut by Cannon Industries. Secondary Members – Internal bracing members for the side impact members, under seat members and firewall are made out of secondary members. At 1” OD and 0.065” wall thickness, they comply with the SAE regulations. The supplier was Aircraft Spruce. Construction - The primary members have 6” radius bends while the secondary members have 3.5” bend radius. A combination of hydraulic and manual bending was used to attain desired geometry. Bends were used in places to give a more aesthetic appearance to the overall structure. Gussets were used in places to enhance structural support. Finally, tubing end caps were welded to open areas of the roll cage to prevent water and dirt entry. Notches – Fish mouth notches were made using a 1” roughing mill supplied by Victor Machinery. The setup time was longer; however it was accepted because the notches were easier to cut and was consistent due to the special cutting geometry of the tool. This ensured a clean and close fit between joined members. Welding - All members are TIG welded together due to the availability of equipment and mastery of technique by student welders. Prior to welding, welders practiced on stock tubing and then checked for weld integrity and penetration by destructive testing the tubes. Rollcage tubing was cut, deburred and cleaned and later setup on
a flat welding table with jigs. Careful attention was placed in restricting the heat affected zone to a narrow area. A wire wheel and sanding paper was used to smoothen the beads post welding. In-house manufacturing - All operations, such as bending, cutting, welding etc were carried out in house at the institution. Contracting an outside establishment as was done in the past is great for time and convenience but little is learnt in the process. Thermal Processing – Heat treatment ensures uniform material property in the chassis after bending and welding operations. It also increases the yield strength and fatigue limit due to the fact that a desired hardness can be specified. Hardness was selected after studying hardness versus fatigue limit plots. To balance compliance with hardness, 35 Rockwell C was specified to the Body Cote facility in Rochester. Oil quenching was also specified to ensure that the metal did not cool too fast which can result in cracking. The team left the rest of the process to Body Cote since they were experts at it. Paint – The paint on the frame features the institutions’ colors of black accentuated with yellow. Power coating is specified for the rollcage. Powder coating chosen because of strong reasons. A) It is environmentally clean B) The paint is durable which makes it a good choice for a rugged vehicle. C) Due to the geometry of the rollcage, paint has to be applied both vertically and horizontally. Powder coating produced a consistent surface with little distinction in both these directions. D) In a plant making 4000 units of this vehicle, paint equipment costs can add up. Capital equipment and operating costs for a powder line are generally less than for conventional liquid lines. Power coating maybe stubborn for removal, however special liquids such as benzyl alcohol and techniques such as sand blasting can effectively remove the paint. The advantages outweigh the cons and this decided its use for the rollcage and suspension wishbones. BODY PANELS – Body panels accentuate the shape of the roll cage. To increase the ruggedness of the vehicle in off-road terrain, the body panels needed resilience and light weight. To attain these features, the best compromise was HPDE plastic. A .125 in. plastic piece was used for the bash plate. .0625” thick panels are used to cover the sides, roof and nose. Panels are made cutting them on a saw and then bent using an industrial heat gun. They are fastened to the rollcage mounting brackets using quarter turn fasteners, which provide a quick, easy way to service the vehicle. Only the firewall on the vehicle is metal to comply with the SAE rules. It is made out of 0.020 inch. thick sheet aluminum and fastened using dzus type fasteners. ALTERNATIVES CONSIDERED: 1. Some time was devoted to researching what is called ‘butted’ metal tubing. This type of tubing has thinner wall in the middle and thicker wall at the ends. This is the de-facto standard used in steel
bicycle construction to cut weight. Attaining tubing of such geometry for a rollcage may only a dream because such tubing is very expensive. Moreoever, suppliers stock them in certain pre-cut lengths mainly for the bicycle construction applications.. 2. The first iteration of the rollcage (Fig. B5) featured aesthetic curves at the front end of the car. Manufacturing considerations told designers that there was no tubing die available in-shop to create these shapes. Moreover, the exercise would be fruitless as bending such sharp angles without giving bend allowance would lead to buckling failure. Hence, a boxier design for the front end was settled upon.
ELECTRICAL OBJECTIVE – The goal of the electrical system is to wire up key safety components on the vehicle such as the kill switches and brake light. The wiring scheme will provide an easy to install pathway between source, load and ground. KILLSWITCH – Two SAE approved kill switches are incorporated in the vehicle. One kill switch is provided for the driver and the other for a crew member. The engine was wired to the kill switches using instructions provided by Briggs and Stratton. In previous cars, the driver side kill switch was located to the right side on the SIM. Common sense told us that in this position, the driver would leave his hold on the steering wheel to extend his hand to the switch. In this year’s car, the kill switch is mounted near the steering wheel on the steering wheel mounting tube. This way, the driver can operate the switch with minimal loss of focus. BRAKE LIGHT – A round 4” submersible LED light was used to signal driver braking intent. LED lights are brighter and last longer. [10] Careful attention was given to the mounting of the light. If the light is too high or too mid center on the rear, it could potentially create visibility problems for the drivers behind our vehicle. Hence, the light was mounted at an optimum distance between the two extremes without interfering with other equipment. POWER - A securely mounted 9 volt battery powers the brake lights. Standard 16 guage electrical wire in red and black color is used to connect the components.
CONCLUSION In this report, we discussed our primary goals in designing a competition ATV for the 2011 SAE Baja Intercollegiate Design series. Key points that can be seen in the report are summarized as below: 1. The main objective was to construct a vehicle optimized for performance at the off-road environment in Illinois. It would comply with the SAE rules and provide a safe, enjoyable experience for the consumer.
2. Major systems and subsystems were explained in a concise manner without information overload. These systems were arranged in descending order of priority. The passenger was given highest priority in the design process. Power train, suspension and tractive performance were very important areas of the vehicle design which followed. Finally, the layout of the inner systems and their packaging provided an idea for the rollcage. 3. Although the SAE regulations afforded design freedom in these areas, many design choices were made from logical reasoning and with safety and cost kept in mind. What we learnt was there is no one single solution to engineering problems. In the Baja design exercise, we noticed that some design ideas were not really bad but simply needed more time to be fully explored. 4. An important goal was to establish an organizational structure in order to tackle design and manufacturing related tasks involved in this project. Project responsibility was shared among 5 major teams. Each team recognized project interfaces, communicated well and operated within time and budget constraints. This results-oriented practice showed the institution that we are a young, but serious engineering organization. It also provides sponsors a financial incentive to support our engineering activities.
ACKNOWLEDGMENTS Certainly, a project of this scope would not have been possible if it weren’t for the students involved in the MCC Mini Baja club in 2010-2011. We wish to thank Justin Woodhouse, John Chiu, George Chiu, Kyle Youngs, Teo Ebratt, Sean Zimmer and several others who had enthusiastically put on the thinking caps for our work. We also wish to extend our gratitude to faculty advisors Richard Bucholz and Professors Bertram Gamory, John Wadach and David Leach for their unhindered support and advice. Lastly but not the least, we also would like to thank Monroe Community College and our sponsors for providing us with the funding needed to carry out this project.
5. Candian Center for Occupational Health & Safety, “Noise: Basic Information”, http://www.ccohs.ca/oshanswers/phys_agents/noise _basic.html 6. Articles Base, “Motorcycle Noise”, http://www.articlesbase.com/sleeparticles/motorcycle-noise-465957.html 7. Olaav, Aaen, “Chapter 2, CVT Tuning Handbook”, Aaen Performance, 2006 8. Aurora Bearings, “ABC’s of Rod Ends”, http://www.aurorabearing.com/Files/articles/ABCsof SphericalRodEnds.pdf 9. Dupont, “Delrin Design Guide”, http://www2.dupont.com/Plastics/en_US/assets/dow nloads/design/DELDGe.pdf 10. Popular Mechanics, “LED vs Incandescent”, http://www.popularmechanics.com/science/environm ent/will-led-light-bulbs-best-cfls-and-incandescents
CONTACT Ronald George (Team Captain) Major: Electro-Optics Email:
[email protected] Phone: (516)300-3102 Carlo Inglese (Team Treasurer) Major: Electrical Engineering Email:
[email protected] Phone: (585)506-5703 Daniel Sze (Team Secretary) Major: Environmental Engineering Email:
[email protected] Phone: (585)610-3819 Richard Bucholz (Team Coach) Phone: (585)503-7104
DEFINITIONS, ACRONYMS, ABBREVIATIONS SIM: Side impact member of the rollcage.
REFERENCES 1. SAE International, “2011 Baja SAE Rules, Part A Article 1” 2. Damon, Albert, “The Human Body in Equipment Design”, McGraw-Hill, Massachusetts, ISBN 0674414500, 1966 3. Autoblog, “Cornell's SAE Baja buggy helps us get even with would-be alma mater,” http://www.autoblog.com/2009/12/04/cornells-saebaja-buggy-helps-us-get-even-with-would-be-almamater/. 4. SAE International, “2011 Baja SAE Rules, Part A Article 2”
Kingpin Angle: The angle, measured in degrees, between A and B. A is the imaginary line drawn through upper wishbone outboard mounting point and the same point of the lower wishbone. B is a vertical line. Scrub Radius: The scrub radius is the distance in front view between the king pin axis and the center of the contact patch of the wheel, where both would theoretically touch the road. Remote Reservoir Shock: Shocks with an external reservoir that increases fluid capacity. This increase in capacity helps lower fluid temperature, thus offering the ability to negotiate rough terrain without experiencing shock fade (aeration) due to overheating.
APPENDIX A
Fig A1: Concept rendition of a baja car.
Fig A3: Packaging logic diagram of baja car. Wheelbase (in) Front Track (in) Rear Track (in) Front-Rear Weight Ratio Weight of car (lbs) Ride Height (in) COG Height (in) Overall Length (in) Approach Angle (deg) Castor (deg) Front Tire Diameter (in) Rear Tire Diameter (in) Width at Widest Point (in) Front Suspension Type Rear Suspension Type Steering Type Drive Type Transmission Type Overall Drive Ratio
Fig A2: Radar plot of desirable attributes in a new MCC vehicle.
60 55 53 40-60 400 12 <20 75 10 10 22 22 30 SLA Double Wishbone Twin-Shock Swingarm Rack and Pinion Rear Wheel Drive Continuously Variable 40:1
Fig A4: Rough specifications that were arrived at before detailed design began.
APPENDIX B
Fig B1: CAD model of the upper and lower wishbones.
th
Fig B5: First iteration of the complete roll cage with a 95 percentile passenger. The design went through another round of small modifications before it was deemed practical for manufacture. Fig B2: FEA analysis of shock mounting bracket design in a 10g landing scenario. The bracket was loaded at the mounting hole. The resulting factor of safety was 6.
Figs B3 (above) and B4 (below): FEA analysis of the aluminum upright in a 10g landing scenario. The factor of safety is 2.9. Shown in B4 is the swing-arm.
Fig B6: Plan view comparison of the old chassis (right) with the 2011 chassis (left) to show longitudinal and lateral decrease in dimensions.
APPENDIX C
Fuel Consumption (Gallons/Hour)
Gallons Per Hour Vs Engine RPM At Specific Load Levels 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 1500
25% 50% 75%
Fig C4: Total theoretical load on the chain when transmitting 10 hp.
Full Load
2500
3500
4500
RPM
Fig C1: Fuel consumption curve of the 10 hp Briggs and Stratton engine used to power the car.
Muffler Type Lo-Tone Standard Super Lo-Tone
3600 RPM Full Load No Load 84.5 79.5 81.5 77.5
3000 RPM Full Load No Load 82.5 76.5 79.5 74.5
Fig C5: A CAD mockup of major powertrain components. Fig C2: Engine noise level in dB(A) measured at 4 m. away from the unit.
Fig C3: Comparison of advertised power and torque numbers with dyno tested values. Due to the variability, all design calculations incorporated the more conservative of values to gain realistic results.
Fig C6: 6061 Al has an ultimate strength of 310 MPa and a yield strength of 276 MPa. The area under the stress strain curve multiplied by material thickness yields the energy absorption at rupture per unit width for the material. It was assumed that the stress strain curve was linear to the yield point and linear from the yield point to the ultimate strength, where strain = elongation at break.
APPENDIX D
Wheel Camber vs Wheel Travel Camber (degrees)
0 -7 -6 -5 -4 -3 -2 -1 0 -2
1
2
3
4
5
6
7
-4 -6 Wheel Travel (in) [Negative for droop, positive for jounce]
Fig D4: Organizational structure of the MCC Baja Team.
Design Spending In 2010-2011 2010
Fig D1: Front ront wheel camber variation with suspension movement.. The results come out of a purely rigid body linkage analysis in CAD.
3%
Engine
0% 5%
Transmission
11%
6%
Drivetrain Steering
10%
19% Suspension
0%
7%
Frame
8% 24%
7%
Total material costs in 2011 was $8300.00
Body Brakes Safety Equipment Electrical Equipment Fasteners Miscelleneous
Fig D2: This optimization plot helps choose the primary rollcage tubing that will pass SAE regulations. The window of choice is bound by the two vertical lines and the horizontal line.
Fig D3:: This optimization plot helps choose the primary rollcage tubing that will pass Rulebook regulations. The window of choice is bound by the two vertical lines and the horizontal line.
Fig D5: Financial resources available to each vehicle system.
Fig D6: Ganntt Chart for the 2011 Baja Vehicle Project.
