Race Car Engineering
2004
Good Handling is • Grip = tires operating at maximum adhesion to the track surface. • Balance = both ends of the car are operating harmoniously, the car instills confidence and is fun to drive. • Control = the car responds quickly and predictably to driver inputs, you can make the car do what you want it to do.
Grip • Those factors which influence the adhesion of the tire to the track – – – – – – –
Tire Temperature Tire pressure Camber Tire loading -- Balance Tire Compound Tire performance Grip enhancers and killers
Balance • • • • •
Weight transfer Relative Front and Rear Slip Angles Car rotation Consistency in fast and slow corners Transitions
Control • • • • • •
Steering response Stability Transition Responsiveness Progressiveness Driver comfort and confidence
Axiom 1 If you want to corner at 2 G’s you have to support the car for 2 G’s forces. Regardless of the individual setups, two similar cars cornering at the same speed will have to deal with the same forces.
Outline • Notes on chassis alignment & Record keeping • Grip = Tires: Slip angle and Grip • Balance = Suspension Geometry Static Analysis = Suspension Dynamics &Weight Transfer • Control = Chassis tuning – without shocks = Chassis tuning – bump rubbers and droop limiting = Shocks • Data Loggers = What have I done?
Myths • • • • • • •
Fix the end that has the problems first If a car pushes soften the front springs or ARB If a car is loose soften the rear More rebound will make a tire grip better Anti Roll Bars increase weight transfer There are soft setups and there are hard setups Increasing spring rates will make the car hard to drive • Just get out and drive or a different line will solve the problem
Facts • Get the car balanced FIRST. • Push or Loose, under-steer or over-steer are balance problems. • An unbalanced car is a car that is underutilizing the tires a one end and over utilizing the tires at the other end. • Setups must be tuned to the prevailing conditions.
More Facts •Most drivers can not tell the difference between an improvement in control and an improvement in grip •A car is supported during cornering through the combination of Springs, ARBS, Shocks, and Dynamic Suspension Geometry.
•The line a car takes through a corner is more dependent on the car setup than the driver’s technique. •Consistently fast cars are easy to drive.
•Good Setups are the product of hard
work done in a systematic manner over time. •Good record keeping of setups, details
of each session on the track, and a record of all changes will tell you what you did to make the car fast and help you correct the car when it is not fast.
94 FC Citation
Accuracy How accurate do you have to be?
Chassis Setups require • Accuracy: 2001 SCCA Runoffs percentage that 10
th
qualifier
was behind pole FV 1.4%, FF 1.2%, FM 3.5%, FC 1.5%, SRF 1.2% An acceptable tolerance should be no more than half the qualifying difference, 0.6%
• Repeatability: Same degree of accuracy, 0.6% • Ease of operation: If it is too hard or cumbersome you will not maintain the accuracy through out an event. The system must be portable.
You should do your work with a degree of accuracy that this race was decided.
Equipment You Make • Ride Height Gauge • Bump Steer Gauge • Alignment Flags • Toe Bars and trammel bars • Trammel Pins • Shock Struts
Bump Steer Gauge
Plate mounts to spindle
As the upright is raised or lowered any change in toe causes the plate to twist relative to the indicators. The indicators rest against the plate and move in or out as the plate is moved. This tool is accurate within a few thousandths of an inch.
Indicators
Base Plate Should be heavy.
Ride Height Gauge
2 Useful Tools Trammel Pin
Procedures - Sequence • • • • • •
Set ride height Zero toe settings Adjust camber Adjustments are inter related Adjust caster Zero toe settings and take new readings Repeat the process until correct
Flag Alignment System Use to set Camber, Toe, Ride Height
1.5 Square Tubing
20 in.
Measure the gap Plate
Alignment Notes • Do setups without driver but with representative fuel load. • Check the setup before and after running the car. • Have an alignment process for home and a checking procedure for the track.
Setup Work Sheet This is an example of a work sheet for recording the settings as you progress through the alignment process
For Those Who are Prepared • Record the lengths of all suspension arms: a-arm legs, toe links, radius rods, push rods, etc. • Keep records of all changes to any of these measurements. • Have these records available at the track in case a suspension arm needs to be replaced
Suspension Build Sheet The idea is to keep good and accurate records of the setup
Chassis Data Sheet This Sheet contains information to help make quick and accurate adjustments.
Corner Weight and Ride Height Once on the alignment pad proceed by:
• Resetting ride height – Front ride should be equal side to side – Rear ride should average the target
• Setting corner weights – Front weight should be equal – Remember that weight moves diagonally across the chassis
• Alternatively set the front weights even while keeping the chassis level (two scale system). • Repeat the process until weights and ride heights are equal with any discrepancy taken at the rear
Corner Weight Discrepancy
Record Keeping
Set Up Sheet Page 1 The Set Up Sheet should contain all the necessary information to reproduce the exact settings. Additionally, the set up sheet can contain any additional information to give a better understanding of the particular setup.
Run Sheet The Run Sheet is the record of all the on-track activity.
Additionally it may show the starting set up as a reference to help decide what changes are appropriate. When the session is over, the Run Sheet gives a complete record, showing the beginning, all intermediate steps, and the ending setup. The Run Sheet is the record of the performance for any given setup.
Run Sheet This is an example of the run sheet This sheet shows where the setup was at the beginning of lap 80.
It shows the lap times for each outing, the amount of fuel, the drivers comments for that outing, and the setup changes that were in effect for that outing.
Driver Debrief Sheet This is the formal and necessary review of the time on the track. Combined with the chassis set down sheet, this sheet is guide to the future changes in the set up.
This is the most neglected aspect of record keeping. The driver always has more pressing things to attend to such as girls and glory. This sheet when properly executed is the best key to a better setup.
An alternative form of the driver Debrief Sheet Using a map to make notes on is easier and sometimes more helpful. It is also easier to leave out important details. Make a check list to be sure that all details of the debrief are covered
Debrief Map
Chassis Set Down / Set Up The sheet records the ending set up and well as provides space to record changes for the next session.
Set Down
Gearing: How to select gears 1. Determine the lowest speed at which you can apply full power. This speed should correspond to maximum torque in first gear.
2. Determine the maximum speed any where on the course. Be sure to allow for drafting other cars and wind direction. 3. Choose the intermediate gears so that the RPM drop between successive gears is constant or declines slightly as speed increases. 4. When going up hill keep RPM higher and avoid shifting. When going down hill use lower RPM by shifting sooner or gear longer.
Gear Ratios This Sheet presents both graphic and analytical tools for analysis ratio selections. The right hand (shaded) portion of the charts shows critical speeds and the RPM for that speed in each gear.
The graph is a way of viewing the RPM drop for each gear change. The top chart has the ratios from the Setup Sheet. The lower chart is used as a work sheet to try alternative ratios.
Tires: Slip Angle and Grip
Tire Distortion – Slip Angle
How does a tire develop GRIP? Tire
Medium Grip
Track Surface Grip is the result of: •The inter locking of the tire and track •Adhesion of the tire to the track •Tearing force required to separate the tread material from the tire.
It takes time and pressure for rubber to conform to the track surface
High Grip
Carpet Plot:
Lateral Force vs. Slip Angle for given Normal Loads
150 kg = 331 lbs
Avon Tires Front Tires
Rear Tires
250 kg = 551 lbs
350 kg = 772 lbs
It is all about Grip Sustainable lateral force More grip is lost by unloading the inside tires than is gained by increasing the load on the outside tires.
Less weight transfer will result in greater total cornering force.
Things to Increase Grip: • Press the tire into the track surface harder. • Increase the time the tire has to conform to the track surface. • Increase adhesion between the tire and the track surface
Slick Zone of Tire Temps. •
•
•
Decrease in total grip
Lateral force due to friction between tire and track Lateral force due to adhesion of the tire to the track Combined lateral force generated by the tire and track.
30
50
70
90
110
130
150
170
190
210
230
The decrease in friction is due to the decline in the rigidity of the tread compound as the compound warms and before the tires begin sticking to the track.
Axiom 2
*When driving a car around a corner* 1. There is no cornering without lateral force 2. Lateral force produces weight transfer 3. Lateral force plus weight transfer produces Slip Angle Therefore Handling is about Weight Transfer and Slip Angles
Slip Angle Contact Patch Distortion Tire Foot Print
Steering: no slip angle Steering is Full Ackerman Geometry
Instantaneous Center of Rotation Balanced or neutral cornering Slip angles are equal
Cornering: 5 deg. Slip angle Corner radius is constant ( 20.7 ft.)
Outside tires have equal Slip angles. Thus neutral or balanced handling
Cornering: 8 deg. Slip angle
The steering angle of the front wheels is nearly constant for all the slip angle conditions (decreasing less than a degree).
Cornering: combined illustrations 0 degrees 5 degrees
This illustrates a balanced cornering condition.
8 degrees This is the path of the instantaneous center of rotation given a constant radius and varying slip angle for the outside tires.
Under Steer vs. Over Steer
Under Steer vs. Over Steer • As you increase your speed through a corner the steering angle should remain constant regardless of corner speed. • An under-steering car requires ever greater steering lock with speed. • An over-steering car requires less steering lock with speed.
Under Steer : unequal Slip Angles Center of Rotation Front Tires
Rear Slip Angle 5 degrees
Center of Rotation Rear Tires and the car as a whole
Front Slip Angle 8 degrees
Over Steer: unequal Slip Angles Center of Rotation Front Wheels
Rear Slip Angle 8 Degrees Center of Rotation Rear Wheels And for the car as a whole
Front Slip Angle 5 Degrees
Slip Angle Notice the tire distortion. The front tires are turned to balance the front and rear slip angles and adjust the corner radius. The car is tracking around the corner.
There is no cornering force without Slip Angle. Toe Settings and Camber Angles preload the Slip
Friction Circle :
Angle.
Friction Circle
2.5
Acceleration
2.0
• Ft max = maximum G of about 1.25 acceleration and 1.25 cornering.
1.5 Ft max
1.0
0.5 Lateral G’s Left Turn
Camber to left
-2.0
-1.5
-1.0
Lateral G’s Right Turn
-0.5
0.0
0.5 -0.5
-1.75
-1.25
-0.75
-0.25 0.0
0.75
1.25
1.75
2.25
-1.0
-1.5
-2.0
Camber Shifts the Circle, increasing the cornering in one direction and decreasing in the other.
1.0
2.5
2.0
1.5
1.0
Braking -2.5
0.5
1.5
2.0
Friction Circle and the
Combined G Graph
Speed Note: Slower and earlier entry to the corner along with the higher minimum and higher exit speed.
Combined Gs
Mid Ohio
Better utilization of the car’s potential
Friction Circle and combined G’s Speed
Note: the early setup and power application
Combined G’s
Throttle
Laguna Secca.
Combined G’s again Speed
Combined G’s Brake Pressure
Steering
Throttle
Note constant speed period and drop in combined G’s as the driver floats around the corner.
Tire management and understanding what they are telling you New tires care and feeding Inflation prior to being used. Set pressure as soon as possible after mounting. Inflate to stretch undersize tires. Use a tire record sheet - track miles, position,cycles. Direction of rotation.
How to read car balance before you hurt the tires Temperature across the tire. Temperatures front to rear. Tire pressure rise is more accurate indication of car balance.
How to maximize tire life Determine the balance of a car before the tires are damaged. Rotate tires after a predetermined number of laps. Dismount and reverse on the rim.
Recommended number of wheels - 14 - to reduce tire costs
Tire pressure management Getting the tire pressure correct is vital to good performance and life. Always use the same gauge. Set pressure of all tires in the morning. Record all changes in tire pressures. Use the record of tire pressure adjustments to adjust new sets.
Tire pyrometer use Where to stick the needle. Consistency is vital. Measure quickly after a run.
Tire Pressure Compensation
Tire Record If you have a tire man, make him work.
Steering Geometry: Balance & Control
Suspension Geometry: Steering • Camber • King Pin Inclination (KPI) also called Steering Axis Inclination • Scrub Radius • Caster • Caster Trail • Toe
Steering Axis Front View King Pin Inclination
King Pin Axis
Camber Angle
Steering Axis Side View
Caster Front Steering Axis
Caster Trail
Steering Axis Bottom View
Scrub Radius KP Axis at ground Steering Trail Steering offset
Pressure Center
Front
Steering Axis Three Dimensions
Front
Steering Axis: So What ? • Caster causes the front wheels to lean in the direction of the turn.
Caster and Steering Axis
Tire Contact Point
Steering Axis
Pure caster causes the tire to lean in the direction of the turn. Plane of Steering Axis
King Pin Inclination (KPI) Steering Axis
Ground
Plane of Rotation
KPI – Pure, no Caster Steering Axis
Spindle Steering Plane
Ground
KPI + Caster Scrub Radius = 1.5
KPI + Caster
Scrub radius causes longitudinal forces at the contact patch to generate torque about the steering axis. Caster combined with trail causes steer dive when the wheels are steered from center.
Steering Axis 1 •Caster combined with scrub radius causes the car to drop as a wheel steers in toward the center and causes the car to rise when a wheel steers outward. This in turn causes a weight transfer from the outside front – inside rear diagonal to the inside front outside rear. •King Pin Inclination (Steering Axis) causes both wheels to gain positive camber as they are steered away from center. •King Pin Inclination causes steering lift as the wheels are steered from center causing a self centering torque.
Steering Axis 2 •Steering Axis may be offset rearward from the wheel center to reduce Caster Trail and thus reduce the steering effort.
Steering Axis Offset
Caster Trail = 0
Example
Lola T97 Indy Lights
Straight Ahead
Suspension as in turn 1 MO 2.5 G’s lateral acceleration
Roll angle
.227 deg
Suspension displacement is taken from the damper displacement data.
Steering is straight
Roll Center is 1.366 Right and -.941 below ground
Steering geometry makes a difference
Roll was .227 Now .417 with steering input
Roll Center was 1.366 to the right Now it is 3.594 left As the car is steered the inside front suspension rises and the outside front falls
Full Ackerman Steering Geometry
All wheels steer about a common point
Any other geometry results in the a front tire skidding as the vehicle turns
The inside front is turned sharper because it must turn about a shorter radius.
20 deg.
25.7 deg.
Example: corner push with exit snap over steer • Driver says “entry all right, mid corner mild under steer, but corner exit the rear steps out”. • Tire temperatures are normal or front tires are cooler than the rear. Tire pressure rise for front tires is same or less than the rear. • Problem is that the car is under steering. But at the exit, as the steering wheel is straightened, the grip on the outside front tire increases causing the front to turn more in the direction that the front wheels are headed. Result the rear steps out.
Why does the grip increase? • As the steering angle is reduced the slip angle is reduced and it approaches the optimum for the load on the front tire. Thus the grip increases causing the tire to follow more closely the direction the wheel is turned. Refer to the carpet plots of slip angles and forces. • The inside front tire increases grip because the lower slip angle and increased load. • The load on the outside front tire increases as the steering is straightened because of the effect of the steering geometry. The inside front tire drops (ride height increases) as the outside front tire rises (ride height decreases). This is the result of the combined action of KPI and Caster. • Other symptoms: – Front tire temperatures lower than rear – High wear of inside edge of outside front tire
• Cure: – Stiffer front end more ARB or Front Spring – sticks the tire into the ground – Stiffer rear spring, not more rear rear ARB – increases the rear slip angle relative to the front – Higher rear ride height – more rear slip angle Stayed tuned for this will achieve the clarity of mud
Under Steer 1 • An under-steering car is slow to respond. – Therefore you must start your turn in early in order to make the apex. You can not drive the fast line with an under-steering car.
• Most drivers feel comfortable with some amount of under steer because their experience of over steer is the corner exit snap out type, unstable. • When a car is neutral, the steering angle is constant for a given radius turn even as the speed and slip angle in the turn increases. • Alternatively: the steering angle of a neutral car changes as the turn radius changes.
More about Camber
• Camber causes an uneven distribution of pressure on the contact patch. – The inner edge of the tire is compressed more than the outer edge. – The tire is composed of two springs (the side walls) supporting the contact patch (tread).
• The uneven pressure results in a lateral force, Thrust, in the direction of the camber. • The thrust preloads the slip angle causing lateral stability. • Camber increases grip by increasing the load on the inner edge of the tire. • Camber Shifts the friction Circle right thus increasing lateral grip. Thrust
More about Toe • Toe preloads the Slip Angle. • Slip angle creates lateral grip and lateral thrust. • Slip angle increases lateral stability. • Front toe out counteracts front camber thrust. • Rear toe out can help corner transitions. • On a driven wheel toe increases rolling resistance. • On a drive wheel toe has little effect.
Toe in
Toe out
Front
Chassis Balance Chassis Dynamics
Chassis Dynamic Movements: 6 Degrees of Freedom Movement Along the three axis's Longitudinal axis – Acceleration & Deceleration Lateral axis – Lateral Acceleration or Turning Vertical axis – Change in ride height Rotation about the three axis Roll about the longitudinal axis Pitch about the lateral axis Yaw about the vertical axis A vehicle suspension system controls the movements of the vehicle in all three axis’s, restraining the six degrees of freedom.
Chassis Dynamic Movements:
Controlling Chassis Dynamic Movement is the key to a
Balanced Setup. This is done by: • Controlling weight transfer, • Controlling slip angle development. Suspension Geometry, Springing and Damping are the main mechanisms to achieve balance.
Suspension Kinematics 1 • Instantaneous Center – The instantaneous point about which an individual wheel rotates in bump (vertically).
• Roll Center – The roll center is the intersection of the two lines formed between the tire contact patch and their respective instantaneous centers. – It is possible for the roll center to be outside the track of the car.
• Mean Roll Center Height – The point about which the body rolls. – The Mean Roll Center starts on the vertical center line of the chassis and at the same height as the Roll Center and can shift within the width of the track.
Suspension Kinematics 2 Suspension Kinematics is the movement of the wheels as constrained by the suspension geometry. Shown is a Single Wheel Front View Swing Arm (FVSA)
Instantaneous Center
Roll Center
Suspension Kinematics 3
• The roll center is the point through which tire forces act on the sprung mass of the car • Rollover moment arm is the distance from the Center of Gravity to the Roll Center. • Roll resistance arm is the distance from the center of the chassis to the pressure center of the tire at ground level because the springs and anti roll bars act at the tire pressure center. Shifts with the mean roll center. • Jacking is the vertical component of the reaction forces of the tire pressure center to the roll center.
Weight Transfer: first look Weight transfer is a function of; Mass Center of Gravity Location Wheel Base Track and the Forces Acting on the Mass
Lbs. transferred = ( lateral G’s) x (mass) x (c.g. height) (wheel base or track)
Springs, anti Roll Bars, and Shocks determine the rate and distribution at which the loads change on the four corners of the car.
Springs and ARB’s do not change the amount of weight transferred.
Suspension Dynamics
•Pull the handle slowly and the glass will move across the table. •Pull the glass faster and the glass will fall over. •Pull the handle fast enough and the glass stays put.
Suspension Dynamics Stationary
Un-sprung Mass Weight Transfer
CG
Pull Lateral Acceleration acts through the center of gravity (CG)
2 Wheels Stationary
CG
Weight Transfer
Lateral Acceleration
Pull Reactive Force / Grip Weight Transfer
Sprung Mass
Lateral Force
Roll Center (R/C)
Pull
Sprung Mass weight transfer with Springs Sprung Mass Weight Transfer
Lateral Force Un-sprung Mass Weight Transfer
Pull
R/C Traction Force acts through the R/C
Geometric Weight Transfer Jacking Force
R/C
Geometric Weight Transfer Jacking Force
Geometric Mass
Lateral Force Sprung Mass
Un-sprung Mass
Weight Transfer
Pull
R/C Traction Force acts through the R/C Traction/Grip Forces exert a horizontal and a vertical force. The vertical component is the jacking force.
Lateral Weight Transfer There are three components of lateral weight transfer • Un-sprung Weight Transfer = (un-sprung mass * Lateral Acceleration * non-suspended mass Cg height ) / Track • Geometric Weight Transfer = ( Sprung mass * Lateral Acceleration * Roll Center Height) / Track • Sprung Mass Weight Transfer = (Sprung mass * (Sprung mass Cg – Roll Center)) / Track • Geometric Weight Transfer is the source of the Jacking effect.
Jacking Effects: • Weight Transfer without roll effects. – Shock movement results from the vertical (up or down) component of the jacking effect. – This will affect roll angles. – Will affect mean roll center location
• Because the weight transfer is immediate, tire slip angles are impacted immediately – This will affect grip – This will affect balance • • •
1 / Chassis Torsion Rate = 1/ torsion rate front + 1/ torsion rate rear Chassis Flex reduces the effective roll resistance 1/Total Front Roll Rate = 1/Front combined rate of Springs and ARBs + 1/ Front Chassis Torsion Rate
Some Examples • Use Jacking to build tire temperatures • Changing the Jacking effect can change the balance of a car because of the rate of slip angle change. • Use ride height and jacking effect to vary handling balance in different corners. • Suspension pickup changes to change RC height and Jacking effects.
Kinematics Illustrated Rollover Moment Arm
Jacking Forces
Tire Reaction Forces
Lateral Acceleration
Roll Resistance Arm
Weight Transfer Lateral Acceleration
Variables for the Distribution
of
Weight Transfer (steady state) Roll Centers – Location plus Vertical and Horizontal movement Center of Gravity -- Location longitudinally (weight distribution) and vertically Roll Stiffness at the wheel -- springs, ARB’s, chassis trosional rigidity Tire Spring Rate – vertically and laterally
Longitudinal Weight Transfer • Anti Dive & Anti Squat = jacking in the longitudinal axis • Anti dive substitutes geometric weight transfer for sprung mass weight transfer. It does not stop or reduce weight transfer. • The suspension does not know the difference between longitudinal resistance due to braking and due to turning.
Balance made easy; just Change chassis rake
Ride Height - What Me Worry You bet • All cornering forces act through the roll center. • Springs, anti-roll bars, tire pressures, and shocks change only the roll resistance. • Ride height changes the rollover moment arm and jacking by changing the roll center location.
Setup Sheet for Mitchell Sim.
Mitchell Simulation
Mitchell Sim. 2 Ride Height -.25 in.
The original was 1.700 G’s lateral
What is .001 G? In a 50 mph corner at 1.700 vs. 1.701 G’s
• • • •
.02 feet per second 1.3 feet every minute 52 feet in a 40 minute race 3.5 car lengths at the end of the race
What is that worth to you? In a 90 mph corner that is over 100 feet for a 40 min. race
What is the cost of imbalance?
The red dot represents 3% unused potential in the right front. That is .14 ft./sec., 8.4 ft./min., or 336 feet at the end of a 40 min. race.
Summary of Sim. # 1 to 4
Deceleration due to turning in
Speed
Lateral Accel. Inline Acceleration 1 0 Gs -1
The car is decelerating at -.3 Gs from turning resistance alone. Steering
Brake Pressure
Mid Ohio
Geometry: no Anti Dive Axis's at chassis centerline Of the upper and lower A-arms
Instantaneous Axis of Front Suspension Upper A-arm Plane
Cg
Chassis Centerline Lower A-arm Plane
Geometry Anti Dive Instantaneous center side view Instantaneous center front view
Upper A-arm plane
Cg
A-arm axis’s
Front Longitudinal Roll Center Chassis centerline Ground Plane
Lower A-arm plane
Front Roll Center
Suspension Geometry in F1 Note the jacking effect on the front suspension
Enough Theory: Numbers you can use. • • • • • •
Spring Rates ARB Rates Chassis Torsion Rate Tire Spring Rate Motion Ratio = Spring / Wheel movement Velocity Ratio = Wheel / Spring movement
Allow us to derive the following:
Ride Rate / Spring Rate at Tire • Wheel Rate = (Motion ratio)^2*Spring Rate • Wheel Rate at the contact patch (WRc) = (Wheel Rate) * (Tire Spring Rate) / (Wheel Rate + Tire Spring Rate) or 1/WRc = 1/WR + 1/TR
• Ratio 1 = WRc / Corner Sprung Mass • WRc / Ground Clearance = Constant (close enough to be useful) – This allows you to calculate new ride height for any change of the WRc. From page 2 of the Setup Sheet Excel Workbook
Roll Resistance Those things that resist roll • Spring Wheel Rates in ft. lbs./degree = (Wheel Rate * ( Sin(1) * Track) / 12) • Anti Roll Bar Wheel Rates in ft. lbs./degree = Vertical Spring Rate of ARB * ( Sin(1) * Track) / 12 • Roll Rate of Tire at Tire Contact Patch in ft. lbs./degree = Tire Spring Rate * ( Sin(1) * Track) / 12 • Chassis Torsion Rates in ft. lbs./degree (this is calculated in ft. lbs. / degree) • Total Resistance at one end is the sum of 7 spring rates.
Magic Ratios Spring Rate Split = Front Spring Rate / Rear Spring Rate Roll Stiffness Split = Front Roll Stiffness / (Front Roll Stiffness + Rear Roll Stiffness % Heave = Front Spring Rate at the Tire / (Front Spring Rate Tire + Rear Spring Rate Tire) *100 % Corner Rate / Corner Weight = (Spring Rate/ Sprung Corner Weight) * 100 % Corner Roll / Corner Weight = (Roll Rate / Sprung Corner Weight) * 100
Set up Sheet Roll Stiffness This sheet is derived from the setup sheet. Here is an analysis of the set up that is represented by the set up sheet.
Spring Rate Change Changing Spring rates without changing Ride Height results in a change of both Spring Rate and Dynamic Ride Height.
Double Spring Rates and Balance the car by adjusting ARBs
Sim. 5 Spring Change
Summary of Sim. # 1 to 5
Sim. 6 ARB Change
The front ARB is increased to max. that would still maintain 1.700 G’s
Sim. 7 ARB change
Rear ARB is removed and the car is balanced by adjusting the front ARB.
Summary of Sim. changes
Set Up Sheet
Page 3
Suspension Friction & Torsional Rigidity 2 overlooked and little understood variables
Suspension Friction • Suspension friction is the resistance of the suspension system to any movement. It can be measured and Friction kills grip. • Grip is lost because small changes in tire loading are not absorbed by the suspension system.
Friction Test the Suspension • Press the chassis down and release the pressure. • Measure the ride height. • Lift the chassis and let it settle gently. • Measure the ride height. • (The difference in the ride height) * 2 * (spring rate at the wheel) = Force required to move the chassis.
Excessive Rebound or Suspension friction • Excessive Rebound does not allow the tire to follow the ground – At speed and over short time intervals the chassis is rigid in space preventing the tire from following depressions in the track surface
• The car jacks down over successive bumps over a short time period. – The frequency of bumps is greater than the chassis frequency of response. – On successive bumps the force required to displace the suspension increases because of the residual energy retained by the shocks and springs. – The car skips from top to top.
• The lack of compliance causes the contact patch rubber to loose grip on the surface of the track. With a under sprung car, adding the rebound feels good because the car gains support and is more controllable and this improvement is sufficient to mask the loss of grip.
Chassis Stiffness • Chassis stiffness determines the amount of roll resistance that can be developed. • The springs and ARB effectiveness is reduced by the torsional rate of the chassis. • A weak chassis requires higher spring and ARB rates. These rates may be too high to give acceptable ride and grip levels.
Chassis Torsion
For a chassis of 1500 ft.-lbs./deg. the roll rates are Front 454 and Rear 315
ft.-lbs./deg.
Control: Requires that driver inputs result in changes in chassis attitude. • • • •
Steering response Stability Transition Responsiveness
Vary directly with torsional rigidity
A weak chassis lowers the frequency of the chassis in roll thus the time/distance it takes for a driver input to result in a change in attitude increases
FEA model of 94 Citation FF/FC frame
94 FF
94 Citation at Mid Ohio
Spring Preload or Droop Limiting • Any spring preload changes only the amount of droop before the shock tops out. • Preload that exceeds the load on the spring to support the loaded car results in zero droop. At this point the suspension moves only when the preload force is exceeded. • When the shock tops out the dynamic loadings change: – – – –
Roll resistance from springs and ARB is unchanged, Roll resistance from tires is unchanged, Mean roll center shifts toward inner tire pressure center, Jacking force decreases and roll center declines.
Spring Pre Load Chart
Thread Pitch = 1 / number of threads per inch Load = ((# of Flats) + ( # of Turns / Flats)) / ( Thread Pitch / Flats per Turn)
Droop Limiting • Physical roll center is inside tire ground level • Additional roll results in decrease in ride height • Change in roll resistance – the decrease in ride height reduces jacking effect • Change in weight transfer or wedging
Droop Limited Rear Suspension Indy Lights at Mid Ohio, Scott Dixon driver Left Front
Right Front Speed Right Rear Shock topping out
Left Rear
Damper Movement in mm. 0 = full droop
Notice Change in Ride Height with Speed
Bump Rubbers: use them carefully • Self dampened springs • Rising Rate springs
The spring rate is a function of diameter, length and the material used. •Larger diameter = higher spring rate •Longer length = less change in rate with displacement •By shaping the race can be altered
Dynamics
Ohlins
Penske
Displacement ( ins.), loads (lbs.), and rate (lbs./ins.)
Disp.
Load
Rate
Load
Rate
Load
Rate
.039
2.2
56
10
254
10
254
.079
16.5
363
32
559
20
254
.118
32.12
397
50
457
28
203
.157
58.96
682
63
330
32
102
.197
91.74
833
80
432
38
152
.236
148.5
1442
96
406
42
102
.276
247.5
2515
48
152
Bump Rubbers
Bump Rubbers and Packer
Packer in 1/8th in.
Fix first problems first Straight line stability Braking Corner Entry Mid Corner Corner Exit Be certain you identify where the problems start. Be sure to analyze the entire corner instead of concentrating on the problem area.
Tuning at track Ground Clearance: Initial setting of the ride height. Spring rates: Road springs, ARB’s, Bump Rubbers,Tires pressures Suspension travel: How much bump and droop – preload Ride height: Adjusting Center of Gravity, Roll Centers, and Jacking Effects
Tire Presentation to the ground – contact patch Camber, caster, KPI, Roll Toe, Tire Pressure Aero Loads and Balance – Flat bottom ground effects Rake and ride height Weight Distribution and Moment of Inertia in Yaw Fuel load, ballast position, driver seating position
Algorithm for chassis adjustments • Identify the problem to be solved. • Identify the handling features not to change. • Identify the attitude change that will solve the handling problem. • List the changes that will lead to the attitude change. • Evaluate the proposed changes. – Will it fix the problem. – What are the adverse effects from the change.
Chassis Changes • Most solutions will require changing two or more variables. • Make sure that when you make a change that you identify all the variables you have changed. – Spring changes require ride height changes. – Wing changes require ride height changes. – ARB changes may require spring changes.
Real World Examples 1 • Corner exit over steer – Is it throttle induced? • Change balance of rear ARB and rear springs • Rear ride height
– Is it steering induced? • More roll resistance front • Run Sheet information: – Entry is good, mid corner is alright, but I can’t apply power without the rear stepping out. The tire temps show a slight push avg. front temps 15 degrees higher than rear. • What to do?
– Entry alright, mid corner same, but as I exit the corner the rear steps out. Temps show a slight under steer. • What to do?
Real World Examples 2 • High Speed over steer / low speed under steer – Insufficient roll control: stiffer springs and/or ARB’s – Goal is to get the car to under steer every where then fix that problem. – Adjust roll center through ride height changes.
• Run Sheet information: – Car is not too good. I have a hard time getting the car to turn in to the slow corners and get down to the apex. The fast corners the rear wants to come out about mid corner of later. Tire temps show push and are not particularly high. • What to do?
Elkhart / Gingerman Springs Example 3 Over-steer in mid corner power on at both tracks.
•Gingerman sweepers
• Elkhart Carousel Track induced Over Steer
Throttle induced Over Steer
Increase spring rate & Lower Rear Ride Height Lower Spring rate & Raise Rear Ride Height
Under-Steer: a second look Re think • Under-steer because the front tires are under utilized. – Stiffer front roll resistance
• Under-steer because the front tires are over stressed. – Less front roll resistance or more rear roll resistance
• Driver induced under-steer. – Does the driver lack confidence in the setup? – Can the driver change technique?
Driver induced Under Steer
Difference in minimum speed
Higher lateral G loading late in the corner
Problem: Carrying too much speed into the corner. Not getting enough rotation or yaw in to the car prior to apex. Too much of the cornering effort is done near the corner exit
Wings
Aerodynamic Devices - basics Ground effects – • All cars are ground effects cars. The larger the plan area of the car the greater the down-force possible.
• Center of Pressure varies with chassis rake (pitch) • Down-force varies with ride height.
Wings – • Front wings and end plates • The drag from the front wings is offset by the reduction in the drag associated with the rest of the car. Front wings frequently stall at angles of 6 degrees.
• Rear Wings and end plates • The air flow to the rear wing is seldom horizontal but is down swept from passing over the rest of the car. Thus the optimum attack angle might be nose high. Rear wing drag increases with down-force.
• Measuring wing angles • Include the Gurney/wicker in the wing angle. Down force will be close when the angles are equal with and without Gurney’s.
Wings:
Rear Wing
Front Wings Dual Element Upper Gurney Flap
Slider
Single Element Lower
Dual Element
Mitchell Sim.: 100 lbs Down Force
Flat bottom ground effects, winged formula car
Ground Effects Flat bottom with diffusers and tunnels
Adjust Ride Height with air density • Standard Atmosphere - 29.95 Hg and 15 C (68 F) • Barometric Pressure – lower pressure is lower air density
• Temperature Adjustment – Higher Temp less down force
• Humidity Adjustment – Higher humidity lowers air density
• Ride height / air density ratio is constant
Aero Adjustment of Ride Height
Real World Example 4 • Push Low to Medium Speed Turns. – Imbalance in high speed and low speed handling. – Goal: better low speed balance without upsetting the high speed balance
• Solution: – More rear wing and higher rear ride height. – The improved corner exit speed offsets the loss in top speed.
Fundamentals of Shock Absorbers
Various types of shocks •Double or Twin Tube •Gas Filled with internal and external Gas Chambers
Types of adjusters •Needle valves adjusters •Spring preload adjusters •Blow Off adjusters
Canister pressure •Why canister pressure •Low speed bump adjuster
Piston Style •Linear or Digressive •Flat or Dished Face •Cupped Face •Bleed Holes &/or Bleed Shims
Adjustment Ranges •Low Speed •Mid range •High Speed
Damper Fundamentals (2) Piston Style 1. 2. 3. 4. 5. 6. •
Linear High-flow linear Digressive / linear Digressive / digressive Velocity-dependent (VDP) Digressive blow-off Bleed holes &/or bleed shims
Adjustment Ranges • • •
Low-speed – piston or needle bleeds Mid-range High-speed
1
2
3 BLEED EFFECT
4 BLEED EFFECTS
5
6
Rebound
Canister in compression or bump
Penske 8760
Compression or Bump
Ohlins T44
Rebound
Shock Movement
Shock Velocity
Shock Histogram Most damper motion is at velocities below 1 in/sec. This is the most critical range to develop grip and handling
Compression ends and rebound stroke starts Crank is at top Compression reaches max. velocity This is the midpoint in the stroke Gap shows gas pressure effect
Compression stroke starts
Force vs. Velocity Gap shows the hysteresis
Rebound reaches max.
Average Force vs. Velocity These curves are the average of the curves in the previous slide 1 in/sec. Max velocity
5 in/sec. Max velocity
These curve shows the same shock but at a different crank rotational velocity
Dyno crank position bottom of stroke
Crank is at max velocity compression stroke Stroke at 5 in/sec.
Note the sloop and sharp brake on curve Stroke at 1 in/sec.
Max velocity Rebound
Force vs. Displacement
Dyno is at top of stroke
The offset shows the nose pressure or gas pressure that the shock exerts on the dyno load cell..
Gas Pressure NOT Zeroed
Rear Shock @ 5 in/sec.
Rear Shock @ 1 in/sec. Front Shock @ 5 in/sec Front Shock @ 1 in/sec.
Front & Rear Shocks
Critical Damping • That amount of damping which results in the quickest stabilization at new position. • That amount of damping that will maximize GRIP
• That amount of damping that stops the chassis over shooting.
Rebound Kills Grip • Insufficient Rebound damping allows the chassis to over shoot as the spring extend. – This results in a loss of grip as the wheel follows the chassis upward.
• Excessive rebound retards the downward movement of the wheel. – This results in a loss of grip as the wheel follows the chassis and not the road surface. More rebound does not make the tire stick to the ground
Shock Tuning 1 High speed canister bump -- is there enough support or too harsh over bumps Low speed Canister Bump – too stiff or too soft , too harsh or not enough support Rebound – always try to use the least amount of rebound possible. More rebound than optimum reduces grip. Low Speed Bump (bleed) – optimizes grip. With too much bleed the car does not respond and feels unsupported. Too little bleed reduces grip and causes the tire alternate between grip and slip. Adjust Low Speed Bump and Rebound together, both stiffer or softer, to optimize damping for track conditions.
Shock Tuning 2 Adjustment Location
Front
Rear
More Compression High-speed
More front unsprung mass control, possible excess suspension loads over bumps or curbs, possible loss of grip over bumps More rear unsprung mass control, possible excess suspension loads over bumps or curbs, possible loss of grip over bumps
Low-speed
More Rebound High-speed
More Canister Pressure
Larger Bleed Area
Low-speed
Less front chassis Better front un- Less front chassis Shallower nose drop, less trailing- sprung mass rise, less power- More front height angle, more front throttle oversteer, control, possible on understeer, control, possibly grip, possible loss possible loss of loss of front grip possible loss of less front grip of low-speed front grip over bumps grip chassis control
Less rear chassis Better rear un- Less rear chassis Shallower nose drop, less powersprung mass rise, less trailing- More rear height angle, more rear on understeer, control, possible throttle oversteer, control, possibly grip, possible loss possible loss of loss of rear grip possible loss of less rear grip of low-speed rear grip over bumps grip chassis control
Data Loggers • What data loggers won’t do: – Data loggers won’t tell you what’s wrong, – Data loggers won’t tell you what to do.
• What will a data logger tell you: – Data logging tells you what the sensors read at various points around the track and in a fixed time frame.
• You must interpret the data. • The data loggers record the result of the interaction of the driver, the car and the track. • The data logger takes a snap shot of all the sensors at a point in time, often recording the readings sequentially. • The driver and the data logger are seldom in sync with each other. The driver operates in anticipation of what he expects the car to do and adjusts according to his senses.
• Car Performance Channels – Time recording hertz – Speed - wheel speed sensor – Accelerometer – lateral and longitudinal
Data Logging 2 Track Map
• Driver Performance Channels – Steering wheel movement – Throttle position – Brake Pressure
Fundamental Analysis
• Other Data Channels – – – – –
Damper Pots Strain gauges Ride height sensors Gyro Vertical accelerometer
Advanced Analysis Math Channels
Math Channels • Wheel Movement = Damper motion * Motion Ratio • Chassis Roll = (L Wheel mvt. – R Wheel mvt.)/ Track • Chassis Pitch = (LF Wheel mvt + RF Wheel mvt) / 2 – ( LR Wheel mvt + RR Wheel mvt) / 2 • Speed Steer = Steering Angle * MPH * (MPH)^.5 • Corner Radius = (1.467 * MPH)^2 / (32.167 * Lateral G’s) • Combined G’s = ((Lateral G’s)^2 + (Longitudinal G’s)^2)^.5 • Gear = MPH / (RPM / 1000)
