Race car suspension Class

In summary,-The stock car suspension is important for understanding the complexity of a Formula Cars suspension.-When designing a (front) suspension, geometry layout is critical.-spindle choice and dimensions, kingpin and steering inclination, wheel offset, frame height, car track width, camber change curve, static roll center height and location and roll axis location are major factors.-The first critical thing to do is to establish the roll center height and lateral location. The roll center is established by fixed points and angles of the A-arms. These pivot points and angles also establish the camber gain and bump steer.-I have used Suspension Analyzer for years on Super late Model stock cars as
  • #1

Ranger Mike

Science Advisor
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references _ Paved Track Stock Car Technology by Steve Smith
Tune to Win by Carroll Smith
Software - Suspension Analyzer by Performance Trends
In order to understand the complexity of a Formula Cars suspension, a basic knowledge of the stock car suspension should first be mastered. When designing a (front) suspension , geometry layout is critical. spindle choice and dimensions, kingpin and steering inclination, wheel offset, frame height, car track width, camber change curve, static roll center height and location and roll axis location are major factors.
The first critical thing to do is to establish the roll center height and lateral location. The roll center is established by fixed points and angles of the A-arms. These pivot points and angles also establish the camber gain and bump steer.

I have used Suspension Analyzer for years on Super late Model stock cars as well as formula cars and it is as critical as a tire pyrometer and stop watch, in my opinion..Saves tons of time figuring the roll center and will show the roll center movement as the suspension moves through its travel.


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  • #2
Hoorays for Mike.

How do you want to run this then, do you want to pick a topic and run with it or get people to ask questions?
EDIT: Nevermind. :D

That list looks good Mike, its got all the bases covered. I'd deffo do the second list at some point, as that's where the juicy stuff is. The wheel rates and leverage for spring rates is where I'm most rusty so i'd request that to be included in the future.

And btw thanks in advance. :D
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  • #3
Topics I will try to cover
Front Roll center ( what it is , and how to calculate it)
Instant Center ...ditto
Roll center offset
Designing the suspension mounting points
Kingpin Inclination
Scrub Radius
Camber ( camber curve too)
Ackerman Steering
Bump Steer ( measuring and setting)

later ifin you want it
Mass Centroid
Equal links and parallel links
Unequal and non parallel links
Long links vs short links
lastly...and a whole other thread is wheel rates and proper spring selection..huge!
befroe i get going...give me yer input so i don't go off the track into the boonies
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  • #4
Roll center height determines what percentage of the overturning moment 9 inside to outside weight transfer) will be distributed onto the tire contact patch a downforce, and wha tpercentage isrecieved as lateral loading against the tires tread face. Vertical laoding creates downfroce on the outside tire so the more vertical loading there is the better the outside tire sticks during cornering. This downward loading is why the tire traction increases as the track banking angle increases. the lower front roll center will create more vertical loading on the outside tire contact patch. The higher roll center will oad the transferred weight more horizontally, which creates a shear force at the tire contact patch.


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  • #5
A car needs body roll during cornering to transfer weight downward onto the outside tire contact patches. This is the result of a lower front roll center. if weight was transferred laterally to the tires the rubber would shear across the track surface and the car would slide out...or , in round track tech terms..it would push like a freight train. No grip!
typical ft RC on paved track stock car is 1.5 to 2.5 inch above the ground and offset 3 inch to the right of the car centerline. Formula Cars have RC about 0.5" above ground or lower and centered. The upper and lower control arms should be placed so that the instant center is 1 to 2 inches inside the opposite lower ball joint.
You can manipulate the instant centers by:
changing spindle height
mounting points of the control arms
length of control arms

Instant Center (IC) width controls how the roll center (RC) acts during body roll. The wider the IC width, the less negative camber gain achieved during body roll. A narrow IC width creates more radical change. shorter IC width also makes the RC height move up and down radically during body roll, which majorly? effects front tire loading during corner entry and mid turn. as with everything on a race car ..it's all about compromise. keep the RC location as stable as possible and the IC from radically changing during body roll.

RC offset- round track cars turning left offset the RC to the right side to add leverage (jacking effect) during turn entry to further stick the tire..this is a no no for road course cars ..you want the RC centerlined and moving vertically up and down and not wandering to the left or right during body roll.

other factors like Center of Gravity (CG) and length between the RC come into account..this is a whole other discussion on engine location and will save until later..but on the old late model stockier we used the camshaft location as the CG which was real close to actual. Next up for discussion is designing suspension mounting points..but am out of beer...gotta go!
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  • #6
Designing suspension mounting points- ifin you do not have access to the software I mentioned and you do not yet have the car built, you can pick up the old Number 2 pencil and start drawing.
1. Use a 1/4 to one scale.
2. draw the ground line ,vehicle center line and center of the left and right tire contact patches. Determine where the outer lower control arm ball joints (BJ) are located by bolting the upper and lower control arms to the spindle and bolting the spindle on the wheel to be used...some round track cars have different wheel offsets so be careful. mark these BJ centers on the drawing.
3. determine desired roll center location such as 2 inches above the ground and 3 inch to the right. mark the point on the drawing.
4. IC of the left and right control arms should be 2 inches inside the opposite lower BJ so draw a vertical line ..will cal this the IC vertical plane.
5. determine location of the pivot center of each upper BJ. with the upper and lower BJs bolted to the spindle measure from the lower BJ center to the upper BJ center
6. before the location of the upper BJ centers can be marked on the drawing the steering axis angle has to be calculated. this is the kingpin inclination of the spindle and the amount of static camber that will be used. we use a 10 degree kingpin angle and the initial negative camber setting at the right front is 3 degrees. So an angles vertical line is drawn from the fright front lower BJ centerline at 13 degrees. at the left front a 10 degrees spindle is used and initial setting is 1.5 degrees ( tilts the top of the spindle away from the centerline) so 10 degrees minus 1.5 degrees is 8.5 degrees. draw a vertical line from the left lower BJ center at 8.5 degrees.
7. draw a line from the center of the right front tire patch through the RC to the IC vertical plane on the left side. the point of intersection is the IC for the right side.
8. draw a line from the right front IC to the right ft. lower BJ center.
9. draw a line from the right front IC to the right frt. UPPER BJ center
10. draw a line form the center of the left front tire contact patch and repeat step 7 applicable
11. repeat step 8 for the left side
12. repeat step 9 as it applies to the left side


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  • #7
13. Now that the lines have been drawn form the IC on each side to the upper and lower BJ centers, these lines which converge to the IC will dictate the planes on which the inner pivot points must be located for the upper and lower control arms on each side. these pivot points must fall on the lines.
14. the only thing left to do is the length of the upper and lower control arms on each side. Note; make sure you check the rules of the sanctioning race association. Some organizations have maximum wheel base and track width as well as minimum wheel base length and width and you don't want to construct an illegal race car...
15. the location of the lower control arm pivot points will dictate by the length of the steering rack being used or vice versa so be aware of this if rules restrict this. The inner pivot mounting points of the steering tie rod must be straight in line with the lower control arm inner pivot points so that the bump steer will be correct.
16. to determine the upper inner pivot points location we have to work out the required length of the upper control arms. we do this by working from the desired camber gain.
First we have to determine the location of the lower and upper outer pivot points when the suspension is moved 3 inches in bump travel. draw a horizontal line 3 inches above the lower outer pivot points. use a compass to swing an arc about the lower inner pivot points making an arc to meet the 3 inch line just drawn. this intersection is the lower BJ center when the suspension travels 3 inches in bump.
find out where the upper BJ is located by first drawing a horizontal line 3 inches above the upper outer pivot points, the desired camber gain for this race car is 4.25 degree at 3 inch bump travel. add this to the steering axis angle (13 degrees) which makes 17.5 degrees. draw a vertical line at 17.25 degrees wit ha protractor, from the line at the lower BJ center elevated 3 inches. where the angled vertical line intersects the 3 inch upper horizontal line is the desired location of the upper BJ at 3 inch of bump travel.
the inner pivot points location for the upper control arm is determined by swinging arcs about different locations of the upper control arm IC line until the correct angular change is found. the correct angular change will connect the starting upper BJ pivot points wit the intersection of the 17.25 degree vertical line and the 3 inch upper horizontal bump travel line. unless you know the most popular length used for your control arm finding the correct upper control arm length is a matter of trial and error.


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  • #8
Reasons for having low Roll Centers ( RC) - I can not say this too often...Racing is about Tires, Tires , Tires. All efforts are to provide the best tire contact patch for the longest period of time and making sure the car finishes. To this end, it s all about planting the tire with enough downforce to permit the fastest corner turn entry, fastest mid turn time and fastest turn exit traction. Tire compound is a critical factor. I could write a book on this but let us assume we are stuck with a hard compound tire..Duometer reading around 85 hardness. Let us also assume we can not manipulate Mass placement in the race car ( can not offset the engine, and rules dictate minimum engine height, percent left side weight, percent front to rear weight. The most critical element is to have the best balance between Mass placement and RC location so that the car turns in the middle of the corners. Sufficient weight must be transferred to the outside tires to create vertical downforce.
Jacking Effect- This is the reaction of the outside tire force transmitted to the RC pushing it up ward during the turn. Imagine a poll vaulter going up over the bar. the poll vaulter is the RC. The pole is planted at the outside of the outer tire patch. The pole vaulters forward motion in comparable to the centrifugal force acting on the cars body during cornering. The greater the forward motion of the pole vaulter, the greater the height attained..comparably the greater the centrifugal force cornering, the more JACKING EFFECT and the higher the RC is raised. the lower the RC, the less jacking effect. RC located at ground level have zero jacking effect.
If this is not enough to make your head explode..there is one more major thing to consider. The distance between the Center of Gravity (CG) and the RC will effect the handling. This is best covered in Spring selection since the springs counter body roll as well as the anti roll bar ( sway bar). Suffice it to say the closer the distance between the CG and RC requires stiffer springs.
Bottom line is that cars with high CG have more body roll. Harder compound tires require lower RC combined with softer springs to create vertical downforce so lower RC creates more body roll and provides the traction and side bite that hard tires require.


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  • #9
time for me to take a break...you guys got any questions?
  • #10
Kingpin inclination is the angle between true vertical and a line drawn through the upper and lower Ball Joints (BJ). On stock cars it is about 10 degrees. Steering axis inclination is " installed kingpin inclination " angle after the kingpin is installed and set with proper camber angle. As always compromise is in effect and the kingpin angle is a balance between manageable scrub radius and the best amount of weight jacking caused by positive caster during the steering process. Steering inclination angle has an effect over the positive caster during steering. Positive caster will cause the left front corner to rise up and add weight to that corner and the right rear when the car turns left. and vice versa when the steering is cranked to the right. the steering axis angle multiplies this weight jacking effect. The greater the steering axis angle the greater the weight loading caused by positive caster. When this steering axis angle is projected to the ground and you measure that point to the centerline of the tire patch of the applicable tire, we have the Scrub Radius.
Scrub Radius- this is the tires turning radius about the steering axis. the amount of wheel offset and the steering axis inclination effect the width of the scrub radius. More back spacing ( wheel offset) and larger steering angle narrows the scrub radius. if a car has no scrub radius the car will act darty and will react too quickly to change. too much scrub radius will heat up the tire and cause premature wear. Scrub radius provides " feel" or feedback to the driver. race cars wit h10 inch front tires usually use scrub radius between 3.5 and 6 inches. less than 3.5 inch means no feed back and a " darty " car.
Camber- Going back to what wins races..it's all about tires. Camber is used to maintain the most tire contact with the track surface. Camber is the Tilt of the front wheel. Zero camber angle means the wheel has zero vertical angle to ground. If the tore is leaning inward ( as viewed from the front of the car) is has negative camber. Now things get hairy.

Camber Curve - This is a graph plotted out showing camber in degrees as the race car goes through suspension travel from bump to rebound..usually 3 inch. We use a camber gage and two dial indicators and a wheel plate alomng with a bottle jack to check this along wit hbump steer. On flat tracks to medium banked tracks ( 0 to 12 degrees) most stock cars go through 4.25 degree camber change over 3 inches. Or 1.42 degree per inch. On high bank tracks ( 13 degree and up) 1.25 degree chamber change of bump travel is the norm. On Formaul cars it is a lot less as the attached illustration showes. Upper A-arm (control arm) length effect this camber build. Shorter arms build radical camber change since the shorter arm moves through a tighter arc.
Camber curve factors-
Roll Center height- lower RC means less camber build per inch of bump travel.
Body roll - the more body roll the more negative camber gain needed to keep the tire contact patch.
Spring stiffness effects body roll etc..
Tire type- the taller the tire and softer the sidewall means more lateral deflection of the tire and this means more negative camber is needed. also lateral displacement of the tire at the contact patch effects this. the relationship of the tire width and the wheel rim effects sidewall deflection. This is why a tire pyrometer is critical in assessing the chamber situation.


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  • #11
Caster - this is the inclination of the steering axis from vertical as viewed from the side. Positive caster is when the top of the spindle steering axis is tilted to the rear of the car. Positive caster puts a positive feel in the steering wheel an is a self centering aligning torque. This is what permits a bicycle rider to take his hand s of the handle bars and the front wheel still goes straight. as am old racer once told me... too much positive caster builds the drives arms..if you ever drove a car with too much caster, your arms would ache after a few laps. Negative caster does just the opposite so never use it. When a car has positive caster and turns left, the left front corner will rise and the right front corner will dip. It is pretty easy to adjust caster so that you have zero gain or loss. To correctly measure caster you need a camber / caster gage and the ability to turn the front wheels in 5 degree increments.
On round track cars we run cater split because we are always turning left. this means we run 1+ degree on left front and +3 on the rt. front on manual l steering and 1+ degree on left front and 4+ degrees on the rt front on a power steering car.

Toe - Out - this is the difference between the vertical center of the front of the rt. and left tire vs. the vertical centerline of the rear of the right and left tire. this is usually 1/6 to 1/8 inch. more toe out scrubs the tires causing excess wear..too little will make the car darty. Toe In was popular with production cars because the rubber bushings used to mount the A-arms would flex when tires rotated and go toward a slightly more toe out position at speed.


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  • #12
Ackerman is the difference in turn radius between the front tires. On oval track cars it can be desirable to create a situation where the left front tire turns faster than the right front tire. The Ackerman effect can help the car turn better through the center of the turn since it builds TOE OUT dynamically, thus eliminating the requirement of running a lot of static TOE OUT That twill make excessive drag. You can measure the amount of Ackerman you currently have by using a set of turn plates. Typically, Ackerman is measured by turning the right front 10 degrees to the left. If you have Ackerman, the left front will travel further than the right front. A typical amount would be three degrees in 10 degrees of steering. To simplify, moving the right front from zero through 10 degrees of steering will cause the left front to move say 13 degrees in this scenario.

Ackerman is created by your front end geometry. Tie rods that angle forward from the inner pivot point out to the spindle will have more Ackerman.

You can usually adjust the Ackerman by moving the left front tie rod end in a slotted spindle arm. Moving the tie rod end closer to the ball joint will create more Ackerman. Offset wheelbases have an effect as well. On 3/8 mile and under tracks more Ackerman is usually more desirable. On 1/2 mile tracks and above less is generally needed. Just like with rear stagger, too much Ackerman will make the car loose on turn exit or will cause premature tire wear. Too much Ackerman can over heat the left front so that it will not perform on the long run. The amount your run depends on your set up and the track.

Sometimes you can see the effects of excessive Ackerman by inspecting the wear pattern on the left front. If you see a graining pattern in the tire surface or if you have very high pyrometer readings in the left front you may want to consider reducing the amount of Ackerman.


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  • #13
BUMP STEER-As a front wheel moves up and down through its suspension travel, unless the steering is directly connected to the A frame, the wheel will tend to turn either left or right when the steering wheel is held firmly in the same position. This tendency of the front wheel to turn during suspension travel even though the steering wheel isn’t moving is called “bump steer”. It is caused by the fact that the steering box is attached to the chassis and doesn’t move while its tie rods are connected to the steering arms, which do move up and down.

Bump steer in a racecar should be minimized but..when measured and correctly set the bump steer it will add to the Ackermann as the car enters a turn and the brakes are applied. The rougher the racetrack’s surface, the more important it is to minimize bump steer. Just as the name implies, bump steer causes the car to turn itself when a wheel encounters unevenness. To measure bump steer, first set up the chassis with caster, camber, and toe-out, with full fuel , and driver...we use tractor weights ..some may argue the weights are smarter than the driver but that is another discussion. Remove or unhook the front shocks, springs and anti-roll bar. Put the car on four jack stands. Lock the steering wheel straight ahead. Remove the tire and wheel and bolt a flat plate to the hub.

With the spindle about half an inch below its normal ride height, adjust the dial indicators on the right and left of the gauge so they are level. Then measure the orientation of the plate with respect to the bump steer dial indicators by setting both dial indicators to zero. This will be your baseline. Jack the spindle and hub assembly up one inch and read the changes seen on the dial indicators. The difference between the dial indicators is the measure of bump steer. Stock car late models , the left front should bump .030” out and the right front should bump .015” out in one inch of upward spindle travel. On cars using stock spindles, such as NASCAR LMS cars, the left should bump .030” out and the right front should bump .015" out. If the cars bump steer is off, it can be adjusted in most cases. If you are working with fabricated spindles and a rack whose height can be moved up and down, adjusting the height of the rack with respect to the height of the spindles through the use of spacers is the solution. If the steering box’s height can’t be adjusted (if it isn’t a rack, chances are that it can’t be adjusted) and if the tie rods join the steering arms with tapered rod ends, adjusting bump is very difficult, it can only be accomplished by heating and bending the car’s steering arms. Even with that kind of effort, bump is still pretty tough to get right on the numbers with non-rack cars.

Adjusting bump is a matter of taking some time, yet it’s worth it. Note the photo of a Bump Steer Gauge. also the charts showing adjustment to remedy a bad bump steer condition.
I am out of beer so email any questions...later


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  • #14
ok Racers
what do you want to cover next..different design of suspension linkages or spring selection and wheel rate calculations?
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  • #15
Good stuff so far, spring selection and wheel rates next please.
  • #16
i better start a new thread on Calculating wheel rates and springs for race cars
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  • #17
Spring rate vs. Wheel Rate - Springs are rated in terms of resistance load placed on the spring. Usually springs are rated by pounds per inch..i.e. 400 pound spring means if 400 pounds were placed on the spring it would compress one inch.
Wheel Rate (WR) is the effective rate of the spring at the lower ball joint (BJ) located on the lower A-arm or control Arm which compresses the spring (modern independent suspension). WR is Spring Rate (SR) actual effective value after the mechanical advantage or leverage. This factor is Motion Ratio (MR) is the linkage squared. look at Wheel Rate illustration - note MR is the pivot point to center of the spring distance A divided by total effective length of the A-arm B.

Wheel Load Rate - racers were concerned about WR because the BJ is located several inches away from the center of the tire contact patch...if we look at the calculations of the Wheel Load Rate we find the difference is VERY SLIGHT. IMO, the ease of calculating Wheel rate vs. Wheel Load Rate and keeping things simple out weight the additional effort.

Coil Over Wheel Rate- this calculation is similar to the conventional A-Arm set up but we have t add in the cosine squared of the shock mounting angle. Again, all things I racing are compromises and the limited space on the front end where you can mount coil over shocks and not limit engine accessibility for maintenance dictates the mounting angle..More angle decreases the leverage...see the angle / cosine table


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  • #18
before we get int oin board suspension lay out and formula car wheel rate calculations..i need to dig out my notes on "how to calculate the proper springs for a given race car" both ..front and rear...because if you do not know where to start as a base line it will take days at the track to trial and error the set up until it gets close..
  • #19
Ok, here is how we determine the proper springs for each corner of the race car. you asked about loads on each wheel..well here is an example of our old door slammer running on a medium banked asphalt track.
Stock suspension with solid rear axle.
We calculated that it is under 1.3 Gs in the turn. " F= ( m*v^2 ) / R " is correct formula
one more piece to ponder..

from our cone killing days in SCCA Autocross..skid pad testing ,,go to parking lot, airport,,what ever, set up circle 200 to 300 feet in diameter, drive around the cirle as fast as you can without spinning out..
G = 1.225 x R / T squared
R= Radius of the turn in feet
T = Time in seconds to complete a 360 degree turn

typical Corvette corners at .84gs
road race sedan like Tran Am 1.15 Gs

if you know the tire performance curve from the manufacturer charts weight (vertical load in static pound) vs Traction (lateral load in lbs) you can calculate the Cornering efficiency.

This particular car weighs 2800 lbs. of 35% of weight will transfer under 1.3 G
and 75% will be on front end due to engine weight and corner loading

2800 lbs. X .35% = 980 lbs. transferring or loading tires

75% of 980 lbs. = 720 front end weight
divided by three to determine wheel rate ( two front springs and sway bar )
so we need wheel rate of 240

Wheel rate = (Length of A-arm divided into inside frame mount point to center of spring mounting point) squared

times spring rate

now the hard part
get out the tape measure and measure bottom front A-arm length
1. inside frame mount point to center of outside ball joint
2. distance from inside frame mount point to center of spring mounting point

stock Chevy A-arm is
16.5 inch inside frame mount point to BJ and 9 inch from inside frame mount point to center of spring pocket
assume you have a 800 lbs. spring
wheel rate = 9 / 16.5 = .54

.54 x .54 x 800 = 233 lbs. spring required to handle weight transferred

run a little stiffer sway bar and tune from here..
Chances are the Gs are off a little but we need a baseline t ostart andthis is as good as any.
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  • #20
We can see the effect of wheel rate regarding spring placement. Conventional coil over shock A-arm layout and the Formula Car inboard suspension which has a lot less unsprung weight.


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  • #21
gotta go wrench on the race car..up next is differnet types of control arm suspension
let me know ifin i missed something and will try to cover it?
  • #22
Great information! I wish I was at the point in my automotive tinkering to design a suspension. Unfortunately I'm at the will of aftermarket parts manufacturers until then! Interesting to read, though.
  • #23
Lets look at the true suspension mounting points on a Formula Car. Note that once you measure the FC, it is pretty easy to determine the Camber change regarding Bump and Droop. Remember..its all about tire contact patch.


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  • #24
Lets look at the suspension during corner entry ..as regarding chassis Roll. You have to control Sprung Weight and its effect on weight transfer, i.e. tire loading...note the change in Roll Center location..camber changes. You have to use the proper springs and Anti Roll Bar ( sway bar in Kentucky) to counter the Roll.

Question - How can we re-design the chassis and make it better at handling the effect of chassis Roll??


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  • #25
Mike....very interesting reading.
Question...Im running a winged pavement sprint car. Something that has always troubled me is the height of the front and rear roll centers on a typical sprint car with front and rear panhard bar. Front roll center height is usually around 11" and level side to side and approx 3" to the right.
Rear panhard height is usually around 13-14" and runs uphill 1 - 1 1/2" and approx 3" to the right as well.
Tire temps seem not bad...with all tires around 115-120 across the tire with the exception of the right rear which usually runs about 140.
I am wondering what effect lowering the roll centers both front and rear would have ?

Also...the more static caster I run...lets say 10deg on the right front and 7deg on the left front ( this is pretty typical of a pavement sprint car )...if I increase this, does this jack more or less weight into the car as I turn to the left ?? In other words...if I increase the amount of front caster will this free the car up in the center of the turn ?

Thanks Mike....I look forward to your response....Kenny
  • #26
Kenny ..sorry for the delay...I just returned from Mid Ohio..got the checker and got a third after the fuel cell foam deteriorated and clogged the fuel filter..ugg
please let me research the answer
1. do you run a front straight axel
2. do you run a solid or in dependant rear axle?
3. do you know how much akermann you have on the front?
4. do you know how much bump steer?

i got to brush up on areo thing regarding sprints
i helped an asphault sprint guy who converted to vintage sprint and the springs were way off after he removed the wing..got to dig my notes out
will reply asap
  • #27
Mike...thanks for the reply.
Sorry to hear about the foam ordeal...been there myself.
ok...Im running a solid front axle and a live rear axle...no independant suspension allowed.
I currently have no ackerman in the front end ( hmmm...I just assumed this since both my steering arms have the same CL-CL where the tie rod attaches)
No bump steer as per the usual with the exception of bump input into the drag link. I would think that this would be minimal since I only have an inch of suspension travel and a 49" long drag link that I run level to start with. 9" CL-CL on the pitman arm
Im running a 375 spring on the LF and a 400 spring on the RF...these are monted on the solid axle about 7" from the king pin CL
In the rear I am running a 250-275 RR spring mounted on the birdcage and a 225 - 250 LR spring mounted on the birdcage. I would assume that there is no motion ratio involved since the birdcage goes up and down with wheel travel the same amount. I allways run 25lbs of split across the rear with the bigger spring on the RR. Left side weight is about 56.5 percent...rear weight is about 60 percent

Thanks again...kenny
  • #28
My reference is " Circle Track Suspension by Forbes Aird published by Motorbooks International Power Pro series

Beam axles were the first type of axle used on racer cars. these were dropped in favor of the independent suspension because of the room required for vertical movement, excessive unsprung weight ( leaf spring) and forces that interfered with steering. Today's spring car beam axle has been refined to greatly reduce unsprung weight thur coil over shocks , light wheels etc...the steering problem remains. For all their problems the one piece of good news is the camber remains unchanged no matter what the chassis does ( as long as the track remains dead smooth). When you drive over a dip or BUMP on the track you encounter shimmy. A bump applied to one wheel on a beam front end would cause the wheel to steer abruptly a few degrees, because of gyroscopic effect, that steering action would be communicated through the tie rod, to the wheel on the other side which was being tilted through the same camber angle at the same time. Under certain conditions, the two wheels together would generate a gyroscopic torque that would pick up the " down side" wheel and slam the first wheel back onto the track surface toed-in. The whole cycle would continue into uncontrollable flapping of the front wheels.
Bump steer with a beam axle is tuff to cure because no single point on the chassis, for either wheel, is a fixed center of movement for both bump travel land chassis roll while cornering. In Bump, each wheel arcs around the contact point of its mate on the other side. In roll, the center of motion is near the middle of the car. So it is a given we got to live with the Bump steer we have.

I assume the Center of Gravity (CG) front (usually the cam shaft height) and rear is Above the Roll Center (RC) front and rear. Now we know that the hot set up is soft springs to minimize chassis stress and track surface irregularities but they lead to chassis ROLL. Why not raise the RC to reduce this chassis roll ( shorten the distance between the CG and RC means a shorter lever ). ?
On a beam axle set up , when a wheel bumps ,the axle tilts. If the RC is located some distance above the ground, ( all sprint cars are) it will be forced to move sideways as the axle moves in an arc around the far side tire. This lateral shove to the car may cause the tire to break traction.
Leaving the RC near ground level would require very stiff springs ( you have a long lever from CG to RC) to control roll. This may l;ead to heavy springs up front and lighter springs on the rear thus causing " porpoising " over bumps...no way..
so we have the classic compromise.
All my research says the typical spring car has a front RC between 8 and 10 inches and there rear is near axle level.
I think your RC is a little high in front and I would experiment on lowering it.
I would look into a J-Bar to replace the rear Panhard bar..Will lower the rear as well. They even make a HALO Bar.

Caster looks good and would not change it,,
RR tire temp is a little high but says you got grip driving off the turns,,maybe get more heat in the lft rear? are you carrying the left front wheel coming off the turns?

that RR tire temp is most likely from your Toe since you wild eyed sprint car type do a lot of steering with the right foot...going like He-- until you see God then turn left!

all and all..sounds like a VERY Close to optimum set up...
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  • #29
thanks for your reply Mike.

Ok...crank hight is 9" cam looks to be about 5.5 " above this, so you ...14.5" CG and ya...I'll go with about a 10" front roll center. Front tires are 24" diameter and the axle is 2.25 diameter with the panhard spud mounted to the bottom of the axle.
The problem I am experiencing is the car allways seams to be tight coming off. and infact did try and carry the LF, if not both front tires. I've got the top wing almost over the front axle to try and keep the front end down
To correct this problem I went up to a 375RR spring and a 350LR spring to try and keep the front of the car down. I am running a short 4 link ( 27.5 CL-CL ) in the rear, and since I've put this in the car hooks up REEL hard and wants to lift the front end. The bigger rear spring definitaly helped to keep the front end down, but I feel that the car wents to step out...not loose...but more like the rear tires are sliding laterally across the track ( if that makes sense to you )

  • #30
oh...forgot...does more or less caster affect weight jacking as you turn the wheel ?

  • #31
Very well done, Ranger Mike. I am just about to enbark on designing and fabing front uprights for a dwarf car to lower the amount of scrub. I have some pictures of an IRL car and their uprights to start from.

My question is the effect of the height of the spindle from the lower ball joint on the front end geometry. I would like to raise it in order to lower the ride height of the car. I have not seen any articles on it (I have the Steve Smith book on race car suspensions) I have a pretty good physics background from GMI. I was going to go and do some 4-bar linkage diagrams on my own, but if you have some information, I would appreciate it.

As for the push off in the sprint car, I would say it is either in your stagger (you did not mention how much you run) or the rear steer. The car is standing up on the rear tires and the front end geometry has little to do in the equation when a is as positive as with you or a top fuel dragster!.
  • #32
I try and run around 4" of stagger...but really...Im stuck with what ever I get from the tire truck...which is usually between 3.5 and 4 "

LR radius rods-
lower @ 1 degree down
upper @ 1" spread at frame...in other words if the bird cage spacing was 5" CL-CL then the radius rods would be 6" CL-CL at the chassis ( I have spuds to run them both down hill 1 degree, but have found that the car hooks up SIGNIFIGANTLY better with the upper radius rod uphill )

RR radius rods-
lower @ 1 degree up
upper @ 1" spread same as LR

I would really like to try and make the rear 4 link work with sum other changes.....I feel I am very close, but would love to hear any input.
Along with the rear spring rate increase I went to a straight 6 valve RR shock, which also helped.
I would like to run as little as 2" of stagger...I feel if I can get the car to turn true the center of the corner with this, that the car should come off the corner MUCH harder...yes indeed...much like a top fuel dragster.


Umm....is the general rule for rear panhard bar angle 10% of total length ? In other words if your panhard bar was let's say 25 " then you would want to be no more then 2.5" higher on the frame ?
  • #33

When a car with positive caster turns left, the left front corner will rise and the rt. ft corner will dip. The amount of these changes depends on the amount of pos. caster used combined with the spindles steering axis inclination angle. The steering axis inclination angle multiplies the effect of the pos. caster and associated corner lift and drop. The greater the steering axis inclination, the more posative caster will change the corner height of the car as the wheel is steered. This effect is caused by the curved path that the spindle pin follows as it is turned about the steering axis.
As the car is steered left and the left ft corner rises, the result is the same as jacking weight into that corner. The chassis gains weight at the left front and right rear corner, and loses weight at the rt. ft and left rear. This effect takes some cross weight out of the chassis. The more positive caster used at the left frt. and the greater there steering axis inclination angle, the greater the loss of cross weight in the chassis as it turns left.
Kenny, whe nyo uscale the car you can see this as you crank the steering wheel.
btw..what is the king pin angle...and steering axis inclination angle you are running??
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  • #34
i am working on it..also kennys question
  • #35
jaybee 17
Overall height of the spindle upright effects Roll Center (RC). shorter spindle height ( upper Ball Joint to Spindle shaft center line) with everything else the same, will LOWER the RC height. This is because the shorter spindle produces a longer instant center. the shorter instant center to the wheel it is drawn from , the higher the Roll Center.

you can change the RC by shortening the lower BJ center distance relative to the Spindle shaft center line as well.
you can lower the RC by raising ( relative to the track surface) the inboard upper A-arm mounting point.
you can lower the RC by lowering ( relative to the track surface) the inboard lower A-arm mounting point.
get performance trends software to take the headache out of the calculations..

I would look at three link rear suspension to cut my teeth on,,,,the four link is very tricky to learn...
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