|Jul22-09, 06:03 AM||#1|
Race car suspension Class
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.
|Jul22-09, 06:14 AM||#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 thats 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
|Jul22-09, 06:25 AM||#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
Camber ( camber curve too)
Bump Steer ( measuring and setting)
later ifin you want it
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
|Jul22-09, 07:55 AM||#4|
Race car suspension Class
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.
|Jul22-09, 08:58 AM||#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!
|Jul23-09, 05:13 AM||#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
|Jul23-09, 05:52 AM||#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.
|Jul24-09, 06:18 AM||#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.
|Jul24-09, 06:21 AM||#9|
time for me to take a break...you guys got any questions?
|Jul26-09, 07:35 AM||#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.
|Jul26-09, 08:13 AM||#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.
|Jul26-09, 09:17 AM||#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.
|Jul26-09, 09:38 AM||#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
|Jul27-09, 06:44 AM||#14|
what do you want to cover next..different design of suspension linkages or spring selection and wheel rate calculations?
|Jul27-09, 07:33 AM||#15|
Good stuff so far, spring selection and wheel rates next please.
|Jul28-09, 07:22 AM||#16|
i better start a new thread on Calculating wheel rates and springs for race cars
|Jul29-09, 08:31 AM||#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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