Race car suspension Class


by Ranger Mike
Tags: class, race, suspension
Ranger Mike
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#325
Feb17-12, 05:54 AM
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If we draw a line through the top ball joint , bottom ball joint to the ground we have the Kingpin Inclination Angle, also known as Steering Axis Inclination. Typically this angle is between 5 and 10 degrees and 7 to 9 degrees being best compromise for Stock cars, big sedans running 12:1 ( most common) 10:1 or the very quick 8:1 which both require power steering. 6 to 8 degrees for open wheel road course cars running 16:1 steering ( two turns lock to lock).
The scrub radius is the distance in front view between the Steering Axis Inclination and the center of the contact patch where both would theoretically touch the road.

The point at which the steering axis line contacts the ground is the fulcrum pivot point on which the tire turns. The location of this point within the contact patch has a great effect on steering effort, feel, and stability. If you have not already guessed, the easiest way to change scrub is by changing your offset with either new wheels, or hub centric wheel spacers. Scrub radius should be kept at a minimum but not Zero. Zero scrub radius (center point steering) means the car will react too quickly and steering is " darty". The car will steer very easily and give no feedback to the driver. It is like driving your car on ice. No feel.

If the scrub is zero, the scrubbing action of the contact patch is equal on either side of the pivot point causing the tire to act like a car with a locked differential, inducing a condition called 'squirm'. In a straight line the tire tends to be stable and tracks well. As you turn though, the portion of the contact patch on the outside of the pivot point moves faster than the portion on the inside of the contact patch. Since the scrubbing area is equal on each side of the pivot point, yet the forces are different, the tire tends to fight itself and it becomes 'grabby' causing tire wear to increase and the steering to become unstable.

Any bump or cornering force that is applied to the tire can exert a twisting force on the steering that is proportional to the length of the scrub radius. Increasing SAI helps to self center the steering. But if too much SAI causes lateral scrub of the tires in the turn and really heats them up. Lot of wear.

When in a left turn, Scrub lengthens the right side wheel base and tends to loosen the car. When you counter steer, it shortens the wheelbase and adds understeer. This is a driver friendly situation as it has a very stabilizing effect to the cars handling in driver feed back. The driver needs feedback in a turn because it tells him how heavy the tires are loading and when they are on the edge of traction.

Positive and negative scrub radii have benefits in different types of suspension. A MacPherson strut assembly typically performs well with a lot of SAI and caster, a system negative scrub works well in. Because both SAI and caster increase the amount of camber on the outside wheel when steering, the fulcrum pivot point is at a point that has more leverage, requiring less steering effort. Negative scrub also helps reduce torque steer in front wheel drive cars. Positive scrub radius works well with suspensions that use dual control arms that use less caster and SAI to optimize geometry.

As with anything else, a little of a good thing is great, but lot of a good thing is not necessarily better. When you have excessive scrub, whether it be positive or negative, steering effort increases and road 'feel' increases, as the steering is more susceptible to road shock. Additionally, if you plan on doing some homework on, and modifying your scrub radius, you must take into account the amount of sidewall flex your tire will encounter under hard cornering. When the sidewall flexes, the contact patch moves in relation to the SAI and can make a slightly negative scrub radius become zero.

Our open wheel formula car runs about 2.5 inch scrub radius. Late model stock cars will run 10 to 12 inch wide tires and 4.5 to 5 inch scrub. The left front will have 1 to 1.5 inch more scrub due to positive camber used on that side. As far as I can recall scrub should be between 3.625 and 6 inches with right front having 5 inch.. being " ideal' for most fast late model cars.

Mechanical Trail - SEE POST # 319 ABOVE - Also know as or caster offset , is the horizontal distance from where the steering axis inclination intersects the ground to where the front wheel touches the ground. The measurement is considered positive if the front wheel ground contact point is behind (towards the rear of the car) the steering axis intersection with the ground. More trail will give the feel of " returnability " of the steering. Less will give the feel of the steering being " floaty". .
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dano844
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#326
Feb17-12, 03:10 PM
P: 3
Mike thank you again for all of the information. I will try to stick with around the 5" scrub radius at both sides of the car. I will be measuring and mocking this weekend to get the front settled. I will let you know if I need anymore advice. Again thank you for your time, it is greatly appreciated.
Thanks
Dan
Ranger Mike
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#327
Feb20-12, 05:52 AM
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we had some discussion about Anti Squat..I found this when I was digging through my note..food for thought
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Ranger Mike
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#328
Feb20-12, 05:59 AM
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I had a question on changing rear end mount brackets..here is one case..it is an example of fabricated mounting bracket for Panhard or J bar..make them beefy and lot if support..car manufacturers spend lots of time and money to design the proper mounts and I don't like to deviate from them unless it is for max performance and can be justified..but you can do it..
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smokin butts
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#329
Feb21-12, 09:51 PM
P: 9
measuring centerline

Do you measure the centerline of your car at

1 between the Aframe mounts.
2 between the tire contact patches.
3 between the moment center where the car rolls which would depend on left side weight .
Ranger Mike
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#330
Feb22-12, 04:24 AM
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smokin..which ever method you use..you have to use it every time. The tire patch method is ok but if you ever go to offset wheels or spacers..you have all readings off form that point. The recommended method is from A-arm mounts..I go off the lower A-arm mounts, drop a plumb line from each to the poster board, find the center line and project it back to the rear of the car with a long piece of aluminum and find my center line from the frame rails at the rear. The drive line will be close to center but I found that components are usually offset a bit. I use a 3 foot carpenters square to project the center line up to the car frame at several points and PAINT a line on the frame, chassis etc...so I don't have to repeat this drill.
1meyra
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#331
Feb27-12, 07:53 PM
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Starting out with a new year traveling from track to track with a super late model offset chassis 2750lb with 10inch tires was wondering what spindle height and degree would be a good start some tracks are flat and others are banked up to 18 degree or should we changing for each race.
Ranger Mike
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#332
Feb28-12, 04:44 AM
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1meyra welcome
I assume you run paved tracks. Spindle height most common on super lates is 7.75 inch. It has lower Roll center which means less camber change through spring compression. Spindle kingpin inclination can be between 5 and 10 degrees. Five years ago 8 1/2 degrees was about the max..now 10 degree is the current hot set up. You can even get 12 degrees now.. Sweet Mfg. even makes a spindle where you cans wap out slugs to change offset by 2 degrees..trick for 2012 in the late model community. I would not change out spindle between tracks. Go with shocks and springs / ARB (sway bar).
these are only starting points and each chassis may differ..

Flat tracks 0 to 6 degree banking you must increase rt. ft. spring rate to control body roll. If you run 350# spring on the rt. ft. on 3/8 to 1/2 mile 10 degree track, swap a 400# spring in to run 1/4 mile flat track. You may need a slightly stiffer rt. rear spring to balance out the car. Higher bank tracks ( 0 to 18 degrees) turn the corning force into more down force than lateral. This means you have to increase the spring rate to counter this added down force. Make the lt. ft. and rt. ft. springs 25 to 50 # stiffer and lt. and rt. rear springs 15 to 25 # stiffer.
Run about 1/8 inch toe out on the low bank tracks and on fast 1/2 mile use 1/16 " to keep down the scrub. On tight 1/4 tracks run 1/8 to 3/16 toe out.
smokin butts
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#333
Mar5-12, 01:10 AM
P: 9
When straight line braking , such as in road racing whatstops wheel hop or rear wheel lock up with out dialing all front brake in. is there any rear measurements that can be adjusted to stop so much weight transfer to the front when braking , or be able to make the whole car lower during braking

SMOKIN
Ranger Mike
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#334
Mar5-12, 06:39 AM
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i am in germany this week..will reply when i get back,,,thanks for the interest

rm
Ranger Mike
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#335
Mar12-12, 04:42 AM
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smokin..
some notes on wheel hop

you said When straight line braking , such as in road racing whatstops wheel hop or rear wheel lock up with out dialing all front brake in.


Rear wheel hop is basically the suspension gets into a resonant situation and doesn't dampen itself out.

Under acceleration, the rear suspension compresses. Due to inertia, the wheel continues traveling upward. This unloads the tire, reducing the acceleration force. The spring then pushes back down on the wheel, and it regains traction, forcing the wheel back up.


Play and movement in suspension arms can exaggerate the problem.

The best way to fix it depends on the vehicle. Vehicles suspension can be designed with "anti-squat" capabilities, but at the expense of ride quality. 4 link cars with solid rear axles can use "no hop" bars to change the suspension geometry.

Angle of upper link on rear end will effect wheel hop. A large amount of anti squat ( down hill angel of top link) will cause rear end lightness and rear wheel hop under braking. The 3rd link shock will effect it as well.
Shocks, shocks shocks..check the shocks...stiffer shocks will help.


Leaf spring cars can use traction bars to eliminate it.

is there any rear measurements that can be adjusted to stop so much weight transfer to the front when braking , or be able to make the whole car lower during braking

Stiffer ARB ( sway bar). Softer rear spring package. Need to keep more weight on rear end.

Also, test the car with a light fuel load because the weight difference (sometimes in the neighborhood of 150 pounds) can affect wheel hop as well. Wheel hop is less likely on production-based sports cars with an independent rear suspension because the center- section of the axle is hung by the chassis, but it can still occur especially if an unskilled driver is punching the brakes and shifting the gears
smokin butts
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#336
Mar15-12, 10:39 PM
P: 9
when working with a roll center program !

1 what dive and roll numbers is a good base to start with?

2 if your roll center migrates on roll ! how much is too much and how much is not enough

3 where should instant centers be on dive ? how close together should they be
Ranger Mike
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#337
Mar16-12, 05:45 AM
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We are going to have to deal with migrating roll centers when we have restrictive rules like stock mount points , metric this and that etc..depending on how you like to set up the car, I would look at 2 inch dive maximum, 1.5 degree roll...I do not personally look at instant centers as such, since i use the software program, the Roll center and location are what I look at and only to see impact on the camber curve. We know the RC will be going all over the place but if we can keep it going to the pavement, and staying around the 3 to 4 inch offset during the first inch of travel ( 1.5" would be ideal) this is about as good a set up as you can get on the tires they make you run on. I have seen RC offset up to 5 inch to the right and still planted the right front well though I believe 5" is too much leverage ( on asphalt).
Kozy
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#338
Mar16-12, 11:23 AM
P: 118
Does lateral RC position actually matter? I have read some convincing arguments that it does not, only the height...
darraten10
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#339
Mar16-12, 04:40 PM
P: 1
Hi sorry to change things up I have just found your forum and up to page 5 and found it to be totally interesting and learnt alot. Im from a micro sprint dirt background and have a question. Previously with castor and things you guys have said i think if the car rolls to the right(left hand turn) the car will loosen through the turn me i have always found it to tighten increase inside rear drive through weight transfer. From reading i believe we run too much castor which will be reduced and too much roll center so with the rolling to the right this would depend on spring weight on whether it was a push not so much the movement due to the amount of weight transfer as a softer spring would allow it to get off quicker and transfer less weight. Our car is like alot of dirt cars panard front jacobs ladder rear. How does a jacobs ladder rear and panard rear change different to ride height changes. Cheers Danny
Ranger Mike
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#340
Mar17-12, 04:45 AM
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Kozy
if you are running circle track you absolutely should offset the roll center. see page 16 post # 215..bottom line is you have to plant the right front tire with enough transfering weight to turn the car..on dirt the boys run 4 inch offset on asphalt 3 inch. I think it is the greatest close hold secret of round track racing...


darraten10

welcome..

good to see new racer!
you have many questions rolled into on..how much caster are you running and where is the Roll Center? See post on page 16 for rear panhard changes and why..I will dig up notes on jacobs ladder..
Kozy
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#341
Mar19-12, 11:47 AM
P: 118
Ah, roundy roundy cars, that explains it. I know nothing about that.
mender
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#342
Mar19-12, 08:50 PM
P: 563
Quote Quote by Ranger Mike View Post
is there any rear measurements that can be adjusted to stop so much weight transfer to the front when braking , or be able to make the whole car lower during braking

Stiffer ARB ( sway bar). Softer rear spring package. Need to keep more weight on rear end.
Mike, a stiffer sway bar won't affect longitudinal weight transfer, and a softer rear spring will allow more rear end lift than a stiff spring. Weight transfer caused by chassis/suspension movement is the only component of the total weight transfer that can be altered with adjustments; the weight transfer due to decel force is determined by the C of G height and is built into the car.

Dialing in more anti-squat will tend to pull the rear of the car down during braking but that usually unloads the tires and can also result in wheel hop.


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