Automotive Race car suspension Class

[1meyra]

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]

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 into 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]

When straight line braking , such as in road racing whatstops wheel hop or rear wheel lock up without 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]

i am in germany this week..will reply when i get back,,,thanks for the interest

rm

 

[Ranger Mike]

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 without 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]

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]

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]

Does lateral RC position actually matter? I have read some convincing arguments that it does not, only the height...

 

[darraten10]

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 learned alot. I am 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 a lot 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]

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]

Ah, roundy roundy cars, that explains it. I know nothing about that.

 

[mender]

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.

 

[Ranger Mike]

Mender, i may have not understood your post ref: sway bar...I always thought the ARB was 1/3 of the proper handling package ( see post 19 on page two above)..
I am with you on the anti squat dial in..but after looking back thru this forum, we never discussed Anti Roll Bar ( sway bars ) and will work on post to do this.

 

[mender]

Mike, I was answering in the context of:
"When straight line braking , such as in road racing ..."

When there is no cornering involved the ARB just rotates in the bushings so it has no effect.

 

[Ranger Mike]

Mender..you are correct...the ARB would have little to no effect on straight line braking..which I never think about unless the car is coming into the pits...excellent advice!
thanks

 

[rick7343]

I have to say, I never knew there were so many things to consider when building a oval track car... lol

It seemed so much easier 15 years ago when I last raced and we weren't supposed to move anything...

So I'm wondering the consequences of less than ideal scrub radius?

To widen the stance of the car (chevelle) the previous owner was using 8" wheels with 1-1/2" back spacing... I have yet to run the numbers... hmmm...

What should I be looking for in scrub radius with 8" wheels? What if I can't get it witout sacrificing track width?

Thanx!

 

[Ranger Mike]

Hey Mender..i am working on ARB post but I been rebuilding the old NAA Ford tractor. Dandelions don't wait on anyone!

Hey Rick...glad to hear from you..I think if you review post 264 and 255 on page 17 about track width...and then post 325 on page 21 about scrub radius..good info..personally scrub radius is drivers choice..
depending on feel if car is too darty or grabby..

me being a non driver and selfish crew chief interested in WINS..screw the driver feel and feed back and make it as wide as you can get away with...

 

[rick7343]

So scrub radius has no adverse affect on grip That's good to know...

 

[rick7343]

So how about the affect of weight placement?

Since the car has a stockish 4 link in the rear it has a high rear roll center... so would placing the weight higher in the rear of the car help the rear end roll?

We always used to keep the weight as low as allowed, but now I'm wondering...

 

[Ranger Mike]

Yes you pick up more roll when weight is placed high on the chassis..but..also you get all the ill effects too..see post 272 on page 17..we discussed fuel cell mounting...

on left turn circle track cars i always remember an old guys axiom..low, left and light..build it as low as you can with as much low left side weight as possible and as light as the rules will let you...guess i better research polar moment for post after i finish the ARB post...

 

[Timaladd]

Ranger Mike,

I want to pick your brain on 2nd gen camaro leaf spring mounting for 3/8 medium banked dirt. Track usually goes slick.
1.-What are your thoughts on raising one or both of the front spring mounts to gain drive off? Any othe adverse affects of only altering the left front mount?
2.-Effects of splaying front mounts inward? Effect of splaying one side more than the other?
3.-Besides changing ride heighth, what other affects (sidebite) do lowering blocks resent. More focused on roll heigth changes than leaf spring wrap.
4.-How much rear steer do leaf spring provide? I have been successful on crate latemodel using rightside bars to reduce right side wheelbase as right rear bumps. Much better to reduce wheelbase in bump than put more angle in leftside bars which reduces leftside wheelbase.

Thanks so much for you help. I have learned a lot reading your posts.

 

[Ranger Mike]

i have just finished rebuilding the old Ford NAA jubilee tractor..got to mow 6 plus acres and the girl friend is blonde...no way am i going to get any help form that quarter..so i got behind in my posts and emails..should be caught up this week..folks

 

[Ranger Mike]

Timaladd..welcome,,good to have new racer.. i grew up on these set ups..even had torsion bar front end..

see post 116 on page 8 for roll steer explanation

Hotchkiss drive rear suspensions
NO rubber bushings belong in any race car chassis set up.Get nylon or urethane or brass bushings but get rid of the rubber stuff.

Been around since horse and buggy days but it is simple and it works. it was found that mounting one shock ahead of the axle and the other behind would reduce torque induced wheel hop during acceleration and braking.
Since the leaf spring is used to transmit acceleration and braking forces, their positioning and configuration is critical. These forces cause the axle to twist and tend to wrap up and distort. To counter this it is necessary to increase the number of spring leafs or thickness of the leaf to get enough force to counter spring wrap. You can add a full leaf or thicker leaf but this also increases spring rate. It was found that if only the front half of the spring leaf is added, axle wrap is decreased while not adding as much spring rate as a full leaf.

Spring eye height of the front spring eye primarily determines how much anti-squat the rear suspension will have. The higher the spring eye the more anti - squat. Generally height between 10 and 15 inch above ground give best results. If you run the spring eye too high you will get brake hop. The rear spring eye also effects anti-squat but not as much.

To work properly, mono leaf springs must be mounted straight and parallel to each other and perpendicular to the axle housing.

The rear Roll Center (RC) on mono leaf setup is located half way between the bottom of the rear end housing tube and the top of the leaf spring.The Rc can be changed slightly by using shorter or taller lowering blocks. Don't go over 2.5 inch because a taller block will create too much leverage on the leaf spring and really adds deformation under acceleration/braking.
Cars with monoleaf springs usually have rear RC at 8.5 to 11 inch. These leaf systems are not as tuneable as Coils/ coil over shock packages. The only way to change rear RC is to move ballast weight around. On a dry slick track mounting ballast higher in the chassis will create more overturning moment which creates more right rear side bite. Typical monoleafs come in .323, .291 and .262 inch thickness. Thinner springs work better on dry slick tracks because they deform and cushion wheel spin better. But this weakens the springs and you must monitor the leafs for deformation. Thicker leaf will last longer but not provide optimum hook up. The .291 inch leaf is best compromise but..you need to replace these at least once a season.
NOTE: The springs must always remain parallel to each other in the fore/aft plane. In other words.. Don't locate the right side spring eye in the top chassis spring eye hole and the left front spring eye in the bottom front spring eye hole. Move them the same and together.

Use slider boxes to mount the rear spring eyes. They eliminate spring bind cause by shackle distortion. The mounting angle determines the amount of roll steer you will have. The best way to amount a leaf spring is with slight downward angle, as viewed from the side, with the front eye of the spring slightly lower then the rear spring eye. The downward angle should be 2 to 4 degrees.

Arch of the leaf and amount of torque warp in the leaf determines the amount of rear roll steer. When cornering, the more arch in each spring will mean more roll steer. The right rear spring compresses and lengthens. The left rear unloads and shortens up. The right rear is lengthened creating rear over steer. The amount of roll oversteer depends on the spring arch and amount of body roll. Three inches of arch usually produce a fair amount of roll over steer. One inch is probably not enough.
When you nail it out of the turn, the torque wraps the front spring and shortens the front segment and pulls the axle tube forward. This depends on the arch and thickness. One last note; see the post # 298 on page 19, on eccentric for right side front spring eye. hope this helps.

 

[rick7343]

Time to order some springs...

How do I calculate what springs I need in the front of my chevelle clipped car? I hear some of the metric guys are as low as 600 lbs...

If there is a benefit to dropping the front end in the corner, I will do it...

Does 600 lf, 650 rf, and a 534lb bar sound crazy for a mid banked 1/3 mile ashalt oval?

Can the anti sway bar be too big?

 

[rick7343]

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.

So am I reading this right? We want body roll to get the outside tires to bite? Doesn't the SSBB theory work against this as it limits body roll?

 

[Ranger Mike]

Rick..you are pretty close to correct on the " soft" springs..I recommend every one re-read post 17 on page 2 _Spring rate vs Wheel rate and post 19 on page 2 - how to calculate front spring rate required for your car. And I can see I got to finish the sway bar/ARB post cause you are all asking for more info on this. I'll do her now!

 

[Ranger Mike]

Anti Roll Bar (ARB) - sway bar - stabilizer bar

Back in the 1960s passenger cars did not have ARB until the end of the decade. I was a kid going to the local asphalt track and none of the hot dog late model cars had these. This was in the day when you could stick a 1200 # railroad car spring in the right front of the car as the hot set up in hobby stock class. Meanwhile the NASCAR boys were going faster and faster on the high speed ovals and found out that low was good ( regarding body to pavement clearance). In fact back then you only had to pass ride height inspection at pre-race tech inspection. Some crew chiefs fabricated chassis stops that would break off during the race and ZHAZAMMM..the car would drop an inch in the front to channel air better and grab some aero advantage. About this time it was found that if you added the ARB, you could run softer front end springs and still corner the same. The softer springs got the car even lower once air started to push the nose down at higher speed. Naturally, everyone started copy catting the set up and after that, every one was running ARB but most did not have a clue as to why.

The same thing was going on in the open wheel formula car world. When the Fc cars went from skinny tall tires to short wide gummy rines the suspension designers didn't keep up. The shorter tires dropped the roll center which added to roll. The idea that soft springs were needed to maintain the tire contact patch so in came the pencil thin ARB used on the front and rear of the FC cars. When the chassis mounted WINGS became effective, the Fc boys had to up the spring rate or replace the belly pans pretty frequently. The wing thing got ridiculous to the point the spring rates were way past 1000# to the point that aero down force was the only game in town. Everyone forgot about the contribution the ARB made to the set up. This insanity continued until the tire was the only spring thing on the car .(See solid axle conversion kit later on in the post.). eventually the ground effects stuff was banned and reason returned to the racing world..for a time and the ARB was back in vogue. What people do not appreciate is that within reason, springs and ARB are interchangeable so far as roll resistance is concerned. We can reduce roll resistance of the ARB if we up the spring rate on the springs, the same amount at the end of the car that is effected.Chassis Engineering by Herb Adams - One excellent source and got a lot of this post from his fine book.

Roll Angle
When a car corners the body rolls to the outside and we have BODY ROLL. The amount it rolls is ROLL ANGLE. When we have this condition many bad things can happen and only one good thing. It is all about tires, tires, TIRES. A tire has max traction when n it is perpendicular to the track surface. During body roll, the camber changes and once we have over 3/4 degree camber gain per degree body roll the geometry is not there to maintain proper contact. Now we can dial in static negative camber of 2 to 3 degrees and this helps keep the outside tire square to the track surface but there are draw backs to this set up. We have to control the body roll, more specifically, the Roll Angle (RA).

We can reduce RA by lowering the Center of Gravity but in most cases this is already done. We can change the Roll Center height and location but this is a tough thing to do on the front end due to all the related suspension components. We can increase the track width with wheel offset but we can only gain a minute amount with this tuning. You can reduce the cornering force that generate roll angle (slow down)..NOT! Or we can increase Roll Stiffness.

Roll Stiffness- One way to reduce roll stiffness is to increase the spring rate. If the spring rate is high enough to limit roll to the maximum, the wheel rate in ride inevitably would be too high for tire compliance.

Anti Roll Bar - These are traversally mounted torsion bar springs, adding spring rate to the chassis during cornering.As the chassis goes into roll, the outside arm of the bar moves up while the inside arm moves down, crating twist in the bar. The resistance to twist is the bars spring rate and this adds to the total spring rate required to handle the weight transfer during cornering. Stiffness of the ARB increases very quickly as itsdiameter increases. Stiffness is a function of diameter to the 4th power, stiffness = D4.The effectiveness of the ARB depends on the length of the swing arm. The longer the arm the less force applied relative movement at its ends. Short arms are "stiffer" , long arms are" softer" , since you have a longer lever.

 

[Ranger Mike]

Calculation of proper ARB rate.

See post 19 on page 2 for the calculation of the correct spring rate. Of the total front spring rate ( left front, rt. front and ARB) the ARB should be 30 to 50% of this. Over 50% and see "solid axle conversion kit" note below. Flat asphalt track requires the most ARB higher bank tracks go to the minimum ARB rate %.

Rate in pounds per inch of arm deflection of solid ARB
1,125,000 x D4 / L x A2note: D4 and d4 is to the 4th power..it did not copy over from my word document so multiply it 4 times

Where

D= bar outside diameter

L= effective bar length

A= effective arm length

For tube or hollow bars use

1,125,000 x (D4 – d4) / L x A2

D= bar outside diameter

d= bar inside diameter

1,200,000 is base number for 4130 C34-C38 material

ARB must be mounted square to the lower control arms. The bar must be perpendicular to the vehicle center line. You have to mount it as close to the lower ball joint as practical. Just as we had to calculate the motion rate of the springs on post 19 page 2
we have to go through the same drill with the ARB. Effective wheel rate is found by multiplying the motion ratio squared by the ARB spring rate. The motion ratio is the arm mounting position length on the control arm (A) divided by the lower control arm length (B).
Then this motion rate is squared.

Too stiff ARB - The ARB has to be balanced with the front end spring package. The less work we get out of the springs, the more the ARB has to do. There is no dampening effect on the ARB by the shocks (dampers) Shocks only work on the springs when compressed or in rebound. We can run into '' rock roll back" where we get oscillation. When we run into a stiff ARB condition, and one wheel goes into BUMP, the two wheels are no longer independent and the load will transfer laterally by the ARB itself. This makes for a very darty car. When we reach the point that the ARB is far stiffer than the springs we get to the point the car is very slidy due to roll resistance of the set up. Avoid making the ARB so stiff that we have a "solid axle conversion kit" going on.

Misc. stuff on ARB - Never drill a sway bar ( ARB ) to soften the rate..it will snap. Never weld on the ARB..even tack weld. Beware of nicks and dings as it will weaken it and may even snap.
Paint a stripe the full length of the effective part of the bar and if you see some twist, replace it. ARB can go on you and you will never know it. One hard to find problem is when handling suddenly goes away.Beware - ARB can yield without breaking. Handling will diminish subtly and you can not spot it unless you pull the bar and measure its effectiveness. When you first scale the car, stick a bottle jack under the right control arm and jack it up and inch. Note the scale reading on the left ft. Write it down.
We used a sway bar loader that had a hydraulic jack and the driver could dial in load on the ARB. Worked great. If you have a left side weight rule this is one thing to get you 50 extra pounds but remember to dial it back to zero before you post race scale the car. Recommended only for the team that has radios to "remind" the driver about this.
No one uses solid ARB any more as you are only adding weight.
If your race class dictates a stock sway bar..look at
http://www.hotchkis.net/_uploaded_files/hollow_vs_solidinstructions162file.pdf
stock looking sway bar but is mucho lighter on the front end.

 

[mender]

Too stiff ARB - The ARB has to be balanced with the front end spring package. The less work we get out of the springs, the more the ARB has to do. There is no dampening effect on the ARB by the shocks (dampers) Shocks only work on the springs when compressed or in rebound.

Not strictly true; the shocks dampen suspension movement, which the ARBs (and springs) are connected to.

ARBs effectively add (or subtract) spring rate due to body roll non-linearly and require a different valving split on the shock. Going too far with BBSS can lead either to oscillations (duckwalk) or overdampened reaction to bumps; harder to get the right mix.

 

[rick7343]

So is the soft spring route worth pursuing? Seems a lot have opinions against it..