Automotive Race car suspension Class

Ranger Mike

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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
thanks
 
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 Im 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
 

Ranger Mike

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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 gotta 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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thanks for your reply Mike.

Ok.......crank hight is 9" cam looks to be about 5.5 " above this, so ya ......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 Im experiencing is the car allways seams to be tight coming off. and infact did try and carry the LF, if not both front tires. Ive 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. Im running a short 4 link ( 27.5 CL-CL ) in the rear, and since Ive 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 )

Thanks.......kenny
 
oh.........forgot...........does more or less caster affect weight jacking as you turn the wheel ?

Thanks....Kenny
 
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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!!!.
 
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 Im 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, wich 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.

Thanks.....kenny

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

Ranger Mike

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Kenny

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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Ranger Mike

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jaybee17
i am working on it..also kennys question
 

Ranger Mike

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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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Ranger Mike

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kenny

Panhard bar should be mounted level at the desired RC height to begin with... don't know about that rule of thumb.
minimum stagger you need is:
D + .5 (TW) x CL \ D - .5 (TW)
where D is the track diameter in feet
TW = rear track width of the car in feet ( divide inch measurement by 12)
CL = the left rear tire circumference

example- race track has 200 ft. diameter
car has 64 inch track (64 / 12 = 5.33 inch)
left rear is 86 inch circumference.
run the numbers and you get 88.33 inch right rear circumference or MINIMUM 2.33 inch stagger.
if you run a spool add 10 percent to minimum stagger..if the track banking is 0 to 10 degrees, add another ten percent..if banking is 15 degrees up..take off ten percent from min. stagger

Four link has advantage of good forward bite because the upper links are mounted up hill. anytime the forward link is up hill the the rear end is trying to go up under the chassis during acceleration and this increases loading on the tires, these 4 links setups create oversteer as the body rolls. the right rear is pushed back and the left rear is pulled forward which helps the car turn going through the corner without breaking traction.

all I can tell you is ball park settings on the links..uppers should be 15 to 17 degrees up hill angle. the right upper max angle is 18 degrees and min. angle is 14 degrees.
the left upper usually has 3 to 5 degrees less angle than the right so the two links will be parallel through the corner. the lower links are usually 2 inches shorter. this reduces roll oversteer. the most common setting on the lowers is 5 degrees down.
 
thanks guys......

Kingpin inclination angle is 10 degrees. Hmmmm.........wut if I was to build a RF spindle with only .........lets say 6 degrees ?
I say I would like to run around 2" of stagger, because the super late models here run that kind of stagger. I figured that if that was good enough for them then I should be able to run the same. Theres guys out here looking for 5 and 6" of stagger for the sprint cars.......They must be nutz.

Dont get me wrong.....my cars not a pail, and I run very well, But Im allways interested in learning and going faster. I appreciate all your inputs.

Hopefully next year Ill have a set of scales, so I can try like you said Mike...............Put the car on the scales and see what it does as you turn left.

I have 1 race left this year.......Ill try a panhard change. I will start with the rear bar level and will lower the front bar..........Ill make a axle bracket with +1 and 2" drop and try them both.

Thanks again........Kenny
 
I am going to measure our track this weekend...............when u say diameter of track Mike..........do you mean at the goove?.............out side of track ? Add inside and out side then divide by 2 to get an average ?
Im interested in seeing what this formula says I should have for stagger

Kenny
 

Ranger Mike

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I am going to measure our track this weekend...............when u say diameter of track Mike..........do you mean at the goove?.............out side of track ? Add inside and out side then divide by 2 to get an average ?
Im interested in seeing what this formula says I should have for stagger

Kenny
Kenny i would measure the dfiameter of YOUR groove you run..or the hot dogs hot groove as this is where you want the stagger to work...i have driven to various round tracks on off days and actually tape measured the turns and took a protractror level fionder to measure the banking...
also we ran nitrogen to make sure the stagger grew minimum amount...
you are right in that 5 inch stagger is HUGE...go with the super late model boys as starting point..

keep the rear roll centers about 1 to 3 inch taller than the front or it will push..and i know you know this but make one small change at a time and document everything.
before messing wit hthe roll centers try adding more up hill angle to the top links like 2 degrees at a time. then when you are really hooked up and start pushing..look at RC changes...
 
Yes.......I use nitrogen in the tires as well as tire reliefs.

Now...........heres an argument Ive had with more then a few people about setting tire reliefs.

If you ask Hoosier they tell u to set them 5PSI higher then your starting pressure..this doesnt make sense to me.........if my RR tire says it wants 15PSI.....should I not set my releif to 15PSI to start with, even when the tires cold ? I know you build pressure as the tire gets hot, so isnt that why we run a relief ??
Typicalaly what I set my bleeders to is

LF 14PSI RF 15PSI

LR 7-10PSI RR 15PSI

I have found that buy varying LR tire pressure that the car is much more responsive the using the adjuster nut on the spring ( Im told that 1PSI change in tire pressure = 1/2 a percent change in cross )

As far as hooked up and pushing..........thats where Im at now.........front end feels real light coming off
 

Ranger Mike

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pop offs

tire relief valves ( pop offs) are tire valves you can pre set to pop offor open at a specific PSI. ( kenny i know you know this,,,i detail this for the uninitiated)
these come with springs set for a PSI range. I have Longacre pop offs and they have a set of springs for 3-15 PSI and springs for 15 and up PSI. they tell us to set the pop off PSI 2 pounds above desired range. Just like a tire valve stem that has a spring to close the valve..these have a spring to keep it closed. The springs are linear and pretty accurate. so you just have to live with the preload..Rebco, i think.. has a higher default rate and wider range (which means the pop off value is not as sensative ) so i would check with the manufacturer. it is all relavent anyway as you will pyro the tire at three places to see the temps and adjust anyway..
 
Mike..............where does rear panhard bar adjustment most effect chassis handling ??
Through the middle and coming off ??
Front panhard bar adjustment gettin in ??

Thanks...........kenny
 

Ranger Mike

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The distance between Center of Gravity (CG) ad the Roll Center (RC) is important. The panhard bar is easily adjusted ( hence the RC). the shorter the distance, the softer the springs because the lever length is shorter. with this in mind let review the three phases of handling problems..note if a car is loose going in to the turn, there are ten adjustments you can do, with brake bias , RC height , stagger, cross weight and spring rate being easiest to change.

Loose going in ( corner entry) oversteer
too much rear brake bias, RC too high, rt. ft. spring too soft, right rear spring too stiff, ft. sway bar too soft, too much stagger, more cross weight ( add wedge) rt. rear tire over inflated, shocks need tuned, wheel tracking- rt. rear wheel moved outward too far relative to rt., front

Middle of the turn car is loose
more cross weight
reduce stagger
lower rear RC
rt. ft spring too soft
rt. rear spring too stiff
rt. rear tire pressure to high
ft sway bar too soft

loose coming out of the turn
stagger is main thing to look at, if loose coming out, reduce stagger
increase cross weight
soften rt. rear spring or stiffen left rear spring
rt. rear tire over inflated

if you have under steer..( pushing..) do the opposite of above

so instead of changing the springs you can tune by the panhard bar..but..ultimately you want to run lower RC than the other guy and this will mean changing to different springs
 
Back again............Mike, Im curious about the front roll center location left to right in a pavement sprint car. Ive noticed that Beast and a few others have the stand off on the right front for the panhard mount quite close to the frame, where as my car has the stand off quite close to the coil over. Lets say for arqument sakes that my front panhard bar is 6" longer then on a Beast chassis. Assuming that the roll center falls in the middle of the front panhard , then my front roll center would be 3" farther to the right then the Beast chassis ( also assuming that they both mount to the axle in the same location relative to the LF ) ??? Now Bob East is no dummy and Im assuming hes done this for a reason. My friends tour car has a front roll center 3" under the left front tire ( Kelly Tanner's old chassis ) I have to assume this was done on purpose as well.

As allways I look forward to your response......Kenny
 

Ranger Mike

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I had to read your prior posts ..you have 10 high Roll Center..and run pavement, not dirt track
i know the Emod guys run RC offset 4 inch bias to right for jacking effect..but these Emod guys use the General Motors Metric frame..A-arms coil springs off big GM cars as cost saving measure..( cost savings and Racing is like saying you get lots of sex after being married for 5 years...yeah right...)
straight axle rockets like you have use leaf springs / panhard bar set up on the front which are least expensive, lightest , simplest form of positive locating method.
the axle does not move purely vertical, it arcs around the chassis pivot with radius equal to the length of the P rod, so either the axle or the chassis has to shift sideways a bit whenever the axle moves vertically up or down over bumps...you got perfectly smooth track , right??
one draw back is the spring tries to fight the panhard bar during movement so to keep the lateral movement to a minimum the longest Panhard bar as possible is used.
finally the roll center is located where the center line of the P bar intersects with the center line of the car so it moves some what when either axle travels vertically or the car rolls.
i would stick with offset to the right..dirt guys run offset to the left..which is their hot setup..exactly where is your front RC offset from vehicle center line?
 
Mike......my front panhard bar length is 31" OAL. From the center of the axle to the panhard mount on the left side of the axle is 11.5"
 
Mike.....one more thing. Should I make a " bolt-on " front panhard bar bracket for the axle so I can tip it up and down to keep the bar level as I move it up or down on the frame ??

Thanks........kenny
 

Ranger Mike

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Kenny
i am in dallas for sales meeting but will post when i get back..
thanks..
rm
 
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*Bookmarks and links to FSAE Forums, if you don't mind me doing so?*
 

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