Automotive Race car suspension Class

[Luke Doolin]

Hello, another newbie here. I run an ex-Cup car in scca road races. 3200 lbs approx. When I have the chassis side to side level at front and rear I have about 250 lbs more lr weight than the rr. I know the car was a converted oval car so I can't move too much static weight around. To get more wt on the rr to balance it I have to jack quite a bit on the screws and then my ride heights get way off. The car gets a bunch of tilt (right higher) from jacking weight into the rr. It looks to be not right for a road race car. A buddy told me not to worry about it because the cross weights are actually fairly close... and to not worry about balancing the rear tire weights. Thoughts?

 

[Ranger Mike]

Welcome and good to have you going left and right!
You have a big problem. That car was made with lots of low left weight. Using jack bolts will not get you handling where you need to be. If i remember correct, the car is build with 1/2 " offset from center line. I suspect the engine, transmission drive shaft are offset. I know you have battery and dry sump mounted on left side and fuel cell is probably offset to the left too. Maybe even the rear end is bolted in offset from the center line. This will be a major rework project. Also you may have lower trailing arms of non equal length and odd angle to each other. Unless you can get 50-50 left to right weight you will have tuff time getting a checker.
Welcome to road racing

 

[Ronorlin]

Hey RR,
Thank you for the reply and the info. I will be back to my car next week and I already have Steve Smiths book! Thing is, I didn't remember about the camaro work he did because my car was all metric when I bought it. I had it clipped. (out of necessity) A wall at the Rock.
So let me get back into this in a few days. And I have a coleman spindle option allowed now. I will get the part number for you and discuss further. I am going to run out of time quick as I run at Memphis first weekend in Oct. Just won't have time to re-vamp it all by then. May need a "band-aid" approach for that race. But rest assured, I will this winter. I look forward to future dialog, and thank you. So i may have to run that 1" taller upper BJ on the RF to get the geometry close.
Ron

 

[Luke Doolin]

What about cross weights, if they are close or very close? Should I worry as much about the lr weight?
The car BTW, has even a arms, engine is evened up as the rear and trans...it's only when I put MY weight in that the problem surfaces...250 lbs is less than 10 % of total car weight.
What about actually adding ballast to the rr? Bad idea?

 

[Ranger Mike]

luke
On a left and right turn car the closer you get the left to right side weight the better you will handle. This is with driver and full load of fuel. Shift the battery to the right side. Add right side ballast. I would center up the fuel cell. You would only change front to rear weight if this is done as you burn off the fuel. Dry sump may be too much trouble to change over but think about this as you have several gallons of oil and it all adds up. One thing you can do is count up the large right and left turns, not the sweepers. Is your track biased to the right or left?

 

[Luke Doolin]

The battery is in front of the lr and that could be changed. The dry sump is in the lf fender behind the wheel and would have to be moved inside the cage on the rh side. Hot! unless I build a box around it. I shudder to think of actually adding ballast weight to a race car where there is no minimum but I have considered it before. Most road courses have dominant right hand turns with the exception of Daytona which has the two important right hand hairpins but the rest is pretty much left hand turns. I am talking about the tracks in the SE US, like Road Atlanta, Sebring, etc.
The cross weights are close but the lf is much heavier than the rf.

 

[Ranger Mike]

this is a great link to read when converting an oval car to road course..
left vs right side weight is 50 to 52% but you want right side bias so should be 52% on right side for the " hot setup".
I would think about re-locating the dry sump, oil cooler, and fuel cell to the right side. when you burn off the fuel you may end up back at 50-50 left vs right side weight.

http://www.hotrod.com/how-to/chassis-suspension/ctrp-0011-race-car-weight-distribution/

 

[drobbie]

Just saw this picture of our race car from last weekend. I softened up the suspension all around and now I see the RF rolling under (blue arrow). Trying to figure out if I should stiffen or soften the RF spring. Any idea why this might be happening? Several cars had this same look.

 

[Ranger Mike]

I believe the original problem could be defined as "lateral tire deformation". A tire is just a bag of air, and it's shape can be influenced by many factors.

When a car is cornering, the tire is attempting to deal with the immense forces pushing sideways. Where the tire meets the track, it deforms.
In the picture, it illustrates how the tire is being forced out of it's inflated shape by cornering loads. Obviously, one fix is just to add pressure. What tire pressure are you using on the rt ft. tire?

Tire temperature should tell the whole story. What is the temp at the three points on the tire? what is the Average temp of all three compared to the other tires averages?

 

[drobbie]

I am away on business right now so I do not have the tire temps in front of me, but from memory, the RS average was about 20 degree more than the Left and the rear about 8 degrees hotter than front. Also they did not look unusual with maybe a 25 degree (?) differential across the face. The pressure in the tires is 17.5 -18 hot (starting at 14). The car only weighs 1925 pounds to begin the race and 58% left and 49.5% rear. The track is a 1/3 semi banked oval with fast sweeping corners so you spend a bunch of time in the corner.

 

[drobbie]

Here is the RF tire temps 164 168 180
Average of the tires are LF 121, RF 171, LR 150 RR 178

 

[loganc_racer]

In a matter unrelated to the current conversation. I put to use some of the information I gathered from this forum earlier this year, and ended up 2015 Champion of the Street Stock division at Sandusky Speedway. Thanks Mike and others for your input.

 

[Jared0020]

Ronorlin are you running that GARS race at Memphis

 

[Ranger Mike]

good jog loganc,
sandusky is a beautiful track..you don't need me to win..you just need to learn this stuff and properly apply the physics..

droobie is the temps you posted with same roll over tire condition?
Tempes show right front tire is toed in too much or has too much camber. 20 degrees on outside edge indicates this. Fix this first.
The left front is way cooler than the rest and is not doing its job. Needs more static weight or needs to start carrying more of the dynamic load.
Even if the car feels balanced it will go faster if you can shift some ballast on left side forward. Right rear is 7 degrees hotter than the right front indicating oversteer. Right rear should be 10 to 15 degrees cooler than right front tire. Both rear tires should be 10 to 15 degrees cooler in fact. Left rear is not carrying enough load either. You are very close to ideal so a little tweaking can go a long way. I would shift more ballast to left side and get the heat up the left rear and left front.

 

[drobbie]

Ranger Mike thanks for the info and yes this is the same car. I can definitely make the frong end changes.
My goal all year has been to get the weight to the left front and currently there is 125lbs of lead at the lf. There is not a lot of room left in there for any more and I am well above the minimum weight now. Fyi, this is the only place i have lead mounted in the car.
I would much rather try to get the Lf to carry more of the weight dynamically if possible. What would you suggest?

Thanks for hosting thisforum.

 

[Ranger Mike]

thanks for the nice words..
are you running ARB (sway bar)? what is it?
what is car weight and what are spring rates?

 

[drobbie]

Yes to the Sway bar. #95 rate
Springs from this picture are LF 325 RF 375 LR 165 and RR 150.
Car weighs 1940 with 58% Left and 49.5 Rear. Our minimum weight is 1900 lbs after the race (25 laps) and max 60% left. We generally burn 2-1/2 gal of fuel for a race.
Let me know if you need any other info.
Doug

 

[Ranger Mike]

we ran an outlaw super late model on year with 2400# weight
had 220# arb, had LF 300, rt ft 325
lft rear 175 and rrear 150# springs...
the heavier car had darn near same rear springs as you run
i would consider ft spring change with only 25 # split and go to heavier ARB
like maybe 300 lft ft 325 rt ft but be prepared for possible too loose condition.
once you get it too loose start going up on spring rates..watch tire temps

 

[drobbie]

Sounds similar to what I am looking for.
How much stagger and cross weight did you run with this car?

 

[Ranger Mike]

droobie, this is straight out of the tech books..3/8 track medium banking, 55 to 57% cross weight
stagger is what works for you..1 to 4 inch typical ..try 3 inch, could be 3.5"

 

[drobbie]

Yup sounds like the numbers we ran in our late model years ago and not far from what we are using now. I'll be softening the front springs for next week's race. The issue I may have with the car in the picture is the ride height is only 2" at the LF and 2.75 at the RF. There really is no significant travel to be had. Too soft a spring and your hitting the race track.

But now, I have a another question we have been kicking around here, one to ponder and I am sure there would require some calculation and testing.
What if you used a lighter say bar and a stiffer RF spring to control roll instead. Could you get away with using less cross weight and possibly less stagger? Less stagger would mean less drag and less cross weight would allow the car to transfer the weight easier.

 

[Ranger Mike]

the photo of the car and tire temps all point a classic push going in-tight off condition. Where is the front roll center height and offset?

 

[drobbie]

Roll center is 3-3/4" above the ground and 1-1/8" to the right.

 

[Ranger Mike]

the front roll center needs to move to the right two more inches. There is not enough down force on the right front tire with current position. We need more down force to increase traction with body roll. The right front is plowing and the rt frt spring is too heavy. The left front can not lift under acceleration to plant the right rear tire. So we have a push going in lose off condition.

 

[Gilbert33]

Is there a way to measure the body roll in cornering..? I have measurements from acelerometers for lateral forces, but I would like to substract the effects of the body inclination when cornering. How it will be possible to measure the body roll degrees...? I'm talking of market vehicles...

A Gyro will provide that body roll measure along the Y axis..?

 

[Ranger Mike]

welcome gilbert,
you need to state the question a little more precise. are you asking how to determine body roll of stock production automobile?
what kind of car is it?
need a lot more info to proceed and this may not be best place for this question.
in general we measure body roll using rubber stops on the dampers ( shock absorbers). This tell us us compression on one side but have fabricated brackets and a rod to measure droop on the left side as well. This means drilling and cutting the car body..not good for you if its not your car...

 

[Gilbert33]

Thanks!

Well, yes I'm talking about stock cars. I have using my 2008 camry for tests. I'm using accelerometer to measure the developed cornering foce. But as the car transfers weight when cornering, I begin to be interested into how much it tilts, so I can compensate the amount of force measured by the accelerometer, due to gravity.

That's a excelent idea I had not in mind about measuring shock absorber displacement!

 

[Ranger Mike]

as cheap as go pro cameras are , you could stick two in the wheel wells and glue up a couple of 1 foot plastic rulers to measure things as well.

 

[Ranger Mike]

In Car Racing I can not stress enough that you have to keep your eye on the Checker Flag. Last race weekend we tried a new set of tires from a new manufacturer. total junk. My driver who has 30 plus years winning drove to second place only with great skill and personal tenacity. It sure was not the tires. The track was terrible on Saturday morning. 48 degree temperature, cold dreary day and NO GRIP. A pencil eraser would not scrub off on that track! Things did not go well in afternoon after dropping the air pressure. We ended up running the back up set of tires that had seen a lot better days. We got another second place and that is as good as the last race of the year did us. It was my call to go with the new brand..I had a couple long conversations with the tire engineer ...all very professional but you would not want to be in ear range when the car was sliding all over the track. Keep your eye on the flag and know the guy who screws up the LEAST will win, caz you will screw up..how much and how bad is the only question..but you will screw up..Top top it off some one backed into the tow vehicle and now I need some body work. At least the car is in one piece and all the wheels are pointed the same way...live to race another season..but them tires...

 

[Ranger Mike]

New car or refine older one??
recent rules changes in SCCA now permit the 1995 and older cars to compete in the class we race. My team has been in deep discussion on changing over our SF88 rocker arm suspension car to the push rod design car, RF95, that dominated the class since its introduction in 1994. We had to spend some time gathering data but the bottom line is as follows.
Both cars must use same power plant and have dimensional limitations on length width height and weight.
Our car has about 1.4” longer wheel base.
Rear width is 2 inch wider in the newer RF95 car.
and RF95 car has wider front by 4 inch and slight aero advantage as it is a narrower car. Is it worth getting a 5 year newer car?
Stay tuned...it sure looks like i will be rebuilding a new car next year

That is the question?