Race car suspension Class

[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?

 

[GoneRacin]

Can someone explain why a vehicle's roll center axis is typically not HIGHER than the center of sprung mass?
Can we discuss some advantages and disadvantages in designing a race car with the roll axis higher than the center of sprung mass?
Thank you, in advance.

 

[Ranger Mike]

welcome goneracin
pls see page 12 post #229
RC over the COG ..not a good idea

 

[GoneRacin]

Thanks Ranger Mike.
I have read the post.
The part I am having difficulty understanding is, "If the roll center goes over the Cg, the chassis will actually "jack down" and start to roll to the inside of the turn."
Why would this be a bad thing?
Please note that the roll center would always be higher than the Cg, not "flipping" over the Cg.
Thanks again.

 

[Ranger Mike]

When the RC is intentionally over the COG you have just turned a cars suspension into a very smooth riding Go Kart.
The lateral force is shearing the tire patch and not being scrubbed off in a controlled manner with the cars suspension springs and dampers (shocks).

Yes, the body rolls, and compresses the springs ON THE WRONG SIDE.

You are helping to plant the left front tire and unload the right rear tire in a left hand turn. Pushes going in loose off the turn.
The ‘”hot setup” on short track cars is to offset the roll center to the right (below the COG) so the body roll produces download of the right front tire and increase grip of that tire. It should be the hottest tire temperature on the car followed by the right rear tire, lft ft, ft rear tire.
If you did have a super sticky tire on the right front that could handle all the lateral shear force because the RC is so high the angle of jacking force from the RC to the tire contact patch would be 55 degrees and you would lift the left front tire.

 

[GoneRacin]

Thanks Ranger Mike.
Let me ask a similar question, but in a different way.
Most of the textbook examples of roll couple distribution, roll axis, etc. assume the the roll axis is centered and the chassis will roll toward the outside of the corner.
Now, knowing that the roll axis can be offset and that it moves dynamically, it seems that it would be possible to roll the chassis INTO the corner. (Similar to a motorcycle leaning INTO the corner.)
An example would be a circle track car with a low Cg and high left side weight % on a very high banked corner with stiff springs on the right side, soft springs on the left side and a roll axis located near the right side tire contact patches.
Certainly this chassis could lean INTO the turn and not roll toward the outside.
It seems to me that there could be some advantages to leaning the chassis INTO the turn.
What would be the disadvantages?

 

[Ranger Mike]

Most of the textbook examples of roll couple distribution, roll axis, etc. assume the the roll axis is centered and the chassis will roll toward the outside of the corner.

My assumption is that it is easier to explain these chassis dynamics with a symmetrical set up. Laziness on part of the books author.
A motorcycle rider can shift his weight ( COG) to counter the effects of momentum ( in this case cornering force). A race car can not shift its COG as it is fixed.
A circle track car with a low Cg and high left side weight % on a very high banked corner with stiff springs on the right side, soft springs on the left side and a roll axis located near the right side tire contact patches WOULD PUSH LIKE A FREIGHT TRAIN.

Certainly this chassis could lean INTO the turn and not roll toward the outside. YOU HAVE TO HAVE GRIP TO DO THIS AND YOU WILL NOT WITH THIS SET UP.

It seems to me that there could be some advantages to leaning the chassis INTO the turn. SUCH AS?
What would be the disadvantages? NOT USING THE SUSPENSION TO HANDEL THE CORNERING FORCES TO THE DRIVERS ADVANTAGE.

I think you have not grasped the basics of handling regarding a left turn race car. Some how you got it in your head that a car will handle better if the chassis leans to the inside on a left hand corner. Take time to draw the force vectors and you will see this.

 

[GoneRacin]

Hi Ranger Mike.
Yeah, I guess your right.
It was just more of a thought process, for me.
I had been told that the car would ALWAYS ROLL and we had to deal with some of the associated "disadvantages".
I was trying to think of a "new" way to get the car to lean into the turn by moving the roll axis.
Some of the advantages ...
It may feel more comfortable to the driver.
Less camber loss on the left front tire.
The front anti-roll bar could be eliminated, thus saving weight.
Better for fluids such as fuel in the carb. bowl, gas tank and oil in the pan.
Areo advantages by keeping the left front down and the right rear up.
In the unfortunate event of a rough track, the chassis would drag on the track surface as opposed to digging in or plowing it up.
Passenger cars... not spilling your favorite adult beverage.

 

[Ranger Mike]

pretty logical thought process..keep up the good work!
rm

 

[no friends at]

Ranger Mike ,#253, when you say left side of the car. Is that looking from the front or rear of the car. Left is driver , right is passenger? I have a 30" pull bar and 23.5" lower control arms. Are these to short for a 107" wheel base and 2200 lbs. of car? Dry sump tank, radiator. trans cooler, battery and fuel cell are all mounted in the rear half of the car. Only the radiator pod is aft of the axle. Engine is pulled back 12" and off set 3" to the passenger side. The chassis is 4" off the ground. Working on the body next. Looking to rake the windshield to super car status for less drag. 23 degrees from stock 35 degrees. Am i missing something besides a brain? lol Thanks RM

 

[Ranger Mike]

welcome no friend,
Left side is drivers side- always ..when I talk about it...to help you remember...when you see a driver who porked up over the winter..just tell him..more left side weight!

it a pit joke..
20 to 24 inch trail arms usually.
I assume you run paved track with three link.
in my opinion you can not change things unless you know what you got.
buy chassis software and bench mark the car.
front roll center – where is it ? how high is it? same on rear Roll Center
what is height of camshaft ( center of gravity )
what is left side weight % rear % , cross weight %?
do you have rear roll steer?
these are just a few data points you need to see where you are and change to a better handling car.

 

[Ranger Mike]

Thank you racers and friends for the interest in this topic. 1000 replies...wow...

We just sold the formula car...lasted 4 hours on the internet...we are getting a newer formula car and will go step by step thru the benchmark process so you can follow along...
Again , thank you for the interest and please support this whole Physics Forums (every small donation helps).
This entire Forum is a very good and very economic (CHEAP) learning tool. These advisors and contributors are a class act .
thanks again..
rm

 

[drobbie]

Thanks for hosting this forum, Ranger Mike. I personally know it has increased my knowledge of racing and almost won us the championship this year. (tied in points and lost on the tie breaker). I look forward to learning even more in the future as I "tweak" the front end of my car. THANKS!

 

[Borg]

@Ranger Mike - You're about to get a new customer. A coworker mentioned that he races cars and was looking for ways to improve performance. I showed him this thread and he got very excited to find such a great resource. I expect he will be asking questions after he has had some time to go through all 51 pages of posts.

 

[the dr.]

Ranger Mike, I was wondering if you have tried the sim programs like Rfactor or IRacing, an if so what do you think? I have not tried IRacing, BUT I know that some of my experiments I have tried on our dirt car, have been tested with Rfactor, and it is scary how much alike they are, as long as your comparing apples to apples, I have stepped out on a limb a few times over the past two years trying caster and camber changes that everyone thinks is nuts on the track we race on but when I tried them in the sim program they seemed to work, and on track in testing they have matched almost exactly what we found in the computer. I think if you can get the same car in the game it could be a great learning tool as well. just a thought..

 

[Ranger Mike]

thank you for the upgrade..i have not used sim programs...not that computer savvy...i would need some data results and comparisons to rely on this..the reason i say this is because of all the variables in racing. On the sim program i assume you have constants like turn radius, length ,banking etc..some constants are vehicle length,track width etc..variables are weight, cross weight etc...but there area lot of other variables i doubt are covered..i would have to look at completeness of the software

 

[the dr.]

actually it is like a game, a racing game, you pick whhich track you want to run, and then pick your car, you can change tons of variables on the car, http://rfactor.net/web/rf1/ this is a link to it, while it is not liike a major info cashe, it has been great way to work thru some hairbrain ideas I have had ...lol it is basic for sure as far as a info / data base thing...

 

[95 Racer]

I recently stumbled onto this forum while searching for a specific solution to a rear suspension question,still reading, I think I'm getting close :-) .
When I noticed the discussion mentioned iRacing I had to sign up and chime in.
I agree with the dr., these "sims" are a great resource. I especially like them for improving my race craft, an excellent way to improve muscle memory.
One of the top iRacers Ty Majeski just finished 3rd in the Snowball Derby and I'm reading more and more about racers using it to learn new tracks, etc.
Another plus side is the ability to run telemetry in the sim, although I haven't been able to do much more than scratch the surface in that regard.
At any rate I'll head back to page 4 of this thread and continue reading and learning.

 

[chop6259]

Can you help with some clarification. Is the svic the point where the projected lines created by the inboard upper and lower control meet, or is it the point where the instant axis intersects the tire center line plane. If it is the later, this would put the IC further back in the car (when talking about the front suspension of course) as compared to the former correct? So is the later or the former the one that should be used in any dynamic calculations?

Thanks,

 

[Ranger Mike]

what is a svic? are you using some kind of software program?

 

[chop6259]

what is a svic? are you using some kind of software program?

Side view instant center. This was actually supposed to be a reply to thread #314. I am writing my own set up application for some specific type of racing. I haven't been happy with the out of the box stuff I have seen, and I can't afford to buy the programs that I think can do what I am trying to do.

 

[Ranger Mike]

The Performance Trends Circle Track Analyzer is a 2 D program with animation. I also use Suspension Analyzer from Perf Trend that is 3-D and has animation to see RC in roll and dive.
both sell for $250 last time I checked..call 248-473-9230 , its worth it.
Suspension Analyzer constructs Instant center from top outer ball joint thru a line between the front and rear upper A-arm or wish bone mounts. This projected line is perpendicular bisector to the opposite IC. The other projected line is from lower ball joint thru a line between the front and rear lower A-arm or wish bone mounts. This projected line is perpendicular bisector to the opposite IC.

 

[chop6259]

Thanks Ranger Mike. I have looked at both of the programs you listed, and they both appear to have good front suspension geometry. Unfortunately, they don't have the rear suspension geometry that I am looking for in order to develop a full vehicle mode. The svic, used to determine anti-dive etc, is described in Race Car Vehicle Dynamics as the point where the instant axis intersects the plane of the tire center. I am simply looking for someone to help verify that this is the point you should use for dynamic calculations, or should you be using the point that is generated by the intersection of the lines generated by the forward and rear points of each inner control arm mount. I believe I should be using the point described in RCVD, but just looking for a second opinion.

 

[Ranger Mike]

I recommend constructing your Instant Center from top outer ball joint thru a line between the front and rear upper A-arm or wish bone mounts. This projected line is perpendicular bisector to the opposite IC. The other projected line is from lower ball joint thru a line between the front and rear lower A-arm or wish bone mounts. This projected line is perpendicular bisector to the opposite IC. I like things that are simple. I like both wish bones swinging parallel and in the same arc. This means the two lines contracted above will meet at a single point in space for the IC. I also like a single center point on each front tire to make the contact patch for calculation. Not a plane but a center point of the tire center line and perpendicular bisector to the pavement. You need this for 3D modeling. I am not a big fan on anti dive anti squat.

 

[loganc_racer]

Looks like this conversation is heading in the same direction i am going. Please excuse me for not using the proper terms or misusing them. My question is do these computer programs allow the mounting points to be put in as they are on the car. In other words the mounting centerline of stock lower arms is not parallel to the centerline of the car nor are the upper arms and the uppers are not parallel to the ground either. I take it that this is the anti dive you are referring to. In the past I did what I believe you were describing in #1013. if I were able to use the actual numbers, 3 dimensional I guess I would describe them, what error would there be to the roll center between the two methods.

 

[Ranger Mike]

from my experience you can modify the mount location a bit from " stock". The programs 2D and 3D will do the math to show roll center. You can blow up the program by entering data that locks up the calculations. I think you will find that the A-arms swing in same arc on both the upper and lower A-arms in spite of being non parallel to the " floor". If this is not the case ...who knows what's going to happen when you have two different swings?