Automotive Race car suspension Class

[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?

 

[loganc_racer]

That's what I am wondering if there is a software that I can put in all the points as they really are on a G body metric chassis. I have just bought another car, its a sportsman car with a stock metric clip front and 3 link rear. I have a little more stuff to adjust than the street stock I formerly ran. If anyone has info on an affordable software. or can tell me if it is really worth pursuing.

 

[Ranger Mike]

By no means is this intended as a reflection on you, Loganc racer.
It is a good time at year end to put things in prospective.
Racing is not cheap. The old saw goes” how to make a small fortune in racing...start with a big one”...attributed to Roger Penske.
Racing is not affordable nor are any of its many requirements. The cost of racing is not as much in dollars as it is time and absence from wife, family, work etc...
You may question why any sane or questionably sane person would want to spend many nights under a piece of s race car on a cold garage floor tirelessly working to wring out another 1/10 second speed. Add to this the real possibility of serious injury or DEATH and you start to see why Racers are different. You start to wonder why the addiction to the sport and fixation on a checkered flag for a simple $13 plastic trophy?
The short answer is - because we are racers and hate to lose.
It goes much deeper than this and one would ask why some sailor would risk sailing to new lands for a shorter way to china and risk falling of the Earth (its flat, I have seen the map).
Racers are different. Second place is still losing and no one is happy with number two.

Back to original question -
I would rank good computer suspension software ($ 250 to $500) up there as a mandatory requirement with a good set of digital wheel scales ($1000) and a good pyrometer to take tire temperatures.
Sell the dog, buy the wife a new Mop for Christmas instead of that dish washer and get a good race software program.
Merry Christmas and remember...every one knows there were three wise men but no one remembers which one got there first. That's what no race software gets you in a camel race.

 

[loganc_racer]

Ranger Mike, Sorry I have trouble communicating what I really need to sometimes LOL. I get that. I spent over $10K of my own, no sponsors, this year. That's counting tires, pit passes, fuel, travel, everything. I brought home about $3500 and a championship. I don't mind crawling on the floor, but it sure would be slick to manipulate the changes on the computer. I've got the scales, pyrometer, and stop watch. The cost of the software is not what I am concerned about, unless its in the thousands then that would cut into my racing money. Being a machinist and having dealt with CNC 2 and 3d, I know that there are soft wares that are inexpensive but don't deliver. I was just wondering if the 2d measurements would really provide accurate enough data vs. 3d. I simply am looking for a recommendation.

 

[Ranger Mike]

Loganc, Merry Christmas...Congratulations , Champ!all things considered, 2D beats the pants off of no software. We used it for YEARS quite successfully. It is quicker to type in and is a lot easier to get a good picture of the suspension activity. 3D is a real pain to get all the data points. You have to be a gymnist squirrel into the nooks and crannies to measure inside rear points on a stock cart with fenders. It is a lot easier to use on an open wheel car where you can remove body panels and accurately measure data points. 3D is more accurate in the true movements of the A-Arms etc... but ifin you are only turning left, go 2D and you will be mire than accurate for Saturday Night warfare. You already did the hard part ( you got the championship). Software just makes it a lot easier next time.

Folks, do not forget..you can have the best damg software program written and it don't mean s*** ifin you can not make the car work.
It took Henry Ford over 25 Million dollars and three years to beat Ferrari at Le Mans. One million was spent on computer programming the entire track into a computer program in 1964 as i remember. They still didnt win that year due to poor heat treat of a drive line component.
Go 2D and you will be ok

 

[HowlerMonkey]

We use "AIM EVO-4 with smartycam" with suspension travel sensors, G data, and measure tire temp across the tires.

Then we put it on an alignment rack and use weight/jacking to simulate the conditions at various times during playback and see "where the wheels move to".

We also try to simulate the side loads and braking loads so we can read tire and suspension deflection.

This is synchronized to a track position overlay on the video with data that we can scroll through back and forth.

Most race car set-up is a compromise determined by the track, car, and driving style and you have to decide what point of the track where you want your car to be best in order to get the best lap times.

Many other factors you may need to include are tire and brake heat management as well as tire wear...which depends on racing series rules and car choice.

Of course, you want to it "to be best" on the entire track but you really have to pick where your car being it's best gives the best advantage...depending on the rules/car selection.

On some cars, it may be exit of the fastest corner to the longest straight and on others it may be on a long constant radius carousel turn.....it varies greatly.