Race car suspension Class

[missile07]

I can get tire readings this weekend. Both rear tires were hot and glazed over after the last feature race. I think they were just sliding and spinning.

 

[mender]

The chain is part of the chassis builders recommendations. Without the chain the left rear would continue to drop to the point of the shock topping out. We have ran without a chain before and pulled the shaft out of the shock body.

Then you either need a shock with more travel or you need to limit the travel via spring rate changes. Using a chain won't cure the problem, just keeps the shock in one piece.

As soon as any part of the suspension binds or locks or hits the end of a stop, the spring rate on the wheel effectively goes sky-high and that tire loses traction. If it is a solid axle like yours, it also changes the spring rate on the other wheel the same way, again causing a loss of traction.

By using a chain or a short shock on the left rear, you're making the right rear act like it has a soft spring when that car is flat and a hard spring when it hits the stop as it roll. This can change the handling rather abruptly with only a small change in roll angle and make the car a handful.

Get the chassis movement/roll under control like Mike is suggesting.

 

[missile07]

I wish I could attach a photo of the suspension on here for you to see.

The shock on the left rear does not currently top out. When the left rear drops it will continue to drop until something binds, whether that be the shock or the driveline(driveshaft into transmission). The chain is mounted from the top frame rail to the top of the axle tubing. When the rearend drops to 18 inches below the top frame rail the chain grabs.

I agree that this going solid effect would mess with wheel weights. What some people have done is make a rubber bushing limiter that when the chain is about to go tight it begin pulling on the rubber bushing. I guess this is sort of a cushion.

Again I understand the logic behind removing the chain but we have done this before and have ruined several shocks & bent drive shafts.

You talk about limiting travel with spring rate changes...how would I do that? Heavier RR springs?

 

[Ranger Mike]

i think that the 600 lbs springs are bottoming too much, upping the frt spring rate should help control the weight transfer to the point you will not run into chain bind. Right now, the weight is coming off the left rear and hammering the right front tire to the point it is skidding..not sticking.. stiffer rt frt spring will compress and then rebound and throw some of the weight back on the left rear where it can help drive out of the turn. right now , like you said,,you got a tetter totter going on..its lifting of the lft rear and sticking and staying up front,,
make sense??

 

[missile07]

I agree that maybe too much weight is staying on the RF instead of transferring back to the rear but the LR is always going to hit the chain, top out the shock, or bottom out the drive shaft through the motion of the 4 link suspension.

 

[missile07]

Here is an example of the chain and suspension I am talking about. This is not my car but one very similar. The chain is red and you can see it between the spring and front shock.

 

[HowlerMonkey]

There is no single process that works with all race car suspension.

The car's class determines ultimate suspension development.

An evil "anti-chapman" once said..."any suspension will work...if you don't let it" so it really depends on your very specific application and rules more than suspension theory.


That said, this thread is full of great discussion and advice for a variety of very different applications.

 

[Ranger Mike]

Thanks Howler,,glad to see another wry cynical racer has joined the fray..

I got a PM..from a racer who wishes to remain anonymous..

He drives his own dirt late model car. He is finding that the farther the roll center moves right during cornering while still staying above ground the better his car performs. He asks for input regarding this...

Secondly he finds that the left wheel camber going negative while turning left does not effect performance. How far right can the roll center move before it stops helping performance?

 

[Ranger Mike]

I got a PM..from a racer who wishes to remain anonymous..

He drives his own dirt late model car. He is finding that the farther the roll center moves right during cornering while still staying above ground the better his car performs. He asks for input regarding this...OK... if you read post 5 and 6 above it is a good review of RC height, location and Weight Jacking..
On dirt we use a very hard tire compound which requires a lower roll center to create more down force and bite. Lower Rc means more body roll and thus more side bite.

On dirt the RC is located to the Right side of the cars center line. See the diagram of weight jacking in post 6 above.

Secondly he finds that the left wheel camber going negative while turning left does not effect performance.
It may not if you are lifting the left front...need more info on this point,,

How far right can the roll center move before it stops helping performance...
We locate the RC to the right by 3 inches to create enough leverage ( using body roll) to plant the right front tire so it sticks on corner entry. This let's the rt front tire steer and turn properly going into and at mid turn. IF we place the RC too far to the right we have too long a lever... thus lifting the left front tire as the body rolls. This will load the rt. rear tire too much under acceleration.
If we locate the RC to the LEFT of center, the car will not turn well..not enuff force to plant the rt. front tire..car will push going in and be loose coming off the turns..

 

[HowlerMonkey]

I typed a long post and it disappeared...am not typing it again as I will be busy downloading data and reducing it from our testing at the space shuttle landing facility runway...maybe later.

I didn't have any counterpoint in that post to what was written by others in this thread.

This forum seems to be governed by logic and scientific method which puts it above most any other forum I have yet seen.

 

[Ranger Mike]

Thank you and I hope you will have time to review our efforts from time to time...very glad to have you notice our efforts here..
RM

 

[Fahlin Racing]

Vertical laoding creates downfroce on the outside tire so the more vertical loading there is the better the outside tire sticks during cornering.

I haven't read the book you are using here, but I have read Herb Adams', and he states that when Vertical Load gets too high (for whatever design is being used) is not good, in turn reduces cornering efficiency of the vehicle.

Good stuff Mike, albeit, thought I would add that into here to add more to the visualization on VL.

 

[Ranger Mike]

thanks Fahlin...good advice..
that is a classic "Push" developed by over loading the tire CF...thats what gets beginners confused when the front end pushes..is it the rear tires hooking up too much or are the front springs too soft?

When i first started " round tracking" I watched all the old timers hot lap and they would always get out of the car and walk around to the right front tire and put their hand on it to see if it was "hotter" than the right rear tire..tire pyrometers were way too expensive back then..they wanted to " make her loose and then tighten her up"...
it was all guess work back then...we had something like 7 cars racing from one small town of 10,000 people..
these were "Hobby Stocks"..big V8 in 1955-1957 Chevy, Fords were popular...we ran a 1964 Ford Fairlane. too much nostalgia..sorry..

racing is a compromise ..the chassis only really works with one set up at one particular time on one particular track for one unique set of tires at one ambient temperature...constant tuning is required to WIN...

 

[Fahlin Racing]

You can also mistake an under-inflated LF tire (in circle track) for causing a 'pushing-like' situation because the contact patch is very small, if there is one. When the RF is gripping during the turn, the inside is just slipping from poor tread contact from the tire deforming.

Not having a temperature gun in the tool box just makes things more fun I think. Checking tires pressures and by touching the tires themselves after qualifying/heats/features is a good teaching tool for beginners in circle track on what part of the tire is used most. It worked for me, we moved up in the field slightly just by adjusting our air pressures when we first started.

I can't remember exactly, but I think it was in one of Smokey's books (and others I am sure)that you want each tire within a few degrees of each other (hopefully possible in the situation that is present).

 

[missile07]

Mender,

I am very interested in learning more about how the spring rates change when things bind & what exactly that would do?

We still have yet to race due to rainouts & flooding here locally, so I haven't been able to test anything out.

I went back and looked at a bunch of the previous posts & reading and this is a really good forum!

 

[rwstevens59]

Northeast dirt modified chassis stiffness question.

Here we go again. Just read a blurb about a new chassis builder up here in our neck of the woods extolling the virtues of his new chassis one of which is a change made to the bracing design under or around the engine area which provides better 'chassis flex'. These cars are a complete fabrication using no stock parts, think big sprint car with a lot of added sheet metal. Solid axle front and rear.

Now the race engineer in me says flex is bad, torsional stiffness is good. If the chassis has good (high) torsional stiffness it will be more tunable with springs, shocks and anti-roll bars (if you run them). Low torsional stiffness means, to me at least, that I have a large undamped spring of unknown rate for a chassis and it does not respond well to changes. The claims that these cars are 'easy' to tune on may be true only because they don't respond much to normal tuning changes.

So what am I missing? Why do a lot of the dirt car builders feel chassis flex is a good thing?
For me, I just don't get it.

Ralph

 

[Ranger Mike]

Unless i missed something..i am 100 percent in agreement...

flexy flyer chassis theory been around since the straight axle was introduced...

in my opinion..Races are won by the car that gets there first. to do this..it s all about TIRES...period...if you have the best tire contact patch..you will stick better than the other guy and out corner him..given a half way decent engine and all things being more or less equal..
so how do we do this...by dealing with the with transferred when cornering..its going to transfer...nothing you can do about it..so you learn how best to deal with it..one way is to understand where the CG is and where the RC is and modify these as the rules permit,,,next is to calculate the amount of weight transferred and understand where it goes and at what time ..during the cornering process...that is why you have to understand suspensions and how they work ,,and even more importantly..how to make track side changes to deal with each situation...

 

[rwstevens59]

Thanks Ranger Mike, at least I don't feel so alone in the chassis 'woods' anymore, talking to myself. LOL :-)

Now if I could just get these boys to listen to me about those tires you mentioned...

 

[emccalment]

Digging through this forum, you guys post valuable information I'm dying to be able to understand better. While I'm still working on that particular aspect, I have questions that differ greatly from the current discussion thread I was hoping you could help with. Likely beginner questions, but then again, I'm a beginner. :)

Upon return from Afghanistan I'm going to be looking to buy a car for purposes of taking it to the track and really learning how to drive. Wilst reviewing a wide variety of cars I began to wonder not about initial potential for a car and its handling abilities, but for the modified potential of the cars I'm looking at.

My current questions are how the wheelbase, track, weight and center of gravity would effect a car's final performance abilities. To determine said abilities, am I going to have to go further in depth to type of suspension and characteristics of each?

I apologize if I'm asking in the wrong place, but I am enjoying this thread thoroughly.

Thanks,
Ezra

 

[Ranger Mike]

Welcome and thank you for the service to our country. I recommend you consider joining SCCA. Sports Car Club of America is 200,000 member organization for the amateur motorsports fan/driver. You can join a region and enter in the PDX event which is professional driving experience where you are given instruction by SCCA official on driving the actual track like Mid Ohio. A licensed driver from SCCA will sit in the passenger seat and tech you to drive the course..Then they turn you loose. My old car crew member burned out a set of brake pads on his Toyota Camry during one of these. Then if you really want to step it up, you can by something like a Miata and compete..various classes in SCCA have strictly stock all the way to Open Wheel like we run. all types of classes for all types of budget..I seen guys drive to the track, jack up the car and change tires and go racing..super economical..there is always a racer who can help you and all you need do is ASK
btw this is how you learn about understeer (push) etc,,,in a stock car and start to modify from there,,

 

[steve09]

hello everyone .
this fourm has a lot of info on racing all kinds of different car classes,and i,ve been reading
through this fourm off and on.my dad has three late models one limited and two super.
I help out on the weekends and we have recently got a shock dyno.not shure what valves
work the best and shims to use any info.we race a half mile oval asphalt with sixteen deg
banking. coilover shocks with adjustments for compression and rebound.also mike what
do you recommend for a good software program.

 

[Ranger Mike]

welcome..Steve 09 having purchased a shock dyno means you guys are very serious about winning. I bought one a while back. Got to dig my shock note up in the car hauler. What type software are you looking for? Usually the shock dyno has software with it?

 

[steve09]

hey mike
we have all the software for the shock dyno.but were looking for some geomentry
software.were using chassis r&d for the roll center but it's not as good at showing
you what to change as we thought.something more in detail than just roll center.
like what to change on the car when it's to tight in the center.with out hurting your
car out of the corner.kind of fine tuning to keep more consistant through the race.

 

[steve09]

hey mike
also we have been talking with some other race car builders and they all same the
same thing about right front camber gain.they want almost no gain from static to
dynamic.whats your opinion on that and other than changing spindles and uper
control arms is there anyway to slow down the camber change.
thanks for the info.

 

[Ranger Mike]

Software - Suspension Analyzer by Performance Trends
shows dynamic chassis changes in roll/bump...you have to input three axis measurement of chassis mounting points but well worth it..talk to Kevin Gertgen of Performance Trends Software to assist in explaining the math used for various set up calculations ( caster , camber gain, Toe Out etc..)they also have Circle Track analyzer that is a little less dramatic but quick and easy to input

 

[xxChrisxx]

Susprog.
It sounds almost identical to Suspension Analyser. I think you can download a trial to have a look at the interface.

 

[emccalment]

Mike,

I appreciate the quick response, and that is very much what I am looking into. Almost exactly in fact. I'm heavily considering a Miata with the LSx swap. But, I figure straight line speed can be had with almost any engine or through a swap, so handling ability is what I should make the basis of my decision on what to get. I'm not the type of person to swap cars regularly, so I'm trying to base my decision on where to start with handling capabilities. :)

Unfortunately I missed the opportunity to get into racing with a portion of my family that we weren't very close with when I was little. Oh how I regret that later into life, lol. Loving the discussion guys. Very informative.

 

[Ranger Mike]

we were racing Gateway International Race way in St. louis once and they had the "Miata Mafia" there..about 40 door slammers and man did they race...the next morning at the hotel there were all kinds of Miatas with bent sheet metal, mashed in hoods,,,a lot of contact...
this was a national event and miatas had towed from california, Canada, all for a cheap plastic trophy...those guys had a ball..perfect way to learn the ropes...you will NOT have this in regional events when you start out...
when do you ETS?

 

[emccalment]

You're not helping my desire to move away from the Miata a little bit, lol. But, as I'll probably need something relatively cheap to learn in when I get back, it might end up being a Miata anyways.

As for ETS, I'm on the officer side so it's really anytime I want after Q3 2012. Still working out when that will actually be.

 

[Ranger Mike]

being a former Captain of Infantry...watch your 6 and hunker down ..short timer...all the way!

 

[steve09]

Hey mike
I was looking at the performace trend software and it looks like something that will always
help.we just raced last week and won and were still trying to improve on the car. was you
able to come up with anything on the shock dyno,are dyno is a roerig and works real good
with the shocks we have been using.but not shure the valving is right for other tracks with
less banking and speed.

 

[Ranger Mike]

Great..checker always helps..shocks are the final stage of chassis tuning...sounds like you got the correct spring package to handle the weight transfer..

my notes are for 2500 pound late model, 3 point rear 58% left, 50 % rear, 56 % cross weight,
shocks are
LF -76
RF 76
LR-946
RR-95

 

[emccalment]

Do any of you fine gentlemen know where I can go to find any kind of statistics on modified production cars? People tend to use the Nurburgring for benchmark purposes on new cars, but I'm curious about benchmarks for modified cars. Any thoughts?

 

[Ranger Mike]

sorry emccalment ..i got negative knowledge of this..

 

[Ranger Mike]

Race Update- we ran road course on 95 degree F day..oth days..we put Hoosier R45 tires on the car..this is a hard compound..it took 4 laps to get the temps up enough to get half way sticky..

The driver noted the car rolled going in and the tires would snap ( at the limit of CF) and he almost lost it. We had just replaced the 3 way adjustable Penske shocks ..back from rebuild and ran on shock dynometer. To correct this we increased the dampening on the compression adjustment ..went from Number 3 setting to Number 4...more dampening,,number 6 being max. This made the shock stiffer when compressing..this helped but still not getting enough traction..we lowered the rear ride height a small amount and finally got the car to really hook up on the R45s. The tire pyrometer readings were right on the money. Won all three races.
Lesson learned _ the R45 compound is not the ideal compound..for our weight car on this track with this temperature. next time out, we go with R35 compound ( most often used..has a little more wear and is softer. We leave ride height as set and see if the car pushes ( understeers). The tire pyrometer will indicate which way to adjust.
one more thing..zero over heat problem since we added the oil cooler..