Automotive Race car suspension Class

[Ranger Mike]

I been thinking about this a lot...i still come back to roll understeer since it sounds like it is pushing all the time..entering mid and exit the turn. it takes a lot of work but i think you should measure the wheel base as the suspension goes through compression and retraction to see if the rear end is cocking toward inside of the track in the turn.
I been involved in getting many many used races from racers who wanted out of the game..besides the safety consideration,,,checking quality of the welds, wiring, fuel cell and plumbing...we always stripped down the , took out the springs and mapped the suspension travel in bump and rebound..checked the bump steer front and rear and checked ackermann when we were rebuilding the steering...you have to do this in event you have a real bad crash and have to replace parts..thats why i love the software program for performance trends..shows the linkage action and roll center changes dynamically...
ifin you don't do this your just putting a bandaid on the situation

 

[rwstevens59]

Sounds like an interesting set-up. You're right, figuring the roll centre when the springs are essentially the suspension pivots can get messy. I don't do dirt, so take my questions with a pound of salt!

What little I know seems to indicate that most of the action is at the back, and your description of the suspension seems to confirm that. The roll oversteer to get the car to point, then power-on understeer to transition into a set for the rest of the corner means that the rear changes position noticably when power is applied, right? Could it be moving too much and unsettling the tires, giving the poor drive off the corners? I'd be tempted to set everything closer to neutral in a test session, then move towards the recommended set-up and see if the car really needs that and if so, how much is helpful before going over the line. Does the car feel unsettled with on-off throttle adjustments mid-corner?

Having the reverse front spring split would seem to be an attempt to help traction off the corner by partially compensating for the torque reaction in the diff when throttle is applied; with a narrow rear track, that effect is amplified, so playing with the track might indicate how much that is affecting things. Again, I'd consider reducing the amount of things happening at the back end of the car, then adding them back in as you get a handle on what each adjustment does. Again, finding the range of front spring split for your car might require a test session. From what I understand not a lot of people scale their cars after baselining for the season; do you?

Have you talked to the chassis builder about how his design is supposed to work? Has the car been wrecked? Have you tried alternate set-ups?

mender,

All of your suppositions are correct as I understand the basic design layout of these cars.

With 63% to 65% rear weight the cars basically drive off the back with the throttle. I'm not necessarily convinced this is the best way but it is what is currently being produced by three different car builders. The right rear will move at a maximum of 1/2" forward on throttle depending of course on the right lower link spring rate and preload (more preload reduces movement. With that much movement all dependent on available traction and 'finesse' with the throttle, as you might guess, dropped throttle response in the middle of the corner is not good.

I agree with your assessment of the reasoning behind the, to me, rather large front spring split. I am moving in your direction, in that I would like to see a more balanced car overall, starting with mid-corner, then entry and finally exit. I would like to get all four tires working as much as possible and right now that is not the case.

All I need now is more time and money, time being the most important.

Thanks,
Ralph

 

[rwstevens59]

I been thinking about this a lot...i still come back to roll understeer since it sounds like it is pushing all the time..entering mid and exit the turn. it takes a lot of work but i think you should measure the wheel base as the suspension goes through compression and retraction to see if the rear end is cocking toward inside of the track in the turn.
I been involved in getting many many used races from racers who wanted out of the game..besides the safety consideration,,,checking quality of the welds, wiring, fuel cell and plumbing...we always stripped down the , took out the springs and mapped the suspension travel in bump and rebound..checked the bump steer front and rear and checked ackermann when we were rebuilding the steering...you have to do this in event you have a real bad crash and have to replace parts..thats why i love the software program for performance trends..shows the linkage action and roll center changes dynamically...
ifin you don't do this your just putting a bandaid on the situation

Mike,

When I can get the car away from its owner and spend some 'alone' time with it all of what you have mentioned about mapping will get done. I'm fighting the usual battle of my slow engineering troubleshooting approach versus the 'hot' fix of the week from the 'rail birds'. I'm sure everyone on here knows the syndrome.

Ralph

 

[mender]

The overriding complaint by the driver is drive off the corner.

When you say the drive off the corner (dry?), is the problem caused by power application or roll? Not planting the tires or becoming a handful? I'm assuming the car is loose coming off, and also that you've played with the anti-squat.

Dynamically, the car will handle the best when all four tires contribute but there can be times when getting that requires a set-up that has a very narrow window of drivability. What is your driver telling you he wants to be different about the car?

How do you like Warren's book? He was my jazz improv instructor at university; versatile guy! I hope he includes a personal section in his next book describing some of the head games he used to play as crew chief!

 

[rwstevens59]

When you say the drive off the corner (dry?), is the problem caused by power application or roll? Not planting the tires or becoming a handful? I'm assuming the car is loose coming off, and also that you've played with the anti-squat.

Dynamically, the car will handle the best when all four tires contribute but there can be times when getting that requires a set-up that has a very narrow window of drivability. What is your driver telling you he wants to be different about the car?

How do you like Warren's book? He was my jazz improv instructor at university; versatile guy! I hope he includes a personal section in his next book describing some of the head games he used to play as crew chief!

mender,

Warren is a really neat guy. We have exchanged a few emails (I have been encouraging him to keep writing, although I realize he went through quite some period of illness and is just now catching up). From a practical and educational standpoint his first book, to me, is head and shoulders above the Millikens, but being the first of it's kind the Milliken book is still considered the bible of engineering texts and of good historical value, I guess. Being a toolmaker I love the two sided nature of his work-OK here's the engineering theory and now let's go out in the shop and build a test rig to see if we really understand this. I don't know where the man found the time.

Well, there's the other part of the problem. I'm not absolutely confident in my driver feedback yet as I am also playing driver coach. From what he tells me and what I observe the car seems to turn in well, but he has a tendency to over use the throttle early and over rotate the car, so he is sort of playing 'pitch and catch' at the apex which makes him late on the exit and then he is experiencing wheel spin off at late exit. So...some is chassis and some or more is driver induced, at least in this old mechanics mind (blame the driver when all else fails :-)).

I have tried more and less anti-squat. The car looks better to me coming out of the hole with the higher anti-squat, but same driver complaint. I want to try taking gear out of the car to reduce the wheel spin and force him to drive smoother, albeit maybe slower, but have not been successful to date.

It's just not that simple! :-)

Ralph

 

[rwstevens59]

mender,

Forgot to mention the 'dry' part. When a dirt track has high moisture content at the beginning of the night during hot laps and qualifying it is much more forgiving on the chassis setup, more driver than car. As the track drys and gets hard packed it can go one of two or more directions. One will be the dirt will start to 'take' rubber just like the groove on an asphalt track, sort of. The other is a condition where the track stays dry, does not take rubber and the dirt continues to abrade into powdery dust. The later condition is what we face most in the northeast in short 25 to 30 lap races. In longer events you can almost guarantee the dirt will take rubber. In the dry abrading condition the setup becomes more like asphalt, but think of running asphalt that is damp or oiled down. Available grip is very low.

Ralph

 

[mender]

It's just not that simple! :-)

Words to live by!

I hear you on the feedback, sometimes what the driver feels isn't what it looks like from the outside. What I find helpful is to have the driver think about what he needs to do to hit the highest speed at the end of the straight, but only if your guy isn't afflicted with late braking syndrome! Is he positioning the car properly on corner entry? Sounds like he's apexing early as well; late is great, early is squirrely! Talking about the speed at the end of the straight can help with that depending on the driver; sounds like he's trying to get through the corner rather than off the corner.

I'm not sure how much good taking timing out would help but it may help point which way to go with the gearing without as much work. Might be worth a quick try but just watch the corner exit, not the lap times.

Tuning the driver can be tricky, but if the driver isn't fixed first ...

 

[mender]

mender,

Forgot to mention the 'dry' part. When a dirt track has high moisture content at the beginning of the night during hot laps and qualifying it is much more forgiving on the chassis setup, more driver than car. As the track drys and gets hard packed it can go one of two or more directions. One will be the dirt will start to 'take' rubber just like the groove on an asphalt track, sort of. The other is a condition where the track stays dry, does not take rubber and the dirt continues to abrade into powdery dust. The later condition is what we face most in the northeast in short 25 to 30 lap races. In longer events you can almost guarantee the dirt will take rubber. In the dry abrading condition the setup becomes more like asphalt, but think of running asphalt that is damp or oiled down. Available grip is very low.

Ralph

I ran a few hot laps in a dirt 360 Sprint car on a track that was good on one end and dry on the other and really enjoyed running it out on the good end. Really late apex on the dry end though!

 

[rwstevens59]

On the subject of chassis engineers who write, I also find Mark Ortiz's monthly newsletter to be quite enlightening at times. My only problem with Mark's writing is that dummy here needs pictures most of the time. Seems I can't talk for more than about half a minute about a chassis, or most subjects I'm interested in for that matter, without breaking out my pencil. Just a visual type of thinker I guess.

Thank you gentlemen for your input. Time to go start making some educated changes and see what happens.

Ralph

 

[Ranger Mike]

Ralph..please let me know if i can help..and i am sure you are headed for that checker..Mender and others here offer priceless advice and you have one valuable asset in your virtual tool box here!
RM

 

[missile07]

I am new to the site and I have been reading on a lot of the setup posts. I have a few questions that I can't seem to get anyone locally to answer.

I run an all 4 bar modified suspension with the spring behind on the left rear and the shock/spring in the front on the right rear. What would happen if I ran a shock/spring on the left rear behind the rearend?

I have read posts where it will will tighten the car & some where it isn't an advantage.

Just from my thinking it should loosen me off the throttle yet tighten me while on the throttle.

Any input?

Also how short would be too short of bottom lengths? I know a huge factor is track size and traction but we are running a 12 inch bottom bar on both sides and it seems to be excessive to jump up on the bars. Would a longer left rear bar give traction?

 

[Ranger Mike]

4 link rear suspension is designed to use dynamic roll oversteer to turn the car. see post 116 above . Usually the 4 link has 2 forward facing radius rods on each side running from the "birdcage" on the axle housing to mount points on the chassis. The Panhard bad is used to keep it all in place . Usually the spring s are mounted on sliders IN front of the rear end and shocks are mounted on the rear side of the rear end. the big points of the 4 link are that it creates huge forward bite because the top link angles are running up hill. any time a link going forward is mounted on the frame higher than the mount point of the axle, the rear end will try to go up under the car during acceleration and this really loads the tires as the car is being pushed forward. Newton again..the upward angled arms are reacting against the weight of the chassis thus providing more tire load..its called axle thrust.there is usually a torque link added to cushion acceleration. Typical set up numbers are top left arm angle 13 degrees , length 17 inch minimum.. top rt. 17 degrees, bottom radius arms are both 2 inch shorter and both running down hill at 5 degrees. running equal length lower arms would create too much roll over steer.
l
As the car enters a left hand turn, the body rolls and the right upper link levels out. this lengthens the right side wheel base by 1/8 inch. the left top arm gains a lot more angle and the left side wheel base shortens by 7/16 inch and we have roll oversteer.

Regarding you set up..do you a have a bird cage? is the left rear spring on the birdcage or on separate clamp on bracket?

 

[missile07]

My car is a setup to the manufacturers specs with the left rear consisting of a shock in front of the axle on the birdcage & a spring on a slider/coilover behind the axle on the same birdcage. The right rear has the shock and spring together on the front of the birdcage. When the car enters the turn the left side bars increase like you said in your post and the bar angles increase. Where would be the point to which forward traction is overtaken by loose roll steer?

I run a 16 in top bar on both sides and a 12.5 in bottom bar on both sides. These are the manufacturers settings for this year of car. Normally throughout the race night I will shorten the right side bars 1/4 inch. How far can I shorten these bars in the slick and benefit before I begin to hurt my setup?

 

[Ranger Mike]

the Manufacturer of this set up probably refined this design over the years. In racing,,ifin it works don't money with it..from an out siders point of view...the shorter radius rod to me mean the car will be moire twitchy than if longer bars were used..but not knowing the mount points and theory of why the manufacturer did it?? hard to say..the difference in rear spring mount locations means the spring motion rates are different. You may be running higher spring rates but t he wheel rate is lower. Look at the spring mount point to chassis mount point distance on the two...regarding hooking up the tires on slick track..are you familiar with 4 link indexing?

 

[missile07]

Yes to the indexing. Our birdcages have two sets of holes on the top part of the cage. We can move the bar down on the holes to index. We don't do this very much.

I agree with you that the shorter lengths make the car unstable. It has mounting holes that would increase my bottom lengths an inch at a time. It seems a shorter length would make the car jump up on the bars quickly but would also fall off quickly.

Is there a such thing as too much understeer? As in leading the RR too much? We usually go to even or maybe 1/4 inch but have never tried more.

Also a lot of the old theory when the 4 bar first began was to clamp the shock in the front to wrap up with the axle in the slick on the left rear.

 

[Ranger Mike]

if i remember correctly the old muscle cars mounted the rear shocks similar..left in front of axle and rt to the rear.. you can set up the car with too much rear over/under steer..what you want is max amount of rear steer DURING BODY ROLL..but zero when the car has straightened out..let me dig up notes on indexing as this is the second best method of tuning..vs angle of radius rods,,,

 

[missile07]

I am afraid that the amount of rear steer caused by the left rear going forward is breaking the tires loose when picking up the throttle.

Also we run a limiting chain on the left rear to keep the rearend from dropping so far...could this be pulling the left rear off the ground? it seems like playing with the car on the jack stands that when you jack up on the left side frame and the left rear tire begins to come forward, the weight moves to the right side and the left begins to come off the ground. in race conditions (dry slick) this would seem like there is no weight left on the left rear with all the steer and weight transfer to drive off of the left rear.

Sorry if that was confusing. If you get a chance look at our cars on www.cherrymotorsports.net[/URL] and how they are responding in photos. This may help

 

[Ranger Mike]

this is just my opinion but i think you turned the suspension into a three legged milk stool when you chain stop a corner...this makes a big go cart out of a flexy 4 link suspension..a few of the photos look like the car is NOT transferring equal load to the front tires. tell me about the front set up..what are the spring rates? you using metric a-arms or straight axle?? front sway bar??

to me if the tires spin coming out of the corner you have to adjust the torque link to cushion the shock of matting the gas pedal...thats what its for..
unless the car came with the chain tie..it is a bandaid and i think we have to look at the front springs to make the 4 link work the way it was designed to..

 

[missile07]

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.

both of our cars are GRT chassis and run the same setup. Chevelle front end with 600 lb springs on both sides as suggested by GRT. we run a stock lower a-arm with a tubular uppers that have been moved from the orignal stock location by GRT. they are adjustable for caster & camber.

I think the general idea of the LR chain is to allow the left rear bars to gain angle and rear steer but stop it from going to far. I was told once before to think of the chassis as a seesaw. If the right front goes down then the left rear comes up and opposite. So i would think instead of going to a lower right front spring we would want a heavier spring to add weight to the left rear?

 

[Ranger Mike]

I am with you on stiffer right front spring.
I would stick a 750 lb. spring in right front... i like making one huge change until the car dramatically changes then backing off until you get what you want.

from my set up notes 750 rt ft..700 left ft
200 rt rear, 225 lft rear. 2300 lb carIs there any way to get some good tire pyrometer readings on a dry track?
alternative is tire wear gage,,.

we need to get all four tires working to carry as much equal load as possible. ..

 

[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.