Race car suspension Class

mender

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?

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

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

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

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

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 lets 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

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

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?

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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 dont 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,,,