Automotive Race car suspension Class

[Ranger Mike]

Mender..i hear you..only prob is this forum would not appreciate it unless...we could state the tip relative to physics and application there of..we " old timers" got a wealth of info that will disappear when we take the ultimate dirt nap...maybe..ifin there is enuff interest, and we had some formal schooling help from the advisors..we could document these speed secrets relative to the particular principle of physics that it applies to?

 

[mender]

Good points; I agree with you regarding the intent to keep things "sciency".

Maybe we could get an indication from one of the admins?

 

[mender]

No comments?

Well, since this thread has a rather remarkable 245 views per post, one might assume that it's in the right place and the people who frequent this forum are interested in the topic and that it's safe to proceed!

 

[Ranger Mike]

fstock34 and the Dr. ...would you like to share your chassis set up information with the public? I will contact ADMIN regarding recommendation on this subject..mean time, can all those racers, Engineers, old hands that have contributed to this thread, please give us input.

Prankster16a ..want to help?
My thought is that we have two project cars available - fstock34 and the Dr. One is a dirt car and the other is an asphalt car. I think the folks at the software program I use would be receptive to assisting with the tech data regarding the math used. The purpose of this endeavor would be to demonstrate the use of Physics when applied to race car set up.
xxChrisxx please let us know..Dr. Dodge? Turbo-1? Brewnog? Input?

 

[Ranger Mike]

ok guys
step up and give me yer opinion

 

[Ranger Mike]

This thread has been viewed by quite a few and was intended to discuss the design of various suspension components. It has started to turn into a set up help line and general racing tips thread. I just received a PM from the PF Mentor Russ Watters and Kevin Gertgen from Performance Trends software. Both encouraged me to start a new Thread to discuss Race Car Set- UP. Russ thinks the Mech Engineering forum is best place so will start to work on a new thread.
RM

 

[rwstevens59]

I'm new here so, hello all.

A question that I keep asking myself and many others, seems to have many answers, so I thought I would consult this group.

Spring rate and wheel rate for an independent front suspension was covered very well at the beginning of this thread and is covered in many references the same way.

Now my question is from wheel rate we can calculate the roll resistance of that independent front suspension (as well as an independent rear suspension) and we can compare roll resistance front to rear. The problem comes in when I ask or try to research what the roll resistance is for a car running solid axles front and rear with spring split, and with the chassis offset to the left for oval racing. There are many simple explanations of solid axle cars as long as a. The spring rates are equal left to right and b. The car is symmetrical about the average track center line. Does anyone have a reasonable answer or method for figuring this out?

Thanks,
Ralph

 

[Ranger Mike]

Welcome Ralph
maybe I'm missing something but you said " Now my question is from wheel rate we can calculate the roll resistance of that independent front suspension"...the only roll resistance i know of is regarding tires..are you talkin about the amount of weight transferred when cornering?

 

[rwstevens59]

Sorry, Mike,
Terms and being very specific about what you mean always seem to get us in trouble in this complicated business.

I am referring to the amount of roll resistance provided by the spring pair at one end of the car. In a steady state corner when the inertia force acting at the CG rolls the car right in a left hand turn the spring pairs front and rear produce a resisting couple (torque) to counter that roll. My use for that is, that in load transfer calculations, the resisting moment produced by the spring pairs front and rear is considered the elastic portion of the sprung weight transfer. The other portions of weight transfer are the sprung geometric weight transfer transmitted through the suspension links, this governed by roll center heights. And the unsprung weight transfer, which is basically uncontrolled, except for tire spring rate, caused by the rolling of the unsprung components such as a solid rear axle housing.

The quick and dirty formula for this is: K(angular)= 1/2 Kroll * Spring track width^2/57.3 which yields an angular roll resistance for the spring pair in lb.in./ deg. The kicker is what is the value for Kroll which should be in lb./in. for a spring pair on a solid axle with split and an asymmetric layout. For simplicity let's assume the springs just sit on top of the axle with no linkage to add a further motion ratio to the mix.

All I really want to know is how stiff is my elastic (spring) resistance relatively speaking front to rear. The stiff end gets the weight transfer and therefore the more unequal tire loading left to right and therefore less grip. If you stiffen the front springs relative to the rear at some point it's going to push.

Hope that makes some sense.

Thanks,
Ralph

 

[Ranger Mike]

as you said at the start - The problem comes in when I ask or try to research what the roll resistance is for a car running solid axles front and rear with spring split, and with the chassis offset to the left for oval racing. There are many simple explanations of solid axle cars as long as a. The spring rates are equal left to right and b. The car is symmetrical about the average track center line...
As I posted earlier in this post, we can calculate the total weight being transferred during cornering if we know the G force in the corner. The left side weight of the car will not change this total. Regarding the equal spring rates- these are ok if you are running a road course but not optimum if your racing left hand turn only. Cross weight or diagonal weight transfer from left rear to right front must be in the mix as well. This is why the right front spring has higher spring rate than the left front spring..rear springs are also not equal rate.
question - what kind of front spring set up do you have?

 

[rwstevens59]

The cars I am referring to here are Northeastern DIRT Modifieds if you are familiar with this 'breed' of car. The solid front axle has coil over front springs mounted just behind the front axle on the radius rod brackets, think of a T-bucket roadster hotrod only without the dropped axle. The cars are 2500 lbs and the average setup calls for a 250 lb/in LF spring and a 150 lb/in RF spring. The rear is a solid tube Winters quick change (not live) with a spool. The rear is suspended on a pair of torsion bars (sorry don't have my setup notebook with me and I forget the exact calculated spring rates at the axle). The rear torsion arms rest on rollers below the axle centerline, so unlike a sprint car they are a spring only and do not play a role in longitudinal location of the axle.

There is a one line mention of calculating spring split roll resistance in Milliken in the chapter on Ride and Roll Rates can't remember the page. And quite a bit in Warren Rowley's book on race car engineering dealing with solid rear axles. Both of these sources state that a pair of springs (or wheel rates on a front independent) provide a roll resisting torque equal to: K(left)*K(right)/(K(left) + K (right)) with K(left) and K(right) being the respective spring rates in lbs./in. This produces an offset spring center located somewhere closer to the stiffer spring. If you plug this value in for K(roll) you get an awfully low roll resisting moment in lb.in./deg. Now springs on any beam axle produce a much lower roll resisting moment than their respective ride rate just because the springs are inboard of the wheels with a spring track that is quite a bit less than the track width. All of the above (spring center calcs) I can prove from first principles just using a simple beam supported by two different rate springs as my model as well as just placing a bar over two small springs with different rates, shim the soft side up to level on a surface plate and find the point where the beam rotates evenly with dial indicators. All well and good, but is that how the car behaves and when you start adding in the asymetries of the car CG centerline being offset 4 inches to the left, I'm starting to get lost in the woods without a compass.
Ralph

 

[Ranger Mike]

I admire all the work you did on the calculations..more than I ever did.

About all I can offer is from all my notes on Dirt modified using metric GM chassis days, I can only reply with the following:
We ran 100 lbs stiffer spring on the right front vs left front...was 3 link rear suspension.
55% left side wt. 56% rear wt. 51% cross weight. if my notes are right we used 200 lb springs on both rears..
why are you using a higher rate left front spring? This is a "reverse Split" and can be used to keep the car flat at tracks with large amounts of banking or for when you want the car to lean over on the right rear...right?

Are you running a big wing on top?

 

[rwstevens59]

That is the chassis builders recommended setup and brings us to why I am scratching my head. The bodies of these cars don't have a big wing on top but do create a fair amount rear down force which leads to an aero push almost always. The rules really don't allow for a proper aero balance. The tracks we run are flat, less than 10 deg. at 5/8 mi. and semi-banked bull ring at 1/4 mile. I am questioning the spring choices and trying to decide which way to go and by about how much. I don't just want to take a WAG at it because the car is close. The overriding complaint by the driver is drive off the corner. Now, I have never believed you can calculate your way out of these situations because it is just too dynamic. But, calculations will give you clues as to whether you are fine tuning or making major changes. The other problems as always are money and time. We are now into the season, hot laps are just that and only tell you nothing fell off the car and the engine is running OK. The first opportunity to see what you have is in the heat race, experiment too much, and you don't qualify well. Doesn't offer a lot of space for test and tune. But if we stick to the 'Golden Rule' and change one thing at a time and only one thing at a time we will improve if for no other reason than finding out what doesn't work.

I have asked this question concerning roll resistance of spring pairs on solid axles several times in different engineering circles and am finding out that it really isn't simple and it is actually easier to calculate on an independent suspension. The seemingly simple beam axle becomes quite complex when viewed dynamically.

Ralph

 

[Ranger Mike]

i would equal up the front springs as change one.
when is the driver needing bite off the turn..on wet track/ tacky track? dry hard packed track?
can you adjust rear wheel track. i forget ..are you running a panhard bar?

 

[rwstevens59]

The car exhibits push in all phases of the corner when wet and sloppy early in hot laps depending on the group you go out in. As all tracks in the northeast the track does not stay wet and sloppy for long. The complaint of lack of drive off on late corner exit occurs when the track is dry. The 1/4 mile track transitions to an interesting condition when it dries out. I would say the clay sand content must be quite high as the track does not get hard packed and shiny but rather drys out and continues to abrade into powder which blows off and it almost never takes rubber.

The baseline setup starts with a 2 inch spacer in both the left rear and right rear, so yes, there is some ability to adjust rear track. The rear currently runs a 14 inch wide rim with a 5 inch back set. Three inch back set rims are also available.

The rear suspension is a torque arm with two lower links and a short panhard bar mounted to the frame on the right side (rear view) and the center section at the rear end on the left side of the pinion (rear view) in front of the axle centerline. Yes, this is the total opposite of what you would see in a current super dirt late model, but that is how all three current chassis builders construct these cars. To best picture the lay out think of the old GM 'F' body cars like the Camaro, the layout is similar but not the same.

 

[Ranger Mike]

now we are getting some where
have you measured the rear track under compression/rebound?
I am trying to determine if there is rear roll steer in play?
is the wheel base on one side growing or shrinking when in bump?
what is angle of third link? what is angles of both trail arms?
i am not too worried about panhard bar mount as much as angle

 

[rwstevens59]

Attached is one of my sons pictures to give you a better idea of what the heck we are talking about here.

The rear suspension on these cars would technically be considered a two link with panhard. The torque arm consists of two plates bolted to the quick change using the side bell through bolts either side of the drive-shaft. The plates go forward about 28 inches and attach to a large heim which attached to a slider plate mounted on two shafts attached to the chassis. The heim is mounted to the plate with a rubber biscuit to cushion rear end wrap up on throttle.
The two lower links are mounted on clamp on brackets at the outer end of the axle tubes below axle centerline. The forward end of the lower links have three mounting holes on the chassis to adjust link angle and therefore anti-squat, which also effects the amount of roll steer. This is kind of hard to picture if you have never seen the design, but it follows all the principles of a torque arm two link with panhard. When I say torque arm do not automatically envision a typical late model torque arm with fifth coil. These cars are laid out like an over grown sprint car with the driver centrally located with the driveline and torque arm between his legs and under the seat.

This rear linkage design has inherent roll oversteer in all lower link mounting positions. This is crutched (in my opinion) by using a spring rod for the right rear link to induce power on axle steer to the left to counter the roll induced oversteer.

The panhard angle is cockpit adjustable for angle, which to me just adds or subtracts a jacking force more than moving the roll center height. If I want to move the roll center I move the bar up or down equally at both ends while maintaining the angle.

Hope this all makes some sense.

Thanks, Ralph

 

[HowlerMonkey]

We're using the aim evo4 with smarty cam on our twin turbo ford GT.

That said, reducing the data from runs or laps is tough because of even tire width and where the contact patch (if not flat and perfect) will affect "spring rate" as seen by the car.

It makes reducing any useful date require a lot more figuring.

That said, look for our current 252.9mph standing mile record to be pushed past 270 in a few weeks.

 

[Ranger Mike]

pushing going in - rt front spring too stiff
too much cross weight
too much rt front tire pressure
not enough stagger
too much panhard bar angle
my opinion

 

[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]

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.