Changes to damping for new spring rates

[alangbaker]

I've done a forum search and I couldn't find a concise answer:

I've just lowered my spring rates from 440f/900r to 250f/440r (I was actually recommended to use 450 on the rear, but I happened to have these 440lb/in springs lying around you see ).

Should I be softening the damper rates as well?

I've got these really nice, triple-adjustable Penske 8760s, but very little idea of how to adjust shocks, as my last car had non-adjustable Bilsteins.

If I go out in practice with the shocks set as they are, and I do need to change the damping, what am I looking for to tell me?

What I thought I would do is carefully count each adjuster from its current position to fully closed before returning them to the original value for the first session, but if anyone has a good rule of thumb for such a large change, I'd love to hear it.

Alan

[S Lathrop]

What is the motion ratio of wheel movement to the shock movement on your car?

The adjusters are just for tuning the shocks. The real changes are in the valving and that requires that the shocks be taken apart. Further, the shocks should be dyno tested when the internal valving is changed to be certain they match.

The shocks are tuned to manage the movement of the mass of the car. You are just changing the spring rates, so your shocks might work as is.

Try the new springs and see what you get.

Given the magnitude of the spring rate change, you are planning to raise the car?

[alangbaker]

What is the motion ratio of wheel movement to the shock movement on your car?

The adjusters are just for tuning the shocks. The real changes are in the valving and that requires that the shocks be taken apart. Further, the shocks should be dyno tested when the internal valving is changed to be certain they match.

The shocks are tuned to manage the movement of the mass of the car. You are just changing the spring rates, so your shocks might work as is.

Try the new springs and see what you get.

Given the magnitude of the spring rate change, you are planning to raise the car?

Well according to the manual I found online, the motion ratio is "+1 turn on spring platform (2mm) is 3mm (front) and 2mm (rear)", so 2/3 (or 3/1) front and 1/1 rear (either way around). However, that being said, the front bellcrank geometry looks very rising rate (angle between push rod and bellcrank already less than 90 degrees (and so losing leverage in bump), and angle between bellcrank and shock greater than 90 (so shock gaining leverage in bump.

And yes, I received a fairly comprehensive setup from David Clubine at BritainWest (if you haven't heard that name, he's been running FFs at a fairly high level in Ontario for a long time) and it included spring rates, ride heights, droop values, camber, caster and toe settings. The suggested ride height is definitely higher than the car was delivered with.

I'll definitely try the shocks as they are, and I suspect that the limiting factor is still going to be me for a while.

Thanks.

[Will Velkoff]

I would say your best bet is to start out with the dampers in their mid range settings, then once you get the chassis dialed in, then start up with making shock sweeps from soft to stiff and see how the car responds. That will give you a feel as to where you stand on valving. Happy tuning!

[TimH]

I would say your best bet is to start out with the dampers in their mid range settings, then once you get the chassis dialed in, then start up with making shock sweeps from soft to stiff and see how the car responds. That will give you a feel as to where you stand on valving. Happy tuning!

That's my plan now that I seem to have solved all my gremlins. I may not get any faster but I should at least learn something.

Oh, and I've been advised to do the testing on fresh tires.

[problemchild]

I would say your best bet is to start out with the dampers in their mid range settings, then once you get the chassis dialed in, then start up with making shock sweeps from soft to stiff and see how the car responds. That will give you a feel as to where you stand on valving. Happy tuning!

Or, when you buy one of the fastest and best-developed cars in the country, don't change a thing until you drive it, or atleast, have Wendal advise on recommended changes.

[S Lathrop]

When I asked about motion ratio, I was looking for a number like 0.6. That would tell me that if I move the wheel 1 inch, the shock moves 0.6 inches. If I have a 440 pounds per inch spring, I have a wheel rate for that spring of 158 pounds per inch. A 250 spring will give a wheel rate of 90 pounds per inch. That is the motion ratio squared times the spring rate gives the wheel rate.

When I make spring rate changes, I run a calculation so that I keep the same relation between spring rate at the wheel and ride height. As an example if I had 1000 pound spring and I was happy with the ride height of 2.35 then for a 500 pound spring my new ride height would be 3.865 inches.

The idea is that with a car in motion, there is a ride height range the car will operate in. When I change springs I will change the amplitude of that motion up or down depending on the spring I am using. Or say I just kiss the ground under very hard braking. My new ride height will still just kiss the ground under very hard braking. Changing ride height by it self changes the handling of the car, especially if you do it at one end only. For me this is the primary chassis tuning tool. I get ride height and rake as good as I can before I do anything else. From that base I will consider changing spring balance as the next step, stiffer at one end relative to the other. Then optimize the rake at the new spring setup. Each track will have a preferred spring combination. And depending on the track conditions like cold, hot , wet, etc. the spring setup will change.

Your question was about shock adjustment. One principal I use when setting up shocks is that any more damping that what is call critical damping (the amount of damping that maximizes grip) will reduce grip. More damping means less grip. That does not say that the car won't go faster or feel better because you are fixing another problem with the shocks. But you are masking the real problem but getting a balanced car with a lower grip level. This is often the issue when a car works well for a few laps and then things start to deteriorate.

This whole dissertation was prompted by your indicating you are going to reduce your spring rates by half . I was trying to show what that large a change would imply.

[BorkRacing]

Steve,
What formula did you use to derive the new ride height with the spring change?

Rob

[S Lathrop]

Steve,
What formula did you use to derive the new ride height with the spring change?

Rob

I use the rate of the spring at the wheel, which is the motion ratio squared times the spring rate. Next I calculate the spring rate of the tire at the pressure I am running. The spring rate at the contact patch is the wheel rate of the spring times the spring rate of the tire divided by the sum of the spring rate at the wheel and the spring rate of the tire. I calculate this number for both the original springs and for the new springs. I divide the old spring rate at the contact patch by the new spring rate at the contact patch. This is a ratio that I multiply times the old ride height. The ride height number is the distance between the ground and the bottom of the car at the axle center line. This number is the new ride height.

Say we have a 2 inch ride height, a motion ratio of .6, a spring of 500 pounds, a tire rate of 1000 pounds. Spring at the wheel is 500*.6^2=180. 180*1000 /(180+1000)=152.5 This is the spring rate at the contact patch. If I want a 300 pound spring I get 97.5 for the rate at the contact patch. The ratio 152.5/97.5 = 1.56. So my new ride height is 1.56*2 = 3.128 inches.

That change seems like a lot but think about it this way, say the sprung weight on one corner of the car is 200 pounds. Just the weight of the car will compress the suspension plus tire 1.31 inches just to support the car. The 300 pound spring will be compressed 2.1 inches. But the new spring is softer so when the car is compressed, it will go down more for the same force/load. Thus the larger ride height change than just a first order change would indicate.

This should be totally confusing. I learned this formula when I was an engineer at PacWest Indy Lights. I use it every time I change springs. If you make a spring change without compensating ride height, you are making two changes simultaneously, ride height and spring rate. Doing that may give the opposite results you were looking for when you decided to change springs.

A good example would be reducing entry over steer by increasing the rear springs. Just change the springs and the over steer will be worse. But if you lower the car at the same time, the over steer will be reduced because as the car starts the corner the rear end is lower and the rate of loading the rear tire in cornering will be lower. That is corner entry. Corner exit the opposite is true as the rear of the car will be higher on corner exit. This formula will get you very close so and further tuning might be very minor.

Alan, this rant does not answer the original question but it shows how big a change you are making and the amount of sorting you will have to do before you think about shocks.

[alangbaker]

When I asked about motion ratio, I was looking for a number like 0.6. That would tell me that if I move the wheel 1 inch, the shock moves 0.6 inches. If I have a 440 pounds per inch spring, I have a wheel rate for that spring of 158 pounds per inch. A 250 spring will give a wheel rate of 90 pounds per inch. That is the motion ratio squared times the spring rate gives the wheel rate.

Understood, but if a car is resting on its springs, and you move the spring platform by x, and it results in a change in ride height of y, isn't that exactly the same thing?

When I make spring rate changes, I run a calculation so that I keep the same relation between spring rate at the wheel and ride height. As an example if I had 1000 pound spring and I was happy with the ride height of 2.35 then for a 500 pound spring my new ride height would be 3.865 inches.

The idea is that with a car in motion, there is a ride height range the car will operate in. When I change springs I will change the amplitude of that motion up or down depending on the spring I am using. Or say I just kiss the ground under very hard braking. My new ride height will still just kiss the ground under very hard braking. Changing ride height by it self changes the handling of the car, especially if you do it at one end only. For me this is the primary chassis tuning tool. I get ride height and rake as good as I can before I do anything else. From that base I will consider changing spring balance as the next step, stiffer at one end relative to the other. Then optimize the rake at the new spring setup. Each track will have a preferred spring combination. And depending on the track conditions like cold, hot , wet, etc. the spring setup will change.

Right. And I've been given a baseline that includes setting ride height (and droop) as well as the spring rates to accompany those heights.

Your question was about shock adjustment. One principal I use when setting up shocks is that any more damping that what is call critical damping (the amount of damping that maximizes grip) will reduce grip. More damping means less grip. That does not say that the car won't go faster or feel better because you are fixing another problem with the shocks. But you are masking the real problem but getting a balanced car with a lower grip level. This is often the issue when a car works well for a few laps and then things start to deteriorate.

This whole dissertation was prompted by your indicating you are going to reduce your spring rates by half . I was trying to show what that large a change would imply.

Got it. But do remember that I'm not just changing the spring rates: I'm changing the ride heights (upward) and droops as also provided by a team that has run RF98 and similar Van Diemens at a high level. But one thing I didn't ask about was what to do with my adjustable shocks.

[alangbaker]

Or, when you buy one of the fastest and best-developed cars in the country, don't change a thing until you drive it, or atleast, have Wendal advise on recommended changes.

I'd do that, Greg, but my car hasn't been Wendell's since some time in 2013, which means there's been a transition to a setup for radial tires that he didn't make.

But I will ask if he has any advice that would pertain to running the car on the (much less grippy) American Racers.

[alangbaker]

Alan, this rant does not answer the original question but it shows how big a change you are making and the amount of sorting you will have to do before you think about shocks.

Understood, Steve, and thanks.

[2BWise]

Got it. But do remember that I'm not just changing the spring rates: I'm changing the ride heights (upward) and droops as also provided by a team that has run RF98 and similar Van Diemens at a high level. But one thing I didn't ask about was what to do with my adjustable shocks.

Any chance you've got the force/velocity curves for the dampers on your car? When I send out shocks I ask that they do sweeps thru the adjustments so that I have at least 3 points along each adjuster (usually the softest, firmest, and median positions). Ideally, with this data I would then calculate the critical damping curves for the current setup. Following a spring rate change my initial target would be to match the critical damping curves for the new rates as best I could; as a starting point. Then adjust as need be to the desired vehicle balance from there.

Most likely you want to drop the damping forces following that significant of a spring rate change, but by how much is tough to gauge without knowing the F-V curves of the dampers.

[S Lathrop]

Understood, but if a car is resting on its springs, and you move the spring platform by x, and it results in a change in ride height of y, isn't that exactly the same thing?

If you know the pitch of threads for the shock, then you can calculate the motion ratio. 1 turn of the spring perch changes the shock length by some amount, how much? And that change moves the ride height how much? The push rod change has nothing to do with the bell crank. If 1 turn is 2mm, and that moves ride height 3mm. Then the motion ratio is 0.667. and a 250 pound front spring will be 111 pounds at the wheel. The original setup were 195.

The lower the motion ratio, the less effective the shocks will be because they just are not moving enough to give strong damping forces at the wheel.

The one VD designed car I have looked at did have very high rising rate front bell cranks. Given that fact, the setups I was looking at made total sense.

[alangbaker]

If you know the pitch of threads for the shock, then you can calculate the motion ratio. 1 turn of the spring perch changes the shock length by some amount, how much? And that change moves the ride height how much? The push rod change has nothing to do with the bell crank. If 1 turn is 2mm, and that moves ride height 3mm. Then the motion ratio is 0.667. and a 250 pound front spring will be 111 pounds at the wheel. The original setup were 195.

Right. I think we were just failing to communicate. I have a manual that gives the amount of ride height change for each turn of the spring perch (originally 2mm pitch on Dynamic shocks, now 0.10" on the Penskes): 1.5 times thread pitch at the front and 1 times pitch at the rear.

The lower the motion ratio, the less effective the shocks will be because they just are not moving enough to give strong damping forces at the wheel.

That one I already knew about.

The one VD designed car I have looked at did have very high rising rate front bell cranks. Given that fact, the setups I was looking at made total sense.

That seems to be exactly the case with my car. Since I have no way to change the shock's fully extended length, I plan to stick very close to the provided ride height and droop figures given, as that is the only way that I can see that the front bell cranks will be in the correct position in their curve.

[jharvey]

Right. I think we were just failing to communicate. I have a manual that gives the amount of ride height change for each turn of the spring perch (originally 2mm pitch on Dynamic shocks, now 0.10" on the Penskes): 1.5 times thread pitch at the front and 1 times pitch at the rear.



That one I already knew about.



That seems to be exactly the case with my car. Since I have no way to change the shock's fully extended length, I plan to stick very close to the provided ride height and droop figures given, as that is the only way that I can see that the front bell cranks will be in the correct position in their curve.

You may easily get yourself down a rabbit hole with all of this. The advice to run it as it is was the best. The bias carcass is stiffer than the radial, resulting in softer springs and lower tire pressures. Many run 12/14 or thereabouts with bias ply, and 17+ on the radial with the softer sidewall.

I would not use the perches to adjust ride height, unless the correct ride height cannot be achieved with the pushrods. If you want to be super anal, get all of your springs tested to see their true rate at given lengths, that way you can know what you need to adjust the perches to so you can achieve even preload. Set preload to get rough ride height, then adjust with pushrods (shims or threaded style). Turning a perch 1 turn with a 900 pound spring will have a greater effect on ride height than it will with a 450 pound spring.

There are many threads on Apexspeed with regards to wheel rates/motion ratios/etc., and they are very useful.

[S Lathrop]

You may easily get yourself down a rabbit hole with all of this. The advice to run it as it is was the best. The bias carcass is stiffer than the radial, resulting in softer springs and lower tire pressures. Many run 12/14 or thereabouts with bias ply, and 17+ on the radial with the softer sidewall.

This was one of the biggest challenges in getting good setups for the radial ply tires. Everything we thought we knew about shocks and springs went out the window.

When I have used spring preloads in a setup, I always calculate the preload as a spring force. So for a 440 pound spring and 4 turns of preload with a 1.5mm spring pitch, you would have 104 pounds of preload. The same preload with a 250 pound spring will take 7 turns. The amount of preload will determine when the inside shock tops out and the that tire looses grip. This can have a big effect on handling for good or bad. My experience is that the effect is more closely related to the force than the spring rate. I have had setups where a 30 pound change in spring preload takes the car from great to bad and 30 in the opposite direction is not enough. And this setting is very driver dependent.

[alangbaker]

You may easily get yourself down a rabbit hole with all of this. The advice to run it as it is was the best. The bias carcass is stiffer than the radial, resulting in softer springs and lower tire pressures. Many run 12/14 or thereabouts with bias ply, and 17+ on the radial with the softer sidewall.

I would not use the perches to adjust ride height, unless the correct ride height cannot be achieved with the pushrods. If you want to be super anal, get all of your springs tested to see their true rate at given lengths, that way you can know what you need to adjust the perches to so you can achieve even preload. Set preload to get rough ride height, then adjust with pushrods (shims or threaded style). Turning a perch 1 turn with a 900 pound spring will have a greater effect on ride height than it will with a 450 pound spring.

There are many threads on Apexspeed with regards to wheel rates/motion ratios/etc., and they are very useful.

I see what you're saying, Josh, but we have to run American Racers up here, so the setup the car came with for Hoosier radials probably isn't optimal. I got good advice on a setup to start off with from an experienced race shop and team, so I'm trying hard to match that.

As for not using the spring perches to adjust ride height, I understand the need not to mess up the bell crank geometry, but since the shocks are a fixed maximum length, I'm using spring perch AND pushrod adjustment to achieve both the correct ride heights and droops, hoping that that means the bell crank geometry will be correct. If my shocks are oddball, it could be otherwise of course, but I can't control everything.

I've read most of the threads here on Apexspeed, as well as devouring Carroll Smith's books, and many others.

[petawawarace]

Very Interesting read. Thanks everyone for sharing your information.

Steve, what do you use for measuring your ride heights? Do you use a surface plate? I'm using strings crossed under the car right now but I'm interested if theres a better way (aside from a surface plate)

[S Lathrop]

Steve, what do you use for measuring your ride heights? Do you use a surface plate? I'm using strings crossed under the car right now but I'm interested if theres a better way (aside from a surface plate)

I try to have a place to take a measurement that is easy to use. Getting under the car is not easy. At the front of the car, I might use the top of the frame to measure from the ground. Ride height is just the difference between the frame rail and the bottom of the car and what I measure. At the rear with a diffuser, I use a rod and pass it through a hole in the bottom of the car and measure from the end of the rod to the top of the transmission. Again a little math and I have the ride height at the rear. On the scales, I generally have a bar that will fit inside the wheels on the pad and measure ride height to that bar. Again easy, accurate and repeatable is the rule for doing any measurement.

I do all my alignments without the driver in the car. and I don't bother to ballast the car for the driver. Some cars I don't have the front wings and nose on the car either. Now once every so often, when I have the car on the scales, I will have the driver in the car and everything just as it would be for going onto the track. I then record weights, and ride heights to make sure they are exactly what I want. These loaded numbers will let me translate my setup numbers (as I do an alignment) to what I will actually have on the track. After going through the process a few times, my setups will be spot on to what I want the car to be when it goes onto the track. Why have someone sitting in the car or go through all the work to load the car with ballast. It is a waist of man power and or effort. Most cars will not go on the alignment pad with the nose and wings in place.

Checking the alignment every time the car comes off the track is important, if you want to run as best as you can. So make it easy to do.

[alangbaker]

I just came across my own thread while seeking information on another topic and I thought I'd provide a brief synopsis of how the car went after I did change the setup to the one suggested and away from what was (I'm sure in another context) and excellent setup by Wendell Miller at Quantum.

The first weekend I had the car, the only changes I had time to address were changes to camber, and making sure the toe settings weren't crazy. Spring rates, ride heights, droop... ...all of that was left as it came to me. I was about 1.5 seconds off the best pace in FF in qualifying on Sunday (I missed Saturday completely), and by race 3 of the weekend (my second), I was only 0.3 seconds off the best pace. So not a bad start.

Then next weekend in July, after getting a baseline setup for American Racer slicks from David Clubine of Britain West, I made changes to spring rates, ride heights, droop, toe... ...the lot. I also asked my questions about shocks just before the race, not so much to try to make changes, but just to put a little more knowledge in the bucket for next time.

In practice, I was 0.227s off the best FF, and in qualifying, I was the best FF, with a new personal best lap time of 1:10.833; a time which would have been good enough for the Mission lap record if done during a race in 2017 (Doug lowered the record in 2018 to 1:10.583, dammit).

Sadly, after that, the car's performance seemed to worsen and I couldn't match my qualifying performance in Saturday's race. What I hadn't figured out (and what took me far to long to figure out) was that the brake bias bar was screwed up, and I very quickly used up my front tires because they were doing almost all the work in the braking zones. The too long part comes into it, because it took me three weekends to finally realize just how bad my rear braking was, and so I threw away two additional weekends flat-spotting increasingly useless front tires.

By the time I'd sorted the brakes, the rainy season in Vancouver had come, and my next races weren't great because I used the rain tires that came with the car as a part of the sale, and they just weren't up to the job. I had one chance to see what the car would really do in October, when we had a perfect cool and sunny weekend with the car prepped with a new set of slicks as well as new rain tires should it be necessary... ...but I missed that weekend entirely due to a bout of vertigo. I really thought I had a good shot of setting a new lap record, too.

But I think that being able to get under the previous year's lap record on an extremely hot July weekend tells me that changing my setup to something more appropriate for American Racers was an extremely good move, and the fact that the bias bar was installed backwards (shorter threaded end of the bar attached to an adjuster and full height jam nut meaning there was no way to move it far enough to the rear master cylinder side) tells me that while the car might have been fantastic had it come straight from Quantum to me, there was very little about the initial setup that could be trusted to be good.

Now that winter is upon us, I will send the shocks out to be dynoed and (if necessary) serviced...

...and I can't wait for spring to get here.

[JerryH]

Some general guidelines:

Use preload to set the desired droop (I like to have about 1/4 inch at the shocks for most tracks.....a bit less if very smooth track)

Use only the push rods to adjust ride ht.

Measure the motion ratiio. You have to know it to do anything.

Set wheel rates (motion ration squared x spring rate) at some multiple of corner weight. ( for FF, same as corner weight is a good starting point.......winged cars higher.......up to about 1.5 x corner wt for FC).

Shock damping should be about 70% of critical damping. Several formulas are available to calculate critical damping. You can find them via google.

Have the shocks dynoed......and rebuilt if they don't match.......and/or they are way off the damping you need.

Use good springs that also match at the same installed ht.

When scaling the car.....level the front of the car......make sure bellcranks are at same angle......then adjust ride ht and corner wts. Driver wt in the car will help in the beginning.......until you know what the change is from driver to no driver.

Use rake and springs for initial ball park tuning......then go to bars and shocks for fine tuning.

If it is a winged car, adjust the wings for high speed corner balance.

Enjoy the process......the challange is part of the fun.

Jerry Hodges
JDR Race Cars

[Tuiguy]

Interesting thread. Alan, what year, make and original configuration is your car?

[alangbaker]

Some general guidelines:

Use preload to set the desired droop (I like to have about 1/4 inch at the shocks for most tracks.....a bit less if very smooth track)

Yup. Did that.

Use only the push rods to adjust ride ht.

Right. I used the spring perches to get the correct amount of droop, and then adjusted the ride heights with the pushrods.

Measure the motion ratiio. You have to know it to do anything.

Fortunately, I was able to find a manual for the RF98 Zetec FC, which I believe has the same suspension as my RF98 (although I THINK mine started out as a Pinto), and it included the motion ratios in the form mm of ride height change for each turn of the spring perch (and included what that turn change was in mm. Front ratio is 3:2 wheel movement to shock movement, and rear is 1:1.

Set wheel rates (motion ration squared x spring rate) at some multiple of corner weight. ( for FF, same as corner weight is a good starting point.......winged cars higher.......up to about 1.5 x corner wt for FC).

Front springs are now 250lb/in which works out to a wheel rate of (2/3)^2*250 = 111lb/in wheel rate (right?). Rear rate would be 440lb/in as those are the springs and the motion ratio is 1:1. I know that sounds like the car should oversteer pretty badly, but it really didn't. I suspect that the rising rate setup of the front end prevents that.

Shock damping should be about 70% of critical damping. Several formulas are available to calculate critical damping. You can find them via google.

Right. I'll do some googling.

Have the shocks dynoed......and rebuilt if they don't match.......and/or they are way off the damping you need.

Use good springs that also match at the same installed ht.

When scaling the car.....level the front of the car......make sure bellcranks are at same angle......then adjust ride ht and corner wts. Driver wt in the car will help in the beginning.......until you know what the change is from driver to no driver.

Use rake and springs for initial ball park tuning......then go to bars and shocks for fine tuning.

If it is a winged car, adjust the wings for high speed corner balance.

Enjoy the process......the challange is part of the fun.

The shocks are definitely going to be sent out for dynoing/rebuilding (if necessary), and I'll try to get some figures for critical damping before I send them out, although I do suspect that given that this car was a winner in F1600 and doubt the shocks have been modified since then, and therefore I doubt that actual changes to the valving will be necessary.

This part of the process I do enjoy very much, and now that I have an engine that should take a lot less time to maintain, I'm looking forward to really getting the car dialed in next year.

[alangbaker]

Interesting thread. Alan, what year, make and original configuration is your car?

The car is an RF98 Van Diemen FC that was (I believe) one of the first cars converted to Honda.

In 1998, Van Diemen produced different chassises for FF and FC. The FCs were a newer design, recognizable by the bolted-on roll bar, where the FF chassis was basically just a small update to the 1997 design which had been shared by FF and FC.

I don't know the car's history as an FC, but it was eventually converted to FF as a Kent-engined car and run in Canada for some number of years. Then Dan Layton, who worked for Honda Performance Development (or Honda Racing; it's not clear), bought the car and had Quantum Racing Services in Oklahoma do the conversion to Honda. He ran it for a while, and then for a while he ran it part of the time and a young driver named Wyatt Gooden ran it in F1600 a bit. Specifically, Gooden took the car out to Mid-Ohio and Watkins Glen and took 3 poles out of 4 and won all 4 races.

Eventually, Quantum sold the car to a guy who wasn't able to get a medical to race it, who then sold it to the guy who sold it to me. Peter Hardsteen was a very nice guy with too many toys (the background of the shot in his for sale thread includes the Provimi Veal Lola that Arie Luyendyk drove to victory in 2 Indy 500s) and he tried SCCA FF racing for exactly one race. Needless to say, he didn't like it. He overheated the engine, and only completed 6 laps at an event at the Texas Motor Speedway in 2014. After he'd replaced the warped engine with a junkyard takeout, AND purchased a second engine to have a spare on hand, he decided to sell the car.

And that's where I came in.

[JerryH]

Had two 97 fc cars. (On pole at 2000 runoffs with first one).

Both had .7 motion ratio front and rear (measured). Almost linier in first 1.5 inches of wheel travel starting at ride ht, which is the most important).

Yours must have been changed....but I would measure to be sure.

Jerry

[alangbaker]

Had two 97 fc cars. (On pole at 2000 runoffs with first one).

Both had .7 motion ratio front and rear (measured). Almost linier in first 1.5 inches of wheel travel starting at ride ht, which is the most important).

Yours must have been changed....but I would measure to be sure.

Jerry

I will check this off season, but having an original manual that lists specific numbers isn't a bad place to start, and it says 0.6666f and 1.0r.

And the geometry of the front pushrod to bell crank to shock is such that it has to be a rising rate suspension. The pushrod and bell crank form an angle of less than 90 degrees that gets smaller as the wheel rises, and the bell crank and shock form an angle of less than 90 degrees that gets larger as the wheel rises. Both of those changes increase the motion ratio when the wheel rises.

All I can assume is that the bell cranks are one of the things that changed between the 97 and the 98.

[2BWise]

I will check this off season, but having an original manual that lists specific numbers isn't a bad place to start, and it says 0.6666f and 1.0r.

And the geometry of the front pushrod to bell crank to shock is such that it has to be a rising rate suspension. The pushrod and bell crank form an angle of less than 90 degrees that gets smaller as the wheel rises, and the bell crank and shock form an angle of less than 90 degrees that gets larger as the wheel rises. Both of those changes increase the motion ratio when the wheel rises.

All I can assume is that the bell cranks are one of the things that changed between the 97 and the 98.

I would measure your specific car just to be sure. I've measured a couple different cars that should have had the same motion ratios and seen significant differences (off the top of my head 0.2; 0.7 vs 0.9). Its also useful to then measure the camber curves while you're at it and evaluate compliance with vertical wheel travel.

[alangbaker]

I would measure your specific car just to be sure. I've measured a couple different cars that should have had the same motion ratios and seen significant differences (off the top of my head 0.2; 0.7 vs 0.9). Its also useful to then measure the camber curves while you're at it and evaluate compliance with vertical wheel travel.

As I said, I'll check this in the off season.

But what do you mean by "evaluate compliance with vertical wheel travel"?

[2BWise]

On my 99 there was slop in the suspension with vertical travel. The hub would move vertically without any movement at the bellcrank (or more importantly the shock).

[alangbaker]

On my 99 there was slop in the suspension with vertical travel. The hub would move vertically without any movement at the bellcrank (or more importantly the shock).

Ah! I see now. I've notice that there is a tiny bit of free play in the system, but I'd say certainly less than 1/8" of vertical movement; probably less than 1/16". I can more feel a little "clunk" if I pull up on the wheel when the car is on stands.

I'd still like to track it down, but with 7 bearings of various types involved, there's always going to be some play.

[DaveW]

Ah! I see now. I've notice that there is a tiny bit of free play in the system, but I'd say certainly less than 1/8" of vertical movement; probably less than 1/16". I can more feel a little "clunk" if I pull up on the wheel when the car is on stands.

I'd still like to track it down, but with 7 bearings of various types involved, there's always going to be some play.

And on the track, that play does not matter. The suspension load eliminates it. You do have to eliminate that play from your measurements when you are measuring motion ratio, etc.

[Bill Johnson 42]

I've got the same issue with "play" on my Van Dieman, it's in the bearings in the bellcranks. New bearings help but there is still a small amount even then,

So Dave, a small amount of play doesn't make any difference even when the wheel goes from compression to rebound?

[alangbaker]

I've got the same issue with "play" on my Van Dieman, it's in the bearings in the bellcranks. New bearings help but there is still a small amount even then,

So Dave, a small amount of play doesn't make any difference even when the wheel goes from compression to rebound?

As long as the wheel is in contact with the track the play will be taken up by the vertical load opposed by the springs.

[Hardingfv32]

1) The mass of the unsprung suspension elements are not always vertically loaded in compression.

2) Any play creates un-damped suspension movement. This creates varying loads at the contact patch and reduced grip levels. Whether this matters to you depends on your desired level of competitiveness.

Brian

[teamwisconsin]

I've got the same issue with "play" on my Van Dieman, it's in the bearings in the bellcranks. New bearings help but there is still a small amount even then

I machine the rocker caps until the bearings are preloaded a bit, then I use old Penske shock shims (they fit perfectly for the ID of Van Diemen bellcrank posts) to take the preload down to .001-.002. This eliminates all the play. Rocker bearings are also a routine service item you should be checking regularly.

Also, replace the spherical bearings on the end of your shocks every other season or so. All this play adds up.

[alangbaker]

1) The mass of the unsprung suspension elements are not always vertically loaded in compression.

2) Any play creates un-damped suspension movement. This creates varying loads at the contact patch and reduced grip levels. Whether this matters to you depends on your desired level of competitiveness.

I agree there are occasional moments where there is no force acting downward on the unsprung part of the system. If you pass over a large enough hole in the road, and the suspension fully extends before gravity brings the car down into contact with the surface again for instance. But being completely out of contact with the road surface means no grip anyway.

But by and large, there will be a force on the unsprung mass in the downward direction almost all the time, and with play measured in a few hundredths, there really isn't a lot to be gained in chasing the last of it out of the system. I'd still prefer to know where it comes from, because you want to catch any component that might be on its way to failure.

[Hardingfv32]

...there are occasional moments where there is no force acting downward on the unsprung part of the system...

This is a complete over-simplification. Even in what you might be calling a downward/compression force you will find constant oscillations in the level of the net compression force. These varying changes in load cause reductions in grip. A lot of this can be viewed using load sensors on various suspension members.

In professional racing where use of shaker irg testing is common, compliance of the chassis and suspension carries one of the highest priorities when developing a race car.

So again, what you concern yourself with is relative how competitive you plan to be.

Brian

[problemchild]

I would rather have minimal stiction and have play in shock sphericals, etc. Sounds bad in theory but most of my mentors believed this. I look for sphericals like Ethan would be throwing away

[teamwisconsin]

I look for sphericals like Ethan would be throwing away

I’ll add you to the list of people to donate the old parts to Greg! Ha!