Chassis Setup - Gurus Wanted!

[Ron Earp]

Alright, the time has come for the JH to go out for a little drive once we fire up this weekend.

I need help selecting the right stuff for this chassis - I've done this before here but it was with a "welded diff" title and I think that scared folks off. So, here goes again, I'm looking for any and all advice to get this thing started off *somewhat* decently as a setup.

Jensen Healey
*Weight will be at 2240lbs
*Live axle in the rear, pan hard rod to locate, with 2 trailing arms, upper two arms have tiny bolts so as to be "non-functional"
*Welded differential - no LSDs available
*Double A arm front suspension
*Drums rear, discs front
*Probably around 160-175hp at flywheel
*5 speed tranny, 3.9 rear end, 13" Toyos

Wheelbase is longish, probably a 1" or 2" longer than a Z. Engine is not centered over the front subframe, it is basically 70% behind the front subframe, car is long and you sit probably only 14" from being over the rear axle.

What spring rates in the front do you think I should try?

Should we run roll bars (have two, front and rear, about 3/4" thick, steel)?

What spring rates in the rear?

It'll want to push, so, how should we set it up to avoid this?

Right now the car has 650lb springs in front and 250lb springs in the rear, is wearing both roll bars - I think this is completely wrong. Probably should have much less rate in the front and we need to make the rear loose so that it'll swing around to compensate for the push of the welded diff.

Alright - fire away!

Thanks for the help,
Ron

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Ron Earp
NC Region
Ford Lightning
RF GT40 Replica
White Jensen-Healey ITS
Silver "Skull" 260Z ITS
Email: "rlearp at gt40s.com"

[This message has been edited by rlearp (edited July 08, 2005).]

[Bill Miller]

Ron,

I'm sorry that I don't have any setup info for you, but I do have a question. I was wondeering why you stayed w/ the 13" wheels, given the new wheel rule?

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MARRS #25 ITB Rabbit GTI (sold) | MARRS #25 HProd Rabbit
SCCA 279608

[racer14itc]

Ron,

Before we can guess at spring rates for your car, we'd need just a little more information.

What is the motion ratio of the double a-arm suspension? And what is the motion ratio of the live rear axle? What is the motion ratio of the sway bars? What are the arm lengths and center bar lengths of the sway bars?

The motion ratio of the rear axle is pretty easy to calculate. It's basically the ratio of the distance between the spring attachment points and the track of the rear tires. The wheel rate is the spring rate time this ratio squared. This is assuming your shocks are vertical, if not, there's a COS function involved. http://ITForum.ImprovedTouring.com/biggrin.gif

The double a-arm motion ratio is a little more complicated to calculate.

Both are pretty easy to derive empirically if you remove the springs. Move the suspension up 1" at the wheel and measure the shock travel (it will be less than 1"). That's the motion ratio. The wheel rate will be the spring rate times this motion ratio squared.

Then it's just a matter of establishing what WHEEL rates you'd like to have. There are base guidelines for front engine, rear drive sedans/sports cars, stated as a percentage of the static corner weight.

My advice? Drive the car as is and see what it does and what the chassis wants. If you want someone to help you analyze the chassis performance through tire temps, I'll be happy to help you.

MC

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Mark Coffin
#14 GP BSI Racing/Action Digital/Airborn Coatings/Krispy Kreme VW Scirocco


[This message has been edited by racer14itc (edited July 08, 2005).]

[David Ferguson]

There are two ways to start on this.

1) Start with any known setup what was basically a balanced car. (factory setup, or something someone else has said works). With this info, you could -- start with the same thing and go testing, or recognize that you would probably want stiffer and make a calculated changes as to starting points to try (basically you keep the ratios the same front & rear).

2) Gather real engineering data on the car such as: Suspension Geometry (motion ratios, roll center heights, camber curves, etc), Weight distribution, and Center of Gravity height, unsprung weight for front & rear corners. Given this we could determine a desired maximum roll angle, and calculate the springs and anti-roll bars that would achieve this goal.

If you just want to start with what you have, and you anticipate a push (due to the locked rear end), here are changes that you may want to try:

- Disconnect front ARB. Stiffen rear ARB if adjustable.
- Install stiffer rear springs (might try Nascar-style spring rubbers as a cheap alternative), Softer front springs
- Lower the front (lowers roll center with double-A-arm suspension). Raise rear roll center (I think raising the end of the panard bar will do this, but I don't really have any live axle experience).
- Change brake bias more toward the rear (softer shoes in rear, harder pads in front, adjust pressure limiiting valve if available)
- Add more static negative-camber to the front.
- Change driving style -- pitch & catch (ie, rotate the car then catch it with power) can work with a solid rear end.

[This message has been edited by David Ferguson (edited July 08, 2005).]

[Tom Donnelly]

Ron,
I would start with test days, just for that; to test changes. And treat a few race weekends as test days. And make one small change per track session. This is one of the things that makes this sport expensive. I think you are right about the springs, you might want to go softer but I don't know how much, it would be more forgiving at first. If I had the money I'd hook up with a shop that has racing experience for advice. OPM has helped me before, and I've received lots of help from katman, Chet, David Spillman and others. Data aquisition would be invaluable here. And you could get a known fast driver with experience in RWD to take the car out and give you feedback.

Tom Donnelly

[racer14itc]

Originally posted by rlearp:

Right now the car has 650lb springs in front and 250lb springs in the rear, is wearing both roll bars - I think this is completely wrong. Probably should have much less rate in the front and we need to make the rear loose so that it'll swing around to compensate for the push of the welded diff.

Alright - fire away!

Thanks for the help,
Ron



Ron,

I think you're confusing spring rates with wheel rates. The wheel rate is the effective rate at the wheel, i.e. how much the suspension deflects for a given load at the tire. On a double a-arm suspension, the motion ratios are typically around 0.5 (given a spring location about 1/2 down the lower a-arm), which would give you a wheel rate (excluding the bar) of 650*0.5*0.5 = 195 #/in. The motion ratio on a live axle is normally around 0.8 (given a spring perch width to track ratio of around 80% which is typical), which would give you a wheel rate of 250 * 0.8*0.8 = 162.5 #/in. So you see, the wheel rates are fairly close, given my assumptions.

Then you have to add the wheel rates of the sway bars to get your total wheel rates.

David F. mentioned that raising the panhard bar will help loosen the car. This is true, but one of the quirks of a panhard bar is the roll center will move up when cornering in one direction (loosening the car) and down when cornering in the other direction (tightening the car). So be aware of that! Ron, who installed your panhard bar? It looks like it was someone who is familiar with stock cars because it is mounted to the chassis on the right and attached to the axle on the left. This is backwards from what you would want on a road racing car. Normally on a road racing car the panhard chassis mount is on the left and the axle mount is on the right side, because our road courses have predominantly right turns. You'll be good at Charlotte though!

If you ever want to discuss this I can be reached at Scirocco14gpATbellsouthDOTnet.

Good luck,

MC

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Mark Coffin
#14 GP BSI Racing/Action Digital/Airborn Coatings/Krispy Kreme VW Scirocco

[This message has been edited by racer14itc (edited July 08, 2005).]

[JeffYoung]

One thing to consider with this car is the total lack of suspension travel. Probably less than 2" in the front.

I'm thinking the high spring rates are going to be needed to keep the car off the bump stops....will read the rest of this very useful information this weekend, thanks guys.

[Daryl DeArman]

Regarding panhard bar:

If you raise both ends of the panhard bar so that it stays level when the car is static then it will be more predictable as the axle will track to the same side under droop and bump.

If you raise only one end (so that the bar runs up/downhill) the rearend will track to one side during bump/droop until it is level [I]then it will track the other direction as the suspension continues to move in the direction (bump/droop).

If building an unique car, I'd try to keep the ratios of the wheel rates pretty close to stock (unless it had a nasty push/or bad oversteer in stock configuration) and keep the ratio fr/rear the same just make it stiffer.

I wouldn't be so scared by the welded diff, unless it is on a wet track. It will only require a little different driving style.

[Ron Earp]

Gentlemen,

sorry for not responding to a thread I started!

Q on 13" - I can't change rear gears, two choices without getting super trick, and I don't want a taller tire to reduce RPM - this little motor will need a lot to do well. Plus, stock are 13", I had 13" Panasports, and Jeff uses 13" tires on the TR8. So I might get some cast offs that won't work on his heavy car but could work on my light car.

Mark, I can try and get some measurements for you but it might be better if you and I work together at the track to sort this joker out - be faster I think that way! David and Tom, thanks for the suggestions there - we will try those.

The Panhard bar was done by Peter Krausse of Krausse and England, who also did Jeff's bar. There is nothing on the read to keep you from doing one side or the other. Lowes here I come in an open top car! Yay.

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Ron Earp
NC Region
Ford Lightning
RF GT40 Replica
White Jensen-Healey ITS
Silver "Skull" 260Z ITS
Email: "rlearp at gt40s.com"

[This message has been edited by rlearp (edited July 13, 2005).]