Chassis Setup for GT40s

Ron Earp · 01-05-06, 12:11 PM

You folks that have tracked and raced your GT40s what sort of chassis setup are you using? That is, front and rear camber, toe settings, caster, and spring rates? Sure, the discussion could bog down with "the XYZ chassis has this setup and the ABC chassis has this setup, so you can use XYZ for ABC" but in general, the cars are far more similar than dis-alike.

At first shot I would probably attempt -2 degrees neg camber upfront, with a tiny bit of toe out for crisp turn in. In the rear maybe -1 degree neg camber and zero toe. Probably get as much caster as I could in the front. Not sure on spring rates except that I felt the RFs I've driven were too soft stock and need to be stiffened. Also seems to be true for a couple of CAVs I've seen on track, got some body roll going on I'd like to avoid.

Anyone have any real life experience and settings to share?

Ron

Andy Besic · 01-05-06, 05:48 PM

I think your caster/camber guestimates are about spot on especially if you are running 15" wheels and Hoosier Bias ply street TD's. I used info found on this site for chassis rigidity and Goran Malmbergs formula's for spring rates etc. and came out with 300# springs all the way around on my GTD with front and rear sway bars. (Rear adjustable) Car is well balanced and while it feels "soft sprung" on the track, I think the tall sidewalls on the tires may have something to do with that.
I wouldn't start over 400# unless you are going to low profile tires, solid heim joint suspension, and lots of track preference over street. [img]/ubbthreads/images/graemlins/twocents.gif[/img]

Ron Earp · 01-09-06, 09:14 PM

Anyone else got any ideas or baselines settings? Or, do we just simply have many more road car drivers than folks that do some track work? Best,

Ron

Gary Gibbs · 01-09-06, 09:43 PM

Here is what I would call a baseline:
FRONT toe IN 0.5 to 0.75 degree,
camber -1.25 degree
Caster +5.5 degrees
Ride height 4.00 to 4.25 inches.
Bump steer at zero of course.

REAR
toe in 0.25 to 0.5 deg
Camber 0.75 to 1.0 degree
Caster (if you can get it ) about 3 degrees
Ride height 4.5 - 4.75 inches (looking for the rear road height to be 0.5 in higher then the front for better aero performance/stability)
Check rear bump steer also. Play with caster to improve bump steer.

ChrisL · 01-10-06, 06:31 AM

Interesting that you should bring this up. I’ve just spent the last Sunday afternoon playing with various settings with the intention of getting a broad feel for cause and effect in preparation for tweaking on the track. Admittedly, there is only so much information that you can glean from spirited drives though winding roads, but almost as important as grip I believe is feel and communication. For what its worth, here are my unscientific and very limited conclusions.

I surprisingly found that the simple act of stiffening the rear end really did improve the feel and significantly raised confidence levels, however stiffening the front end did not have the corresponding effect on feel that I would have thought. Front end castor could be adjusted to also improve feel and also (for whatever reason) reduce skip through bumpy corners. I did not give enough camber adjustment to draw any conclusions, and toe did not get a look in at all.

Howard Jones · 01-10-06, 11:17 AM

First set up would look somthing like this.
Front: camber -1, caster -4ish, 300pound springs 3/4 stiff on shock setting. Toe, in about 1/16" total.

Rear: Camber -1/2, 450 pound spring, 1/2 stiff on shock setting. caster about 1/2 of front setting, toe, straight ahead.

Antiroll bars. Front 80% full soft, rear full soft.

The idea is to set the car up to push a little rather that oversteer if possible. Then go stiffer at the rear, Antiroll bar and stiffer at the front shock setting, to get closer to netural.

Track setting same except, Front camber -1.5 rear, -1.

By the way the thing I learned from my track day was. NOTHING learned at 75% on the street is useful on the track at 90%+. You really need to run through the same set of corners at near full chat over and over to get a feel for chassis set up.

The one thing you can do before you get to the track that will save time is get the brake bias correct. The fronts MUST MUST MUST lock first. You can mess with the bias after that more towards nutural but at first the brakes must be 95% correct befor you will be able to drive the car hard enough to work on chassis setup.

Ron Earp · 01-10-06, 12:01 PM

[ QUOTE ]
REAR
toe in 0.25 to 0.5 deg
Camber 0.75 to 1.0 degree
Caster (if you can get it ) about 3 degrees
Ride height 4.5 - 4.75 inches (looking for the rear road height to be 0.5 in higher then the front for better aero performance/stability)
Check rear bump steer also. Play with caster to improve bump steer.

[/ QUOTE ]

Positive caster on the rear? Is this something specific to your car or something you need? Just wondering, I've never run positive caster on anything I wanted to handle well.

I'm not sure how I'll like toe in on the front. It is supposed to improve stability on the straights but when I've messed with it I can tell a lot more positive benefits from toe out on turn in than I get negative benefits from the same. That is, I don't seriously notice a decrease in straight line stability with toe out but definitely can tell how much sharper the car turns in.

R

Howard Jones · 01-10-06, 03:31 PM

Ron, I think that everyone's car will end up with different settings if for no other reason than we all are on different tires. And the tires are all over the place on width, wheel diameter, pressure, etc.

On first day , first session, setup I think most people will feel more confortable with a unsteering car rather than a tail swappin monster. My car has a honest 350-370 hp and good Z rated 17X 285's on the rear and I can turn it around with power in any corner through about 60% of third gear. AND I have tried to set it up with a little understeer.

If someone with another 100hp goes out with a tight setup and isn't careful they could endup with a really hard hit before the paints dry.

I'll plug brake bias again because I really want to be clear about this. If you have the brake right you will be able to "save" a dumb move, or at least have a chance. But it the rears are locking first AND you have a tailhappy setup, AND you get in there a little hot. I think the result will be pretty clear.

Toe out, at least a little, would help turn in I guess, but would tend to make the rear just a little lighter. Again I'm guessing.

I just find my car a lot more fun to drive fast if I know the front end will stay pointed forward. Well at least most of the time. But then I haven't really enough track time to get deeper into the setup beyondgoing for, safe and comforable.

Hell I'm not fast enough to really know the difference anyway. It's been a long time since I was in my 20s.

Gary Gibbs · 01-10-06, 06:42 PM

[ QUOTE ]

Positive caster on the rear? Is this something specific to your car or something you need? Just wondering, I've never run positive caster on anything I wanted to handle well.

I'm not sure how I'll like toe in on the front. It is supposed to improve stability on the straights but when I've messed with it I can tell a lot more positive benefits from toe out on turn in than I get negative benefits from the same.
R

[/ QUOTE ]
Here I am defining positive as inclining the upright rearward at the top. Most (all?) cars run positive caster in the front. In the rear it is going to be geometry dependent with the idea being the bump steer tends to slight toe in in bump and droop. Thus you never get toe out in the rear which for a high speed car is something you do not want. I will admit I have not tried toe out on this car. I used it extensively on my lower speed Legends race car. This car has great turn in and was not wanting to sacrifice straight line stablilty on a car with this speed capability.

Ron Earp · 01-10-06, 06:50 PM

Sorry, I meant positive camber. You list camber as positive on the rear setup which I've never used. That would mean if you look at the car from the rear the tires would be each pointing inward toward the middle of the car at the bottom.

Keith Hardy · 01-10-06, 07:04 PM

Just like a VW Beetle! And they handled like, er, [img]/ubbthreads/images/graemlins/censored.gif[/img] I can't believe that a GT40 would run well with positive camber on the rear, rather neutral at the very least and most likely some negative I would have thought...never thought to check my KVA but I believe it was set up with neutral camber.

Gary Gibbs · 01-10-06, 10:11 PM

Sorry missed that. Should read Negative camber in rear [img]/ubbthreads/images/graemlins/banghead.gif[/img] [img]/ubbthreads/images/graemlins/beerchug.gif[/img]

Ross Nicol · 01-11-06, 08:08 AM

Here's my setup
Front Rear
Toe 2mm in 1mm in

Camber 3.5 deg neg 2.5 deg neg

Caster 2.5 degrees 7 degrees

Ride height 80mm 120mm

Ross

Andy Besic · 01-11-06, 07:57 PM

With regards to camber/caster at both ends wheel size needs to be taken into account. The more side wall (15" wheel) and tire grip (race compound) the more camber you will want and need to keep a flat contact patch.
Ross, I don't recall what size wheel you are running and what type race tire. Bias ply or Radial. Radials will tend to naturally flatten the contact patch where the bias ply roll more progressively.

Ron Earp · 01-11-06, 08:25 PM

Ross, your setup looks pretty decent (as do the other suggestions, but I'm leaning toward more track than street). I still might try some toe out - I really like the sharpness of turn in on the race cars and I don't feel much straight stability is compromised.

What spring rates are you running front and rear (I won't tell anyone!)? I don't have a lot of experience with spring rates in a 40 but felt the stockers on the RF and CAV I've driven to be far too soft for track work.

flatchat(Chris) · 01-11-06, 09:21 PM

Way back when I raced [img]/ubbthreads/images/graemlins/cool.gif[/img]and there was still plenty of room for improvement.
Using Dunlop 265/590R16 SS12M front and 265/625R17 D11 rear
Springs:- 250lb front and 450lb rear
Sway bar:- 1 1/8" front 3/4" rear
Shockers:- AVO coil over adjustable
Toe:- 1mm out front 0-.5mm in rear
Castor:- 4 deg. neg.front and 0 rear
Camber:- 4-5deg neg all round

This worked well for me at the time
Spring rates depend on angle of mounting (vertical is good)
Anti roll bars depend on chassis torsional stiffness
Spend money on shocks at least double adjustables ie bump and rebound -- gas assist [img]/ubbthreads/images/graemlins/beerchug.gif[/img]

Canuk40 · 01-12-06, 11:18 PM

Hi Ron,

A couple of things come to mind here with alignment settings, the intended use and the suspension geometry dictated by the pivot points in the suspension.

You're more into track use than street so more negative camber front/rear is required than a street driver. On the street the tires will quickly wear out with big neg camber numbers.

About the castor, on the front it's to bring the steering wheel back to center and may ad camber to the static number as the tires turn in, another good thing. Steering effort goes up which is not good for low speed street driving.

The rear castor plays an important roll (no pun intended) in the percentage of anti-squat geometry available. Good for both road and track, however a big number may test the limits of rod ends before binding and may contribute to rear toe steer in bump. Toe out is bad news here.

All of the above is highly influenced by the manufactures choices in pick-up points and upright design so what works for one car may be too much or not enough for another...

On the front toe-out, you mentioned preferring a quick turn in to a minimal improvement in stait line stability. I'll go along with quick turn-in feel however a better way to get it may be to manipulate the ackerman angles to give more lead to the inside tire before weight transfers as the cornering load goes up. A toe-out condition could make the car twitchy and nervous on corner exits. Anyways, minimal toe on the front is best for tire wear and race tracks are flat.

We run street settings, however they do work fine for short lapping sessions if you increase the tire pressures.

Castor F/R 4.0 degrees
Camber F/R .5 degree negative
Toe F/R 1/16" in total

Very conservative but the car is light and lively, strait as a laser beam. Plus the tires last forever

Oops, that was more than a couple of things...

Cheers

Ron Earp · 01-13-06, 07:47 AM

I assume that everyone is using a "standard" sort of bushing on their cars, correct? That is derlin, poly, rubber, etc.? Just wondering if anyone had gone over to a complete race/track set up with spherical bushings.

Ian, I think your settings would work well too for a car with street and strip. The camber needed is also going to be determined in part by the tire of course, and I've not figured out what sort of tire I can use since I don't know what size wheels will be on the car. In fact, I have no car, so it might be mote anyhow. I've not had any problems with a little toe out on any of the track cars, but, I also don't have a lot of options due to rules to change anything more than adjustable alignment specs.

R

Gary Gibbs · 01-13-06, 09:05 AM

I have finished up a new front suspension that includes quick/easy adjustment rod ends for the upper arm and polyurethane for the lower. Adjustable rod end for the tie rod permits bump steer adjustment. Shown HERE.

The rear that I have been running for the last year has now seen the one rubber joint upgraded to nylon. It can be seen HERE.