GT40 Weight Distribution

Hoona · 01-02-04, 04:46 PM

My buddy and I were discussing the optimal weight distribution (front/rear). I understand the GT40 is close to 50/50 but if that is true, is that ideal for cornering? For instance, would you want more in the rear if you had a dragster - ie straight line acceleration?? Up front for a front wheel drive? Equal for a 4 wheel drive? We also wondered if wheel width also made a difference, ie, while cornering, the wider rear tires would have more surface area than the fronts - would you change the weight distribution for ideal maneuverability? Love to hear some theories out there - Thanks Mark

Kevin M · 01-02-04, 06:14 PM

GT40s don’t have 50 – 50 weight distribution. They are about 44 – 56 to 43 – 57 front & rear. Like any mid engine car they carry a good deal more weight on the rear wheels.

Optimal distribution is different for different things; a Porsche 38 – 62 is great for braking. 50 – 50 gives better feel in cornering, but is not as fast as something like 44 - 56 to 40 – 60 from a mid engine car. For acceleration in a straight line you want enough weight on the rear wheels so that they don’t spin. Once you have that, the farther forward the CG the better. (In a high power drag car to keep it from lifting the front wheels too far).

**J What?** · 01-02-04, 10:59 PM

We have been doing chassis setup on our MK4 and with 1/2 tank of gas and my fat arse the car weighs 2650 with a 38-62 weight distribution. Center of gravity is right at the tip of the water pump, 14.8" off the groung with a 3" ride height.
I remember a discussion regarding a computer program that has the capability to enter a twin trailing arm suspension like ours. We have been fudging the figures for a double a-arm but that uses a single point for the upper and lower ball joint.Help?
Bill

Brett James-McCall · 01-03-04, 06:03 AM

J What,
The program you are looking for is Susprog3D. You can buy or go and get a trial download from www.susprog.com

Here's the example they give:

Brett

Gary Gibbs · 01-03-04, 09:17 AM

My understanding is that F1 uses something like %53-%54 rear weight and would appear to be optimal for their racing. 50/50 makes sense if you are running the same rubber F/R. Certainly acceleration and braking will be better with slightly more rear bias. Accel is better because you have more total weight on driving wheels under accel. Braking is better because you will always have weight transfer to the front (about 400 lbs on a GT40) and will have close to 50/50 under braking utilizing all four wheel, etc.

Howard Jones · 01-03-04, 11:37 PM

I had always thought that a grand prix car had about 60-40 rear to front. The two parts on the car that weigh the most for the area they take up are the crank and the gear cluster. They are both between the CG of the car and the rear axle center line. Also the fuel load is bias towards the rear. The only large mass not biased toward the rear is the driver. I would guess that a GP car's center of gravity is about between the fuel cell and the engine. The center of gravity on a GT40 is somewhere behind the driver and should be about the same as a GP car. Aprox 60/40 R/F

Corner weights on a GTD should be about 700 on the rears and about 550 on the fronts. Thats about 57/43 R/F

Kevin M · 01-04-04, 11:24 AM

Formula 1 cars were about 40 - 60 for many years because that was what they found to work best.

A few years ago (In an attempt to slow the cars down) new rules were put into place that limited the width of the rear tires. This caused them to move the CG forward to try to get the best performance they could with the tire sizes they were forced to work with.

Formula 1 is one of the most heavily regulated classes out there. You have to be very careful about looking at Formula 1 and drawing conclusions about what is “optimum”. In many cases what you see is only optimum under the rather bizarre rules they have to contend with.

Gary Gibbs · 01-04-04, 11:27 AM

Very good point about the rules significantly impacting the outcome.

Julian West · 01-05-04, 05:11 AM

A couple of short threads I dug up with the search;

http://www.gt40s.com/ubbthreads/show...true#Post21472

http://www.gt40s.com/ubbthreads/show...true#Post21472

Whilst we're on this topic, does anyone know how to calculate the height of the C of G using corner weight scales??

Kevin M · 01-05-04, 12:05 PM

Just to complicate things a little, there is no such thing as an ideal ratio of weight distribution. It varies from car to car and even from one corner to the next for the same car.

As an example of some of the issues involved with getting the car to accelerate off a corner. You want to shift weight to the rear wheels on corner exit to allow better acceleration. But, if you shift too much weight, the front tires no longer have sufficient traction to turn the car and you find your exit speed is now limited by power understeer.

Here are just a couple of the things involved:

The power to weight ratio of the car. The more power, the more weight is shifted to the rear under acceleration.

The average corner speeds. At low speed the car accelerates much harder and transfers much more weight than at high speeds.

The vertical center of gravity. The higher it is the more weight transferred.

The distribution of roll stiffness. You can use a stiff front bar and soft rear bar to keep the weight more even on the rear tires so that the inside tire can contribute more drive, but then you have the problem of loading the outside front tire very heavily and you are back to the power understeer problem if you go too far with this.

While not an issue for a GT40, aerodynamic downforce is a major factor for most modern cars like Formula 1 (to the extent that a 200 mph car with open wheels can be called modern – another bit of rule enforced stupidity). The weight distribution changes with different speeds because you never get (and don’t want) perfectly even downforce front to rear at all speeds. (You always want more rear downforce than front downforce the faster you go).

This is just scratching the surface as far as what is involved in trying to select an optimum weight distribution for a particular car. In truth, you try to get a figure that gives you the best average results on the particular tracks that you have to run.

Julian West · 01-05-04, 12:12 PM

Fair comment, it's a complex subject.

I think what we're all after here is a static weight distribution front/rear split [img]/ubbthreads/images/graemlins/smile.gif[/img]

Kevin M · 01-05-04, 12:27 PM

You have to calculate dynamic weight distribution in order to get a meaningful static number.

As an example: Take a high power car with a high vertical center of gravity and a 50 – 50 static weight distribution. Then take a low powered car with a low center of gravity and a 40 – 60 static weight distribution.

They sound completely different. But when they are both exiting the same corner, they could very well have exactly the same weight distribution front to rear.

Remember, static weight distribution is meaningless in itself. No car handles well or poorly while standing still. Static distribution is just one part of getting the right momentary dynamic distribution, which is a large part of what we call “handling”.