Corner Weighted The Gt3

[clord]

I corner weighted my GT3 this week. First I measured the ride height. The front is toward the high end of the 5mm tolerance of the spec and the rear is toward the low end....but all within spec. On the scales without me in the car the diagonal totals were within 9 pounds of each other (!!) and with me in the driver's seat they were still within 13 pounds. Nothing to fix here! This is an amazingly good setup straight from the factory. :clapping:

Now on to the alignment tomorrow.

[viken]

That's great, Craig. Keep us posted with the alignment check. Mine was way off on the right side. Both front and rear was toe'd in too much.

Edited April 8, 2004 by viken

[mds]

FYI, a correctly corner balanced car has equal diagonal weights only if the car is weight symmetric front to rear or left to right. The GT3 with half tank and driver is neither.

As an example, my car has a front/rear weight ratio of 36.5%/63.5% and a left/right ratio of 51.9%/48.1%. Total weight with me and a half tank is 3256lb.

A correctly corner balanced car should have at each corner a weight equal to the total weight of the car times the appropriate ratios at that corner. In other words for my car, the left front tire should weight 3256*.365*.519=617lb, ie the total weight times the front ratio times the left ratio. Similarly, the right front would be 3256*.365*.481=572lb, left rear 3256*.635*.519=1073lb, right rear 3256*.635*.481=994lb. In this case, you can see that the diagonal weights 1611 and 1645 are not equal. This is correct, they should not be equal.

No matter how far off your car is from a correct balance, the weight ratios will be the same. So the balance procedure is this: take your car as it is and weight each corner first, figure out the front/rear and left/right ratios, compute what each tire should weight, and then adjust the car appropriately.

Edited April 9, 2004 by mds

[macfly]

Mike, that is very interesting, but now I am a little confused. When I took my car to Johnson's in Torrance to have the full alignment and ride height adjusted I watched the guy there very carefully thru the 6 hours he spent on the car. It got weighted 3 times with driver weight in the process, but the guy doing the alignment was also looking for equal diagonal weights. The difference is 35lbs, or 2%, how critical or badly incorrect is this, and what am I risking/loosing by not correcting it?

The car feels much much better than it did before the alignment, but I had a slight rear toe out amongst other things, so the huge improvement I'm feeling is the camber & toe much more that the balance.

Edited April 10, 2004 by macfly

[mds]

Andrew, cars are usually so nearly weight symmetric left to right and that equal diagonals works fine as a rule of thumb. To do it right the ratio method should be used, but IMO it will be hard to feel a handling difference between the two methods on the GT3.

[Speedfreak]

What is the reason/s for alignment to be off from the factory? It seems that a considerable number of GT3 owners are reporting discrepancies with the listed tolerances. :unsure:

[BrianPA]

Why would you want the "correct" ratio left to rigth to be different? I understand front to rear you're going to have different weights especially in a Porsche but why would you want right and left to weigh differently?

[mds]

Brian, corner balancing doesn't change the left/right ratio, just as it doesn't change the front/rear ratio. So it is not a matter of wanting or not wanting a particular left/right ratio, you are stuck with whatever you have - as long as the modification is just a corner balance. Of course if you move weight around in the car the ratios will change. But they won't change when you move the spring seats while doing a corner balance.

Another way to say this is that the sum of the front two wheels is fixed. So is the sum of the rear two wheels. Also the sum of the right two wheels, and the sum of the left two wheels. Corner balancing will change weights on individual wheels, but in a funny, constrained way such that these four sums don't change.

Speakfreak, according to the latest rumor, the factory does not align and corner balance the cars. They just come off the production line however they were assembled. Apparently the factory does check alignment occasionally as a quality assurance measure on a few cars, but as a rule they are not aligned and not corner balanced.

[Nordschleife]

FYI, a correctly corner balanced car has equal diagonal weights only if the car is weight symmetric front to rear or left to right. The GT3 with half tank and driver is neither.

Mike

This assertion of yours puzzles me, as, in the real world, one does indeed set up a correctly corner balanced car with a half tank and driver's weight in place.

I quote from Porsche's GT3 Cup instructions:

Suspension Alignment

General

The suspension alignment of the 911 GT3 Cup corresponds almost identically with the procedure used on the 911 GT R.

Suspension alignment should be carried out using an optical or better still an electronic axle measurement device. The measurement procedure of the specific axle measurement machine should be adhered to.

The following conditions must be followed before alignment can begin:

• Car to be measured with 1/2 Tank of fuel and 75 kg in the drivers seat.

• Joint and wheel bearing play checked (wheel bearing play can not be adjusted).

• Front and rear tyre pressure set to 2,0 bar.

If the car is going to be measured front and rear, first check the rear axle values and readjust if necessary.

Camber value on the front axle applies when the wheels are in the straight-ahead position. Steering wheel and steering rack should be in the middle when adjusting toe.

Before the wheel alignment of the front and rear axles can be set, it is necessary to check the ride height. The wheel weight (corner weight) difference is adjusted by changing the ride height within the tolerance. The smallest possible difference in wheel weight left to right should be achieved. Therefore, it is absolutely necessary to align the suspension with the car on corner weight scales.

When I look at set up data, I can see that the ultra light drivers have noticably different car setups.

R+C

[mds]

R+C, I agree that you should corner balance the car with a half tank and the driver in place. I did so on my car. Sorry, I never intended to imply otherwise. On my car (with 195lb driver and half tank) the left side ended up weighting 124lbs more than the right side. After corner balancing the diagonals differed by 32lbs. This is correct. On a left/right and front/rear asymmetric car (with driver and half tank) such as mine, the diagonals should not be equal.

Edited July 27, 2004 by mds

[Nordschleife]

Mike

I follow what you mean now. I think it is worth emphasising your point that the aim is to minimise the difference between the left side weight and the right side weight, rather than equalise the diagonal weights.

I have watched good race crews get their adjustments right first time, they do get used to reading a computer screen upsidedown, however.

R+C