Rear toe on a track car?
10-08-2007, 07:59 AM
#1
Burning Brakes
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Rear toe on a track car?
I have a 964 turbo running a 964 cup chassi with mono ***** and MPSC's.
Had my alignment measured the other night and did a corner weight.
The corner weight was off with about 35 lbs, it's now within 7-8 lbs.
Totalt weight empty is 2.765 lbs.
My toe measured to 1mm total toe-in front and the same for the rear.
I'm running -3.0 camber front and -2.5 at the rear.
I would like less toe-in at the front for a faster turn-in, would think I would be able to do that just by adjusting equal amount on both sides? The plan is to start with 0.5mm on each side and see how that feels.
Currently my car is moving sideways back and forth during breaking, when accelerating out of corners a get an uncomforting feeling that the rear is coming loose, actually it's more than a feeling, in my mind the rear do come loose "very" easily. Heard that these cars needs lots of static toe-in at the rear (3-4mm).
When I get the money I will install ERP bushings at the rear (having a feeling the rear stock bushings are toost) and go 18 inch wheels (having trouble getting enough temperature and still have a stable tyre sides). Think this would improve the handeling.
Feel free to comment my analysis.
Now to my question, I never adjusted my camber and toe at the rear, is it possible to only adjust the toe without changing the camber and what toe-in at the rear should I aim for?
Had my alignment measured the other night and did a corner weight.
The corner weight was off with about 35 lbs, it's now within 7-8 lbs.
Totalt weight empty is 2.765 lbs.
My toe measured to 1mm total toe-in front and the same for the rear.
I'm running -3.0 camber front and -2.5 at the rear.
I would like less toe-in at the front for a faster turn-in, would think I would be able to do that just by adjusting equal amount on both sides? The plan is to start with 0.5mm on each side and see how that feels.
Currently my car is moving sideways back and forth during breaking, when accelerating out of corners a get an uncomforting feeling that the rear is coming loose, actually it's more than a feeling, in my mind the rear do come loose "very" easily. Heard that these cars needs lots of static toe-in at the rear (3-4mm).
When I get the money I will install ERP bushings at the rear (having a feeling the rear stock bushings are toost) and go 18 inch wheels (having trouble getting enough temperature and still have a stable tyre sides). Think this would improve the handeling.
Feel free to comment my analysis.
Now to my question, I never adjusted my camber and toe at the rear, is it possible to only adjust the toe without changing the camber and what toe-in at the rear should I aim for?
10-08-2007, 09:49 AM
#2
Nordschleife Master
What do you mean you have a 964 Cup chassis? The tubo chassis is different from the RS chassis the Cup car is built from in several ways, including the seamwelding.
The problem with the front of the 964 is the front upright geometry which has too much toe change per suspension movement (bump steer) and lowering the car aggrivates the situation. As the suspension compresses, the front will toe out from is static alignment which can help on corner entry and is aided by the proackerman geometry already built into the front suspension. I would not recommend running toe in or toe out at static ride height, but rather 0 toe which seem to be compromise with the stock geometry.
In the rear, I generally run RS specifications or slightly more which is 15minutes of toe-in per side and in some cases I'll run as much as 20 minutes of toe-in.
The rear camber and toe are adjusted by two eccentrics and they are interrelated, if you move the toe, the camber will change.
The problem with the front of the 964 is the front upright geometry which has too much toe change per suspension movement (bump steer) and lowering the car aggrivates the situation. As the suspension compresses, the front will toe out from is static alignment which can help on corner entry and is aided by the proackerman geometry already built into the front suspension. I would not recommend running toe in or toe out at static ride height, but rather 0 toe which seem to be compromise with the stock geometry.
In the rear, I generally run RS specifications or slightly more which is 15minutes of toe-in per side and in some cases I'll run as much as 20 minutes of toe-in.
The rear camber and toe are adjusted by two eccentrics and they are interrelated, if you move the toe, the camber will change.
10-08-2007, 10:17 AM
#3
Burning Brakes
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What do you mean you have a 964 Cup chassis? The tubo chassis is different from the RS chassis the Cup car is built from in several ways, including the seamwelding.
The problem with the front of the 964 is the front upright geometry which has too much toe change per suspension movement (bump steer) and lowering the car aggrivates the situation. As the suspension compresses, the front will toe out from is static alignment which can help on corner entry and is aided by the proackerman geometry already built into the front suspension. I would not recommend running toe in or toe out at static ride height, but rather 0 toe which seem to be compromise with the stock geometry.
In the rear, I generally run RS specifications or slightly more which is 15minutes of toe-in per side and in some cases I'll run as much as 20 minutes of toe-in.
The rear camber and toe are adjusted by two eccentrics and they are interrelated, if you move the toe, the camber will change.
The problem with the front of the 964 is the front upright geometry which has too much toe change per suspension movement (bump steer) and lowering the car aggrivates the situation. As the suspension compresses, the front will toe out from is static alignment which can help on corner entry and is aided by the proackerman geometry already built into the front suspension. I would not recommend running toe in or toe out at static ride height, but rather 0 toe which seem to be compromise with the stock geometry.
In the rear, I generally run RS specifications or slightly more which is 15minutes of toe-in per side and in some cases I'll run as much as 20 minutes of toe-in.
The rear camber and toe are adjusted by two eccentrics and they are interrelated, if you move the toe, the camber will change.
Chassi is the wrong word, I have installed a set of stock 964 cup dampers and springs on my car.
Will start with a 0 toe at the front and see if that makes turn-in better.
I need to convert your numbers to something I can relate to, I measure between the front and rear end of my 17 inch wheels.
Geoffrey, another "problem" I've noticed in comparision with other cars is that I need to turn twice in fast corners, first to set the car and then point the car to where I want to go. Could this be related to the fact that the 964 cup springs are progressive?
In general I guess I should get rid och as much old rubber bushings as possible, still have rubber bushings in the lower fitting of the rear damper and the rear control arm. At the front I installed Power flex bushings, regret I didn't go "all the way" and installed ERP bushings.
And of course the "bicycle wheels" (17 inch 205/50 and 255/40) is a culprint for moving sideways.
10-08-2007, 10:40 AM
#4
Nordschleife Master
I'm not so sure the Cup dampers and springs are correct for a turbo car. Which Cup dampers do you have? They appear to have made valving changes over the course of the 964 Cup model run as I've found 3 different valving specifications on the Cup dampers. The Cup car was a 2450 car (2525 wet) and you are another 200lbs heavier, and your weight distribution is different as well. You may be experiencing a fundamental problem with your damper/spring package on the car.
If you install the ERP bearings, ensure that you have measured them against the control arm and that when they are installed, they are tight and not loose. I have had issues with the ERP parts fitting in the control arms and have machined my own collars to exact press fit into the control arm. If these are moving, you'll never feel confident with the handling.
If you install the ERP bearings, ensure that you have measured them against the control arm and that when they are installed, they are tight and not loose. I have had issues with the ERP parts fitting in the control arms and have machined my own collars to exact press fit into the control arm. If these are moving, you'll never feel confident with the handling.
10-08-2007, 12:34 PM
#5
Burning Brakes
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Thanks Geoffrey!
I didn't have the money to get something new specially build for my car, so I'm aware this isn't perfect. But they made a huge instant improvement on my laptimes (4 seonds on a 1:15 lap), so they are not "all bad". My gut feeling is that this is to soft for my car.
I doubt these pictures will help...
Anyway the dampers have been tested, first picture:Red is left front, blue is left rear.
Second picture refers to left rear.
Third (and last) picture refers to left front.
I had someone looking at these graphs a while ago, he responded that my dampers/springs where far from a race setup and that the 964 cup chassi had a dialed in understeer and some other "features" to make the cup racing in the 90's more even and safe/easy for guest drivers.
I didn't have the money to get something new specially build for my car, so I'm aware this isn't perfect. But they made a huge instant improvement on my laptimes (4 seonds on a 1:15 lap), so they are not "all bad". My gut feeling is that this is to soft for my car.
I doubt these pictures will help...
Anyway the dampers have been tested, first picture:Red is left front, blue is left rear.
Second picture refers to left rear.
Third (and last) picture refers to left front.
I had someone looking at these graphs a while ago, he responded that my dampers/springs where far from a race setup and that the 964 cup chassi had a dialed in understeer and some other "features" to make the cup racing in the 90's more even and safe/easy for guest drivers.
Last edited by Staffan; 04-24-2008 at 06:30 PM.
