Eric_Oz_S2

Does anyone know the effect of suspension compression (eg through cornering loads) on rear toe?

My car has been feeling a little nervous at entry and mid corner with the rear feeling loose. This is more on high speed corners. The rear toe in is set at 9' (minutes) total toe in (about 1.7mm) at static load. There is also some oversteer on corner exit on tighter corners.

The car has reindexed tbars with helper springs and the ride height is level fron to back. The car has been corner balanced. Front toe is 0 to slightly toe out.

I'm wondering if it is getting too much rear toe in during cornering leading to instability.

Dave W.

The toe is not abnormal, but it's getting there. Stock specs are 0' +-5' for MY 86-89 and 10'+-10' for MY 1990. If it's not too much trouble, I'd set the toe closer to zero and see if it helps.

Does the car still have the original rubber bushings on the rear trailing arm and spring plate bushing? Those two bushings were designed to flex a little when cornering. If they're worn the rear wheels might be pointing in different directions when cornering.

chrenan

I agree with Dave, I run zero toe, give it a try and see if it helps.

Droops83

No, you want to run MORE rear toe-in to help rear stability during hard braking and cornering. The outside rear wheel wants to toe-out in these conditions due to bushing deflection, and the best solution (besides solid bushings) is to run a fair amount of toe-in on the rear to counteract this. Porsche designed a passive rear toe-in system in the 928 (using a flexible control arm) and in the 993 (using a special soft bushing) from the factory for this reason.

My 951 is nearly stock to stay in class for the Porsche club that I run in, so I have tried a number of alignment settings to get what I have to work. I found that when I dialed in a lot of rear toe-in (I run nearly 0.2 degrees per side!) the rear was much more stable, and if it did slide (which it does a lot!) it was far easier to control. I'd say 10' per side in the rear should suffice.

TonyG

I run a lot of rear toe even with spherical & Delrin rear suspension bushings on the A arms as well as solid bushings on the torsion bar carrier. I run just under 1/4" on both sides.

This, combined with the Guard 50/80 differential, makes for a rear end that is very easily thrown into hard core trail brake situations with no drama. Any sliding that occurs is always in "slow motion". It really lets you drive the car at 10/10ths.

TonyG

Dubai944

+1 on the above comments for running more toe in on the rear. Solid bushings will help a lot too.

Tony is spot on about the Guard diff. I also run one and it allows me to run a bit less toe than I could otherwise. I once swapped a 6 speed in with as standard lsd for a race meeting while my gearbox was being rebuilt and had to add a lot of rear toe to regain the same stability under brakes.

Compared to your settings Eric, I typically run 6 to 10 minutes of toe on each rear wheel, so almost double what you are running and that is with the Guard diff to help as well.

Eric_Oz_S2

Ah. Thanks all for your contributions. PS I have a standard LSD. The above commentary would probably explain why when I reduced the toe in amount a little recently from about 12' total to 9' total it started to fee a little more tail happy on the limit. Next chance I get I'll put it back to about 12'. It is interesting the factory suggests 0 is OK. Probably good for tire wear but lousy for handling.

I thought it may have been getting too much rear toe, but it seams the opposite is true.

Does running solid bushings on the trailing arms make a significant difference?

I gather with more rear toe in I will have to compromise the Autocross/Motorkhana performance?

Dubai944

Solid bushings really improve things by eliminating the unpredictable handling that comes from bushing deflection. You can assess the effect of your alignment settings much more accurately and the car will be more responsive and predictable. They will make things a bit harsher however, maybe not what you want on a car you daily drive. As long as you use alignment settings to compensate, like extra rear toe mentioned by everyone here you can still have a good handling car with softer bushings.

Relying on changes in toe settings to adjust your steer characteristics for different types of motorsport is a bit limited. Alignment settings are great for fine tuning but you will do better sticking with toe settings that make the car stable and predictable and using roll bars and springs to change steer characteristics.

For a car like yours which you probably don't want to realign all the time or change spring rates on, a set of adjustable roll bars would probably be the best bang for buck. Or even just have a bigger rear bar you just swap in for autocross.

mikey_audiogeek

Hi Eric, just to answer your original question: compression of the rear suspension (or lowering) adds toe-in.

Cheers,
Mike

Eric_Oz_S2

Thanks Mike. So the opposite must be true - rear lift (such as under heavy braking) must reduce toe in or add toe out - hence the instability under braking unless there is insufficient toe in to compensate.

mikey_audiogeek

Yeah, in theory, although on my setup (with 850lb/in springs) this will be minimal, and toe-out due to deflection of the bushes under braking is probably the dominant factor. Measuring the torsional stiffness of the suspension about the vertical axis (steering stiffness) is on my long list of things to do. It reveals a lot...particularly at the front...

I got into the habit of high vertical rate and low roll rate on semi-trailing arm suspension long ago - minimising rear toe variation, and minimising jacking effects due to lateral load, are high on my list of priorities.
Cheers,
Mike