AVID-928

Can someone explain what the effect would be of changing front and rear toe settings?

I track a modified 965 with a PSS9 suspension set at the hardest for both front and rear. My best times have been clocked running this set up however I'm experiencing heavy understeer.

Tire setup: 225/40/18 and 285/30/18
Camber: -2.0(F) and -2.6(R)
Toe: 0.15d F & R

Would a toe setting of 0d give me better turning?

GB
1991 C2T
480 RWHP
1.3 Bar
S Cams
Light Flywheel
111 Octane Gas

SundayDriver

Suspension 101...

You are chasing the wrong stuff to tune the car. Tune it first with springs and anti-roll bars, then work on the shocks. Toe in/out can be used also, but you really need to differentiate what the car is doing in different situations.

What is the car doing under braking? Corner entry? Mid-corner? Corner exit?
What happens over bumpy sections? How is the turn-in feel?

What does the person/company who selected the shock valving say the ideal setting is? Stiffer is NOT better. Generally speaking, you want the stiffest setting you can get away with for compression and the softest for rebound. With single adjustable shocks, you better hope the balance is right in the valviing, because you can't get it right otherwise.

Do you have adjustable bars? If so, start there, assuming the springs are reasonable rates (compare with others running a similar car).

As far as toe settings. If you add toe out in the front, you will get a crisper turn-in. You will also suffer straight line stability issues so you have to balance that. Some toe out in the rear will certianly give you oversteer - I can almost promise it will be way more than you want. When I have seen cars set up that way, the driver usually does not understand why he is spinning 4 times per lap.

fatbillybob

Can someone tell me how do I convert toe in specs from minutes to mm. I understand how to set mm of toe but how do I set minutes of toe? I don't have a giant protractor nor can I find the exact center line of the front wheels to any degree of accuracy when toe is spec'd out in minutes.

mds

Here are some toe conversion charts:
http://www.smartracingproducts.com/p...conversion.pdf
http://www.smartracingproducts.com/p...e_settings.pdf

Steve Weiner-Rennsport Systems

GB:

Mark gave you some great advice.

The very first thing you should do is to soften the front shocks to #3 since #1 is much too hard.

Then, you need to install a set of adjustable swaybars as thats THE prime tool for changing the understeer/oversteer balance of the car. These give you the ability to change the roll stiffness at each end so you can make the car as neutral as you like.

Only after those measures would I change your toe settings as they look fine to me.

fatbillybob

THANKS MDS! You guys are awesome.

Cupcar

On my 964 I run more negative on the front than the rear, you have the reverse. This helps understeer I find.

I find a little toe out in front, around 2 minutes each wheel helps turn in.

Toe out in the rear is the ticket to an off track excursion.

That's my experience for autocross anyway FWIW.

Geo

Mark, can you explain this for me? It's rather counter to what I have understood. I'm not challenging you, but trying to learn something.

The following is what I have always understood:

According to my good friend who works for Roehrig Engineering, compression damping is no where near as critical as rebound. I also have understood that generally compression would want to be fairly soft relative to rebound, the thinking being that the damper should control the oscillations of the spring rather than add to stiffness, thus you would want the suspension to move relatively easily under compression (with the resistance determined by the spring, not the damper), but to be much more heavily damped on rebound lest you get to bouncing.

SundayDriver

I had that same view until late last year. Then I saw some results from a 7 post rig and spoke to a real shock expert - IMO (Chris Billings from The Shock Shop).

The role of the shocks is to maximize the change in contact patch or load at each corner. If you think about a bump, you want the shock to be stiff as it compresses so there is resistance to the tire moving upwards. When the backside of the bump comes, you want the tire to quickly drop and follow the road surface. (Now this is far from a complete model but gives and idea as it was explained to me.)

I will add that when rebound is too stiff, then in transitions (Esses) you end up feeling like you are always waiting for the car to settle from one direction in order to start the turn to the other direction. If you make the change before the car is settled, it is going to get really ugly when it sets. Try very stiff rebound through some Esses and feel the car. Then soften the setting more and more and feel how much quicker the car comes back.

At the RunOffs, I started with pretty stiff settings for both bump and rebound. throughout the week, I learned a lot and ended up with very soft rebound and firm compression. That was worth over 2 seconds per lap. A large part of that was the ability to get on the throttle much, much earlier with the soft rebound setting where I was getting wheelspin with firmer shock settings.

Geo

Wow. Very very interesting. I get the basic gist of this. I'll have to absorb it some more and ask some better questions.

One issue/concern/point of confusion is using the damper to add resistance to compress. Wouldn't we want to just use a stiffer spring? This should also have the added benefit of "encouraging" the wheel to drop and follow the road surface.

Anyway, like I said, this is going to take some digesting. It's a very new way of looking at things (at least for me). Cool.

SundayDriver

Here are a couple of thoughts - I find it helpful to think about what would happen at extremes. So, imagine that we had very, very high spring rates. You cross a bump - pretty sharp rise and very short - maybe 1" tall and 1" wide. The spring is so stiff, that there is virtually no compression. So when you cross beyond the top of the bump, the tire loses contact with the road until the entire car has time to fall. This is because there was no compression.

OK - so the stiffer we make the springs, the closer we get to that behavior. Now obviously the car is undriveable like that - you have to get some compliance through suspension (or chassis flex). So now let's think about what happens with softer springs. The wheel tries to move upwards at a very high velocity from the upward side of the bump. Now it really doesn't matter how 'soft' the spring is - it is going to move. Without a shock (damper) it is going to keep travelling upwards well after the bump is gone. Again, there is loss of contact with the road. So we need damping to stop the upward travel as quickly as possible (hence stiff compression setting) and then we want the wheel to come back down quickly on the downside of the bump. The spring - does that - we do want the springs as stiff as possible, but there are limits. Too stiff and we can't stand the ride - even in a race car. Too stiff and we get that first effect I discussed of the whole car leaving the ground.

So with the limits of how stiff we can make the spring, in order for the wheel to come back down and remain in good contact with the road, we need a soft rebound setting on the shock.

Hope this makes some sense.

Geo

Sure does make sense Mark. Just exploring questions that come to mind on the topic.

Like I said, very interesting. It requires a rethink of previous ideas about chassis set-up. Cool topic.

SundayDriver

Yup. My head was spinning after discussions with Chris.

Larry Herman

Excessively stiff rebound was something I found out about years ago when single adjustable Konis first became the rage. I put those puppies in my 914 with some stiffer bars/springs and cranked them up all the way. Can you say "undriveable"? I learned that too much rebound was keeping my tires from returning to the track (like Mark was explaining). In an undulating sweeper, the steering was twitching like the car had huge bump steer. Thatr was because the steering was loading and unloading with each bump. I kept softening the rebound until the car tracked smoothly. End of problem.

joseph mitro

yes, this is very interesting. i'm a complete novice to performance driving, but would like to learn more about it.

my 944 has single adjustable shocks with coilover springs on all four corners. so what's the best setting for the shocks? fairly stiff or fairly soft? i have them at full stiff front and 80% stiff rear but i'm thinking about softening them up, esp in the rear. it seems that if the car is bouncing with bumps in the road, it's probably too stiff, but then my springs are pretty stiff for road use.

oh yeah, we started with toe-in vs toe-out, didn't we? my mechanic suggested 1/16th" toe-out both front and rear, but i'm getting vibes like toe-in is better for the rear. is that what most of you experienced drivers run?