beentherebaby

With less rear rebound the rear of the car will rise. When the back of the car rises under braking the weight quantity transfer changes as well as the rate of transfer changes. If the height did not change then yes the weight transfer rate would be all that changes but in the real world everything is dynamically changing which is why it's an art as much as science.

Larry Herman

That's why I hate stock suspensions.

SundayDriver

Sorry - I was not precise. The general wisdom is to make changes on the end of the car where the problem is felt. As I said, testing may change that approach, but it is better to start at the end where you feel the problem.

Shock setting does not affect weight transfer. That is CG and G loads. Shocks affect rate of transfer. Even the small effect of the back end rising is not a shock function, but how fast it rises is.

beentherebaby

The point being that a more rapid rate of rise in the rear body via lower rear rebound, means more weight transfers to the front sooner, i.e. in a given time span. If you increase the rear body height you increase the qty. of weight transferred to the front.

The rear rebound affects both the rate and the rise height until you reach steady state, i.e. max lift of the body or you release the brakes. The higher the rear the more weight transfer to the front under braking. That is why it's preferred to maintain a stable platform without a lot of pitching. Lowering the rear rebound may or may not help depending on where the damping force is to start.

You can achieve the desired results via a combination of springs and shocks. Finding the optimum combo requires testing because it's some what driver subjective in addition to the stopwatch.

Veloce Raptor

Yes that is what I meant: more free rebound or more rebound flexibility to maintain better rear tire grip.








Professional Racing and Driving Coach

Larry Herman

Thank God. I would have been so disappointed if you had it the other way around.

beentherebaby

NOW we know his speed secrets!

It's pretty good for stock suspension. You could always buy the RSR suspension...

atr911

Yes, so it's definitely a balance issue. One of the joys AND challenges of an adjustable suspension is that you can dial each end to compensate for the others deficiency. Usually this can just bring to light another problem when different forces are at work.

For instance, the car may be neutral now, but in an effort to dial out braking wiggle, I may upset the balance. At the end of the day, I'll have to find a compromise I think. Alternatively, I can always add more parts. Most will agree an LSD is a valuable addition to any sports car and it is coming. Once it's in, I'm pretty sure I'll be having to change the settings again to accommodate for the added 'push' the car will exhibit under power.

Here is what I think I will do:

-decrease rear rebound -2 clicks
-increase front and rear compression 1 click
I'll report on findings after. I may need to get into stiffer springs for optimal performance.

On another note, why hasn't anyone commented on rear toe? If under braking the toe goes positive (out), that would give a heck of a wiggle. The setup was done by a guy who trained with one of the top setup guys in the country so I'm assuming it should be okay... comments?

Larry Herman

Probably an assumption (at least on my part) that with 3mm toe-in per side and stock suspension links, it should not be an issue. 996/7 rear suspensions with stock geometry have surprisingly little toe change throughout most of their range of travel. The only way to really know if you have enough toe, other than by feel on the track, is to check the bump-steer.

beentherebaby

OP-

Suspension settings are always a compromise. I don't like to use toe changes to fix a problem because it usually bites you somewhere else unless it's way off to begin.

Remember... One change at a time or it's very easy to get lost.

kurt M

One at a time AND make notable changes. Go 30% or even 50% of scale to be able to feel the change and then add more or subtract to dial in. Sometimes if making many tiny ajustments in a raw setup the changes gets lost in the backround noise and you end up wondering if anything is changing.

I had an early car that tracked well start to get twichy in the rear. Looked everything over and found nothing loose, missing or broken. Burped a little bit of fluid from the rear brakes and everything tightened right back up. A little tiny bit of air in the rear caused the bias to shift. Pedal was 95% firm before.

mark kibort

Actually, the weight transfer will be a function of the deceleration rate (and ride height). the rate of weight transfer can be altered, but the ride height can be altered creating some instability as well, especially with driver input) So, there is some instantaneous forces that could be helped or hurt by shock bump and rebound settings , f/r. I think one of the posters had it best, when asking the question, "when is this a problem in the braking cycle?" All these solutios are knee jerk answers and may make the problem worse depending on its cause. If its a real fast straight in to a very hard consistant braking zone, the more than likely answer might be found in the alignment or tires. If its on initial brake, too much rebound might the issue as VR has suggested, but if its late, or initially in the braking zone at a slower speed, it could be rear braking bias. Driver input has a play here as well. I had a friend that had this issue into the hairpin at Laguna. I was behind him on some practice laps, and his issue was a suspension linkage washer installed incorrectly. that rear wheel was doing the "dance " under braking.
It will be interesting to see the fix on this one.

M758

Is the wiggle slowing you down? Is it preventing trail braking?
Most any car will have the rear end get light on very hard braking. One challenge is all the changes you make to stabilze the rear will have an impact all over the track. You can make the rear more stable and turn slower lap times because the balance is worse some place else.

The real answer is to know where you need the to work for this track and figure that solution that works best overal. I can see any number of changes inproving this, but making the car worse other places.

atr911

It's early in the brake and messes with my ability trail brake. It only happens on really fast straights ending in hard braking zones. There are three particular spots on my home track.

It feels like an unweighting that doesn't settle in time to turn in comfortably.

I'm still running quite quickly, just want to nip this in the butt as I'm sure that even though it isn't rearing it's head on all braking zones, I'm sure it's playing it's part.

atr911

I'm also looking into some pro driver to come out and show me the ropes for a day.