How does rebound adjustment affect handling?

[banjomike]

I have konis all the way around on my car. When one of the front struts died on my earlier this year I replaced them with the easy to adjust sport konis. On the back I still have the type that has to be removed to be adjusted.

I'll be going to an Autocross at the end of the month and I'd like to know what I am doing going into it. My problem is that despite being a mechanical engineer I just can't wrap my brain around how adjusting the rebound only will affect handling.

I have adjusted the front struts and noted the differences, but I rarely get a chance to really test it out. I started with it about 1/3 from the loosest setting, and it match the handling of the old struts pretty closely. I turned the adjustment one half turn in the tight direction on the front. Now it doesn't rotate around quite as loosely as before, or maybe it feels like it is pushing a bit more and a bit later through the turn. I'm going to bump it up another half turn, but it is pretty balanced right now. One thing to note is that the body roll is pretty exagerated. I measured the swaybars... front 19mm and rear I measured either 17 or 18mm, both stock bars.

I'm wondering if it would be worth removing and adjusting the rear struts, to match the front.

that's my friend driving last summer in a very low speed turn.

Any thoughts on any of these issues would be appreciated.

[jluetjen]

Front low speed rebound does a few things. It determines how fast the weight transitions back when accelerating. Softer will let the front rise faster without lifting the front wheels. Once the front wheels start to lift you will either get understeer or the front will stop lifting. When transitioning into roll, it will make the front act "softer" initially which will provide more grip on turn-in. stiffening up the front rebound will make it easier for the front end to transition weight off of the inside tire at the entrance to a turn which will reduce the front grip somewhat.

Now by adjusting the rear struts in the opposite fashion, you can offset the change you made to the front, so stiffer rear shocks has a similar (but not the same) affect as softer fronts. This especially applies across opposite corners, so LF and RR, and RF and LR.

[Cosi1]

Mike,

I have a couple points to add to John's very educated and knowlegable comments.

First, the Koni sports have only one adjustment feature, which effects valving for both compression AND rebound simultaneously, although the rebound mode is effected about twice as much with each adjustment ( 2 rebound : 1 compression ). That means that you are not able to fine tune for compression resistance, or rebound velocity or resistance. (By the way, low speed rebound refers to the velocity with which the damper controls the spring oscilation, not to the speed of the vehicle or the corner.) The Koni Sports simply get universally "stiffer" or "looser".

You are obviously sensitive to nuances in your 911's handling to detect the difference in balance when making "clicks" on these front dampers. And you are on the right track when you say that stiffening the front makes the tail less "lively".

Looking at the photograph, your analysis of body roll is also "right on". To state the obvoius, many items can help cure this, including lower ride height, heavier torsion bars, heavier (adjustable) stabilizer bars, more negative camber, a roll cage, reduced vehicle weight, corner balancing, AND those Koni Adjustables.

Sticking just with the shocks, here are some tuning generalities:
-Especially for autocrossing, you want the suspension to be as stiff as possible without experiencing wheel hop. So, I would probably make certain that your rears are set to "full hard". Then,
-If you are experiencing oversteer, stiffen the front.
-If you are experiencing understeer, loosen the front.
-If it rains, get rid of all stiffness front (and rear, if possible).
-Record all settings for future reference.

It will take a little experimentation, but if you learn how to tweak these shock adjustments (and eliminate some of that severe pitch and roll), you will be miles ahead of the Bilstein "set it and forget it" mentality.

Good luck,
Ed LoPesti
RacePro Engineering
New York

[banjomike]

Fantastic, thanks John and Ed! These were the kind of things I was looking for. I've only just had a chance to skim over the answers but this should help.

Naturally some of the 'hardware' upgrades are planned/hoped for in the future. At this point I am limited to oportunistic upgrades. So I hope that the other rear swaybar mount breaks this summer, then both will be updated and I'll be in a good position for adjustable swaybars!

Time to go and do a search on adjusting those rear konis...

[Cosi1]

Mike,

If those older rear Konis are the ones I think, you simply disconnect one end - probably the one attaching to your rear trailing arm is easier to get at - and rotate the shaft. The hitch is, I can not remember which direction - counter-clockwise looking at it from the bottom, I think. There will be major resistance to turning once full hard (or full soft) has been reached. Be certain to really torque the bolt / nut holding the bottom of the shock to that trailing arm.

Good luck,
Ed LoPresti
RacePro Engineering