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Tuning Dual Adjustable Shocks

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I have had Penske Duals for some time now. I recently had them revalved with the new 8700 canisters (20 clicks) that are supposed to have a much better low speed compression range than the older 8100s that adjust more high speed compression rather than low speed.

I think the most common problems that Lotuses face are corner entry (lift throttle) oversteer and corner exit understeer.

I have gone through alot of setup changes this year, and the car has transformed from a massive exit understeering pig, to being in the ballpark now. A shock revalve, higher rate springs, and offet bushings to get -3 camber have really done wonders.

But the car is still a bit loose on entry, especially on low grip surfaces.

I was recently going though all my notes and resources and have found alot of discrepancies (both in literature and online discussion) regarding rebound adjustments.

This is what I have come up with:

Corner Entry oversteer:
Decrease rear rebound = turn rebound clockwise
Can also increase increase front compression (less effect)
Decrease rake (lower rear ride height)

Steady State (mid corner) oversteer:
Stiffen front bar
Lower Rear Tire pressures
More negative rear camber
More rear toe-in

Corner Exit understeer:
Decrease front rebound =-turn rebound counterclockwise
Increase rear compression = turn compression clockwise.
Increase rear rake (raise rear)

However, I am uncertain if the rebound adjustments are correct here.

Many sources explain that more rebound (resistance to extension) results in less chassis rise. So many sources, including chassis engineers, that I have read state that Increasing rear rebound (less rear chassis rise) would result in less corner entry oversteer. And the same logic would state that increasing front rebound (less front chassis rise) would result in less corner exit understeer. Dave Weitzenhof, who is a F2000 driver and puts on a seminar on car setup explains it this way: car prep Handout 2005a.pdf

However, many other sources I have read explain that lower rebound damping gives more grip (and less feel). i.e. Less rebound means less weight transfer. So this logic would indicate that you would decrease rear rebound to control corner entry oversteer, and decrease front rebound to control exit push. Neil Roberts, and CART engineer, explains it this way: Shock Tuning - Neil Roberts

These discrepancies make it difficult for the novice with Dual adjustables.

My experience with my Penskes seems to be the former, in that decreasing rear rebound resulted in more lift throttle oversteer. However, i don't have enough data to confirm that at this point. I'd like to see what others real world experiences are with this.
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· Registered
218 Posts
Wow -- listen to those crickets. You may have shell-shocked the troops.

In reading the two explanations I found one commonality: Phase 3B, which looks like the least important phase. Very curious. To further muddy the waters, racers I have talked to and at least one set-up article I read took the position that increased bump adds grip, up to a point. Perhaps these two articles are assuming the car is already set up (maximized bump for the chosen test scenario), and the tuning guide picks up at that point for the purposes of optimizing the vehicle in a particular environment.

I am also dubious about the 'added weight= less grip' axiom both these articles are predicated on. For one thing, the Lotus exhibits more tenacious grip (most notably in slow turns) when I have passenger onboard. Also, aerodynamic downforce increases grip. It may be that the force vector has a lot do with how the contact patch responds.

In any case, these two contending tuning philosophies need to be reconciled. Maybe we can find a tie-breaker.

· Registered
1,086 Posts
how stiff you can go directly correlates to your tire's maximum grip potential, which is directly related to the total weight of the car.

More weight = less grip in transitions, however, more weight = more grip in straight line.

You just have to find the balance with your track weight, and tire compounds.

Also, tuning tire pressure with you IN the car makes a difference too, but im sure you know that.

· Vendor
3,792 Posts
When we modeled the XP car suspension we made assumptions on peak lateral Gs and then tuned the spring rates etc. for a rough roll angle based on optimum suspension geometry. This also yields a natural frequency of the car front and rear. We're about 2.4 - 2.5Hz. You get much stiffer than this and the car becomes hard to drive. Any softer and it isn't as responsive as you would like.

When BWR changed to the 8300s we softened the compression for the fronts for the reasons that Trompazo mentioned above.

· Registered
9,783 Posts
I am also dubious about the 'added weight= less grip' axiom both these articles are predicated on. For one thing, the Lotus exhibits more tenacious grip (most notably in slow turns) when I have passenger onboard. Also, aerodynamic downforce increases grip. It may be that the force vector has a lot do with how the contact patch responds.
Just curious... do you have a Track Pack car, or aftermarket dampers? If so, was you car corner balanced for a single driver, or driver & passenger?

If you don't have an adjustable suspension, it just might be that the car is better corner balanced with a passenger on board... (not to mention the fact that you'll get less body roll on right hand turns with a passenger)

EDIT: Also, aero downforce increases the downward force vector on the tires without increasing mass... that's why it works...

· Registered
494 Posts
Discussion Starter · #6 ·
I did some testing last night with rebound settings.

This was not on the Exige. It was on a 997tt with KW V3s on a road course. I was on street tires so it was not too hard to push the limits.

I left compression static, set in the mid range. It was a bit too harsh over bumps, but decent.

I tried multiple rear rebound settings, and did about 4-5 laps at each setting.

Car felt pretty good with rebound in the middle of it's adjustment range. Pretty neutral, with touch of mid corner oversteer if I went in too hot. Overall the grip level didn't seem that high, but I am not used to driving on street tires.

Increasing the rear rebound a few clicks felt better. Still neutral, and car felt more direct and responsive. Grip felt better, and the mid corner oversteer was less. Upping the rebound to near max and the car was way too jittery over bumps and when coming off curbing.

Then I tried almost full soft on rear rebound. Rear had zero grip and was unmanageable. It was loose every corner.

So, what I can say in this circumstance, is that my results correlated much more with Dave Weitzenhof's suggestions above and that reducing rear rebound resulted in LESS rear grip.

However, the caveat here is that the car was getting loose mid corner. Because it was on a road course, I didn't really test lift throttle conditions like I would in autocross. The car seemed to turn in fine, and then slide through the apex.

In autocross, the car gets loose on my on entry/lift throttle. So this data may not be applicable.
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