Corner exit damper settings question

[adambrouillard]

Can anyone explain why on corner exit
(OF Rebound + and IR Bump -) causes oversteer. This seems to be the popular wisdom, but it doesn't make sense to me and seems like it would cause understeer.

[phoenix]

Increasing bump stiffness on the rear will increase the rate of weight transfer (from acceleration) to the rear, which would tend to add oversteer.

Increasing front rebound would tend to reduce corner exit understeer, which is the same as increasing the tendancy to oversteer. This is because the stiff damper rapidly takes load off the front tyres under acceleration, giving them more grip, assuming that you were using the maximum grip available from the front tyres up to the point in the corner that you begin to accelerate.

I'm not sure I would have different bump and rebound setting from left to right on the car. It is more usual to have the same settings on both left and right hand sides of the axle, just as it is usual to have the same spring rates.

If you are oval racing, that would be a different matter... oval racers do all kinds of stuff that 'circuit' or 'road' racers would never consider.

[tejay]

I don't think you can apply a rule in general like this. It would depend not only on the car and dampers in question, but the corner profile/speed as well.

If this is indeed the result you are seeing, then try one of the damper changes on its own, then the other on its own, to see what is actually causing the oversteer. You may find that one causes a lot of oversteer, the other causes mild understeer, and so the net effect if oversteer.

On the cars I work with at the moment, which are very stiffly damped, we stiffen front rebound to give exit O/S. This primarily works because stiffer front rebound will jack the front down, keeping the front roll center low on corner exit, reducing the geometric weight transfer. The evidence to prove this is in the damper data, which show the front ride height being lower with stiffer front rebound.

But on the rear, on a slow power-down kind of corner, we actually use a combination of stiffer IR bump and softer OR bump on the dampers for improved power-down (less OS). We find that on these corners, the power is applied as the car is still rolling to the outside of the corner, so softer outside bump will slow the lateral weight transfer down, and stiffer inside bump will keep more of the weight on the inside wheel as the car pitches (also helping to slow the lateral weight transfer). But this effect is mild, not as agressive as the stiffening of front rebound, so if I do both of these simultaneously, I will end up with mild O/S.

Theories are just that...you need to test to find out what really happens.

As for asymmetrical damper settings...we use them often on our cars (circuit racing)...we can get good adjustment for corners in one direction, without negatively effecting corners in the other. This is one of the advantage of running dual shocks on each end of the car. If you are never going to run offset damping, then you may as well save some money and stick mono-shocks on the car! (for open wheelers anyway)

[phoenix]

Originally Posted by tejay

On the cars I work with at the moment, which are very stiffly damped, we stiffen front rebound to give exit O/S. This primarily works because stiffer front rebound will jack the front down, keeping the front roll center low on corner exit, reducing the geometric weight transfer. The evidence to prove this is in the damper data, which show the front ride height being lower with stiffer front rebound.

I am intrigued. I understand that the front ride height will be forced lower due to jacking down, but how does less weight transfer to the rear induce oversteer?

How much jacking down do you see from your damper data?

I am pretty certain that a few millimetres of lowering at the front from jacking down will make only a very small change to the height of the c of g and therefore keeping the front lower by a small amount will not reduce the rearward weight transfer under acceleration by any significant amount.

The explanation I gave is in line with what damper manufacturers including Koni and Penske say about damper settings and what the late Carroll Smith has to say in 'Tuning to Win'. I know that these changes work in practice having used them myself and until now I have believed the explanations I have read from these people. Very specifically, they all agree that changing damper rates does not affect the magnitude of weight transfer, in roll or in pitch, only the rate at which weight transfer from the sprung mass to the unsprung mass occurs.

[tejay]

Originally Posted by phoenix

I am intrigued. I understand that the front ride height will be forced lower due to jacking down, but how does less weight transfer to the rear induce oversteer?

The difference in longitudinal weight transfer due to the lower CG is negligable, but at it effects lateral weight transfer distribution considerably due to the lower front roll center.

Originally Posted by phoenix

How much jacking down do you see from your damper data?

We can see 5mm.

Originally Posted by phoenix

I am pretty certain that a few millimetres of lowering at the front from jacking down will make only a very small change to the height of the c of g and therefore keeping the front lower by a small amount will not reduce the rearward weight transfer under acceleration by any significant amount.

Pretty certiain??? You seem very confident given you don't even know our cars? We typically change roll center heights 2-5mm for adequate balance changes on our cars, so up to 5mm change due to jacking caused by damper adjustments is very adequate!

Originally Posted by phoenix

The explanation I gave is in line with what damper manufacturers including Koni and Penske say about damper settings and what the late Carroll Smith has to say in 'Tuning to Win'. I know that these changes work in practice having used them myself and until now I have believed the explanations I have read from these people. Very specifically, they all agree that changing damper rates does not affect the magnitude of weight transfer, in roll or in pitch, only the rate at which weight transfer from the sprung mass to the unsprung mass occurs.

Like I said in my previous post, theories are only theories....no two types of car are the same. There are some may variables that go into the way tht a car behaves, that you cannot possible come up with such simplistic rules that will always apply. To think that you can only shows inexperience. Damper rates will not directly effect the amount of weight transfer, but if they can control your roll center heights, then they will most certainly effect the amount of weight transfered, in both pitch and roll!

[phoenix]

What I was saying I was pretty certain about was that a small change to the front ride height - 5mm you say - will have little effect on the height of the c of g. In the first part of your reply you say the effect is negligable so we seem to agree with that and that there is very little change to longitudinal weight transfer as a result: that was all I was saying so I think we are in agreement.

I understand that the lower roll centre will reduce weight transfer at the front. Surely in exiting a corner the load will be coming off the outside tyre anyway, as the car begins to straighten up after the apex?

The change in balance from understeer to oversteer much surely come as the result of the change in load distribution between the front and rear tyres, rather than the change in load distribution between the front two tyres, doesn't it?

Please note - this is not an argument! It is a discussion