User 121020

Close - but it's more about how the rear rebound affects the front grip and less about the grip of the rear tires (though I'm sure that would come into play if the damping forces were too extreme).

Continuing with the corner entrance scenario where we're loading the outside front the most and reducing load on the inside rear the most, increased rear rebound is going to reduce the rate of pitch and roll towards the outside front tire. This is why I disagree with the linked articles that make generalizations like, "Increased damping makes the car take a quicker set." In this case, it doesn't. Additional rear rebound _damps_ the transient motion, slowing the rate of pitch and roll. By reducing how quickly the weight is transferring from the inside rear to the outside front, the rebound is limiting how quickly the normal force generated between the outside front tire and the ground increases. The normal force between the tire and the ground multiplied by the coefficient of friction between the tire and the ground is what determines the friction force, or grip. The increased rear rebound is essentially reducing the rate of grip increase on the outside front tire by slowing (damping) the motion of the chassis.

Very good point. There is a bell curve to all of this - and that is very difficult to convey through text conversations, like this one. It's possible to take any of the damping forces too far in either direction. When that happens, the result is generally loss of traction, loss of tire contacting the road, skidding due to overloading, etc. Tires have a window (relative to many variables - camber, normal load, tire pressure, slip angle, etc.) where they work optimally.

SlowTeg

Very interesting points nmrado. So when getting on the throttle out of a turn, front rebound damping affects initial rear grip as well because the front dampers are resisting the weight transfer to the rear.

Definitely handy to understand the dynamics and trying to think of different "stages" of a turn and how it affects the chassis/tire loading. It's certainly tricky changing a suspension/tire setup as one change can affect multiple different "stages" of how the car acts in a turn.

Gottabfast

Ideally low speed compression damping would decrease the load transfer in the front while rear low speed rebound does it's job during initial weight transfer. Low speed compression damping in the front will help turn in, then as a driver contribution releasing the brakes slowly/smoothly so as not to throw weight back off the front and cause understeer during initial turn in. As the car gets turned low speed rear rebound controls the rear body motion and keeping weight from suddenly transferring forward. Then as you feed in throttle, weight transfers rearward, making front low speed rebound and rear low speed compression damping work together to keep weight transfer predictable as the car is straightened out. The set up is different for each driver because they're capable of controlling different rates of weight transfer and yaw.

It's not about preventing weight transfer, it's about managing it and using it to your advantage to increase traction.

SlowTeg

Ya very true. I think many folks may not encounter nmrado's scenario of rear rebound damping initial turn in as much if say you're on a road course vs auto-x. For auto-x, everything is very quick compared to a road course. Nothing quite compares to a slalom auto-xing on a road course. Hopefully if you're deliberate and smooth you will be dealing more with low speed compression handling the damping like you said.

ZDan

The two italicized statements do not jibe. Slowing the *rate* of pitch and roll motions *increases* the rate of load transfer. Softer damping => FASTER rate of pitch/roll motions, but DECREASED rate of load transfer. Stiffer damping => SLOWER rate of pitch/roll motions but INCREASED rate of load transfer.

If you have no damping, only the springs react the load and they're governed strictly by deflection, more motion is required to transfer load. Stiff damping transfers loads more quickly than the springs can and provides more rapid weight transfer.

User 121020

The whole "if the car has no dampers" is nothing like the scenario I'm trying to describe. Compression damping is left unchanged in my scenario. I'm only talking about how changing rear rebound affects the roll/pitch rate. The two statements you italicized and bolded do not conflict. Additional rear rebound damping is limiting the rate of roll and pitch. Therefore limiting the rate of load increase on the front outside tire.

Your statements are correct for compression damping. All of my statements have only referred to what happens if you change rear REBOUND damping.

Softer compression damping ==> faster rate of pitch/roll motion and decreased rate of tire loading at the corner where the adjustment is made
Stiffer compression damping ==> slower rate of pitch/roll motion and increase rate of tire loading at the corner where the adjustment is made
Softer rebound damping ==> faster rate of pitch/roll motion and increase rate of tire loading at the opposite end/corner
Stiffer rebound damping ==> slower rate of pitch/roll motion and decreased rate of tire loading at the opposite end/corner

Again, if you have a set of shocks with good low-speed damping adjustment, try it for yourselves. It is difficult to envision how all of these forces are playing nicely together. Base your findings off of your own observations and don't believe every article you find. I have found many conflicting "expert damper tuning" internet articles.

Yes, 99% of my findings are based on auto-x experience. You'll see me at every auto-x using my hand to visualize how the chassis is pitching/rolling and which forces are active in determining tire loading and chassis motion. I do the same thing at the track.

ZDan

The statements do conflict. Additional rear rebound damping does limit the *rate* of roll and pitch. But this does the OPPOSITE of limiting the rate of load transfer. Because there is greater rebound damping, the rear is being unloaded more quickly with less suspension motion.

Reducing the rate of pitch/roll motion by increasing damping (whether rebound damping at a corner that is being unloaded or compression damping at a corner that is being loaded) INCREASES the rate of load transfer. Reduced damping => increased pitch/roll motion and reduced rate of load transfer. Increased damping => Decreased pitch/roll motion and increased rate of load transfer.

STatements are true for rebound damping as well. Either way, compression or rebound, if you go stiffer, you decrease motion and increase load transfer.

Yes, yes.

No, no.

If you were to do something stupid like get on the brakes and turn in at the same time (panic situation), loading the outside front and unloading the inside rear, whether you stiffen outside front compression damping or stiffen inside rear rebound damping, you'll decrease pitch/roll rate and increase rate of load transfer either way. On the outside rear, you're INCREASING the rate at which it unloads.

But that's not what's happening at corner entry. As you turn in you are getting off the brakes. Increased cornering forces are unloading the rear but at the same time reduced braking is loading it back up.

With increased rear rebound damping and no other changes, what happens at turn-in becomes very much dependent on how quickly you're adding cornering and how quickly you're getting off the brakes. If you're heavy trail-braking, turning in while still heavy on the brakes, the outside rear will try to extend and with greater rear rebound it will be able to unload itself more quickly, which will load the outside front more quickly. Relatively more understeer from overloading the outside front more quickly.

Agree that you have to try different things independently and see how they work for you individually.

ZDan

I haven't contradicted your assertion that more rear rebound tends to give understeer and that less rear rebound damping gives more oversteer. I agree, but the reason is NOT that weight transfer has been slowed with increased rebound damping.
In a slalom, reduced rear low-speed rebound damping will increase oversteer because of decreased weight transfer to the outside front. [edit: I'm full of it here, see below]
As the car starts to lean, with reduced low-speed rebound damping the inside rear will rebound/extend more quickly, increasing roll rate but reducing the rate of load transfer to the outside front, which will act to INcrease overall front grip (outside front less overloaded, inside front contributing more).

And increased rear low-speed rebound damping will increase understeer because of increased weight transfer to the outside front.

Increase rear rebound damping and roll rate will decrease while rate of weight transfer off of the inside rear and onto the outside front increases.

[EDIT]Actually, I blew it in the above analysis. More rear rebound damping should unload the inside rear more quickly in cornering which should increase load on the outside rear more quickly, and reduce the rate at which the outside front loads up. So it *should* give more initial oversteer.

But I totally agree that what *should* happen and what *does* happen are often two different things. Leaving the above for anyone else who might wanna poke holes in it

jaboyd

The whole "if the car has no dampers" is nothing like the scenario I'm trying to describe. Compression damping is left unchanged in my scenario. I'm only talking about how changing rear rebound affects the roll/pitch rate. The two statements you italicized and bolded do not conflict. Additional rear rebound damping is limiting the rate of roll and pitch. Therefore limiting the rate of load increase on the front outside tire.

Your statements are correct for compression damping. All of my statements have only referred to what happens if you change rear REBOUND damping.

Softer compression damping ==> faster rate of pitch/roll motion and decreased rate of tire loading at the corner where the adjustment is made
Stiffer compression damping ==> slower rate of pitch/roll motion and increase rate of tire loading at the corner where the adjustment is made
Softer rebound damping ==> faster rate of pitch/roll motion and increase rate of tire loading at the opposite end/corner
Stiffer rebound damping ==> slower rate of pitch/roll motion and decreased rate of tire loading at the opposite end/corner

Again, if you have a set of shocks with good low-speed damping adjustment, try it for yourselves. It is difficult to envision how all of these forces are playing nicely together. Base your findings off of your own observations and don't believe every article you find. I have found many conflicting "expert damper tuning" internet articles.

Yes, 99% of my findings are based on auto-x experience. You'll see me at every auto-x using my hand to visualize how the chassis is pitching/rolling and which forces are active in determining tire loading and chassis motion. I do the same thing at the track.
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I absolutely agree with your comments as to how the car/tires reacts to damping changes. Rather than debating chassis response vs handling characteristics I really only want to know that if I'm pushing then I can either soften the front bump or stiffen the rear rebound to gain grip for example. These are great guidelines and simple to use/understand.

Rockweiler

Been a while but was also a dreadfully long winter for us up here in the Great White North. Just thought I'd come back and revisit the issue and give feedback to the many who took time to help me with my rear grip problem.

So I was able to get the car up on a hoist in mid-May and started with the easy stuff: bring the car up in front (was close to bottomed out on the PSS9 height adjustment and drove like a tractor!), slightly lower the rear to bring it more in line with the front so that I am about half an inch lower all around and adjust the damping to your suggestions: stiff up front, loose in the back. Street I run the fronts at 6 on the adjustment dial and on 1 for track and leave the rears in 9 for both.

It definitely helped on the street, especially comfort wise but didn't cure the tail wagging completely ... But who cares ... On the track it is simply amazing!! I just came back from a thoroughly enjoyable day at Calabogie Motorsports Park where my instructor couldn't stop gushing on how well my car was set up and handled, said if it was up to him wouldn't change a thing: tire size, ride height, etc... and he drives the new Z/28.

So thanks to everyone who chipped in and helped me set it up this way since I knew nothing of how to do it properly.