Wheel frequencies VS track surface

[Rustem 1988]

I've read in this forum that more stiff tires require more work for shock absorbers. Does this apply more to compression or rebound shock absorbers?

[Rustem 1988]

Then dyamically, Mu*D*D(Xu) = (Ks+Cs*D)*(Xs-Xu) + (Kt+Ct*D)*(Xu-Xr). Here D*D(Xu) is the acceleration of the wheel, and Mu is the wheel (unsprung) mass. Wst disappears because it is balanced by static offsets of the springs. A similar equation can be written for the acceleration of the sprung mass, and the two can be solved together to compute the various responses of the vehicle per unit road input.

It is a little more than that, because the sprung mass connects the front & rear suspensions (so the sprung mass has "heave" & "pitch" inertias).

So we need to achieve such a combination of the values of tires, springs and shock absorbers so that the acceleration for sprung and unsprung masses during rebound and compression is the least?

[Greg Locock]

If your objective is maximising grip at all times, on rough surfaces, that might seem to be a reasonable path. But imagine a wheel that did not accelerate vertically. Every variation in height in the road surface would have to be absorbed in the tire. This would be a very bad thing. So I think it is the wrong way of thinking. A skyhook active suspension does almost the opposite, it tries to keep the vertical load on each wheel constant. That maximises the overall grip of the vehicle (you can't really talk about single wheel models).

[Rustem 1988]

If your objective is maximising grip at all times, on rough surfaces, that might seem to be a reasonable path. But imagine a wheel that did not accelerate vertically. Every variation in height in the road surface would have to be absorbed in the tire. This would be a very bad thing. So I think it is the wrong way of thinking. A skyhook active suspension does almost the opposite, it tries to keep the vertical load on each wheel constant. That maximises the overall grip of the vehicle (you can't really talk about single wheel models).

Thank you Greg.

I assume we need the wheel to absorb the roughness before it bounces off the road. I also read that it is necessary to try to reduce the relative displacement between the sprung and unsprung mass.

[Greg Locock]

"I assume we need the wheel to absorb the roughness before it bounces off the road."

What is your objective?

"I also read that it is necessary to try to reduce the relative displacement between the sprung and unsprung mass."

True for aero cars, not so true for anybody else.

[Rustem 1988]

"I assume we need the wheel to absorb the roughness before it bounces off the road."

What is your objective?

Yes.

[Rustem 1988]

If your objective is maximising grip at all times, on rough surfaces, that might seem to be a reasonable path. But imagine a wheel that did not accelerate vertically. Every variation in height in the road surface would have to be absorbed in the tire.

So it is necessary that the rough surface be absorbed with the help of a tire and a spring for maximum efficiency?

[Greg Locock]

What do mean by efficiency?

[Rustem 1988]

What do mean by efficiency?

Fast absorption of road roughness with minimal variation of the load on the wheel.

[DaveW]

Fast absorption of road roughness with minimal variation of the load on the wheel.

...But I wouldn't mind if the load increased, maximizing minimum load would be the key for me.

Also there is a time element, minimizing the time that a vehicle remains in a disturbed state is important, perhaps?

[Rustem 1988]

I made some graphs for the vibration velocity with damping and I got at a lower natural frequency the deviation of the velocity from zero less than for the for a large natural frequency. It turns out that for smaller mass velocity is changed in a larger range, if we have free oscillations?

[DaveW]

I made some graphs for the vibration velocity with damping and I got at a lower natural frequency the deviation of the velocity from zero less than for the for a large natural frequency. It turns out that for smaller mass velocity is changed in a larger range, if we have free oscillations?

I imagine that you have used a vehicle model, and the graphs were generated from model results. It would be very helpful if you could list the model, the parameter values used and the annotated graphs you generated....

[johnny comelately]

Dave, you are going to love this , from late 70's (seem to type that a lot) TZ750 and Kenny Roberts/ Kel Carruthers:
Later, Roberts was testing inJapanand finding it hard to get grip. Often the problem is too much compression damping, which lets bumps knock the whole machine upward instead of just the tire. A tire in the air means no grip and no stability. But reducing the compression damping didn’t do anything, so, at length, Roberts said, “Is compression damping in a part of the damper that we can just leave out? Can we just do away with compression damping completely?”

After some “discussion,” a damper was assembled with a spacer in place of its compression valve. When Roberts tested with that, his lap times immediately improved. Very quickly, having a much-bigger compression valve, capable of letting the rear wheel move really fast, became essential to good suspension. Geoff Fox was an important pioneer in this, but everyone quickly adopted the idea. With a bigger compression valve, it was no longer necessary to gear your bike so you could upshift as it hit the banking out of Daytona’s Turn 5. You could keep the power on across steps or rough pavement. More control, more of the time.

[johnny comelately]

and this (talk about having skin in the game for development/testing)
The instability had a cause, and it was not 750cc but rather little wire circlips, whose grooves were not deep enough to keep them in place. When these circlips popped out, either one or both fork damper units stopped damping and terrifying high-speed weave began—a violent Dutch roll at three cycles per second. Don Castro’s Yamaha team bike threw his feet right off the pegs just before start/finish in Friday practice. On Saturday morning, Carruthers had the front ends off all three team bikes, found the problem and fixed it.

[DaveW]

Interesting couple of posts, and a healthy reminder that everything I have contributed assumes that a vehicle has four wheels...

I know nothing about two wheelers, though I did get to sample the Millikens' remarkable simulator, and I often wonder how the tyres remain attached to the road in turns. I read once that it is common to have control of rebound damping in one, and compression damping in the other, of the front forks - how does that work, I con't believe that the structure is that stiff?