Wheel frequencies VS track surface

[Greg Locock]

@DaveW - there is one case where small motion ratios is a good idea - seal friction etc in the shock. For smooth road ride the high leverage of the small MR makes it easier to break the stiction (in particular) and reduce the effect of friction in general. There are other ways to reduce the stiction/friction, but they involve $$.

OK that's irrelevant in the context of this forum!

[DaveW]

You're right, but I could argue the case (fixes are seldom win-win)....

[Rustem 1988]

So you might wonder why it would advantageous to have a wheel (tire and rim assembly) working against a high damper force in rebound.

Do I understand correctly with the shock absorbers in the rebound we can reduce the load on the wheel that can increase the coefficient of adhesion with the track?

[DaveW]

Do I understand correctly with the shock absorbers in the rebound we can reduce the load on the wheel that can increase the coefficient of adhesion with the track?

Possibly.

I'm not sure what you mean by "the coefficient of adhesion with the track"? If you mean "grip" then I wouldn't think it a good idea to reduce the load on the wheel - if a good lap time is required.

What do you think?

[Rustem 1988]

Do I understand correctly with the shock absorbers in the rebound we can reduce the load on the wheel that can increase the coefficient of adhesion with the track?

Possibly.

I'm not sure what you mean by "the coefficient of adhesion with the track"? If you mean "grip" then I wouldn't think it a good idea to reduce the load on the wheel - if a good lap time is required.

What do you think?

I meant the tyre grip coefficient. I think if there's not enough tire grip, then how can one of the tools be to increase tyre grip coefficient with the shock absorbers rebound. Not only for a good time, but for good handling yet. I do not forget that the time of suspension response to changing load or road irregularities can be increased with a large damping ratio.

[Tim.Wright]

Be careful in assuming that a higher coefficient of friction means you have more grip (force) capacity because this is only valid when your total vertical wheel force is coming from the static weight for of the car.

When you speak about dynamic road inputs or load transfer this assumption isn't true anymore. In these conditions, if your wheel load reduces, your available grip reduces.

[Rustem 1988]

I mean, different forces in the suspension with the same static weight load for different values of the shock absorber.

[Rustem 1988]

Be careful in assuming that a higher coefficient of friction means you have more grip (force) capacity because this is only valid when your total vertical wheel force is coming from the static weight for of the car.

Yes, I understand the difference.

[DaveW]

I meant the tyre grip coefficient. I think if there's not enough tire grip, then how can one of the tools be to increase tyre grip coefficient with the shock absorbers rebound. Not only for a good time, but for good handling yet. I do not forget that the time of suspension response to changing load or road irregularities can be increased with a large damping ratio.

Apologies, Rustem, I do not understand your reply - I am not even sure if it is a question or a statement..

However, rebound-biased damping is a "comfort", rather than "performance" strategy. As a result, some dampers (basically intended for road cars) are not capable of generating significant compression damping without issues. Also some race series have a minimum static ride height rule - then rebound-biased damping is a way to reduce running ride height to improve roll & pitch load changes in manoeuvres and, possibly, improve aerodynamic forces. Both of these are reasons why rebound-biased damping might be preferred.

Otherwise, compression damping can improve vehicle performance. On a rig, every performance indicator can be improved (at the same time). This translates fairly faithfully to the track, and performance is likely to be further improved by lowering static ride height.

[gambler]

What is the most effective means of displacing the shock shaft as it enters the shock body? Some shocks have a gas piston some have a gas baggie and some have a remote gas canister. It sounds off topic, but I can't help but wonder that it might have an effect on high speed actuations. I guess a better question would be what is the ultimate shock design to achieve good tie down and yet get over the rough sections and gators?

[Rustem 1988]

And yes, a typical 'rule of thumb' is to start of at 70% of critical wheel damping because it gives a good balance between response speed and overshoot.

Overshoot can affect the suspension travel? I think that If the damping ratio is zero, the overshoot is 1 and the suspension travel should be increased.

[Rustem 1988]

Rustem, I do not understand your reply - I am not even sure if it is a question or a statement..

However, rebound-biased damping is a "comfort", rather than "performance" strategy. As a result, some dampers (basically intended for road cars) are not capable of generating significant compression damping without issues. Also some race series have a minimum static ride height rule - then rebound-biased damping is a way to reduce running ride height to improve roll & pitch load changes in manoeuvres and, possibly, improve aerodynamic forces. Both of these are reasons why rebound-biased damping might be preferred.

Thank you Dave.

[DaveW]

What is the most effective means of displacing the shock shaft as it enters the shock body? Some shocks have a gas piston some have a gas baggie and some have a remote gas canister.

You omitted "through rod" dampers, though even those have a reservoir to cater for fluid volume changes.
.

I guess a better question would be what is the ultimate shock design to achieve good tie down and yet get over the rough sections and gators?

I suspect that there is no single answer to your question...The actual performance of a damper in service depends on many factors both inside the damper, as you suggest, but also on the structure of the vehicle, the way the damper(s) are connected to the vehicle and its tyres. So the first task is to decide how to quantify damper performance so that different damper designs can be "rated".

A damper dynamometer is attractive as an idea, but won't be useful in this exercise because the "impedence" of the dyno is generally quite different from that of a vehicle (crudely, if a damper wants to stop, it can on a vehicle but not on a dyno).

Data gathered during a rig test can be analyzed to provide estimates of work input by the rig, and work dissipated by the dampers & tyres, all as functions of frequency. Arguably the two should balance in a "perfect" set-up.

Just occasionally I get the opportunity to "rig test" a vehicle with two types of damper. The results are often a surprise. In one case, with relatively conventional three way adjustable dampers fitted, only 74 percent of energy input was "accounted" at 10Hz - that rose to 95 percent when Multimatic DSSV dampers were fitted. Both dampers had remote reservoirs and were similar in size. Multimatic DSSV dampers better? Often, but not always....

[gambler]

Thanks Dave...Ill read up on those type dampers. Im playing a little with a DSC sport Magnetic? active set up now so I look forward to seeing those results at the track. I have had the notion of a lo-buck set up of trying to remount a "gas" shock in a method where the gas extension "pulls the body down rather than lift?. Just have not had time to get my head around it........ It is amusing, as I have hours stacking shims... then dyno with that ureka moment..... only to install them and have them shake my teeth out before I even get out of the paddock... It's all good!
Thanks again, Ill listen as long as you want to talk.

[Rustem 1988]

What parameters associated with shock absorbers are important for driving on the road with irregularities?

I have so far decided that the shock absorbers of compression tune for the performance of vehicle, and the shock absorbers of rebound to reduce vibration and pitch, roll.