Tuning Ride Frequency and Damping for Tyre Compliance

[CWAnthony]

Hi all, I've been reading up on ride rates and damping and am struggling to figure a few things out.

As far as I can understand, the 'ride frequency' of the vehicle is the *undamped* frequency that the body oscillates at when disturbed by bumps. We can tune this frequency to match a certain application (limo, performance street car, aero car etc), so that when the body is disturbed by the moving of the suspension over bumps, it oscillates only at this set ride frequency for the benefit of it's passengers.

Then what is it that affects the compliance of the tyre with the road? Is that what the damping does?

Lets say I have a really light car, and very stiff springs. The ground is bumpy (bumps are say 1mm in aplitude) and the car is going quite fast so the frequency of the bumps are say 40Hz. With no damping, will the very hard springs keep the tyre in contact with the road? I presume the problem with having such a high wheel rate is that the body will experience lots of shock and movement transmitted by the hard springs. In the same situation, but with very soft springs and no damping, will the tyre have improved compliance with the road, or will there be no difference?

I only ask because when selecting a ride rate, the only consideration seems to be driver comfort, and possibly the amount of travel you are limited to (according to Milliken). When selecting a ride rate, compliance with the road is never mentioned, and the only time I have seen the frequency of bumps in the road considered is when it comes to damping on a transmissibility graph, and that is still only for driver comfort as far as I can tell.

If anyone can add some information to try and make this clearer for me I would be very grateful.

 

[GregLocock]

Rather a problematic post. I don't know why you emphasise undamped, on a typical passenger car there isn't much difference between the damped and undamped frequency.

The car does not respond only at that frequency.

yes shocks help to control the contact patch loads, but so do the springs.

40 Hz is way outside the region affected by ride tuning

Yes soft springs will improve the grip



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[CWAnthony]

Thanks for the reply Greg.

Do you know of any material I can read that recommends how to choose a spring rate based on the particular surface you are racing on? Or is this not normally a common practise?

Is there any way how to actually calculate the maximum spring rate you can run on a particular surface before grip becomes affected, by analysing the bumps on the surface? Like by looking at the amplitude of the bumps, and the frequencies they will cause the wheel to move at at certain roadspeeds. Or is it literally just done by choosing a certain rate, then going out and testing and tweaking it according to driver feedback and logged data?

As you can tell by my 40Hz guess, I've not seen any realistic values for road bump frequencies. Can I find these in a well known dynamics book, or is the only way to find these out by testing a car myself with very good suspension pots?

The damping I am not so worried about, as it is alot easier to adjust if you get it wrong to start with. Just need to know the ideal spring rate to get an idea of the roll rate and how to set the bar up.

 

[GregLocock]

Simplistically if you are worried about traction in a non aero car, and you are not riding the kerbs too much, then you want the lowest possible rate on each wheel, for an independent suspension. That means you should be just hitting the travel limits on each wheel once per lap. The use of rebound and jounce bumpers modifies this approach.

Milliken usefully discusses the difference for a beam axle.

Spectral analysis of road surfaces show a diagonal line on a log log plot, so there's lots of low frequency and then less and less at high frequency.

eg figure 6 in this

http://www.pavement.com.au/downloads/papers/6th PMS paper--Wei Liu.pdf

However you can ignore everything above 20 hz, in fact most of your problems will be at or below wheelhop, say 12 Hz.

http://forums.autosport.com/index.php?showforum=8

is a good place to ask technical questions about race setup.






Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?