Flat Ride and Center of Percussion

[GEspo]

Hello eng-tips. For vehicle suspension/dynamics design, how can I relate the concepts of flat ride and center of percussion?

Discusses COP at the rear axle:

Shows the bounce center, Olley:

Also, I need to get a Milliken or Gillespie book, which would be better suited for off-road dynamics?

 

[BrianPetersen]

I cannot speak for your specific racing application, only my own ... roadracing motorcycles, and in my case it's generally on pavement of sub-optimum condition. And in that application ... ride quality is not a consideration. Maintaining the best balance of optimum grip and rider feedback is the prime consideration. Grip tends to be optimum with a slightly compliant set-up, feedback tends to be optimum with a slightly firm set-up, rider personal preference is certainly a factor in this, and the damping matters more than the spring rate. But what matters much much more, is keeping the suspension off the jounce bumpers (bump stops). Hit the jounce bumpers and you force all further "suspension" to be done by the tire (causes excess tire wear) and it tends to lead to loss of grip due to inability of the tire to further follow the pavement.

I/we don't look at "flat ride" or ride frequency or anything of the sort. It isn't a consideration - although, it will generally be found that going by feel (have someone hold the bike upright and then rider supports himself off the seat using both arms and both legs and moves their upper body as a unit up and down to see what the bike does in response) the suspension movement should be pretty balanced front to rear.

I/we DO look at how much of the available suspension travel is being used. I tend to optimise my own bike to use all available travel while remaining just a few millimeters short of hitting travel limits. That has meant, in my case, to go a couple steps in spring rates firmer than what Ohlins (suspension supplier) originally recommended, front and rear.

Racing vehicles shouldn't be bouncing and wallowing in any sort of cyclic manner ... period. If it is, you have insufficient low-speed damping, and whether that's compression or rebound damping, is for you (set-up person) to figure out.

 

[GregLocock]

For off road, and racing in general, the flat ride criterion is not usually a consideration. For normal road cars the usual tune to achieve this is for the rear wheel rates to be 120% of the fronts - but this is not cast in stone, and is speed dependent. For off road we have a variety of humps of various dimensions and attempt to tune the dampers and springs and jounce bumpers to maximise the speed over them. This is not complex, but it is very non linear (especially with remote valve shocks), so we just simulate it rather than calculate it. I haven't done this for offroad, so I don't know what they use, but it only needs 4 dof really.

Cheers

Greg Locock


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[GEspo]

Thanks for the responses.