Suspension Questions

[Suspension Guy]

I have been looking at a book about suspensions are am curious what acceptable roll rates, tire vertical rates, roll gradient, and front and rear ride frequencies are for a FSAE car. Any help would be much appreciated!

[Suspension Guy]

I have been looking at a book about suspensions are am curious what acceptable roll rates, tire vertical rates, roll gradient, and front and rear ride frequencies are for a FSAE car. Any help would be much appreciated!

[steve]

I can tell you what figures I finally settled on (after one of the judges told me that the figures I were going to use were too low)
Front suspension 2.7Hz
Rear Suspension 2.4Hz
Roll allowed 1 degree per lateral g
Roll resistance split 60/40 Front/Rear

Don't know if these figures are any good, as we never actually built the car in the end

[Nigel Lavers]

Steve,

What sort of weight distribution went with these numbers?

And why would the rear ride frequency be lower than the front? I would assume you would want the opposite?

Nigel

[steve]

We used a 47/53 Front/Rear weight distribution. Frequencies were selected as a compromise between tyre grip and car response when turning into a corner. On a less than smooth surface, such as the car park at the NEC where we were holding Formula Student at the time, a softer suspension is beneficial to grip as it means as bumps and dips compress and extend the suspension , the load (vertical force) acting on the tyres does not change so much. Due to tyre load sensitivity, it is beneficial to keep the load as near to constant as possible on the tyres to keep the level of grip as high as possible. Eric Zapeltal wrote a good article explaining this in Racecar Engineering magazine a few years ago. Having a rear supension thet was too stiff therefore hurts tractionn during acceleration. The front suspension, however, is stiffer as a stiffer suspension means the car responds quicker (softly sprung cars always take longer to settle when turning into a corner). A further reason for the front suspension being stiffer than the rear is that the front roll stiffness was planned to be stiffer than the rear.