3 replies to this topic

[Ben]

I was having a think the other day about how to specify the damping coefficient for the third damper in a modern 3-damper setup. The basic role of this damper seems to be to provide adequate heave mode damping without effecting the roll mode (obviously some cross coupling occurs, but that's the idea).

I started off with the basic assumption of the sprung mass moving on the suspension and the basic undamped natural frequency equation (1/2*pi*(sqrt(K/m)) and the critical damping coefficient (2*sqrt(K*m)).

What occured to me though was what is the effect of the aerodynamics on all this? Obviously the suspension heave stiffness is designed to cater for the downforce generated at the maximum speed, but this is a steady state value. What I'm getting at is if our downforce is very sensitive to ride height (which it is) can I model this as an additional spring in the system? If I plot downforce vs. ride height can I take the gradient of this curve and add it to the suspension stiffess to gain a more accurate stiffness to derive my natural frequency from?

Ben

[RDV]

....effect of the aerodynamics on all this? Obviously the suspension heave stiffness is designed to cater for the downforce generated at the maximum speed, but this is a steady state value.

steady state is non existent in racing cars, you are accelarating in at least one axis all the time,

the closest you would come to steady state is in ovals at terminal speed on the straights ,
so third damper controls pitch and heave, leaving side dampers to cater to roll transients,

and third damper ( usually coupled with bump rubbers or third spring) enable vertical rates to be different to roll spring rates, from a given rideheight , also enabling preload to be used ,

and most importantly with brubs to have rising rate springs in heave ( to try to follow aero load square power rise ) and linear in roll . this takes care of car attitude , enabling nose down attitude on slow corners and feathering whole car on straight , hope clear enough..

What I'm getting at is if our downforce is very sensitive to ride height (which it is) can I model this as an additional spring in the system? If I plot downforce vs. ride height can I take the gradient of this curve and add it to the suspension stiffess to gain a more accurate stiffness to derive my natural frequency from?

this is mainly what third damper/spring combinations are used for , controling aero effects,

as to gradient yes and no, must take in account tyre rate and deflection and the fact that trying to move a heavily aero loaded body at speed enables you to run lower values in damping , for aero loads have a damping effect in themselves,

except if extremely sensitive to rake and ride height , think of " porpoising " or "galloping" in 70`s & 80`s wing cars , where the flow though varying section venturi (ride height or pitch induced) would produce uncontrolable chassi movements due to varying aero loads , when at same frequency as tyres car would actually "hop" at very high speeds ....most unpleasant to drivers

[RDV]

... missed out the last bit, often third dampers are run just as convenient place to put "vertical" spring , or bump rubber but without any damping effect ....

[Ben]

Excellent thoughts.

The steady state thing was a bit of a slip, what I meant was that the heave rate rises to match the aero load as you say. A basic calculation here would just use a steady state model of downforce vs. ride height but obviously in reality it's a highly dynamic system.

The point about the damping effect of aero is interesting, I got the idea for the original question at work when I was thinking about a helicopter in hover (in ground effect). I think some of our aero guys are gonna get their brains picked on this

Ben