Bump Stops vs Damping Ratio

[rikgray]

Using bump stops to increase the spring rate to keep the car off the ground with aero loading,

Dampers are typically linear or digressive.
When the bump stops are on the shaft of the damper with coilover, the spring rate rises exponentially,
This leads to a decreasing damping ratio and an under damped range on the end of suspension travel (or over damped on beginning of travel). Since I haven't seen a damper that has different damping at different stroke positions.

With rising rate from rocker design the spring and shock stay matched. But not on my car.

Some cars use rubber bumpers as a third spring without damper.

I'm looking at a wheel rate going from 400 lb/in to 700 lb/in. from ride height to full bump.

Is this a noticeable effect?

[R. Pare]

Bump rubbers are self-dampening.

[DanW]

There's some interesting ones out there.

[rikgray]

Doing a bounce test:
Chrismas tree foam bump stops don't bounce much, and probably have some damping.
Urethane donut bump stops bounce. Probably not much damping.

The Christmas tree bump stop has a steep parabolic force curve and not very useful as a spring. If is is used as a spring and had significant damping it would heat up, soften, and be useless.

The donut bump stop has a linear to square to cubic force curve, with a useful spring range, but not much damping. So the problem remains.

[DaveW]

Take a look at these. They're what I use in my 3rd "spring' setup and on the dampers. They have enough damping so they do not bounce. They are very progressive and so do not give a sharp increase in stiffness at contact.

[patman]

Aero loading does not require much damping. It is a steadily applied force that increases very slowly as you gain speed.
Bump forces are totally different, being either cyclic, or sudden, or both.
Inertial forces (roll etc) are somewhere in between.

As long as the only time you are getting into this range of travel is during high aero loading, you should not need to worry about damping.

Your primary concern should be smooth transition so that you are not upsetting the car (ie 'bottoming out'). Correctly shaped bump stops should be fine for this...

[rikgray]

The aero loads change slowly, with road speed, but.
Since the suspension is not bottomed out, there is bump movement at high frequency.
At high aero loads the bump stop is a large percentage of the spring rate.

I've added more rebound to the shock settings, and it works.