# Damping of my car

1. Jan 13, 2013

### zacharoni16

I was wondering how I can estimate the damping coefficient of my car by doing the hand bouncing the car body and watching the motion of the car?

Mechanics usually bounce the car by applying hand force and watching how the car moves in harmonic motion. Is there a way to estimate the damping coefficient by using this technique?

Or is there another technique like dropping the car a few inches or something to estimate it?

2. Jan 14, 2013

### CompuChip

The motion of the car is of course (intentionally) overdamped, to keep you from bouncing around forever. How fast it returns to equilibrium depends on the damping coefficient quite directly. So with a bit of math, you could derive a relationship between the damping coefficient and the time it takes, for example, for the amplitude to reach some fraction of the initial amplitude. Then you could easily test this by bouncing the car by hand and measuring this time. Then you can just plug in the time and calculate the damping coefficient. Note that in the theoretical calculation, the damping is exponential so it will never quite reach zero. However, the car quite obviously stops bouncing after two or three periods. This introduces a bit of uncertainty in when you decide which fraction you will take. I guess something like 1% could do, in physics 1/e is a common factor.

3. Jan 14, 2013

### sophiecentaur

Car dampers are not symmetrically damped. There is purposely less damping on compression (to allow the wheel to ride sharp bumps and to give a softer ride) than on recovery. The have a flap valve inside which closes some of the holes in the piston on the down stroke.
This makes the 'damping coefficient' a bit less accessible than for a simple damped oscillator.

@CompuChip
The damping has another function. It stops the wheel overshooting when it is thrown upwards by a bump. The wheel-spring combination has a much shorter time constant than the car-spring combination and it is easy to get a resonance with the wheel and tyre. You can get an oscillation, particularly when braking and the wheel can actually leave the road in a series of bounces. It also plays hell with cornering as your traction can be halved. I had a Renault 4 that did this until I replaced one of the rear dampers. Very dramatic but no noticeable affect on the 'ride' as the torsion bar suspension was very soggy anyway.

Last edited: Jan 14, 2013