It is fair to say that these terms are used frequently but, for many like me, they seem to creat confusion.

Can someone explain (i) rebound, and (ii) compression?

Why are certain shocks designed to adjust only compression and others to adjust compression and rebound?

 

Compression is just what you think, the compression of the shock. Rebound is how much damping the shock has when it extends.

Compression deals with unsprung weight (everything under the springs, not held up by them). Rebound with sprung weight (everything including and above the springs).

Compression damping is NOT spring rate. Too much compression causes a very flinty, crashing ride. Too little causes terrible impact harshness. Too little rebound causes the "Cadillac ride", the float, the ponderous change of direction and inability for the car to take a good set and be stable. To much holds the spring down and might cost you a bit of wheel travel on the next bump, and that can hurt ride (mx bikers call it packing as you pack the suspension down).

Rebound is the big daddy and most important, but doesn't mean the compression doesn't matter.

GM shocks have a lot of compression damping, and too little rebound. This is worst in the stock rear shocks. That's why the cars ride sharp bumps like crap, but also aren't the most precise, well controlled cars in the world.

 

Thanks for the explanation Sam. Could you add any more about too little compression leading to impact harshness? I would have thought it to be the opposite, in that if it was too "stiff" on compression, the wheel would not roll over the bump, but try to go through it (since the car has much more interia than the wheel), leading to a harsher impact, whereas less compresison damping would be less resistance to wheel rise, allowing it to get out of the way.

I take it I'm wrong, I'm just looking for more detail to better understand it.

 

There are different kinds of "harsh" There is the bang-bang kind, and the thud and sudden stop kind.

If you have way too much compression, the shock won't want to let the wheel move to absord the smaller hits. If you have way too little, you hit the bump and the wheel moves too quickly and you might well run out of working travel before the wheel/spring has slowed down and you get a big bang when you ram onto the bumpstop at 400 miles per hour.

 

Good info Sam!