Originally Posted by JIS
The trouble is I am not clear at all what that other little adjuster on the right hand side is for; eliminating pogo sticks ?!!
Yup...eliminating the "pogo stick" effect. You know, one good bump in the road, and you're on a rocking-chair-ride for a bit.
Idealy, the shock should absorb all the deflection energy given to the wheel, and return the suspension to it's normal (rest) point asap. Compression damping is used to handle the absorb-the-energy part
so you feel as little of a bump as possible. The oil passages are manipulated with an adjustable tapered (or some other magic with externally adjustable shocks) rod that either slows the oil or allows it to go through the piston faster depending on how the rod is set. With no adjustment, there are just the spring washers on the piston that flex with pressure, allowing the oil to flow through the piston at a wanted rate. Adding spring tension or removing it allows for different a different compression rate. Toss in the tapered rod, and you now have a range for the compression rate.
When the shock goes the other direction (rebound), there's another set of oil passages that get the same treatment. That triangular knob allows you to adjust the rebound. (There's no compression adjustment on the stock BMW shock).
On rebound, you want the shock back home to rest so the shock is fully ready for the next bump. Too much damping and it's slow to recover, and the next series of bumps could bottom out the shock. Too little damping, and while the shock is certainly ready for the next bump (and there's little chance of bottoming out), you could "top" out, too.
If the shock goes past the normal rest point, then it has to come back down, taking time away from the recovery (and bouncing you unnecessarily as well). In some instances, a little overshoot is ok, as it gets the shock back to rest the quickest (which is what you're after).
Too much compresion damping jars the rider and the bike is a bit more unstable as well...which is why, even if there's no adjustment, it's a good idea to get a (replacement) shock that's easily adjustable by a suspension shop.
As far as I know, most suspension shops have a shock dynometer such that they can dial in your weight, the bike weight, and your riding style to get you set pretty close the first time. If you do street riding and track days, it would probably pay to get the additional compression adjustment on the shock. But, for riding on the street, it's pretty much set-and-forget at the initial rebuild.
Probably a bit more than you asked for...
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