It took me a while to get a handle on just how the pre-load works.  As I see it now, it adjusts the range of movement of the suspension to match the load.  

At first I couldn't understand how shortening the spring would make a difference, but then I realized that I was only looking at the situation when the shock was unloaded and so fully extended.  When you're sitting on the bike, the suspension sags under the weight of the bike and its load so the shock is not fully extended.  Adding pre-load compensates for the sag and lets the shock operate in it's proper range of motion.

Concrete example:  Let's say a shock is 20 inches from eye to eye.  The spring is 18 inches long, fully extended with no pre-load.  There is 8 inches of travel from fully extended to bottomed out.  (No, these are not from a specific bike, they're just theoretical to see how things work out.)

OK, when you sit on your bike, the suspension sags 5 inches.  That means the distance between the mounts is now 15 inches, at which point the spring is 13 inches long.  At this length, the spring is "pushing back" with the enough force to balance the weight applied to it.  5 of the 8 inches of travel has been used up, and the shock is 3 inches from bottoming out.

Let us say the pre-load is set to 3 inches.  Unloaded, the shock is still 20 inches, but the spring is at 15.  Now when you sit on the bike, the suspension sags until the spring is at 13 inches, when it again is "pushing back" with the proper force.  That happens at only 2 inches of sag.  That leaves us with 6 inches of travel until bottoming.  True, there's less travel until the suspension is fully extended, but that's not what spanks you in the butt.

So, in effect, adding pre-load doesn't shorten the spring so much as it extends the shock when it's under load.  This allows us to tune the suspension to the load.

OK, all you experts -- do I have it right?

drharveys - FSO wrote on 12/29/07 at 22:19:52:


That is the idea - and in the case with the multi rate springs like Hagons or Progressives you get into the heaver spring coils much quicker with a little pre-load - likely can measure the unloaded seat height lifting

Talking suspension? I'd just as soon go to the dentist.

Call me on your picture phone. You can check my teeth while we talk suspensions. Sounds fun Huhh?

Ok - Let me see if I have this right,

I have '97 Savage, I'm 6'1" and weigh 250 lbs, I currently run my tire pressures between what the spec's read for single rider and with a passenger, and I have the pre-load on "5".  

My question is, should I consider changing the preload?  My previous experience with motorcyles is over 24 years ago when I was a young lad riding dirt bikes.

It does seem to ride ok, but a hard bump will knock your teeth out.  

When I bought the bike last summer, the pre-load was on "2" and the rear end seemed to move around alot in the curves.

Thanks for the tip - I'll move it down to "4" and see how it rides.  Gonna have to wait awhile thou - we have snow here in Maryland and it is too cold to ride.