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Old 01-22-2008, 04:06 PM   #52
NC Rick
Cogent Dynamics Inc
NC Rick's Avatar
Joined: Aug 2007
Location: Asheville, NC
Oddometer: 645
Excellent post Kneepuk! I'm just a dummy trying to sort all this out. Your correct the spring is what supports the weight of the bike and rider. As far as absorbing bumps and irregularity's, the spring mostly stores energy and the damping converts the energy to heat. If the spring absorbs and stores all of the energy of a given bump then the shocks rebound has a big job to control the springs release and converting that energy on the rebound stroke. That is the way the old bikes worked. With more sophisticated damping, we can "help" the spring by absorbing and dissipating some of that initial bump energy using compression damping. This allows us to control the absorption rate based on speed as well as by stroke as the spring will. Doing this also allows us to design a shock that is more balanced and actually will generate less total heat within the damper. How do we determine the right spring rate? A rule of thumb would be to set the sag to where we want (dirt bike general number 100mm or 4 inches?) and then check the unladen sag looking for a number (25mm or 1 inch for a typical dirt bike?).

My DR right now has 1.5 inches or 38mm of unladen (only the weight of the bike) sag and with a rider of 224 lbs with gear sitting on the bike sags 3.9" or 99mm.

Using this data, and comparing it to the "rule of thumb" I sited, this indicates that the stock spring is too stiff for the 224 lb rider because the unladed sag is not enough for the "rule".

It would be great to see what some of the rest of you are seeing for your sag settings.

More later. Fire away, I always learn from these kind of discussions and really enjoy the process.
Cogent Dynamics Inc.
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