Boxerworks.com

The spring free length above should help identify if they are BMW springs as aftermarket springs are usually shorter with a spacer. Yours should be what are described as BMW late. If by some chance you do have the original springs then I would adjust the length of any pre-load spacer until you get sag in the right ball park.

I've had a few Airhead forks apart and I've yet to find any preload shims. I'm presuming stock springs in all, but one had the San Jose BMW treatment with bottoming springs replacing the rubber bumper in the bottom cap.

Edit: Barry's comment made me check--the springs presently out of my /6 have a progressive winding and are 22 1/8" (56.2 cm) long. The set from a supposed 1977 R100S are 21 1/2" (54.6 cm) long and there were no spacers on them, but the fork legs may have been apart before I got it. Also not as progressive a winding.

Were the springs truly progressive as in the coil spacing continuously varying along the length of the spring or were they dual rate as in having a number of widely but equally spaced coils and then a smaller number of tightly but again equally spaced coils. When the tightly spaced coils close up and touch, the spring transitions from it's soft rate to it's hard rate as listed in the table on page 1.

The problem is, the market place tends to call any spring with more than one rate, progressive which I suppose they are of sorts. The Progressive brand of springs claim to be truly progressive. If they are truly progressively wound then I can see that as an advantage.