I have an older OHLINS BM 4412 shock, which has a black knob which can be turned for adjustment of some feature.  Can anyone explain what this knob does and does anyone have a manual for this shock?


Thanks,
Jim

That's the damping adjustment. Try turning it to about 60% of its range from the bottom. If the shock reacts nicely to that setting leave it. Or, go up or down a click at a time until you find the best 'feel' for the desired preload you've already set.

Usually if a shock has just one twiddly knob, then it's for the rebound adjustment. A good base setting for rebound damping is to screw the adjuster right out (minimum damping) and then sit on the bike and bounce it with enthusiasm. If the bike rebounds so much that it goes 'down-up-down' then you need a bit more damping. Keep screwing the adjuster in until the bike just goes 'down-up'. Ideally, its super nice if you can get the front and rear suspension to do this in harmony - that's the sweet spot and you'll be a riding god!

The lower adjuster is for rebound only. Usual the only setting on basic shocks.
The upper knob is for compression setting. Not common on road bikes, it enables separate settings. It's usually designed for off road or track bikes.