Caveat – I’m no suspension engineer!
Ok, so you’ve splashed out on that brand new fork, or complete bicycle even, and have run out of things to play with. So your mind turns to the fork. The problem is, playing around with the fork can turn something that works well into something that barely works at all.
Where to start
I’d suggest the best thing to do for a start is a service on the fork, if you feel comfortable doing it yourself it isn’t very hard. Assuming you have the manual follow the instructions to do this. One point I’d like to make is that a change to a lighter weight fork oil can help make a fork more responsive – especially if you are fairly light, say <75kg. In this case you might want to try 5wt oil.
If you’re a heavy brute then you might want to consider heavier weight oil, but bear in mind sometimes heavier weight oil can have problems being circulated through the damping cartridge. You probably don’t want to exceed 15wt oil. Any good synthetic motorcycle fork oil such as Fuchs (Silkolene) will do.
Start at the factory recommendations found in the handbook. This way you at least have a reference, and if you totally bugger up this stage can go back to something you know worked ok.
Sag is important, but sometimes with an air fork it is a compromise between the correct sag level and the air pressure required to allow the fork to work properly through the compression stroke. Your handbook has a guide for sag, but you generally want around 25% of the total travel available to be taken up in sag. Use the preload adjustment on a coil sprung fork to set this measurement properly.
Well, this is fun. If you have an air fork, the compression rate is infinitely variable. Basically you set it up so that the fork uses all of its travel occasionally. But what you don’t want is the fork to bottom out on every medium sized undulation in the trail or gully.
On the other hand, a fork that is only getting two thirds of available travel is not working effectively either. Let some air out man!
If you’re doing a XC race though, for example, you might want to run a bit more air pressure. This will make it less responsive and reduce ‘bobbing’ which some riders feel gets more of their power to the ground, but at the expense of some comfort. For a shortish XC race this is no problem.
The same applies to a coil sprung fork, but changing the compression is a bit more difficult here. Yep, you’ll need new springs. This isn’t a problem, as manufacturers make a range available. As a rough guide, Marzocchi forks generally have too heavy a spring for the average rider and Manitou forks have too light a spring.
Rebound is where problems can occur. There’s a range of methods to get the correct rebound, but the one I use is to find a favourite, mid speed corner and ride around and around.
When you’re doing this, change the rebound slightly for each try. If the front wheel want to ‘push’ away, you have too much rebound (it pushes back too quickly).
If the front wheel wants to ‘tuck’ under or ‘fold’ there isn’t enough rebound damping.
How do you know when you’ve got it right? That’s easy, you don’t think about it any more…
To sum up, slight adjustments can make an enormous difference, for good and for bad. Don’t go changing multiple things at the same time or you won’t know what did work and what did not.
Get the compression right first, and the rebound adjustments last. Go west, young men and adjust.