in message <[email hidden]>, David
(') said:I am sure I am not alone in being confused about the setup and
adjustment of front forks. I Have some Rockshox SID XC front forks
which I can pump up to-taste with my little high pressure pump.
My question is, I see that other forks have a red switch on the top
which allow "lock-out" which I presume stops them being effective. Is
this an On/Off thing or do forks exist with variable lock-out ?
Some guy in the shop mentioned forks with a variable rebound
adjustment too. I'm not sure whether this is desirable or just OTT ?
Different suspension units (this includes suspension forks, but also
rear shocks) have different adjustments and on the whole more is better
provided the mechanics are adequately sorted. Essentially the problem
is that the rider makes up a very significant proportion of the total
weight (unlike motor vehicles where the passengers are usually at most
a third of the total weight). Consequently, one suspension setting will
not suit all riders, irrespective of what conditions they ride, and
different riding conditions may merit different tuning as well.
As you've noted you can alter the spring rate of air suspension just by
pumping it up. So you can make it more or less plush without bottoming
out under your weight. If you have spring forks instead of air ones,
it's sometimes possible to change the preload on the spring and to
adjust the spring travel, but to change the spring rate you have to
actually take the whole spring out and replace it with one of a
different rate. With oil damped forks you can adjust the damping either
by opening or closing the apertures in the damper through which the oil
is forced (the usual way on adjustable forks) or by changing the
viscosity of the oil. Some sophisticated suspension units have a
pressurised air chamber on each side of the piston, and this is where
the adjustable rebound comes in. And there are variants on stable
platform valves, which are damping valves which open differently for
different speeds of suspension movement, and (if fitted) these are
usually tunable too. Finally as you note there's lockout, which is
useful if you're riding on tarmac as on good surfaces long-travel
suspension tends to bob a bit and actually hold you back.
General advice is to pressurise the system so that with you just sitting
on the bike, not moving, the suspension compresses about a quarter of
its total travel (this is called 'sag'😉. To adjust damping/rebound,
find a smooth road with a sleeping policeman (ideal) or kerb (OK
provided your tyres are hard enough). Pedal up to your normal riding
speed and then go over the obstruction not pedalling and sitting as
still as possible. If the suspension smoothly returns to its normal
'sag' position after the obstruction that's ideal. If it overshoots and
bounces around its sag position for a few bounces before settling down
you need more damping. If it doesn't make it back to its sag position
then you need less damping, or else you need to service your seals and
bushes.
I should say that having serviced some cheaper forks with lots of
adjustments I'm pretty unimpressed with the mechanical construction.
While I said above more adjustment is better because you can tune the
forks to your weight and riding style, it's not better if it isn't
robust enough to do the job. Good forks are expensive to make and
involve a lot of precision engineering, so don't buy a cheap fork with
lots of knobs to twiddle.
--
[email hidden] (Simon Brooke) http://www.jasmine.org.uk/~simon/
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