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Q.
If I lower the air pressure in my
rear shock will it speed up rebound damping ?
A.
Running lower air pressure in a
reservoir is questionable, the gas behind a floating piston is there for
several reasons: displacement, to aid reabsorption of gas bubbles that are
produced in the oil, in some shocks it aids in reducing bottom out, it is
a form of assistants during the rebound stroke, and in some shocks
produces the threshold for compression damping.
A recommendation
for high performance while decreasing ( faster rebound/compression )
damping is to use a lighter weight oil, retain a high pressure in the
reservoir, they are designed to tolerate a very high pressure ( dampers
that don’t have Schrader valves often utilize around 200-300psi static,
moto shocks use around 400psi static )
Advanced tuning requires
reconfiguration of the shim stack on the rebound or compression side of
the piston ( Fox, Marzocchi, Rock Shox ) SPV dampers like Swingers and 5th
Element are limited to shim changes on the rebound side only, the latest
trend on these shocks is to remove the stable platform valve altogether
and replace it with a compression shim stack, for coil overs on DH bikes
this appears an improvement ( more traction ) and adjusting the IFP
position in the reservoir ( reduced air cavity ) can be beneficial in
aiding the spring during bottom out as well.
Manitou shocks like
the Metel use a full shim stack, no SP valve, the platform on these shocks
are in the form of a check valve ( like less sophisticated propedal Fox
shocks - Vanilla R and Van R ) the single ( external ) platform adjustment
effecting an oil route on the compression stroke, ( via a stationary
shimmed valve ) these shocks utilize up to 300lbs of gas pressure behind
the IFP ( the pressure non adjustable and not user friendly! ) but can be
hot rodded to become an active damper ( removing the check valve ) with an
external compression and rebound adjust, then add the reservoir cap from a
Swinger, and its now user friendly, ( or pinch the piston, shims and bolt
from a Metel and install in a Swinger )
Those rear shocks with
Schrader valves allow the rider to disassemble and custom tune their
shocks for their requirements, reducing air pressure behind the IFP should
only be considered a short term fix if at all, in my view the pressure,
which is significant to the rate of movement in a floating piston, should
be more than consistent to the action of the shock shaft and coil spring,
too low a pressure and the IFP cant keep up, this causes all sorts of
issues, and none beneficial to damper performance or
longevity.
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