i just bought a used dhx2, so testing the dials,and it seems the rebound is too slow, even on the fastest settings, with the spring removed
So what is the normal time in seconds, for the shock to return from fully compressed to full length?
Assuming a coil shock, Without a spring there is no force to extend the shock, so it'll take forever.
In reality maybe there is some pressure built up inside the damping mechanism from trapped gas(?), or gas behind the floating piston, which might extend the shock a bit but very slowly, and probably not fully.
I dont think you can tell whether a damper knackered by the time it takes to extend with no spring
That'd be my assumption, in fact, if you held it compressed for a while to let the pressure equalise between the chambers, I reckon it'd just stay put entirely.
Its damping the spring. No spring, test not valid. If you are cycling the shock by hand it still wouldn't act as if there was a spring in there.
Knackered damping is more likely to result in fast rebound rather than slow.
When setting up a shock, setting rebound is easier because it's controlling the spring which is a known quantity. Compression however, is much more personal and variable and therefore harder to set up.
looking at online service videos, it still looked a bit slow, about 2seconds minimum, so i checked the pressure which was a bit low. Pumped it up to 125psi and it seems good now.
Assuming a coil shock, Without a spring there is no force to extend the shock, so it'll take forever.
Only if the shock is knackered and the pressure in the IFP has gone.
Without (quite a lot of) pressure in the IFP the oil would cavitate and the shock would lose damping, there's a good demonstration here.
OHLINS Controlled shock cavitation! - YouTube
In simple terms the oil will boil at very slightly above 0psia (i.e. near absolute zero pressure, atmospheric pressure is 14.7psia). So unless you pressurize the damper then the maximum pressure difference across the damper in rebound 14.7psi because the pressure on the high pressure side is atmospheric (plus a bit as it ramps up) and the low pressure side can't go below zero.
If you add 250psi to the IFP, then at rest you have that pressure on both sides of the main piston. And when it starts to move you will always have that pressure because i's a liquid and therefore incompressible, this prevents it boiling meaning the damper can now apply a force = pressure x area upto that 250psi x the pistons area in both directions.
Remember back to the old original Fox DHX-5, there was no tuning guide for the air pressure, it was just a case of inflate it to the max and go ride. The DHX-4 was an identical shock for OEM customers where the valve was inaccessible anyway so it was just factory set. Turning the big cap on the end of the reservoir adjusted bottom out resistance by varying the volume of that small 'air spring' behind the IFP.
Even cheaper suspension with no IFP (usually known as an emulsion damper) is often pressurized to avoid cavitation. The exception that proves the rule is budget forks. It's hardly an issue in forks because you have this huge ~32mm diameter damper moving very slowly relative to the ~20mm damper (or even worse, the tiny little adjustable dampers on the neck of the reservoir) that moves 3x quicker in the rear shock.
The answer should be a few seconds, but as spooky said it's not really a valid test of the shock, tuning shops would put it on the dynograph to measure how quickly it moves with no spring, but that's applying a known force (or moving at a known speed).
