[djjohnr]

I know enough about the effects of shock adjustments to set-up my bike for different courses. However I’d like to get some more information on exactly HOW some of these effects are being achieved.

1) How does the Boost Valve on DHX coil shocks work? It obviously affects the entire range of compression. I know screwing in the bottom out dial reduces the reservoir chamber volume creating a ramp up from air pressure earlier in the travel, but what is it doing physically to affect the rest of the compression range?

2) What is physically happening with ProPedal on the DHX coil shocks? Is it simply a poppet valve doing all of the damping duties?

3) Is there only one compression shim stack? If so is there a high speed valve controlling oil flow to it?

4) Are there any bike shock manufacturers using multiple shim stacks (ie separate stacks for high/medium/low speed)?

 

[Steve M]

The boost valve is a pressure relief valve that is held CLOSED by oil pressure (this is why you need a certain minimum pressure, below that value it can pop completely open), remembering oil pressure in the reservoir = air pressure. Basically, as the air pressure increases (as more oil is displaced into the reservoir), it applies more pressure to the boost valve - this is where the progressive damping comes from. Progressive damping is unrelated to the air spring effect of compressing the air/gas charge. When people say a shock is position sensitive or progressive, it means the DAMPER is position sensitive/progressive. Anyway, more air in the chamber has the same effect - increases oil pressure which tries harder to close the boost valve. The propedal is actually a reverse threaded adjuster that REDUCES preload on a spring that holds the boost valve open. More propedal = less preload on the spring = valve is harder to open. As a result, the propedal and air pressure BOTH affect the whole range of damping - in fact, dyno testing has shown the effects of each adjustment to be pretty well identical on a DHX5.

There is only one compression shim stack on the main piston - shim stacks ARE the high speed valves, it's the one controlling the oil flow.

By multiple shim stacks, do you mean multi-stage stacks or entirely separate circuits?

 

[djjohnr]

Ok - so more oil pressure = less oil bypassing the compression valve (shim stack)?

By multiple shim stacks I mean a multi-stage stacks.

 

[djjohnr]

That can't be right, let me try this again-

- Shock compresses - piston moves in.
- Compression shim stack provides both high and low speed damping (by nature of shim deflection)
- Oil displaced by the shaft enters the reservoir
- Boost valve closing restricts oil flow based on oil pressure which is determined by the opposing air pressure in the reservoir + oil pressure from displacement (as more displaced oil enters the reservoir pressure goes up - hence the progressiveness). More oil pressure = more damping. Propedal adjuster just sets preload on the valve.
- Compression damping from boost valve closing is additive to damping from the shim stack

Do I have it right?

 

[davep]

pretty much...

The propedal adjustment actually adjust a spring pre-load in the opening direction of the propedal valve... it acts in the opposite direction that the internal pressure does (on that valve).

The more you back off the propedal, the higher the spring force opposing the internal PSI force. The more you increase propedal, the less opposing spring force there is.

I would guess that with pressures at the minimum, and with the PP 'off' that the PP valve would be closed with only a very slight preload...There would be no real reason to have that valve open or floating.

Of course there is some LSC comming form the main piston, but most of that comes from the PP valve. I am pretty sure there is some free bleed in the main piston (I have a main piston/shims/piston bolt and a PP valve assembly in the garage, but it has been a long while since I looked at the parts...)...so at very slow shaft speeds, the displaced oil going through the PP valve is the only LSC

 

[djjohnr]

pretty much...

The propedal adjustment actually adjust a spring pre-load in the opening direction of the propedal valve... .

So is the Propedal Valve separate from the Boost Valve? From Socket's post I understood that there was only the Boost Valve...is this not the case?

 

[davep]

Same thing...differing terminology. The spring force is adjusted via the 'propedal' knob..the air pressure is adjusted via the 'boost valve'....


The one on the right with the blue knob.. http://www.pinkbike.com/news/2006-Sea-Otter-Push-Industries.html