Installing PD Valves

[evo550]

Phew!!!! It's good to know my tree barking hasn't been in vain
So how does the instillation of the pd valves remove the ability of oil weight as a compression damping adjustment tool ?? or does it just allow you to adjust comp damp without changing oil?

[Lozza]

More or less yes but mainly you no longer soley rely on the oil wt v's orifice size for damping.I still run the 10wt but would like to set up for modern 5 or 2.5wt

[brent j]

Lozza is spot on, with standard damper rods you force oil through a fixed hole. Thicker oil flows slower so more damping.
With PD valves the opening is spring loaded and variable. It removes the spike when the forks try to move too fast. It also gives resistance (damping) at fork speeds that are too low to give damping through a fixed hole. This adjustable damping only works one way though, on compression.

Lozza try heavier oil as well. I was running 10 until Conondale last year. My bike was bouncing up from jump landings. I changed to 15 and fixed that problem and found a big improvement in ride. What I never realised was that most of the jarring I was feeling was not the impact of the pump but the front wheel returning too quickly.
I'm actually waiting on some 20wt to try

I found a big improvement is ride quality, how the bike takes bumps and lands off jumps etc. But the biggest difference came in control. On an undulating corner the bike stayed very stable and even when sliding I could still steer it where I wanted to go.
And a big improvement in comfort and safety.

Brent

[evo550]

So is there any advantage in fitting these to forks which already have adjustable comp damping?

[Lozza]

I want try the light oil as I think there is improvement to be had with high speed compression by using the light oil, if it is detrimental to low speed I don't know yet.Evo the PD is for a damper rod fork if the fork has a cartridge or a piston/shim stack arrangement then no 

[brent j]

To fit PD's to forks with asjustable damping you usually need to remove what ever adjustment and valving mechanism they have and replace it the new valves.

As I understand, most of the early adjustable systems just gave an increase or decrease in damping overall. There was no high or low speed adjustment. What sort of forks so you mean?
Using PD's you lose the ability to adjust the forks externally but gain in having adjustable high and low speed damping.

Lozza, I found I needed more low speed comp damping being a heavy bike (XT500) but I needed a bit less high speed. I went to a lower rate valve spring and preloaded it more. This gave much the same low speed (static pressure) but the lower rate meant less of an increase in plate pressure as the valves opened and so less high speed damping.
I've gone as low as 5wt oil and as high as 15 at present and felt no difference in comp damping.

To work out how soft I could go I removed the PD valves and rode the bike with the normal oil level (topped up after removing the valves) in the forks. I found some bumps to generate high speed movement and figured this would be as soft as I could ever get them. From there I refitted the valves and started to play until I got more control.

I've also found that different brands of fork oil seem to have different viscosity. I know they are to standard but one brands 20 is lighter (much lighter rebound damping) than another brands 15!!

And my old favorite. Get the right fork springs rates first so the PD's (or whatever brand you use) are only doing the damping and not trying to cover another problem.

Brent

[evo550]

Brent,
 I had contemplated fitting the valves to a set of 40mm husky legs, although my first port of call was to get the spring rate sorted (Way to soft from the factory and with the correct rate on the rear it only magnify's the problem) but I have since came across a  White Power USD front end, so the PD valves are no longer needed.(although some springs might)
Ji's statement caught my eye first which made me think my suspension tuning knowledge was a little bit off.