Installing PD Valves

[Ji Gantor]

After speaking to quite a few Pre 75 bike owners they all tell the same story when using the front brake under heavy braking conditions.
The forks just about bottom out due to a lack of compression control.
This is okay if the track is smooth but if there are braking bumps before a corner than it is hard to control the front end at speed.
As we all know rebound is controlled by the rebound valve and the weight of oil we use while compression is controlled by valving and the amount of oil we pour in ( the weight of oil plays no part in compression ). The air gap between the oil and the top of our forks plays a big part in how our forks perform. Air will compress while oil will not. The best example of this is the reason we bleed our brakes. If we leave air in the line and step on the brakes all we do is compress the air without applying the brakes. If we fill our forks to the top with oil they will be near impossible to compress. We need the forks to compress but we also need them not to dive under brakes. A PD valve or pressure differential valve was invented to improve compression in old forks.

I started this topic so our questions can be answered.
Questions like;
How do they work ?
How do they perform ?
How much do they cost ?
How do we install them ?
Where do we get them ?
Who will install them for us ?
Are they legal ?

So lets discover these and much more about PD valves.

All those that have already installed PD valves I would love to hear from you with what worked and what did not. Please explain what modifications you had to do for the install.

I will be doing my typical "How I Installed My PD Valves" after CD6, so before than lets hear from those that have already done the job.

Ji 
 

[Lozza]

Sometimes you just machine a simple adaptor to sit atop the damper rod,other times you machine the damper rod a bit. Next set the damper rod up in the mill to cross drill the extra holes at the bottom of the rod.That's the only hard job, deburr and assemble forks. We found there is 3.5 turns not 7 as in the manual, but I run mine just preloaded.
Performance is as you would expect miles above any plain damper rod fork, as they can only ever be set for high or low speed comp/rebound never both, riding with them you will feel the damping increase with stroke.

[Ji Gantor]

Thanks Lozza,
Can you let us know what bikes you have installed them into and what modifications were required for each bike from memory.
Every bike fork needs a different treatment for PD valves and this info would be important to those thinking about it with the bikes you have experience with.

Golden reply
Thanks mate

Ji

[husky61]

I have machined up many adaptors for people to allow the fitment of PD valves to pre-75 and even evo bikes.

They work like a simple adjustable fluid metering device

I have ridden some of the pre-75 bikes fitted with the devices and my opinion is as follows. i hope to ride one of the EVO bikes soon.

They perform very well , suprisingly well

Reasonably priced

Very easy to install and i will post some pictures tommorrow of the minor components that you need to machine up to allow capture of the existing damper rod and capture of the PD valve.

YSS sell them and they are readily available. The instrctions supplied with the PD valves are fairly comprehensive.

You can install them yourself or if you are not confident about doing it yourself you can take the components to your local suspension technician for installation. This will add considerably to the cost if you include the machining of the required components , dismantle and reassembly of the forks for installation of the valves .

Not sure about the last question but having said that i dont see why they woudnt be.

The same result can be acheived though modifying the existing damper rods etc , its just a lot more work (trial and error) but once it done , providing your have the same brand of bikes its a pretty uniform change to acheive the same outcome on all your bikes.(with minor variations).

All up i feel that they are very good investment , provide a safer and more managable ride in rough conditions.

Shoey

[Ji Gantor]

Hi husky61,
Thanks for your golden input.
I look foward to seeing the photos.

Ji

[Graeme M]

I have YSS PD valves in my 1975 RM125 forks and highly recommend them. I had Fernando at YSS machine up adapters to fit them and then set them up as per the instructions. I have to admit to not having fiddled with them since and wish I had the time and inclination to spend a day working with various oil weights/heights and valve adjustements. That said, the improvement is very noticeable. They seem to resist bottoming very well, and the overall stroke feels significantly more... ummm... 'fluid' than before. They really do work well for older forks. I think the RM has about 7.75" travel which isn't much, but I am using all of it and it's all nice. Matched to my YSS shocks and I reckon that little RM has to be one of the best handling and suspensioned vintage bikes I've ridden (pre 78 I am talking here). My suggestion to anyone racing an old bike is to set it up with really good modern shocks and PD valves. It makes the whole exercise much more fun, and saves your wrists and kidneys a heap!

[Ji Gantor]

Hi Graeme,
Thanks for sharing your experience.
Did you take any photos of what Fernando made for your forks?
What oil weight are you running?

Ji

[Ji Gantor]

The valves are precisely machined aluminium.
They come in different diameters to suit almost every fork.

Ji

[Ji Gantor]

The valves have a HT plate that covers the entry ports just like a reed valve.
There is an adjuster bolt to set the pre load on the HT plate. This allows the rider to adjust the amount of compression. The PD valve has to be removed to be adjusted.


Ji

[HuskyPete]

Shoey,
I wonder who you got that info from 

[husky61]

I do some machining works for a fellow husky fanatic i know and he let me ride the bike and he also gave me all this great information, including drawings of the components.

I wonder who that was.

[Ji Gantor]

To both Husky fellows,
Thanks for sharing.


Ji

[HuskyPete]

To make it easy to adjust the PD valve you have to make a extended magnetic pickup to lift the valve out without dismantling the forks. You have to make the extension out of plastic with the magnet in the middle. I bought a cheap magnetic pickup from supercheap and glued it into the end of a piece of 25mm electrical conduit, if you dont use plastic the magnetic pickup tries to attach itself to the internal fork tubes as you push it down to the pd valve screw. You just let the magnet hold the adjustment screw and lift it out slowly, adjust it and drop it back down. To easy

Patent Pending

[Ji Gantor]

Hi 67Husky,
That is a great tip.
I will be making that handy little tool.
Keep it coming.

Ji

[Lozza]

From memory RGV VJ21, Some Marzocchi's road and mx, various OEM japanese and a clever Kiwi put them in his CZ forks. Nothing special for mods just adaptors to fit to the rod. Measure twice cut once. A Supercheap 3 pronged 'grabber' beats any patents