OzVMX Forum
Clubroom => Tech Talk => Topic started by: Ji Gantor on May 27, 2009, 09:22:33 am
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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
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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.
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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
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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
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Hi husky61,
Thanks for your golden input.
I look foward to seeing the photos.
Ji
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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!
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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
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The valves are precisely machined aluminium.
They come in different diameters to suit almost every fork.
Ji
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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
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Shoey,
I wonder who you got that info from ;D
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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.
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To both Husky fellows,
Thanks for sharing.
Ji
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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 ;D ;D
;D Patent Pending ;D
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Hi 67Husky,
That is a great tip.
I will be making that handy little tool.
Keep it coming.
Ji
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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 ;D
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I have them (in 414). Walter put them in along with my new springs when I sent him my forks about four months ago. I think he did some adapptors as well. I haven't even pulled my forks apart since or even adjusted the valves (I was gunna but can't be fugged now). Along with my new rear shocks (YSS), I'm very happy. I couldn't be happier if I had a second willy. ;)
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Hi All Things,
What bike have you got them in?
How much did YSS charge for the conversion?
How well does it work, twice as good or even better than original?
Ji
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A 414 Montesa. You probably haven't heard of them. An evolution bike. I think he charged me two goats and a chicken at the time, it's all a little foggy now. Compared to my other 414 which is bog stock, it soaks up the square edged stuff a lot better. Mind you the beefier springs probably made a difference as well. I just sleep better knowing that there's a little bit of Walter in me (well, my bike).
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Hi All Things,
Do you remember what spring rate, oil weight, PD valve size or oil level Walter used?
I am sure the other Montesa owners would love to know.
Ji
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Well......the spring rate I wouldn't know but they pretty light before-hand. :-[. The valves are for Marzoochi 38mm forks and we used 10 wgt oil. :)
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Thanks All Things,
Ji
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I am sure the other Montesa owners would love to know.
There's other Montesa owners?! Bugger! I'll have to get that Carrabella now......
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I swear by these things!
I have them in my XT500.
I'm using PE175D fork legs and they fit straight in, no mods to the damper rods. The spring spacers (hate to use the word preload) need to be shortened by the length of the valve body.
I played with oil weights and found 15 to be exellent and I'm waiting on some 20 to try.
By varying the valve spring preload and/or changing the rate of the valve spring you can alter high and low speed damping.
Changing the oil height has a marked effect as well.
It does take time to try these different settings but I think it is worth it.
I know they will drop straight in to the following forks. PE175D, IT465 (I think G) YZ250G. They will fit a lot of others without mods but that list is from my experience.
Suzuki TM125, fitting them at present. The TM has a "capped" damper rod. The cap screws on and holds the piston in place. I machined most of the cap off and made an adaptor to sit on the shortened cap. The valve sits on the adaptor. I didn't allow the adaptor to sit on the piston as it is retained on the underside by a circlip and this would take the weight of the bike.
I do plan to make a new piston to screw onto the damper rod and it will use a seal ring for better control. The new piston will accept the PD valve directly with no need for an adaptor.
I have tried to keep the overall height of the assembly as low as possible as there is limited room inside the forks with regard to length for the spring. I may yet need to machine the fork cap shorter to preserve the space for the spring. As I have mentioined earlier I don't like to preload springs.
It looks as though standard XT/TT500 forks will need an adaptor as the "C" forks I have are capped like the TM.
I'm also in the process of fitting them to 43mm forks on my other XT 500 race bike and another set of 38's.
As good as these things are I still think they should be your second stage AFTER you have the right springs.
Cheers
Brent
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Hi Good friend from NT,
It is always a pleasure to read your words.
And what words, great advice.
The PD valve compresses the main spring which means you run more pre load, you have to shorten the spring or reduce the spacer or cap bolt by a similar amount.
Great stuff mate.
Ji
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I do shorten springs but only to increase their rate, not for clearence.
Just a few coils off a spring can increase the rate by quite a bit.
I made a cheapskate tool for removing the valves, a piece of wire, about 3mm dia. Bend a 180 degree hook into one end so it fits around the spring on top of the valve. Then bend the hook sideways so to speak. Make the wire long enough to reach down the fork leg to the valve and then bend the other end about 90 degrees but away form the open end. This way you know which way the hook is facing when you go fishing.
I also use a mickey mouse tool for keeping the oil level static. Every time you remove the valves you lose a bit of oil. Do it a few times and you have changed the oil level enough to change the characteristics of the forks. If you are going to tune the valves you need all other variables static other wise that softer feeling could be lower oil level rather than working PD valves.
I have a piece of old brake tube, it's stuck into the end of a syringe and has a movable clamp to act as a stop. I support the bike with the front wheel off the ground and remove the fork caps. Slide the tube into the fork leg and find the oil level, lock the clamp at this point. When I rmove the spring and valve I loose some oil so on reassembly I add a bit more. I now slide the tube in and suck up any access oil so my oil level always remains constant. I use this method to set my fork oil all the time now. Each bike has it's own level and it's recorded. At a recent race meeting I had a badly leaking seal, I was able to top up the oil between races and know it was right each time.
Cheers
Brent
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Hi Brent J,
You really are a man to respect.
Your constant vigilance on all things with springs makes me bow to your knowledge.
You are my kind of compulsive.
Keep up your experiments.
Many thanks
Ji
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Ji I'll take compulsive as a compliment ;D.
Usually it's names like "sad", "time waster" and such.
I think it's just that in things that interest me I'll keep trying to find that extra improvement. Rather than just have an improvement and leave it at that I want to know just how good I can get it.
And my long suffering wife will attest to that. Now if she would just let me have a play with her CRF230, I'm sure I can make it smoother..................................
As for knowledge Ji, you are the one who worked out the spring rate claculations. I just stuff around with things till they either go better or break ::)
Brent
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I have posted a 'how to' on the IT Forum. With pictures!
http://www.yamahait.com.au/forum/index.php/topic,1428.0.html (http://www.yamahait.com.au/forum/index.php/topic,1428.0.html)
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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 ).
Ji
I thought oil weight controlled both rebound and compression action( heavier oil slower damping/lighter oil faster damping)........................or have I been barking up the wrong tree.
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Ji's statement is correct when PD valves are fitted. With standard damper rods the oil weight does affect rebound AND compression.
Brent
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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?
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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
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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
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So is there any advantage in fitting these to forks which already have adjustable comp damping?
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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
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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
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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.