Judy XC / DH differences and how to do a coil conversion the right way ?

[Thias]

Hi there.
I'm currently in the process to resurrect and modify a '96 Judy.
One of its legs - the one with the neutral shaft - is badly corroded. But I have a spare one.
I also have a set of steel springs taken out of a '99 Hydracoil Judy and a second aluminium dampening cartridge.

The last time I did a spring conversion I had the feeling that the springs were "overpowering" the dampeners, even when using 10WT oil.

Which all made me think of using a dampening cartridge in both legs. But there is a problem.
The neutral shaft is there to limit the travel of the fork. With it missing, the springs would tug on the cartridges each time the fork fully extends. They would be pulled apart eventually.

As far as I know, the DH-Version of the Judy had a cartridge in each leg. Looking at pictures online, those cartridges seem different. Also I get the impression that one was for compression dampening. But I'm not sure. I didn't have one of these apart, yet. Does anybody know the difference between the XC and DH cartridges? And maybe even have pictures? How did the DH Judys limit the extension travel?

I'd also appreciate your thoughts about the coil swap and how to do it "right"

Picture, so the thread isn't that dry looking

 

[Baulz]

The DH has a separate cartridge for compression and rebound. They were 80mm travel only so maybe there was no need for something like a neutral shaft to limit travel.

Sorry I can't be more help, I'm sure others can add more.

 

[ishaw]

Dh versions had both rebound and compression carts, one was red, the other silver iirc.

I don't think the neutral shaft did travel limiting, having had a few sets of these forks and taking them apart etc, the neutral shaft appears to be the same as the damping shaft, without the outer case and gubbins that makes it work.

The travel is limited by the damping cart. It's screwed into the lower leg and circlipped into the uppers, if you wanted to change travel, you needed a cart with a different length shaft. At full extension, the damper shaft and internals would hit the bottom of the cart and travel no further (unless it all goes horribly wrong and it breaks. Having the neutral shaft mimics this on the other side, as would a damper.

In terms of spring conversion, you don't need to worry about much other than the right spring weight for you and the length. And the adjuster has a way to seat the spring.

 

[kokies]

Your DH is mostly a 1995 model (Arch is a 1996) with 75mm of travel. In 95 the DH only had a compression damper with most of them were the plastic body. The rebound dampers were not added until 1996. If you do have the plastic damper and the shaft is still in good shape I would recommend our hardbody conversion kit. It will convert your damper to an alloy tube and comes with everything needed to replace the seal and service it at the same time.
https://specialtyretroproducts.com/...rdbody-kit-for-1995-1996-rockshox-judy-forks/
I would not recommend using a hyracoil spring. From what I remember the lengths were different and I have no idea what the spring rate is. The spring rates need to be tuned to the fork travel and riders weight. You can check out the chart we have for matching the springs to the fork travel and rider weight here. If the fork is properly set us there is not need for a second damper.
https://specialtyretroproducts.com/...rock-shox-judy-dh-sl-xc-fsx-quadra-speed-efc/
I also would not recommend using 10wt oil. The seals are just a friction fit and 10wt oil is going to build more pressure on those seals.

As ishaw stated the travel is controlled by the damper and neutral shaft. You have to change both to change the travel. You can swap dampers and neutral shaft between any 96 or 96 judy forks if needed.

If you send me the neutral and damper shaft length I can confirm the travel.

 

[Thias]

@kokies I'm sorry, but I think, you did not fully understand my question. I don't have a DH, I have a XC. I should have pointed that out more clearly in my text. It's a '96 with 60mm travel and Aluminium cartridge.

I am quite sure, those forks limit travel by the neutral shaft, because if you put a 75mm neutral shaft in a fork in conjunction with a 60mm dampener, the dampener will pop open after some time. I had that happen two times until I realized my fault.

Hydracoil coils can be used no problem. But you would have to add a rubber ring for topout dampening and you would have to adjust (aka cut) their length correctly. As the Hydracoil and Judy share the same dimensions regarding the casting and the legs, I don't see a problem there. You can basically put all the insides of a Hydracoil Judy into any older version Judy and it will basically work. The only problem ist the lower bolt screw being M8 instead of M6.

I agree that WT10 oil in a cartridge will maybe put too much strain on the seals. All that while still not dampening enough. Hence my idea to put in 2 cartridges. That would solve that problem for sure. But I think, those cartridges would still pop open over time because of the lack of some kind of travel stop.

Here is a picture of the DH cartridges taken apart: (taken from this thread)

The lower, black one is for rebound dampening. It's the same as the XC one. Except for length, obviously. Note the position of the circlip. It holds the upper (here left side) seal in place. When the fork is fully compressed, the piston (with the blue seal) would be pressed against the seal and stop the fork from compressing any further. The seal would stay in place because of the circlip.
When moving the other way round, the fork going to full extension, the piston will press against the lower (right side) seal. Because there is no circlip on that side, the seal would be pushed out over time. Which is why the neutral shaft inside the other leg normally prevents the fork to extend that far. So if using the correct length neutral shaft the forks extension would stop before the piston even touches the seal.

Now looking at the red cartridge that is in the DH-version of the Judys in place of the neutral shaft. The picture above seems to suggest, that this (compression dampening) cartridge has the circlip on the lower (right side) seal. Which makes me believe that this is basically the extension travel limit. But I might be wrong, because I don't know if those parts were layed out correctly in the picture. And as I said I haven't seen a DH compression cartridge in real life yet.

 

[ishaw]

Sorry, but it really isn't the neutral shaft that limits travel. What you have done I'd create an imbalance in size between the damper and neutral shaft which has ultimately allowed the damper to get over extended and pop open. If you had a damper of the same size in both sides, it wouldn't happen as their limits are the same, and no neutral shaft. The neutral shaft is a less refined, bodiless copy of the damper shaft.

The fundamental principle is both sides, whatever is in them, need to mirror each other or you'll have problems like the one you've mentioned.

 

[slackboy]

slightly tangential, but does anyone know of a source for replacement brace mounting bolts ( not for the vbrakes, but the ones that bolt the crown to the legs), I have a set of lowers with badly corroded bolts

 

[Thias]

@ishaw "create an imbalance in size between the damper and neutral shaft which has ultimately allowed the damper to get over extended and pop open"
Maybe I really don't understand. But isn't what you're saying the same to what I'm trying to say? I'm not talking about compression travel. I'm talking about the extension maximum. With the neutral shaft missing (or too long), the fork would be allowed to extend more than the cartridge can handle. Which results in it being pulled apart. No?
My brain hurts

Edit: I think, now I get it! You are absolutely right! If there is no neutral shaft, that is too long, there is no lever/force that would push (or rather pull) on the cartridge more than the max. range.
Ok. Now I feel a little bit stupid. But at least I learned something. Thanks!

@ slackboy. I would search for (titanium? ) flat head countersunk allen bolts M"somethingtherather"and of correct lengh. M6 would be my guess, but I can confirm later.