I have a forking problem!

[CJF]

Can use some help. The forks on my GT500 race bike are not rebounding and seem to be stuck in the compressed state. I already swapped out the replacement springs thinking they have been too high a spring rate because I couldn't compress the forks initially. After putting the stock springs back in, I pushed down on the forks and they sort of made a pop kind of noise and compressed, but now they don't seem to want to rebound. I have 15 wt fork oil in them at this point at the recommendation of the guy at YSS Suspension. Not sure if that may be causing an issue. Or, does it sound like there may be a problem with the fork seals being installed wrong? Wondering if anyone has run into a similar issue.

[netbikes]

When you dismantle the forks you need to remove the 8mm threaded alen head bolt at the bottom, that alen bolt is connecting to the damper tube but before that it goes through a alloy cup that slips over the end of the damper tube, this piece is a oil lock, if that is worn it will alloy too much oil to pass and affect the damping and rebound, sort of like an air leak effect in a brake line.

check your oil quantity are well to ensure you have enough in, can't think of the amount just now but it will be on line somewhere.

15wt is ok, I use atf which is about 10 weight but back in the day we just used 30 grade engine oil. The oil wt has no affect on the working condition, just how it handles and even that you just play witj the quantity till you get it right no matter what the weight.

[tz375]

Not good.

That alloy part that Netbikes described so well at the bottom of the damper is there to stop the forks from bottoming out and basically progressively fills the central hole which restricts oil flow and softens the hit. It's an anti bottoming device.

In your case there is something that stops the forks from compressing. With the heavy springs you hit that "stop" and with lighter springs you managed to push past it and not can't reverse out. Did you fit YSS emulators or just a set of their springs? If you have emulators, how did you fit them into the tops of the damper rods? If they are a tight fit, the oil cannot pass and they are hydraulically locked. - I did that on one of mine. They need a spacer so that oil can flow easily around the nut on the adjuster screw.

If there are no emulators, the question is which two parts are interfering with each other. I would pull the bottom bolt and drain the oil and then remove the leg and I suspect the damper rod together. It may be that there's interference between the bottom of the leg and the top of the damper rod.

[CJF]

Are you guys talking about the small rounded off aluminum piece at the bottom of the damper rod in this picture? There isn't much to that damper rod, so it is hard to understand how it could be causing the problem I'm seeing.

[tz375]

I was trying to explain what that alloy bit does and why it isn't the problem unless the forks are completely bottomed out. Those damper rods are really odd though. Where does the emulator sit on those? It doesn't make much sense unless you have an adapter with a piston ring seal in it. what do the springs sit on? And does that piston on teh bottom of teh legs move up and down through 1-2mm partially uncovering a ring of holes by any chance?

The forks have got to come apart to fix the problem. Have you tried removing the allen bolt from the bottom after you drain the oil? That should allow them to come apart.

[diamondj]

Emulators won't work on the GT500 fork as the dampers are solid through the middle. I think we talked about this on Chris' build thread in customs a while back.

Working on the assumption there's no emulators in the forks, I'm wondering if:

1. The fork seals are causing stiction - I know people have reported the "Leakproof" brand causing issues but it looks like you've got the Parts Unlimited OEM replacement seals. Did you have any difficulty reassembling the forks past the seals?

2. Is the oil too heavy or is there too much oil in the fork? Technically, you should need heavier oil with higher rate springs so I'm doubtful it's the oil weight. Try filling the legs with 5wt or 10wt and see if that helps. Capacity is listed as 220cc's or 7.4 fluid ounces per leg.

3. Failing that - Something may be assembled incorrectly in one or both fork legs. Test them independently and see if they are both sticking or if it's just one leg. You may need to tear one or both legs down to figure out what's wrong.

I have a spare set of GT500 fork legs and I could always tear one apart and photograph it since the parts diagrams are not always that clear.

http://www.temeculamotorsports.com/fich ... FRONT+FORK

Hope this helps!

Jim

[tz375]

Jim, all good points. I wasn't sure what Chris got from YSS - whether it was springs or if he got a set of their emulators and managed to convert the forks.

Presumably the "damping rods" just a standoff for the springs and don't actually do any damping. That means they must be a shuttle valve design. Does that bottom piston come off the leg and are there holes underneath or is it basically fixed?

In Chris's case, it sounds like there some sort of blockage or dent in the slider or something restricting movement.

Chris, Did the legs slide in and out evenly before you rebuild them and did they get dropped off the bench or did you fit a front fender and have too long mounting screws. That's always an issue on CB160's for example.

[CJF]

The forks worked ok before i did the rebuild, so it had to be somthing that I did with the rebuild. I think I'm going to tear them down again to to be sure I know what is going on. For some reason, I have a feeling that the seals may be causing too much friction against the tubes. I can't think of any other reason for them to not rebound and stay compressed like they are. I guess the other option could be if there is in fact some kind of damping holes at the bottom of that "damper" if you want to call it that. It think it looks more like a bar to hold up the springs than anything else, but will take a closer look when I tear them down. The other thing I need to check is the front fender mount bolts...I had no idea that those holes could impact movement of the inner fork tube? If they can, wouldn't that allow for fork oil to exit through those holes? In any event, I'll check that first.

[tz375]

Chris, I think you are right that the damper rod is actually just a post for the springs to rest on.

What I was alluding to in the fender reference was that on some for sliders, if a fender bolt is too long it bottoms in the hole and further pressure to tighten it displaces metal out of the bottom of the hole and extrudes it into the bore. I'm not familiar with that particular fork so I have no idea if it's even possible with those sliders, but I have several non-sliding sliders from other bikes .

You are right, that if the bolt was really long, it would probably cause cracking and potentially leakage. I thought it might be worth checking - just in case.

My interpretation of the description of the symptoms is that the slider went on so far, until it reached a tight spot and now it's been pushed past that tight spot, it won't rebound upwards past that same point. That seems to indicate something causing it to bind rather than just sticky seals. When that happens, we sometimes have to remove the wheel and insert the axle into both legs, tighten all the hardware and hit the axle with a large rubber mallet to drive it apart. Crude but usually effective.

Look for smear marks on the bushes and on the pistons and look inside the slider for signs of rubbing.

[CJF]

I figured out what it was. I have a tarozzi fork brace installed and I guess I tightened up the bolts too much. When I loosened them up a little the fork popped back and rebounded like it should. Turned out to be a lot easier fix than I was thinking. Thank you guys for all of your help. Now it is down to one last thing hooking up the oil lines and filling her with gas!!!! Leaving for a vacation with the family, so I guess it is going to have to wait till we're back. Can't wait to fire it up for the first time and maybe even get out for a track day.

[tz375]

Well that's interesting. That brace must be pinching the sliders.