Rebound issue or lack of

[Storm Trooper]

Have a fork issue. Heading back on Sat from the Alps, after discovering a serious judder under heavy braking I found that I had no rebound left. I rebuilt the forks over a year ago with new K-tech springs and the damping rod mod, and they were working perfectly till last week. Definitely a rebound issue as I have just have some rebound left when wound all way in. Any more than a turn out it is just springy. All internals seem ok. Oil quantity good albeit, very discoloured. No leaks, all seals good. Any ideas?

[VTRDark]

PM sent.

(:-})

[bazzashadow]

PM sent.

(:-})

spoil sport carl

wanted to know the answer in case it happens to me

[VTRDark]

wanted to know the answer in case it happens to me

I was on the wrong path anyhow.

But after re-reading the first post.

after discovering a serious judder under heavy braking

You should rule out brakes and bearings before stripping down the forks.

(:-})

[bazzashadow]

wanted to know the answer in case it happens to me

I was on the wrong path anyhow.

But after re-reading the first post.

after discovering a serious judder under heavy braking

You should rule out brakes and bearings before stripping down the forks.

(:-})

sorry carl
well-done

[Storm Trooper]

Brakes and wheel bearings are fine. Have checked discs calipers, headstock, and all these were overhauled and replaced last year at same time as the fork rebuild. Issue I believe is that the suspension has gone soft. Lost its rebound.

I have stripped down and everything seems ok. The only thing I can find is that the oil has seriously discoloured and has possibly thinned. Could it break down this quick and for what reason?

[Cadbury64]

All the rebound damping is generated by trapping fluid between the rebound piston and the seal on the shaft at the top of the cartridge, which then forces the oil through the needle-controlled bypass and the shim stack. If either seal is worn then oil will bypass here instead of going through the rebound valve.

If those seals are OK then I'd take a close look at the shim stack on the nut side of the piston, you may have some crud caught under a shim that is stopping it sealing against the valve body. The same thing could happen on the check valve on the top side of the valve, or the check valve spring might have broken. You should be able to lift this with a screwdriver and have it snap back to seal against the top of the piston.

So my best advice would be to disassemble the valve stacks and make sure they're all clean, inspect the rebound piston ring for wear, and then try reassembly with fresh oil. I've found that when the rebound needle is screwed in fully, the forks are close to hydraulic lock on extension so it will be clear whether your rebound system is working or not.

One other reason you might have no rebound is if the fork cap had loosened on the damper rod; was it all tight when you removed it? The fork cap height on the rod is critical to getting the needle in the right place. On reassembly, I set the needle adjuster screw fully in (flush with the top of the cap) then wind the fork cap down until the needle just bottoms. Hold the fork/rod in place, back the needle screw out a bit, then tighten the locknut (this avoids accidentally mashing the end of the needle when you tighten the locknut).

I hope something in all that rambling is helpful!

[Storm Trooper]

Ok guys, I am seriously pissed off and not exactly oozing confidence after our 2 day Trip up the West of Scotland. The shudder issue is still there, from back in June when it started in France. Even after all the overhauling and eventually replacing the discs and pads. This was after I rebuilt the forks, with new seals, new bushes, new O-rings and oil, also checked and re-greased the steering head bearings. New wheel bearings. New Tyres (Dunlop D209's) Both Calipers were stripped and rebuilt. The seals and pistons were all good. When trying to watch things as and when I brake, from what I can see it is the lower half of the forks that are vibrating/shuddering a lot. I have rebuild them and have also used another pair of younger lower fork stations than my originals when it all started so this is really bamboozling me. Can the upper tubes wear and cause the vibrations as the tolerance allows for movement? It honestly feels when it happens that you are trying to brake while riding over corrugated iron. I am totally stumped on this one and its scary as sh1t!

[grumpyfrog]

Trooper, do you want to borrow another set of forks to rule them out? Pair hanging off the garage roof in Forfar.

[Cadbury64]

I feel your pain, I have a similar issue on my VFR800.

I have checked my discs for run-out, and cleaned the pads, pistons, replaced the pads, but no real change. I took to the discs and pads with an abrasive cloth and this did help somewhat, so I'm pretty sure that it is uneven brake force that causes the shudder.

I've read (but not tried myself) that unbolting and rotating one of the discs relative to the other may change the way the shudder starts.

[Storm Trooper]

Trooper, do you want to borrow another set of forks to rule them out? Pair hanging off the garage roof in Forfar.

Would be brilliant if you were willing to lend me to see if that would at least eliminate the issue I am having with my front end. Can you PM me your details and mobile number and I will contact you to arrange a time to meet. This is much appreciated.

Cheers
Nigel