Fork Knock

My forks knock at full extension, as when I go over washboard; its loud and I thought, at first, that my triple clamps got loose, fortunately this wasn't the case.

I did some searching and found a referece to a Covenant kit by RGM and article by someone on this forum that explores the effect of different length damping rods.

I also found a few entries that say this is entirely normal, and based on the components and my experience rebuilding the forks I'd have to say that I agree.

But ,is there a good fix? I've overhauled a lot of forks with similar designs and never had knocking issues.


RS

Bill, hi. You may want to look into Greg Fauth's (our own Norbsa) modification kit - http://decentcycles.com/store/index.html. A bit cheaper than the Cosentino kit (http://www.cosentinoengineering.com/ind ... ge0019.htm)! No personal experience with it - it's on my list, but it's still a lengthy list - but others here surely have, and Greg I'm sure would be happy to answer any questions. FWIW. - Brian

RoadScholar said:

I also found a few entries that say this is entirely normal, and based on the components and my experience rebuilding the forks I'd have to say that I agree.

But ,is there a good fix? I've overhauled a lot of forks with similar designs and never had knocking issues.

Click to expand...

If the knock happens at full extension when you are riding then it's not normal, and is generally caused by too much clearance between the damper rods and damper rod caps?
If you haven't replaced those parts recently, then they probably need replacing, as the clearance between those parts affects rebound damping.

Also what grade of fork oil are you using? 20W is the recommended grade of oil, or at least it should be your starting point, I know some members prefer ATF, -I don't.
I use the Covenant kit (and progressive springs) and that seems to be an improvement in my view, although other members disagree.


If the knock happens when the forks are at full extension, (when on the main stand for instance) and are then quickly compressed, they will rebound undamped, and that is normal, because when the forks are at full (damper) extension, the damper rod pistons are then more-or-less hard up inside the tops of the damper tubes, therefore oil cannot be drawn into the tubes quickly enough, as the pistons drop over such a short movement to act as the rebound buffer, so the forks spring back undamped.

RGM motors now offers a longer top bush which blocks off the holes creating a hydraulic buffer stop to avoid forks 'topping out'.
I used an old bush which I removed the flange on a lathe, opened the bore a touch and inserted dirctly below the top flanged bush.
Stu.

L.A.B. The forks had 6300 miles and 28 years of idle storage, the fork oil had never been changed. I replaced the bushings, seals and springs (Progressive's), the action was stiff so I but 10 weight oil in intending to replace it with 20 weight when they eased up, perhaps now is the time, I have the 20 weight fork oil.

Ludwig: Nice touch with the spring, what is your spring? I was thinking about a valve spring from a small bore single??

BrianK: Thanks for the links, good seeing you at the British Iron meet last month.

bigstu: Thanks, I'll be going back to RGM's website, I like the idea of sa hydraulic lock

RS

It is a new part I saw in their new catalogue, you may have to contact them..
Stu

I did several things to stop this happening.
1) I opened up the bore of the lower non flanged bush a touch on a lathe to ensure it did not tug from stanion contact.
2) Locktite was used on the outer diameter during installation.

This mod is from the article about improving the action of the forks......http://www.nocnsw.org.au/forks.html
I did both mods to the forks on my Triton, both the top bush and the hole blanking and repositioning.....
However, the biggest improvement I made to the action of the forks on my Triton was to ditch the external springs and revert back to internal springs.

Stuart

bigstu said:

RGM motors now offers a longer top bush which blocks off the holes creating a hydraulic buffer stop to avoid forks 'topping out'.
I used an old bush which I removed the flange on a lathe, opened the bore a touch and inserted dirctly below the top flanged bush.
Stu.

Click to expand...

Did the same thing but I havn't tried the forks yet.