Fork Clunk at Extension RD400E

Hemi · April 12, 2024, 11:47:59 AM

I'm looking for assistance with a disturbing clunk at the top of fork travel in my 1977 RD400E, even when going over small bumps at low speed.
The clunk is disturbing to the point of not wanting to ride the bike.
I've been refurbishing the bike, tip to tail, over the winter, added Vape ignition, many parts, paint, top end.
New fork tubes, have now added progressive springs, tried different weights and levels of fork oil.
The clunk is at the top end of travel when forks extended after a bump in the road, nasty clunk felt through the bars.
The steering head seems fine, new bearing, adjusted to specs.
I've owned 20 bikes over the decades, including an RD400 back in the day, and have never experienced anything like this.
Any and all thoughts would be much appreciated.
I'm stumped, as is John at Economy Cycle......
Hemi

85RZwade · April 12, 2024, 02:04:32 PM

What kind of tubes did you use? I had a similar complaint with a set of tubes from Forking by Frank that I never was able to resolve.

m in sc · April 12, 2024, 02:15:36 PM

reduce the preload.
if theres a spacer, use a shorter one. (can make one out of PVC if need be)

Jspooner · April 13, 2024, 01:33:08 PM

I don't think it's any of the above. I ran into the same problem. There's some hardware at the bottom of the fork tubes that you needed to swap, correct? It's very important that you get that back in the right way. It will go together either way but I'm pretty sure it has something to do with dampening control in that direction. I couldn't tell you which way is right or wrong because I just figured it out by trial and error. It wasn't my bike so someone else did the swap, I noticed the "top out" problem and just started swapping and flipping parts. I would bet that's the problem. Maybe one side is right and the other is wrong or maybe both are wrong.

Hemi · April 13, 2024, 02:37:12 PM

Thanks Jspooner,
Yes, the previous owner installed the new fork tubes.
Perhaps the damping rod was reassembled incorrectly?
Any other ideas out there would be appreciated.
There are no spacers with the progressive springs.
Either way, with progressives or the stock springs and spacers, there is about a 1" extrusion out the top of the forks before screwing the fork cap back on, which I believe is about the correct amount.
Hemi

Jspooner · April 13, 2024, 02:42:46 PM

It's not the dampening rod. Can't really do anything wrong there. What I'm talking about are actually at the bottom of the tube on the inside. If you pull it all apart and remove the dampening rod and look at the bottom of the tube you'll see s9me stuff held in with come c-clips. That's what I'm talking about. Normally you leave that stuff in tact when working on the forks but when you replace tubes you have to remove then and reinstall them in the new tubes.

Hemi · April 13, 2024, 02:43:42 PM

Also,
The fork tubes were supplied by HVC Cycle- part number 1A0-23124-00-00, 1A1-23124-00-00
Hemi

Hemi · April 13, 2024, 02:45:20 PM

Ok,
Thanks Jspooner,
Looks like it time to disassemble the forks.
Hemi

Jspooner · April 13, 2024, 02:52:10 PM

In all honesty, I thought it was all of the obvious stuff too. Before you remove that stuff see if you can fine a diagram that shows it. If I remember right it can go together a few different ways as far as the actual order of the parts and which way they are flipped, but only one way is correct.

STLMike · April 13, 2024, 10:11:05 PM

Hemi, I'm reassembling my fork tubes right now and i'm also using the progressive springs. On both Economy and HVC site it says to cut the springs 1.5". I have cut mine.

However, there is no mention of not using the spacers. With the top fork tube fully compressed into the lower tube, I have about 4" of spring sticking out. With the top fork tube fully extended, my spring is not exposed at all. In fact, it's about 1.5" down inside the tube.

Hemi · April 14, 2024, 12:21:25 AM

STLMike,
That's very interesting.
Let me know how that works out for you when you ride your bike after reassembly.
I've been talking to John at Economy Cycle. He sold me the progressive springs.
He has 2 personal RD400's, one with progressives and the other with stiffer springs for the track.
He hasn't mentioned cutting the springs, and I believe the spacer is supposed to be removed with progressives, leaving about an inch of spring above the tube with forks extended prior to replacing the caps.
Before I switched to progressives, the forks had the spacers and stock springs, and had the same problem, a nasty clunk at full extension.
Hemi

m in sc · April 14, 2024, 12:28:41 AM

1" extension, or less, sounds about right coming out of the top. lack of sag on ft end would be a big indicator of an issue. 4" is WAY too much to have to compress to put the cap back on.

STLMike · April 14, 2024, 12:58:54 AM

I cut the 1.5" off mine and just reassembled. There was no way the spacers were going back in. I also emailed John to be sure.

1976RD400C · April 14, 2024, 07:58:54 AM

Quote from: Jspooner on April 13, 2024, 02:42:46 PMIt's not the dampening rod. Can't really do anything wrong there. What I'm talking about are actually at the bottom of the tube on the inside. If you pull it all apart and remove the dampening rod and look at the bottom of the tube you'll see s9me stuff held in with come c-clips. That's what I'm talking about. Normally you leave that stuff in tact when working on the forks but when you replace tubes you have to remove then and reinstall them in the new tubes.

I think it's called the top out valve. It causes real strong damping the last 1/4" of topping out. Remove the snap ring on the inside of the fork tube and pull the piston/bushing out and it should be in there, aluminum valve and spring.

Jspooner · April 14, 2024, 10:34:05 AM

Quote from: 1976RD400C on April 14, 2024, 07:58:54 AM
Quote from: Jspooner on April 13, 2024, 02:42:46 PMIt's not the dampening rod. Can't really do anything wrong there. What I'm talking about are actually at the bottom of the tube on the inside. If you pull it all apart and remove the dampening rod and look at the bottom of the tube you'll see s9me stuff held in with come c-clips. That's what I'm talking about. Normally you leave that stuff in tact when working on the forks but when you replace tubes you have to remove then and reinstall them in the new tubes.

I think it's called the top out valve. It causes real strong damping the last 1/4" of topping out. Remove the snap ring on the inside of the fork tube and pull the piston/bushing out and it should be in there, aluminum valve and spring.
<a href="https://ibb.co/BsT7108"><img src="https://i.ibb.co/9YyfMQR/20240414-074827-1.jpg" alt="20240414-074827-1" border="0"></a>

Thats what I'm talking about.

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Fork Clunk at Extension RD400E