RGM (extra long) fork leg top bushes

[olympus]

Hi all
I see on the RGM site they sell longer (than std.) fork leg slider bushes to stop "topping out"
Do i take it this refers to the opposite bottoming out?? and what it the theory behind reducing the fork travel & how are they suppose to stop it??

 

[L.A.B.]

Do i take it this refers to the opposite bottoming out??

Yes.

and what it the theory behind reducing the fork travel & how are they suppose to stop it??

They don't reduce fork travel because the fork extension travel is already limited by the damper assemblies (rods and tubes).


http://atlanticgreen.com/forks.htm

 

[olympus]

So if the topping out is simply cured by replacing the damper tube caps, why produce the longer bushings?

 

[chris plant]

glad someone brought this up as im thinking of doing this mod,as I understand it, it creates an hydraulic stop instead of a mechanical stop and stops the clunking when the forks are fully extended ?

 

[L.A.B.]

So if the topping out is simply cured by replacing the damper tube caps, why produce the longer bushings?

It can help, but I wouldn't go as far as to say it's a total "cure" because the forks still have no proper hydraulic extension buffer.
Personally, I found the extended bushes didn't prevent the topping-out 'clunk' either.

 

[cash]

So if the topping out is simply cured by replacing the damper tube caps, why produce the longer bushings?

The hydraulic stop port in the fork leg is in the wrong place. For it to work the port is meant to be covered by the top bush on full extension, but the damper unit stops extension before the port is covered; clonk! The covenant conversion kit includes a pair of aluminium sleeves. These sleeves should be located below the top bushes to cover the port, sadly in some kits they float. The extended bushes simply do that and cannot float. However, I am convinced that primary or secondary cause is wear in the damper cap rod bushes #06,1347. To cut a long story short, over the years I have tried everything to reduce topping. Fixed covenant sleeves and later extended bushes did work well but the topping returned. After fitting new damper cap bushes the topping stopped only to return years later, and on stripping I found 0.002"wear in the cap bushes bore. I have since fitted a pair of cast iron damper cap bushes to reduce the wear rate and the topping has once again stopped.

One other observation. The fork action began to feel a bit vague though everything appeared okay. A very good TT rider confirmed there was something wrong but what? at the same time topping began to show its ugly head. It turned out to be wear in the damper cap bushes. They are machined to a very tight bore tolerance and any wear allows excessive oil to leak past the damper rod, clonk! and poor damping.

I hope this might be of some help.

Dave

 

[PeterJoe]

I installed the extended RGM bushings in my bike and like LAB, it didn't stop the forks from topping out. What I figured out is that the extended bushings stop short of covering the 1/8 inch holes in the fork tubes when fully extended. So I plugged one of the 1/8 inch holes with an aircraft NAS 1097-4 rivet. When I reassembled the forks the forks still topped out. So I got ready to disassemble the forks again to plug the other 1/8 inch holes but decided instead to try putting two 1/16 inch washers between the damper rod's check nut and the fork cap bolts. What this does is allow the forks to extend an extra 1/8 inch. This fixed the problem and the forks no longer top out.

The only problem I have noticed by allowing the forks to extend the extra 1/8 inch is that the rear tire may not lift off the ground when the bike is parked on the center stand. Especially if your center stand parts are worn. I rebuilt the center stand and pivots on my bike and have no problem.

I recall another person on this forum that plugged all of the 1/8 inch holes by tapping the 1/8 inch holes and using short set screws along with Loctite instead of using rivets.



The fork tube before installing rivet:

Fork tube with rivet installed

Peter Joe

 

[kommando]

If you fit longer damper rods and the longer bushes you get a proper hydraulic bump stop.

 

[jseng1]

kommando - longer damper rods will let your Norton forks extend so much that there can be binding if there is side load on the alum sliders an full extension (because the bushings are so close together). The alum sliders can even break if stressed when extended that far (wheelie or pothole).

Peter Joe - you have the right idea to prevent topout by plugging the holes in the fork tubes with rivets so the longer bushings will work. I'm the other guy on the forum you mentioned that threads and plugs the holes with set screws.

Cash - see the link below to see an easy way to cure the worn damper cap problem (this makes it rebuildable). The alum sleeves in the link below have been slit and expanded and have a tight push in fit inside the fork leg so they don't "float". But you can't see the slit in the photo.

https://jsmotorsport.com/js-motorsport-technicalmore-norton-fork-upgrades/

 

[kommando]

Not if you fit longer bushes and longer damper rods, try reading what I suggested, I did not say only fit longer damper rods which does give issues.

 

[Esmerela]

2 years ago, I made 1" over 10mm diameter damping rods and opened up the damper tube caps to suit.
I turned up larger od valves to take out the slop in the damper tubes and machined replacement valve seats with two flats instead of four.
I run 5wt fork oil and fitted std springs from Andover.

The forks work really well I may get one topping out clunk if the bike has stood for a while after a couple of pumps the system is primed and no more clunks, manhole covers and pot holes, no longer trigger the butt clenching teeth chattering sensations of the past. The centre stand no longer lifts the wheels of the ground so I carry a couple of 80 mm x 25mm discs of board in the lid of my tool tube. I have yet to try a set of plastic bushes , I believe they would help on the stutter bumps.

 

[jseng1]

kommando

Yes the longer bushes with the longer rods will not extend too far (I was just thinking about the longer rods).

peter joes idea of a couple washers under the top nut to lengthen the damper rods 1/8" is a good easy solution.

But you can still have a "clunking" problem with the longer bushings along with longer rods because the 1/8" bleed hole is too close to the lower bushing and when the alum slider is fully extended there can be air bubbles or pockets that don't give an adequate reliable hydraulic bump stop. Plugging the 1/8" holes gives you a little more hydraulic distance and guarantees that the top out "clunk" is gone forever.

Here's the slit sleeve and the 6-32 set screws for plugging the (threaded) bleed hole.

The sleeves installed on the leg

 

[cash]

kommando - longer damper rods will let your Norton forks extend so much that there can be binding if there is side load on the alum sliders an full extension (because the bushings are so close together). The alum sliders can even break if stressed when extended that far (wheelie or pothole).

Peter Joe - you have the right idea to prevent topout by plugging the holes in the fork tubes with rivets so the longer bushings will work. I'm the other guy on the forum you mentioned that threads and plugs the holes with set screws.

Cash - see the link below to see an easy way to cure the worn damper cap problem (this makes it rebuildable). The alum sleeves in the link below have been slit and expanded and have a tight push in fit inside the fork leg so they don't "float". But you can't see the slit in the photo.

https://jsmotorsport.com/js-motorsport-technicalmore-norton-fork-upgrades/

jseng1,
I did indeed look at bushing the damper caps and even making them out of bronze. RGM stocks them in cast iron so I went the lazy way and nipped down for a pair, so far they are holding up okay.

 

[olympus]

So I plugged one of the 1/8 inch holes with an aircraft NAS 1097-4 rivet. When I reassembled the forks the forks still topped out. So I got ready to disassemble the forks again to plug the other 1/8 inch holes but decided instead to try putting two 1/16 inch washers between the damper rod's check nut and the fork cap bolts. What this does is allow the forks to extend an extra 1/8 inch. This fixed the problem and the forks no longer top out.







The fork tube before installing rivet:

Fork tube with rivet installed

Peter Joe

Could you please clarify exactly where these washers need to be fitted??

 

[kommando]

Under the fork top nuts on the damper rod, they just effectively extend the rod by the thickness of the washer, The 5/16 nut that locks the damper rod to the fork top nut bears down on the washer and fixes it to the bottom of the fork top nut.

 

[olympus]

HI Kommando, so the washers are effectively on top of the damper rods, and are pinch between the top damper rod nut and the underside of the top tube cap bolt??

 

[kommando]

Yes, but nothing to stop you screwing the rod in until it bottoms in the hole and then turning it out 2 or 3 turns or even more and then locking it with the nut against the underside of the top fork nut. The effect will be the same as adding washers but more difficult to equalise the 2 sides.

 

[Deets55]

Yes, but nothing to stop you screwing the rod in until it bottoms in the hole and then turning it out 2 or 3 turns or even more and then locking it with the nut against the underside of the top fork nut. The effect will be the same as adding washers but more difficult to equalise the 2 sides.

Do you still need to plug the small holes or does this help resolve the problem by itself?
Pete

 

[Esmerela]

Under the fork top nuts on the damper rod, they just effectively extend the rod by the thickness of the washer, The 5/16 nut that locks the damper rod to the fork top nut bears down on the washer and fixes it to the bottom of the fork top nut.

 

[kommando]

Pictures

Fork op nut fully screwed down and lock nut nipped up against top nut

Rod screwed out and then locknut run up and then nipped tight

And alternatively washer added between fork top nut and lock nut

All never to go inside a set of forks as all replaced.

Do you still need to plug the small holes or does this help resolve the problem by itself?

If you extend the rod so the extended top bush goes fully over the smaller hole before the damper valve touches the top of the damper body then it will trap oil and act as an hydraulic bump stop.