Wondering if anyone else has had similar issue &, better still, an explanation/solution to this problem.

Both fork cartridges from by bike have oil leak down from the rebound adjuster, if the damper rod is left in the one position for a period & then either pulled or pushed, it has no damping action for part of the stroke. It takes a while for the oil to flow back up the damper rod to the rebound adjuster. Once the oil flow has reached the adjuster at the top of the fork normal control returns.

I believe this is the cause of an audiable clunk I sometimes hear when the bike moves off after it has been standing for a time or when I move the bike a little and then apply the front brake compressing the front suspension. This condition has no affect when riding, as the constant movement of the forks keeps the path through the damper rod up to the adjuster full of oil.

Is this a design deficiency from having the low speed rebound control needle outside the cartridge or do I have a leakage parth that needs addressing? Newer low speed rebound control adjusters have the needle at the rebound valve in the cartridge & therfore avoid a long path up to the fork tube top.

Hi Magg,
Any oil eak = bad news, but I don't think its too bad. Have you ever had the fork tops off from the damper rod? I seem to recall there being an o-ring near the top of the rod, which from what you say is where the leak is. I dont have a part number as the fiche I have is for uk spec, but davidsilverspares should be able to supply this.Seems odd that both are doing it at the same time unless someone has recently disturbed them/ left them out?
Best Of Luck Buddy. Guys on the forum can answer most things so hopefully you'll get a few more replies.

Believe I have resolved this issue.

It would appear the leak down is the result of the design used to obtain external rebound adjustment. After failing to find any fault within my forks to explain the loss of damping on initial movement, I gave some thought to the various oil levels within the fork leg, damper cartridge & damper rod.

It now seems logical that oil must drain back down the damper rod when the fork leg is motionless. This action is simply the oil level within the damper rod returning to the same level as that between the outer tube and the damper cartridge, “water finds its own level”.

When the fork leg is then placed in motion, the oil has to be pumped back up the damper rod until it reaches the fork cap, where the rebound adjuster needle is located. The oil flow restriction of the rebound adjuster needle restores normal damping control.

The diminished damping level at initial movement of the damper rod does not occur on newer forks because the adjuster is located within the rebound piston in the cartridge and is therefore below the oil level between the outer tube and the damper cartridge. In the newer designs the “rebound control” at the top of the fork tube connects to a small rod that passes down through the damper rod and engaes with the adjuster needle in the rebound piston. No oil has to be pumped up the damper rod.

So, although there can be a disconcerting clunk on initial fork movement, it is not life threatening and does not affect the performance of the front forks because, after travelling a few metres, oil will have reached the rebound adjuster needle at the top of the fork tube and normal rebound control will take over.

Been doing more investigation into the mechanics of the NC30 cartridge forks, trying to reduce both the amount of dive under brakes and the impact harshness. Because of the OEM design of the rebound adjuster, changes with the rebound adjuster also affects the compression damping. This in theory should allow one to increase the compression damping, to help reduce brake dive, by adjusting for more rebound damping. However, adjusting rebound damping to control compression damping gives a very poor outcome. Too much rebound damping and not enough compression damping or visa versa.

A better solution is to preload the compression valve shim stack to increase the low speed damping and reduce the number of shims to reduce the high speed damping. This reduces both brake dive and impact harshness, provided the rebound adjuster remains within a narrow band of adjustment. This is the stage I am at at the moment.

Best solution, I believe, is to arrange for independent compression and rebound damping adjustment, like most modern (<10 year old) bikes. With the NC30 cartridge forks, a compression adjuster at the bottom of the fork legs is a née on impossible engineering challenge using OEM fork legs. However, a compression adjuster at the top of one fork leg and a rebound adjuster at the top of the other is an elegant solution and will retain the OEM fork legs. This can be achieved by making one fork leg compression damping only and the other leg rebound only and is the system I am presently developing. Other manufactures, including the latest Ohlins cartridges, use this system.

Should anyone be interested in this development I can provide more detail.

Hi there,

You got it, well almost, because there is some problem with the oem forks that you won't be able to easily work around.
- The rebound adjustment is a bad design, and even if you separate the circuits in two forklegs, you will still get high levels of cavitation due to the design, it's only made worse if you want use the adjuster as a compression adjuster.
- The rebound cartridge will need to be changed, it will hold a too small amount of oil to give consistent damping, you really need a 25mm cartridge to get oil volume up.

Both of these issues can be worked around, but at a price, that is why most people put a different fork on, it's cheaper all in all (plus there is other benefits, as increased stiffness).

I made a setup like you are working on last year, I used a suzuki TL cartridge (25mm) for rebound and modified oem honda cartridge for compression, I had it working okay, but forktops/adjusters was limiting damping quality and I still had trouble getting the cartridge to build up pressure fast enough.

I think there is a good reason why yamaha and ohlins cartridges using this tech are bigger (30mm), since it helps in keeping damping response time down.

But I don't want to discourage you, if you want to go for it for fun of development, go ahead, I learned alot. But if you want better "all out" performance, go for a different fork altogether.

Ronni

Ronni,

Interested to hear what you have done. Although I am playing with the OEM forks to improve their performance, I am also looking forward to the engineering challenge of developing separate compression and rebound systems. Have done re-valving of off-road forks in the past and now wish to have a go with the forks of my NC30, all be it much more than just a valving change.

The issue of the small size of the OEM cartridges I have accepted as a compromise due to cost and to some degree the available space within the OEM forks, although I still expect the final result to still be superior.

Care to elaborate regarding the limitations you had with the top cap adjusters.

My design will use a pair of CBR600F3 cartridges with hollow damper rods and place the flow control needles at the compression/rebound valve ends of the rods. The NC30 OEM top cap adjusters will be modified to use the inner shaft to drive the adjustment needles. The placement of the “rebound” flow needles at the top caps, together with resulting loss of isolation between the rebound and compression damping cycles I believe are two major flaws in the OEM design. Interesting that Honda claimed that the F3 cartridges, with the relocated flow needles, were a great leap forward from the F2 design which is basically the NC30 design.

Have re-configured the original CB600 compression and rebound valves of each cartridge to perform only compression or rebound and a bench test has been most satisfactory. Just waiting on delivery of more material to finish machining the new flow needles and modifications to the top cap adjusters and then will be ready to assemble and road test.

Anticipate that using each cartridge to perform only one function will be a significant improvement, even with 20mm cartridges, and the increased valving area for the compression function must make tuning an easier task.

I used the NC30 topcaps, with the rebound needle in the top cap and that didn't work very good. I have no doubt that with lowspeed moved down in the cartridge and topcaps modified to suit, performance will be good.
I do think that 20mm cartridge is on the small side for rebound, but I would like to be mistaken.
I ported the compression piston to smooth out flow and that worked fine, nothing radical, just removed all the sharp edges, also did this on the backside of the redundent rebound piston, to aid cartridge refilling (this was my primary concern, as I wanted to get as fast response as possible).

Ronni

RonniB have done further work on the separate rebound and compression damping fork legs and have now reached the stage of setting the shims stacks. Have engineered a system that will allow me to withdraw the cartridge from each fork leg without needing to remove the lower leg bolt. This will mean I do not have to remove the wheel and axle and can also keep the oil in the fork leg, thus greatly reducing the time it takes to change the valving. Also have moved the bleed needles down inside the cartridges.

Would you be willing to share the shim stack configuration that you used in your version of separate compression and rebound and same me a little development time.

Good luck, post up some pictures of what you create!

On a side note, if it doesn't work out too well, the ZXR400 USD fork conversion is quite popular but if you get the ZXR400 SP forks, these come with all 3 adjustments as standard.

Rebound, Compression, Preload



You can see the rebound adjuster at the bottom just below the caliper brackets!

Just an idea if it becomes too costly with your build!

Want to play, so not intending to replace the forks and can always go back to OEM internals if I have to.

Cost so far has been limited to a pair of S/H GSXR-600 fork cartridges and this is only if you do not want to use the OEM cartridges. The small amount of machining required has been managed on my home lathe.

I am using the GSXR-600 cartridges because they have the bleed adjuster needles inside the cartridge unlike the OEM system.

I expect that the quick removal cartridge system, with slight modification, could be applied to OEM cartridges also, but have not tried it.

The basis idea is to retain the cartridge retention bolts in the lower fork legs and screw the cartridges onto the "studs" formed by the bolts protruding into the lower legs. I have made a tool and modified the fork spring bases so that the cartridges can be tightened from the top of the forks.

Because I am using GSXR-600 cartridges that use a 10mm thread retaining bolt, I can tap the lower fork leg holes 10mm and screw the bolts in to seal the holes and provide a stud to screw the cartridge onto. OEM cartridges would need an 8 mm tapped base plate placed at the bottom of the lower leg into which the cartridge retaining bolt could be screwed to seal the hole and again form a stud to screw the cartridge onto.

The special tool is a long tube pin spanner that locks into holes in the fork spring base so that the cartridge can be screwed onto the "stud" in the lower fork leg.

In practice it is simpler than it might sound, the pictures below should help.

The long tube pin spanner.


Modified fork spring base.


Long tune pin spanner coupled to fork spring base.


A picture of the lower fork leg "stud" is not included because of the poor picture quality.