Soggy front end

Yes thats the correct type/year, they work by the washer being pushed off its seat and the oil is forced through the rod centre and out of the hole near the base...very basic ...bmw used the same type on their early K 750/1000

daveh said:

Here is the arrangement of components in my Ceriani fork leg (1974).

Some had in addition a plastic piston ring which sat in a groove in the piston and sealed against the bore of the stanchion.

P.S I have tried to get the image larger on screen but without success, having followed the instructions in the Stickie from LAB. Has anyone else had this problem, and how did they solve it?

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Well here's mine Dave, assembled in the same manner. All the same parts bar the extra seal (I think it is) under the piston that you have.



Comparing the two I'd say mine is just an earlier version.

Analysing what I've got, I can't see any sign of a tapered damping rod, so I reckon my aforementioned expert is barking up the wrong tree. As an engineer I should know better - don't form conclusions until you've got all the data, so why should I let others do the same? Fair enough, he didn't have a fork leg in front of him in bits, but I gotta say it's caused me some concern that I could've done without.

I'm swinging towards the thinking that there's nothing basically wrong with this, unless the part that Daveh's fork has under the piston is essential to operation. A poor seal here isn't going to make much difference on compression as the bleed rate is set by the damping orifices down the bottom of the damping rod. It might make a bit of difference on rebound, but I'm seeing the front end bottom out easily.

davamb said:

Well here's mine Dave, assembled in the same manner. All the same parts bar the extra seal (I think it is) under the piston that you have.

Comparing the two I'd say mine is just an earlier version.

Analysing what I've got, I can't see any sign of a tapered damping rod, so I reckon my aforementioned expert is barking up the wrong tree. As an engineer I should know better - don't form conclusions until you've got all the data, so why should I let others do the same? Fair enough, he didn't have a fork leg in front of him in bits, but I gotta say it's caused me some concern that I could've done without.

I'm swinging towards the thinking that there's nothing basically wrong with this, unless the part that Daveh's fork has under the piston is essential to operation. A poor seal here isn't going to make much difference on compression as the bleed rate is set by the damping orifices down the bottom of the damping rod. It might make a bit of difference on rebound, but I'm seeing the front end bottom out easily.

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Dave, from what you said, the fork dive is scary and unsafe. I doubt you can achieve an acceptable level of damping unless you modify. I can give you precise dimensions of all my parts if you need them so that you can do a valid comparison. I would go along with Graeme's view. There are differences, e.g. only two holes in the damper rod, a longer piston, and the addition of a 'diaphragm' - a thin shim tack-welded to one side of a washer with radial holes in it and which acts as a flap valve, part 3, from left to right, bottom, in my pic. When I had these original dampers, they were just about OK by the standards of the early 70s, crude by modern standards, but I don't remember them having excessive dive.

Some people have used Racetech emulators: http://racetech.com/ProductSearch/2/For ... /1970-2010. Maybe worth emailing them?

daveh said:

I doubt you can achieve an acceptable level of damping unless you modify. I can give you precise dimensions of all my parts if you need them so that you can do a valid comparison.

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I totally agree with you Dave and what I've got has got to be made to work better than it does. To that end I'm thinking I'll probably end up with cartridges, but in the meantime I'm experimenting with different oil weights. Still seems under-damped with SAE10 so far, I'll try SAE15 next. One thing I observe is that it looks like the main compression damping orifices toward the base of your damper rod are larger in diameter than the ones in mine. Would you mind measuring one for me and letting me know what you observe? Thanks.

OK, Dave, here goes -



There are two holes near the wheel spindle end, each 5.0 mm in diameter, i.e. the rod has been drilled through. The two smaller bleed holes near the top of the rod at 180 degrees to one another are not 'through' holes. The positions of the latter holes will be in relation to the sequence of components on the damping rod in my earlier photo, which are different. The bore of the damper rod is 10.5 mm.

It may be that the hole diameters are not directly comparable because the assemblies are different - I don't know. If your rebound damping is acceptable and you try to increase your compression damping with thicker oil, you will make the rebound damping firmer as well and that might give a harsher and less responsive ride. If you have really thick oil, the forks might not return fast enough and could 'pack down'.

You could try smaller compression damping holes. You could plug the existing holes with some alloy rod and drill smaller holes above or below, at 90 degrees, not TOO close to the old holes in case you weaken the tube. If that doesn't work, you can plug the new holes and revert to the original set up. Ideally, you would have the forks on the bench and play around with different weight fork oils by extending and compressing the legs without the springs. Leave one fork leg alone and use it as the reference so you can compare. You would normally measure the height of the fork oil in the leg with the stanchion fully compressed and without the spring. The volume of oil in the fork will also have an effect on fork action.

You may find that you can improve low speed compression damping and thereby reduce fork dive but you may also find that it will smack back at you more over sharp bumps. So, I would say, no harm to play around with trying to make things better but at the end of the day, you are stuck with a primitive damping design.

Good luck and keep us posted!

Thanks Dave, much appreciated.

daveh said:

You could try smaller compression damping holes. You could plug the existing holes with some alloy rod and drill smaller holes above or below, at 90 degrees, not TOO close to the old holes in case you weaken the tube. If that doesn't work, you can plug the new holes and revert to the original set up. Ideally, you would have the forks on the bench and play around with different weight fork oils by extending and compressing the legs without the springs. Leave one fork leg alone and use it as the reference so you can compare. You would normally measure the height of the fork oil in the leg with the stanchion fully compressed and without the spring. The volume of oil in the fork will also have an effect on fork action.

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All my testing thus far has been with one leg sans spring in the bench vice, and what you describe is exactly what I intend to do. I was thinking of tapping the existing holes and then making some smaller diameter inserts - large ID "jets" if you like - from thread stock and trying them. I could drill them out to different internal diameters and evaluate without making any change really to the damper rod.

daveh said:

You may find that you can improve low speed compression damping and thereby reduce fork dive but you may also find that it will smack back at you more over sharp bumps. So, I would say, no harm to play around with trying to make things better but at the end of the day, you are stuck with a primitive damping design.

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Yeah, understand. I'm not terribly optimistic about achieving an excellent outcome at this stage, but still worth trying.

daveh said:

Good luck and keep us posted!

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Will do and thanks again.

The compression damping holes I measure at 5mm, so same there Dave. (I'm ignoring the rebound holes at the moment.) The other major difference is the way the damping rod seals against the inside of the fork tube, on mine it just relies on the machined finish to the piston sides at the top of the damping rod, yours has another part that I presume is used to make the seal. Hard to evaluate the effectiveness of my arrangement, but the piston does seem to be a pretty good fit.

I then re-assembled the fork and tried 200ml of 15 weight oil. It seems to me to give a substantial improvement in feel compared to the 10 weight. (Castrol Motorcycle Fork Oil, in both cases.) At this stage I think it worth pulling the other fork apart, checking it over and giving it some 15 too. Then time to try it on the road. If no real and worthwhile improvement I'm going to start looking for cartridges. I'll also see if it's possible to fit emulators.

davamb said:

The compression damping holes I measure at 5mm, so same there Dave. (I'm ignoring the rebound holes at the moment.) The other major difference is the way the damping rod seals against the inside of the fork tube, on mine it just relies on the machined finish to the piston sides at the top of the damping rod, yours has another part that I presume is used to make the seal. Hard to evaluate the effectiveness of my arrangement, but the piston does seem to be a pretty good fit.

I then re-assembled the fork and tried 200ml of 15 weight oil. It seems to me to give a substantial improvement in feel compared to the 10 weight. (Castrol Motorcycle Fork Oil, in both cases.) At this stage I think it worth pulling the other fork apart, checking it over and giving it some 15 too. Then time to try it on the road. If no real and worthwhile improvement I'm going to start looking for cartridges. I'll also see if it's possible to fit emulators.

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Yes, that's a sensible approach.