1973 Commando new fork upper bushing almost locks forks when tightened into place

D

DennisMo

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Jun 6, 2023
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I have rebuilt a front fork with a new chrome fork, bushings and seal.
All is installed but the spring is not compressed with the upper bolt.

The upper brass bushing looks like it is that sintered porous bronze material

When I very lightly tighten the collar the locks the oil seal and the brass bush into place, the fork
can be slide up and down with very little hand pressure.

But when I fully tighten the collar, the fork gets very stiff but can be just moved in some spots with significant hand pressure. In some spots on the chrome fork it is essentially locked if I fully tighten

Is this normal? Have you any advice on this?

Dennis
 
Not normal, either the slider is bent/distorted or the holes in the bushes are not concentric to their OD in one or both bushes. Or the tube is bent.

I have a distorted BSA slider on a shelf awaiting a repair I have not worked out how to do.

If its the bushes then take them out and measure the wall thickness in several places and see what the variation is.

Roll the tube on a flat surface to check its not bent.
 
It sounds like the seal is being crushed by the collar?
Is the bush and seal fully seated in the tube?
The collar should retain the seal, but it sounds like your collar could be distorting it.
 
"But when I fully tighten the collar, the fork gets very stiff but can be just moved in some spots with significant hand pressure. In some spots on the chrome fork it is essentially locked if I fully tighten"






I have observed the same thing. (When tightened really tight) I used Sealant on the the threads of the nut, so gently snug would stay, seal. It was leak free, didn't come loose.


And yes, as mentioned above, could be from bushings being machined out of square (flange to sleeve).
 
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I’ve had some headaches getting my forks to work properly and one of the issues was a set of oversized stanchions which caused a similar issue, not as bad as being hard to move but excessive stiction. Measure the stanchions and make sure they’re not oversized. If the individual forks are not smooth before full assembly you will have problems so it does need sorting.

Dave
 
Thanks!
I like the idea to just snug up the collar.
Did you use a rubbery sealant on the threads or a loctite type sealant?
Dennis
 
Each element of the front suspension is simple by themselves to check. If you are putting the whole assembly completely together and then checking, you can only guess as to which element is the problem. So, the answer is to go backwards. Remove the dampers and springs and just concentrate on the sliders and fork tubes with no wheel installed or axle connecting them.

Seat the bushings and tighten the slider cap. Does each slider move freely?? If not, then your problem is related to the bushing/fork tube interface. As was said, that could be an out of square bushing being distorted when you tighten the cap OR improperly sized bushing or fork tube. It could also be a bent fork tube or some debris in the slider recess distorting the the bushing when it's pressed home...

If your sliders move freely when assembled alone, then remove the spring from the damper assembly and check that the damper moves freely (without oil in it since you are just checking for some sort of blockage or broken part) If they seem good then assemble the springs but don't install yet.

Next, I would assemble the wheel and axle temporarily on the sliders. Push the wheel up to compress the slider upward and then tighten the pinch bolt on the primary side of the axle. At this point your 2 sliders are supposed to move together without binding. If the sliders moved freely individually, but now there's binding, then you have narrowed down your possibilities to a few things. Bent fork tube, twisted fork yokes. You should be aware that a common mistake is forgetting to pump the forks a few times before you secure the axle pinch bolt. Pumping them helps align the sliders before you secure the pinch bolt and will cause binding if it's misaligned when tightening the bolt.

Your damper cartridges and springs will cause resistance because that's what they are designed to do, but they shouldn't be the cause of binding,... which if you followed my statement above the only way they could is if something was maybe broken inside the damper and was jamming the movement somehow.... but not likely...

If nothing is binding then pull the axle, assemble the dampers and install them, add fork oil, then reassemble the axle and wheel, being carefull to pump the forks before you tighten the pinch bolt...

The big issue is you have to go backwards and test fit and measure all the individual pieces and inspect their fit to see if you can find the issue. good luck
 
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o0norton0o said:
Each element of the front suspension is simple by themselves to check. If you are putting the whole assembly completely together and then checking, you can only guess as to which element is the problem. So, the answer is to go backwards. Remove the dampers and springs and just concentrate on the sliders and fork tubes with no wheel installed or axle connecting them.

Seat the bushings and tighten the slider cap. Does each slider move freely?? If not, then your problem is related to the bushing/fork tube interface. As was said, that could be an out of square bushing being distorted when you tighten the cap OR improperly sized bushing or fork tube. It could also be a bent fork tube or some debris in the slider recess distorting the the bushing when it's pressed home...

If your sliders move freely when assembled alone, then remove the spring from the damper assembly and check that the damper moves freely (without oil in it since you are just checking for some sort of blockage or broken part) If they seem good then assemble the springs but don't install yet.

Next, I would assemble the wheel and axle temporarily on the sliders. Push the wheel up to compress the slider upward and then tighten the pinch bolt on the primary side of the axle. At this point your 2 sliders are supposed to move together without binding. If the sliders moved freely individually, but now there's binding, then you have narrowed down your possibilities to a few things. Bent fork tube, twisted fork yokes. You should be aware that a common mistake is forgetting to pump the forks a few times before you secure the axle pinch bolt. Pumping them helps align the sliders before you secure the pinch bolt and will cause binding if it's misaligned when tightening the bolt.

Your damper cartridges and springs will cause resistance because that's what they are designed to do, but they shouldn't be the cause of binding,... which if you followed my statement above the only way they could is if something was maybe broken inside the damper and was jamming the movement somehow.... but not likely...

If nothing is binding then pull the axle, assemble the dampers and install them, add fork oil, then reassemble the axle and wheel, being carefull to pump the forks before you tighten the pinch bolt...

The big issue is you have to go backwards and test fit and measure all the individual pieces and inspect their fit to see if you can find the issue.
Click to expand...
Exactly.
 
o0norton0o said:
Each element of the front suspension is simple by themselves to check. If you are putting the whole assembly completely together and then checking, you can only guess as to which element is the problem. So, the answer is to go backwards. Remove the dampers and springs and just concentrate on the sliders and fork tubes with no wheel installed or axle connecting them.

Seat the bushings and tighten the slider cap. Does each slider move freely?? If not, then your problem is related to the bushing/fork tube interface. As was said, that could be an out of square bushing being distorted when you tighten the cap OR improperly sized bushing or fork tube. It could also be a bent fork tube or some debris in the slider recess distorting the the bushing when it's pressed home...

If your sliders move freely when assembled alone, then remove the spring from the damper assembly and check that the damper moves freely (without oil in it since you are just checking for some sort of blockage or broken part) If they seem good then assemble the springs but don't install yet.

Next, I would assemble the wheel and axle temporarily on the sliders. Push the wheel up to compress the slider upward and then tighten the pinch bolt on the primary side of the axle. At this point your 2 sliders are supposed to move together without binding. If the sliders moved freely individually, but now there's binding, then you have narrowed down your possibilities to a few things. Bent fork tube, twisted fork yokes. You should be aware that a common mistake is forgetting to pump the forks a few times before you secure the axle pinch bolt. Pumping them helps align the sliders before you secure the pinch bolt and will cause binding if it's misaligned when tightening the bolt.

Your damper cartridges and springs will cause resistance because that's what they are designed to do, but they shouldn't be the cause of binding,... which if you followed my statement above the only way they could is if something was maybe broken inside the damper and was jamming the movement somehow.... but not likely...

If nothing is binding then pull the axle, assemble the dampers and install them, add fork oil, then reassemble the axle and wheel, being carefull to pump the forks before you tighten the pinch bolt...

The big issue is you have to go backwards and test fit and measure all the individual pieces and inspect their fit to see if you can find the issue. good luck
Click to expand...
Good post!
Mike
 
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