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jimq Offline OP
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Can anyone comment on the importance of the location of the two little holes. particularly in relation to whether they are above or below the stanchion end plug.

As stock at full extension the upper hole is inside the end plug and the lower is approximately 21mm below. If i run a 30mm lowering spacer under the top out spring the top hole is 21mm above the cap and the lower hole is inside the end plug and if i run a 50mm lowering spacer both the holes are above the end plug and the top hole will be partially obscured by the spacer.

Should i move one or both of the holes or perhaps replace them with one of an equivalent size?

My forks are stock except for emulators and 0.7kg springs.

Between 30-50 mm is the range of lowering i expect to be working in but i will not be sure what the final figure will be until i have tested and tuned the bike.

The bike is to be a road racer.

Cheers.
Jim.

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Read DMadigan's second post with the diagrams, in the text he explains the purpose of the holes.

Quote
When the top of the end plug moves past the holes there is a drop in damping because some oil flows directly into the rebound volume bypassing the damper head.

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jimq Offline OP
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So If I want a close to constant rate rebound damping I could move both the rebound holes so they are always open in the rebound Chambers then adjust the rebound volume with the oil viscosity. This would help me get around difficulties with wanting to experiment with fork lengths.

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jimq Offline OP
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Or leave all the damping up to the check valve

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If your road racing wouldn’t a T140 disc front end be a better change? You could also pull the forks up through the top yoke to alter your ride height.


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Only a guess but because these dampers were designed by test rider "feel", the small holes might have been put in to reduce the damping when under hard braking since they do not have the pop-off valve of modern dampers. I think you would be better off replacing the dampers with something more modern that can be tuned.
Vintage rules here allow up to 38mm forks. I would look at that route. Search on line for "motorcycle fork diameter list" and there are lots of 38mm forks to chose from. Pick a road bike used for racing made for a large range of years such as the FZR-600 89-97. Those would likely have good dampers already.
Modern forks typically have a separate damper tube for the head to slide in rather than the fork tube itself. This allows for a closer tolerance without having to hone the stanchion itself. It also is not affected by the slider/stanchion clearance or bending of the stanchion.

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