Modernizing a 5/16" fork dropout
04-03-23, 01:38 AM
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Modernizing a 5/16" fork dropout
Many older forks come with dropouts fitting a 5/16" (about 8 mm) axle. Many shimano front hubs come with a 9 mm axle, and therefore cannot fit. To widen the dropout, a cylindrical rotary burr fitted to a die grinder can be used. A 3/8" (~9.5 mm) burr with a 1/4" shaft (~6 mm) can be used. Make sure that enough material is left on the dropout before you decide to ride it. The OLD of the fork maybe different from say 100 mm as well. Hopefully the axle can be respaced to a good dimension. In the case of a large OLD in the fork, a solid axle can always be used instead of the more common hollow axle.
04-03-23, 05:39 PM
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This has been done is every shop i have worked in over the decades. Not often but still done. But a less costly and more easily reversable method is to file/grind two slight flats on the axle to reduce the axle "diameter". Andy
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04-04-23, 10:15 AM
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Whats your take on unscrewing cones etc. for maintenance on a hub that has been modified in that way?
04-04-23, 10:27 AM
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Andrew R Stewart is undoubtedly speaking from experience with the suggestion of filing flats in axles. It's not a problem. As I recall (dimly---it's been decades since I worked on one), Sturmey Archer internally geared rear hubs had or have axles with flat sections from the factory, as did or do their dynamo front hubs.
In both those cases, the flats are used to keep the internals of the hubs from rotating---the rear so that the planetary gears rotate properly around the stationary sun gear, the front so that the generator's resistance won't force the axle to rotate in the dropouts.
So, you can safely file the axle. You won't have to take much metal off at all to fit the dropout. Just be sure to file off equal amounts on opposite sides.
In both those cases, the flats are used to keep the internals of the hubs from rotating---the rear so that the planetary gears rotate properly around the stationary sun gear, the front so that the generator's resistance won't force the axle to rotate in the dropouts.
So, you can safely file the axle. You won't have to take much metal off at all to fit the dropout. Just be sure to file off equal amounts on opposite sides.
Last edited by Trakhak; 04-04-23 at 10:31 AM.
04-04-23, 11:50 AM
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IMO this job is about method rather than tools.
The key is to protect thr axle reference points, so work on only one side of the slot, and be careful not to cut from the top.
I use a flat file, and touch up with a rat tail. It's a two minute job, not needing special tools, nor eye prorection.
The key is to protect thr axle reference points, so work on only one side of the slot, and be careful not to cut from the top.
I use a flat file, and touch up with a rat tail. It's a two minute job, not needing special tools, nor eye prorection.
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An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.
Last edited by FBinNY; 04-04-23 at 12:49 PM.
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