Shimano FH-M756 freehub body compatibility - can I upgrade?

Question

I've got a Deore XT rear hub (FH-M756-A) on my tourer, and twice the freehub has let me down. The first time it was full of mud mixed with grease but I managed to get it working just enough to ride home. I replaced the whole freehub body - that was a year ago.

I recently serviced the new one. It was pretty muddy inside, so I cleaned and regreased it. That failed on me yesterday, luckily near a station so I only ended up with 11km to walk/roll downhill.

Clearly the seals aren't up to my (mainly road) riding despite being MTB parts. Rather than rebuild the wheel on a new hub, I'd like to upgrade the freehub body, but is that possible? I can't find a compatibility matrix, and Sheldon says "Most Shimano Freehub bodies are interchangeable" before listing a lot of exceptions. In my particular case it's QR versions, of which there are many - so is it possible to look at the specs and work out whether a given body will fit? Even better, is it possible to work out which would have a good chance of keeping the mud out?

Answer 1

There are many other freehub bodies (FHBs) that can physically fit on that hub shell, because it uses the most common of the Shimano splined interfaces between the FHB and the shell. All of the ones whose native shells show the same pattern in their EV documents can probably be made to at least bolt on.

If you go off-spec and use one from some other hub then here's what you have to look out for:

• The presence or absence of washers in the spline area needs to be right so that the FHB isn't too sunken into or extended out from the shell.
• You'll need to transplant the pressed-in labyrinth seal you have into the new one so it plays nice with the seals on the FH-M756 cone.
• It's not guaranteed that the ball track on the new FHB is precisely the same distance from the splines as the old one. This probably isn't enough to matter for any purpose other than wheel interchangeability.

There's no guidance available in any official document about whether the above issues might arise with any other particular freehub. That part is case-by-case experimentation only. Usually it goes pretty easily unless drop-in wheel interchangeability is important.

This hub is usually resistant to contamination, but clearly you're finding its limits, at least as provided by the manufacturer. I would agree that it's unlikely you're going to find anything better per se. It's possible you could have better luck by changing up the lubrication inside, for example by running a heavy oil or oil/grease slurry.

It sounds a little like you're riding in an area with the kind of fine, silty mud that is simply very hard on FHBs. It might be better in that case to go with a hub that is built to be pulled apart and maintained more easily. The problem with that plan is then you'll have to do it and the interval needed will probably be pretty frequent.

Answer 2

Given that you already have the XT hub, you are at the top of the tree as far as premium consumer components go from Shimano — I don’t think you’d fare better going to XTR.

Perhaps there is another way to reduce mud ingress, mudguards/flaps etc, but really it sounds like you need a hub that is easier to service from another manufacturer. The Shimano freehub is difficult to disassemble and not really a serviceable component when compared to other designs.