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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?

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  • Which model hub do you have?
    – MaplePanda
    May 3 at 11:30
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    @MaplePanda Edited - I couldn't find the order earlier, on my work machine
    – Chris H
    May 3 at 11:33

2 Answers 2

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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.

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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.

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  • This has covered quite a lot of the UK, and there's plenty of farm runoff on our roads in the winter, so yes, fine silty stuff in water more than actual mud. Bu tit turns out the failure mode was more interesting this time: the inner part of the body, onto which the cup threads, was broken in two, and the body came off as soon as I removed the axle and cone, without waiting for me to remove the hollow bolt. I'll rebuild the original, and maybe mix some oil in with my grease - but I've got my eye on another wheel
    – Chris H
    May 4 at 20:41
  • @ChrisH I don't know how old the replacement is but that is so weird that it makes me want to jump to the thought it might be a COVID-era QC hiccup. Parts that get heat treatment like that are the most prone to that kind of thing. May 4 at 22:16
  • It's almost exactly a year old, after the first failure last April, but of course I don't know how long it was in a warehouse. I stripped it recently so I was surprised at the failure, and it's not a part that can be cross threaded.
    – Chris H
    May 5 at 5:43
  • The strange thing is that it's just the rear hub - my other bearings are fine. I rebuilt the wheel on this hub (2 years ago) after the original (Joytech) needed new cones and ideally cups, and I couldn't get them
    – Chris H
    May 5 at 5:46
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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.

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  • I've already got full mudguards, and while that bike might do 8000km a year most of that is on road - it does seem strange that I'm seeing these failures when MTB use should be far dirtier. This looks to be not the latest XT given that it's 9/10 speed compatible. OTOH they're not that hard to work on after a couple of times - with my workshop tools
    – Chris H
    May 3 at 11:31
  • @ChrisH If you wanted to service a Mavic freehub, a Miche freehub, a DT Swiss freehub, a Bitex freehub, I think you would redefine "not that hard to work on." Of course, Shimano are not complicated, but it is not my idea of fun.
    – Noise
    May 3 at 11:50
  • The trade-off is a wheel rebuild vs a few services or replacement freehub bodies, and I'm not quick at building wheels - though at least it's a clean job. But I'd be more likely to buy a pre-built new wheel and service this for a spare (I need a spare front anyway, and a spare back could be useful, but I was going to keep this heavy touring wheel as primary and get a lighter spare.
    – Chris H
    May 3 at 11:56

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