Shimano XT 780 Hubs.

[Gattonero]

Definitely agree with the design flaw.
It makes sense especially on an adjustble-bearing design like the Shimano.
On cartridge bearing design, their fit is quite tight with the axle, and the good ones being oversized to 15 or 17mm (although this means smaller cartridges...) gives less to zero flex in the hub axle so the pawls will work quite straight in the ratchet ring.

[NATURAL ANKLING]

Hi,
All good stuff.
The ears should never see the load they are just for keeping the pawl in place, so a design fault which Brucey has given a possible explanation for.

No ultimate solution can be given unless you can get the parts in your hand so the design and parts can be scrutinised.

There is clearance in the bearined part which (always needed with balls / clearance will happen as part is loaded and clearance is generated) as said could allow the outer part to skew, then I am thinking with precession the problem that Brucey is suggesting is exasperated, even the pawl is rocking between the ratchet faces with pedal strokes...................

[Brucey]

...On cartridge bearing design, their fit is quite tight with the axle, and the good ones being oversized to 15 or 17mm (although this means smaller cartridges...) gives less to zero flex in the hub axle so the pawls will work quite straight in the ratchet ring.

that the ratchet is engaged or not does not appreciably reduce the bending loads in the axle in cartridge bearing hubs. 'Zero flex' is an aspiration rather than a reality.

What does happen is that any free play in the bearings and/or flex in the axle, and/or error in original machining can allow incomplete pawl engagement, i.e. instead of three pawls engaged, two or even one will try to transfer the entire load. This puts some horrendous loads into the various parts and is often what causes the ratchet to fail.

cheers

[Gattonero]

...On cartridge bearing design, their fit is quite tight with the axle, and the good ones being oversized to 15 or 17mm (although this means smaller cartridges...) gives less to zero flex in the hub axle so the pawls will work quite straight in the ratchet ring.

that the ratchet is engaged or not does not appreciably reduce the bending loads in the axle in cartridge bearing hubs. 'Zero flex' is an aspiration rather than a reality.

What does happen is that any free play in the bearings and/or flex in the axle, and/or error in original machining can allow incomplete pawl engagement, i.e. instead of three pawls engaged, two or even one will try to transfer the entire load. This puts some horrendous loads into the various parts and is often what causes the ratchet to fail.

cheers

Uhm, almost all the 3-pawls designs I've seen are made to not have the pawls engaging at the same time.
In fact, good freewheels for trials have double number of pawls engaging in opposite points, right because of the very high instant loads they get

[NATURAL ANKLING]

Hi,
Wouldn't that mean unequal loading on each pawl or a possibility that one pawl does not engage?
When worn in all the pawls would engage at same time.

Its true that if you turn a new freehub multiple clicks will be heard, I am not sure on the actual dynamics of a multi pawl freehub and how engagement would happen in service.

[MarcusT]

it is all down to the revised freehub design in those hubs with aluminium rear axles. I think it is a design cock-up, but I can't prove that (not even to my own satisfaction) unless I can examine a few more failed units (that are not completely converted to shrapnel within... )

AFAICT all the shimano freehub designs that use steel rear axles are still reliable in normal use, just like they always have been. I earnestly hope not be proven mistaken, even for the sake of my own usage!

Ironically the new XT design was clearly intended to be stronger; there are more, larger, pawls within it.... hey ho....

cheers

Hi Brucey
I was wondering if you have any updates on this hub problem. I want to replace my current wheels and there are several models with this hub set. I asked the vendor and he states "the older hubs had that problem, not any longer"

I would appreciate any input

[Brucey]

I've not heard of any further breakages myself, and no-one has sent me any bits to look at either. But then again I have seen comparatively few people actually using these hubs, either.

So, no real update to report, sorry.

cheers

[Fred80]

I just had a failure in this hub literally just bought a secondhand bike that had only done a few hundred miles the bike hadn’t been ridden for a year and when I tried it out it worked but once I bought it abd got home and went fir a five mile ride it just stopped freewheeling.. not the guys fault who sold it it just happened. I have bought another hub XT fh780 and my LBS has replaced it,, now after reading this post I am a bit concerned this will happen half way in to my tour from st Malo to Bordeaux next week! Or does anyone think that as I have just bought a new hub the problem may of been resolved!

[fixerupper]

I'm very interested in this thread ...Quite by chance today looked at the rear hub on my new (to me) 2004 specialized Rockhopper that I just got last week .
The rear hub is a Shimano FH-M475VIAM . So what do folks think about these ? Shall I look for an up grade and which one shall I go for ?...I have to say this is my first MTB and to me a total newbee the bike rides like new ....

[Brucey]

....I have bought another hub XT fh780 and my LBS has replaced it,, now after reading this post I am a bit concerned this will happen half way in to my tour from st Malo to Bordeaux next week! Or does anyone think that as I have just bought a new hub the problem may of been resolved!

should have been resolved, but you never know....

BTW if you have the broken parts I'd be interested to see them.

cheers

[Brucey]

I'm very interested in this thread ...Quite by chance today looked at the rear hub on my new (to me) 2004 specialized Rockhopper that I just got last week .
The rear hub is a Shimano FH-M475VIAM . So what do folks think about these ? Shall I look for an up grade and which one shall I go for ?...I have to say this is my first MTB and to me a total newbee the bike rides like new ....

perfectly functional hub, that. If serviced regularly it will outlast most other parts on the bike, in normal use.

cheers

[fixerupper]

Sounds good to me Brucey..thanks for that ..

[MikeDee]

I just built a set of wheels with these hubs. Ugh...

In my opinion, Shimano freehubs I have exhibit a lot of play in the bearings and come that way new.

I broke an old 9 speed LX freehub so these problems are not limited to current offerings.

I've never heard of anyone bending or breaking an aluminum axle in these XT hubs. Anyone seen that?

[Brucey]

I just built a set of wheels with these hubs. Ugh...

In my opinion, Shimano freehubs I have exhibit a lot of play in the bearings and come that way new.

I agree they are often like this, and this increases the chances of pawl breakage. IME they give their best performance when the freehub bearings are adjusted to give no play. IME this only needs to be done once, provided you then keep the freehub body properly lubricated; the wear rate is least when there is no free play in the bearings.

Note that when there is play in the freehub bearings, the rear seal in the freehub body can leak, allowing water inside. In a failed freehub It isn't always clear exactly where the water has got in, but I recently found a 'smoking gun' that finally proved that it can be from the rear side (only) to me; a freehub that had failed, that leaked oil out of the back when oil was added, that was only rusted on the rear bearings, that didn't leak oil when rebuilt, even though I used the same seal and other parts.

I broke an old 9 speed LX freehub so these problems are not limited to current offerings.

but it is (outside of trials riding etc) quite a rare thing. Any details?

I've never heard of anyone bending or breaking an aluminum axle in these XT hubs. Anyone seem that?

Not yet, but then many of the hubs with aluminium axles have had lives that are limited by freehub body failure.

BTW a version of the 'long pawl' now appears in other hubs, eg various road hubs. For whatever reason it appears to be more reliable in those applications.

cheers

[MikeDee]

I noticed that my 780 freehub has very little free play in the bearings so perhaps Shimano is paying attention to this problem.