Shimano XT 780 Hubs.

[hayers]

Have these hubs on my bike - ok so far - about 3.5 years... having read above will plan to replace when i wear the rims out assuming they last that long!

Has anyone seen any warning signs of imminent failure i should look out for?

[Gattonero]

No apparently not. There is a post for Shimano LX rear hubs, which is a bit dated at 2010 but seems to indicate the LX are better than XT. Seems everything is better than XT from what I can glean now with hind sight.
Thanks to you Brucey as well. I will try to be patient and wait if I can get a freehub under Warranty but the long term reliability issue will still be there. You are probably right about the pauls chipping as it sounds correct and the bits to jam the hub have to come from somewhere.
I am not a fan of cup and cone hubs but allowed price in this instance to rule my decision. I may decide to remove the hub and send it back for a refund which is an option with Chain Reaction and have a sealed bearing hub, one recommended by my LBS. The loss for this mistake will amount to about £50 to £60 for the labour and spokes. Buy cheap buy twice wins again.

In fairness Shimano hubs are usually reliable. All my touring and commuting has been done on Shimano freehubs and I've never had a failure. My weight varies from 15-17 stone and I use a low gear of 22x34 so with a loaded tourer I'm getting a fair amount of stress on the hubs.

I've used a mix of 2008 XT 770s and various lower end hubs.

I suspect if there has been a huge increase in failures then it was a recent design change. You may just have been unlucky.

I have read that the LX is a steel axle rather than the aluminium axle in the XT which was why I went LX this time. I prefer steel for highly stressed components.

The problem is not the alluminium axle, it is the freehub itself.
XTR or D.A. class hubs are unlikely to suffer the same failure, they still use the O/S alluminium axle but have a Ti freehub

Buy cheap buy twice wins again

Quick assumptions are never very useful. In fact, LX hubs are relatively cheap, the difference is in the proven original design, not in the price.

[hamster]

Quite: LX / SLX are cheaper than XT and are proven in the field to be more durable.

[NATURAL ANKLING]

Hi,
There is nothing wrong with most of the cheap shimano freehubs even alivio, breaks with freehubs.......just put them in the draw with capag ones
You have to go back to freehubs with no hollow bolt, this was short lived as shimano added the bolt very soon, but they still did not break just got a bit drunk as the freehub body fretted the aluminium hub and no steel washer there too.
As said even the copys are robust.

If it ain't broke................
I find it frustrating to constantly update designs just for marketing's sake, and you've met them types who always have to have this years model just because then can.........................

[Gattonero]

More than just marketing, I think it was weight reduction.

For example, Campag hubs got a lot better (wider surface of the cones, easier maintenance, lighter) when moving from steel to alluminium axle, because of a better design

[Brucey]

The problem is not the alluminium axle, it is the freehub itself.
XTR or D.A. class hubs are unlikely to suffer the same failure, they still use the O/S alluminium axle but have a Ti freehub
.

Indeed no-one is saying that the aluminium axle is itself liable to fail (although it does come with a very crappy adjustment system that encourages the wrong setting, thus often vastly reducing the life of the bearings), but those hubs which use the aluminium axle (rather than the steel one) are the ones which are likely to have the freehub bodies which are liable to fail. Anything with a steel axle does not have this particular freehub problem.

IIRC the XTR/DA freehubs use a variant of the same design as breaks in the XT models. If/how/why they don't break, I cannot say for sure, but if my theory is correct, it may be as little as a tiny additional machining operation on the pawls.

As to 'why aluminium axles?' -yes, weight, but also it facilitates the manufacture of hubs with through-axles.

cheers

[Gattonero]

Well, for sure there is less space: the OD of the freehub is the same so an O/S axle leaves less room.
I wonder if there is something related to this and the pawls travel and engagement? IIRC the new XT hubs have a faster engagement like the XTR/D.A.?

[Brucey]

Well, for sure there is less space: the OD of the freehub is the same so an O/S axle leaves less room.
I wonder if there is something related to this and the pawls travel and engagement? IIRC the new XT hubs have a faster engagement like the XTR/D.A.?

yes they are (IIRC) very similar inside; four pawls, each with two 'ears' that the springs sit in.

In terms of pawls and loading, one would expect the new design to be stronger (than the old design with two pawls as used in the hubs with steel axles), so I think they screwed up with some small detail.

cheers

[NATURAL ANKLING]

Hi,
I don't have the privilege of seeing all this stuff / work in a shop.
But this section is rather thin, this must have a heavy bearing on the reason for failure! this edge is bottom of the pawl scoop and sees all that load from the pawl.

[hamster]

The problem is not the alluminium axle, it is the freehub itself.
XTR or D.A. class hubs are unlikely to suffer the same failure, they still use the O/S alluminium axle but have a Ti freehub
.

IIRC the XTR/DA freehubs use a variant of the same design as breaks in the XT models. If/how/why they don't break, I cannot say for sure, but if my theory is correct, it may be as little as a tiny additional machining operation on the pawls.

IIRC the Dura Ace ones have the same hub concept as Campagnolo (and most others) with the pawls inside the hub shell and a long unsupported axle. It's lighter but suffers the similar weaknesses of Campag axles.

[Brucey]

....But this section is rather thin, this must have a heavy bearing on the reason for failure! this edge is bottom of the pawl scoop and sees all that load from the pawl.

aye, but they are all like that (even the ones that come from hubs with steel axles). I think the 'bursting' of the freehub body happens after the pawls break up; huge forces can be generated by the wedging action.

cheers

[NATURAL ANKLING]

Hi,

More than just marketing, I think it was weight reduction.

For example, Campag hubs got a lot better (wider surface of the cones, easier maintenance, lighter) when moving from steel to alluminium axle, because of a better design

Do aluminium axles exist without a steel through axle If its got a steel though part then wheres the weight saving
Maybe yes there are aluminium through axels, you used one.
As I said I have little experience with all this modern stuff (stuck in the 80-90's)
http://www.cycleworld.com/2014/04/18/as ... inum-axles

[Brucey]

IIRC the Dura Ace ones have the same hub concept as Campagnolo (and most others) with the pawls inside the hub shell and a long unsupported axle. It's lighter but suffers the similar weaknesses of Campag axles.

IIRC FH-7900 and FH-9000 use the same scheme as the XT hubs.

http://www.shimano.com/media/techdocs/content/cycle/EV/bikecomponents/FH/EV-FH-9000-3327_v1_m56577569830805688.pdf

http://bike.shimano.com/media/techdocs/content/cycle/EV/bikecomponents/FH/EV-FH-7900-2875A_v1_m56577569830728255.pdf

I think that shimano have gone to a different design for some of their DH kit, but not elsewhere?

cheers

[hamster]

You are right, it was 7800 that was the Campag-like design.
http://si.shimano.com/php/download.php? ... -2252A.pdf

Interesting that they keep chopping and changing!

[Brucey]

indeed.

There is also FH-7850, which is another bolted freehub

http://www.shimano.com/media/techdocs/content/cycle/EV/bikecomponents/FH/EV-FH-7850-2714A_v1_m56577569830648615.pdf

Now, also, one of the 10s Dura-Ace hub designs developed a reputation for cracking the DS flange, but I don't remember which one....

cheers