Shimano XT 780 Hubs.

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MikeDee
Posts: 724
Joined: 11 Dec 2014, 8:36pm

Re: Shimano XT 780 Hubs.

Postby MikeDee » 9 Jun 2018, 7:22pm

I just went through my WH-6800 Ultegra wheel freehub. Took out one shim and now there's no play in it and it doesn't bind. I took off the freehub from the wheel and found a lot of thread that my Continental tires had shed under the freehub by the rubber gasket seal.

MikeDee
Posts: 724
Joined: 11 Dec 2014, 8:36pm

Re: Shimano XT 780 Hubs.

Postby MikeDee » 17 Jun 2018, 1:55am

Gattonero wrote:I don't get where this is going :roll:
We've started from a Shimano (steel) freehub failure, and now it comes that alluminium alloys are not suitable for bicycle manufacturing because "no fatigue life" (sentence that sound like an absolute statement) ?
We're comparing oranges to apples here, as clearly my
"weight like per like". Of course, alluminium alloys are not suitable for applications where space is a constraint and a lot of shear force is exerted
has not been considered. This shows how a discussion can be manipulated to provide only one's user opinion. Not exactly what Forums are for, exchange of opinions.

Anyway, let's cool off. I'll try to give a more useful contribution to the OP with this faulty Shimano 780 freehub that came into my hands right yesteday.
I've been lucky to be allowed the use of a proper workshop. The hub has been quickly laced to an old rim with old spokes, for easy removal of the jammed freehub.

The freehub would rotate only by a few degrees, and won't move even with strong force applied.
Once opened, the upper race looked okay. I'm not sure if one of the ball-bearings is missing?
Image

No real contamination to be seen, but that pawl is not coming up, not good
Image

Wow, the inside of the freehub has deep marks on the ratchet ring
Image

Image

Think I've found the culprit.
That piece of pawl is supposed to be down in the pocket and attached to the pawl, to get the pawl spring acting on it
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Here it is, broken
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And the opposite one too
Image

It looks like the design can be improved, the pawls have a tab where the spring acts on.
The freehub core has 4 pawls, and two springs. Each spring acts on two pawls only, I assume this is to due to space constraint, as the spring(s) won't be able to act on 4 pawls at the same time.
It is no surprise that most of the 4-pawls freehubs designs I've seen, they use a spring per each pawl.

But I'm not engineer, so this is only my view on it. It could be just a batch of faulty material, wrong hardening of the pawls, or else.
Having a few friends in club rides that do use Ultegra 6800 hubs (similar to this) with no issues even after many long rides and Audax in all weather, I think they've sorted the issue?


We are focused on the broken pawl being the cause of the failure, but what if one of the ball bearings (one looks like it's missing, as you said) fell inside and jammed and broke the pawl? Excessive play in the freehub might cause this. Only those thin shims keep the balls from going in. Did you see a crushed bearing in there?

Brucey
Posts: 43048
Joined: 4 Jan 2012, 6:25pm

Re: Shimano XT 780 Hubs.

Postby Brucey » 17 Jun 2018, 9:47am

MikeDee wrote:I just went through my WH-6800 Ultegra wheel freehub. Took out one shim and now there's no play in it and it doesn't bind. I took off the freehub from the wheel and found a lot of thread that my Continental tires had shed under the freehub by the rubber gasket seal.


that is a common issue with conti tyres; the chafer material, er, chafes away and it can get into all kinds of stupid places. Slack in freehub bearings is invariably a bad thing; it is good to have got rid of it, and once play-free, the wear rate also drops dramatically and the sealing improves; if the freehub body isn't flapping about the seal can work properly, whereas it can leak lube out and water in if is moving about.

We are focused on the broken pawl being the cause of the failure, but what if one of the ball bearings (one looks like it's missing, as you said) fell inside and jammed and broke the pawl? Excessive play in the freehub might cause this. Only those thin shims keep the balls from going in. Did you see a crushed bearing in there?


IMHO there isn't a missing bb (that gap is normal and probably isn't big enough for another ball and smooth operation anyway) and once the freehub is assembled the balls cannot escape; the curvature of the races holds them in position. I've dismantled hundreds of freehub bodies and I have never seen a ball that has escaped in service. If there were a crunched ball I'd expect to see lots more metal particles in the grease.

FWIW I have recently stripped a shimano road freehub body and I found within a conventional pawl layout (two pawls set at an angle of ~120 degrees) and each pawl was the 'long' type (more or less as seen in the 780 freehub) but with only one 'ear' and only one spring. This unit had survived being run with water in the bearings (i.e. with lots of slack in the bearings) and the pawls hadn't broken. I am not sure what if any detail differences there are in these pawls or their seatings; I may strip it down again and take another look at it.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

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SimonCelsa
Posts: 793
Joined: 6 Apr 2011, 10:19pm

Re: Shimano XT 780 Hubs.

Postby SimonCelsa » 8 Nov 2018, 11:06am

Ceased using one of these hub in July this year after approx 2yrs/8500 miles.

It was making increasingly frequent loud 'cracks' so swapped it out for the LX version.

Just got around to overhauling it but found one of the pawls bust.....from my limited google searches it seems that a whole new rear hub is cheaper than obtaining spares.

I was once on a medium sized ship with a 4 bladed prop, leaving Venezuela one of the blades dropped off & the whole ship nearly vibrated itself to pieces from the imbalance. I was wondering whether I could rebuild the freehub with just the 3 pawls!!!???

XT780 fhub.jpg

Brucey
Posts: 43048
Joined: 4 Jan 2012, 6:25pm

Re: Shimano XT 780 Hubs.

Postby Brucey » 8 Nov 2018, 12:15pm

the idea behind all shimano freehubs (apart from the one that was fitted to FH-HF08 tandem hubs etc) is that two pawls share the load at any one time. So from their inception freehubs had two pawls (that were offset to one side, closely resembling two from three evenly spaced ones) that both engaged at the same time.

This offset arrangement has the advantage that it is moderately tolerant of a small amount of slack in the freehub bearings; under pedalling torque the whole lot gets pulled to one side and the freehub body doesn't move around and make funny noises.

With the newer freehubs (pretty much all of them which have large diameter axles) they have a different arrangement; on the offroad versions there are four pawls but they are only ever engaged two at a time, in opposing pairs. This gives half the lash on engagement, which some folk (IMHO mostly with weird needs and/or an uncultured pedalling style) say they prefer. Anyway this means that if only three are fitted, sometimes only one pawl will bear the full load.

Probably it would be better to rebuild it with two pawls (a single opposed pair), which would give more lash, but a more secure drive.

BTW the 'new' designs of freehub and hub appear to be borne out of a naïve lack of understanding about how these things actually work and what is needed to adjust them correctly. Hence the hub bearings appear to be easy to adjust, but are actually not, not if you want to do a proper job, and the placement of pawls desgned like that in opposed pairs leaves the freewheel mechanism vulnerable to faults should the slightest slack appear in the bearings. Slack in the bearings is almost inevitable; most hubs have some slack in from new and the rest soon develop some in most cases. The slack causes two things;

1) in combination with the opposed pairs of pawls, the freehub body to move around under load (which makes a clunking sound, which varies with the gear selected) and

2) the ears to break off the pawls. This arises because the pawls are loaded at a funny angle and the back of each pawl is not relieved in the right way. This results in loads on the pawls which break the ears off.

So in play-free freehub bodies, it sort of works, but in freehub bodies with the slightest free play, breakage is quite likely under high torque conditions. Such conditions are seen in MTB and touring uses. The design in comparable 'road' freehub bodies is slightly different and seems less vulnerable to breakage, in part because the conditions of use are different too.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

mgurski
Posts: 1
Joined: 22 Apr 2020, 3:28am

Re: Shimano XT 780 Hubs.

Postby mgurski » 22 Apr 2020, 3:45am

I am thankful for all the analysis of the xt 780 rear hub. However, my experience with this hub has been totally problem-free, after 10 years of fully loaded touring through the mountains of France, and Italy, hills of England and Germany and the coast of California etc. Two things I do each season is take apart, clean and repack the hub and then adjust adjust adjust. Also I run a 22 front 36 rear sprocket. I am at the point where my rims need replacing due to wear but the hub is great. My wife has the same hub for past 5 years, same thing, annual cleaning. So sad to hear all the problems others have had. I also ran a bike repair business for 6 years and saw a few failed rear hubs, but not this one.