Shimano XT 780 Hubs.

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Gattonero
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Re: Shimano XT 780 Hubs.

Postby Gattonero » 19 Jan 2017, 8:19am

hamster wrote:
Brucey wrote:
Gattonero wrote:
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.


To be fair, I never saw or heard of a broken or bent Campagnolo o/s alluminium axle!
There may be premature wear of the Rh cone/race
It is by riding a bicycle that you learn the contours of a country best,
since you have to sweat up the hills and coast down them.
Thus you remember them as they actually are...

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NATURAL ANKLING
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Re: Shimano XT 780 Hubs.

Postby NATURAL ANKLING » 19 Jan 2017, 11:59am

Hi,
Well I know nothing as I said earlier :)
I would of thought alu axels like what shimano use would not fair to well with your diy mechanics.
Its been said before the saving in weight of a few even hundred grams pales into insignificance if you just get a little fitter, so unless you are an off road Olympian moneys wasted.

If you run a shop I am sure they are queuing round the block for new lines :)
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Brucey
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Re: Shimano XT 780 Hubs.

Postby Brucey » 19 Jan 2017, 12:18pm

Gattonero wrote: To be fair, I never saw or heard of a broken or bent Campagnolo o/s alluminium axle!
There may be premature wear of the Rh cone/race


that may be because

a) no-one is mad enough to fit one to a loaded touring bike/tandem

and/or

b) something else will break first e.g RH bearing , freewheel body, etc etc

They are not very strong hubs, not in a touring context.

cheers
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Gattonero
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Re: Shimano XT 780 Hubs.

Postby Gattonero » 19 Jan 2017, 10:19pm

It's all relative, isn't it?
I've been out for 4 days, unsupported, on Campagnolo Neutrons. I'll have no problem to use Record or Chorus hubs, too; I pack light and bring only what I thing I'll really need, all in all is about 90kg (72kg rider +11kg bike +8kg bags with food) so it's all a matter of weight and use made, as I am quite "gentle" with my equipment, although I don't chicken out whatever the road throws.
Then, of course, some people can be quite "heavy" not in weight (no disrespect intended) but rather as far as riding style. Nothing would really survive with them :(
It is by riding a bicycle that you learn the contours of a country best,
since you have to sweat up the hills and coast down them.
Thus you remember them as they actually are...

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NATURAL ANKLING
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Re: Shimano XT 780 Hubs.

Postby NATURAL ANKLING » 20 Jan 2017, 12:17am

Hi,
A basic steel freehub will survive anyone.
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Gattonero
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Re: Shimano XT 780 Hubs.

Postby Gattonero » 21 Jan 2017, 10:10am

Yes and no.

I am surprised that people would still believe quick assumptions like "alluminium does always crack, steel is eternal, carbon fiber always cracks...". A quality alluminium alloy is far better than a cheap "steel" when properly designed, manufactured and used.
I've lost the count of how many cheap steel freehubs, most shimano-copies, I've seen implode in one year or less :?
It is by riding a bicycle that you learn the contours of a country best,
since you have to sweat up the hills and coast down them.
Thus you remember them as they actually are...

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Re: Shimano XT 780 Hubs.

Postby Brucey » 21 Jan 2017, 11:30am

Gattonero wrote:Yes and no.

I am surprised that people would still believe quick assumptions like "alluminium does always crack, steel is eternal, carbon fiber always cracks...". A quality alluminium alloy is far better than a cheap "steel" when properly designed, manufactured and used.


Aluminium is a wonderful material but it has no fatigue limit, and certain grades corrode horribly in some circumstances. Using it for things like axles and spokes in lightweight parts is fine if you don't mind them having a relatively short life.
I've lost the count of how many cheap steel freehubs, most shimano-copies, I've seen implode in one year or less :?


Not sure what you mean by 'implode' but I too have seen very many freehubs (of all makes in the shimano style) fail. However (outside of the XT debacle) I've seen very few that actually broke for no good reason; nearly all of them went bad because they were not lubricated properly and/or the water got in.
If such hubs are lubricated and adjusted correctly they can last for many years, even non-shimano ones. Of allegedly failed ones, the number I have seen of genuinely unrepairable freehub bodies is still in single figures, despite the fact that I have overhauled hundreds of them.

cheers
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NATURAL ANKLING
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Re: Shimano XT 780 Hubs.

Postby NATURAL ANKLING » 21 Jan 2017, 11:37am

Hi,
I for one would like to see the remains of the imploded assemblies, so would someone else here.

You could post some pics for us?
Edited-
I was going to say the ones that have poor lubrication...............but someone beat me too it...........................
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Bowedw
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Re: Shimano XT 780 Hubs.

Postby Bowedw » 21 Jan 2017, 5:59pm

With regard to aluminium axles I have ridden thousands of miles on the old Mavic 501 with the screw on blocks carrying fairly heavy loads and never had any trouble with them. Those hubs had far more length of unsupported axle than modern hubs.
Also can anyone explain why a solid axle is more prone to bend than hollow quick release one. I have been told it's because the QR compresses the axle but on a solid one the nuts stretch the axle, is this a reasonable assumption.

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Re: Shimano XT 780 Hubs.

Postby Brucey » 21 Jan 2017, 6:54pm

Mavic 501 axles are not too bad but they can break without doubt.

They actually have somewhat less unsupported axle than modern cassette hubs built in the campagnolo style, i.e. with the freewheel body mounted to the axle directly on its own bearings. In fact such hubs often have the RH hubshell bearing in an almost central location (i.e. further left than you might expect, because the pawls and the ratchet live in the RH end of the hubshell, and the bearing lies to the left of them); the bending loads on the axle are horrendous.

Solid axles are usually built in 3/8" x 26tpi size whereas QR ones are typically 10x1mm threaded. The solid axles are often found in cheaper hubs and the materials used may well not be of the same quality. The compressive preload of the skewer means that the axle is not loaded in pure bending; since the worst fatigue damage is caused by tensile stresses (rather than compressive ones) the compressive preload helps to stop fatigue cracking.

3/8" is a gnats over 9.5mm. It is easy to overlook such a small difference in diameter, but a solid 10mm axle is actually 21.5% stiffer and stronger than a 3/8" one.

cheers
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Des49
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Re: Shimano XT 780 Hubs.

Postby Des49 » 21 Jan 2017, 10:31pm

Brucey wrote:Mavic 501 axles are not too bad but they can break without doubt.


Mine used to break all too often. Lovely hubs, apart from the Al axle! I broke more than a couple of rear axles and still have a wheel hanging up in the shed as it became tricky to get a spare. The wheel was only used for training purposes, not for loaded use.

I personally don't think Al axles have a place for long term usage, especially for touring reliability.

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Gattonero
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Re: Shimano XT 780 Hubs.

Postby Gattonero » 22 Jan 2017, 11:22am

Brucey wrote:
Gattonero wrote:Yes and no.

I am surprised that people would still believe quick assumptions like "alluminium does always crack, steel is eternal, carbon fiber always cracks...". A quality alluminium alloy is far better than a cheap "steel" when properly designed, manufactured and used.


Aluminium is a wonderful material but it has no fatigue limit, and certain grades corrode horribly in some circumstances. Using it for things like axles and spokes in lightweight parts is fine if you don't mind them having a relatively short life.
I've lost the count of how many cheap steel freehubs, most shimano-copies, I've seen implode in one year or less :?


Not sure what you mean by 'implode' but I too have seen very many freehubs (of all makes in the shimano style) fail. However (outside of the XT debacle) I've seen very few that actually broke for no good reason; nearly all of them went bad because they were not lubricated properly and/or the water got in.
If such hubs are lubricated and adjusted correctly they can last for many years, even non-shimano ones. Of allegedly failed ones, the number I have seen of genuinely unrepairable freehub bodies is still in single figures, despite the fact that I have overhauled hundreds of them.

cheers


Well, to say that "alluminium has no fatigue limit" is, in fact, to confirm what I said: quick assumptions based on taboos, "alluminium always cracks" seems some people worse fear? I don't get it.
Alluminium alloys DO have a fatigue limit, just like steel alloys there are several levels of those limits, which are always to be considered with the "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, a pedal axle comes to mind.

In this specific case of Shimano XT780 hubs, ironically we are talking about a freehub that gives problems, and is made out of steel :mrgreen: while the alluminium axle is only suspected to be culprit until proven wrong: if had "not fatigue limit" how comes that cracks or other catastrophic failures are unheard for those axles??

Regarding cheap steel freehubs, yes most of them they do fail, imploding pawls and springs due to failure of the bearings.
Again, this magic steel material does rust just as some (not all!) alluminium alloys do corrode. It's a shame that your fine education tends to give a biased opinion on this.
It is by riding a bicycle that you learn the contours of a country best,
since you have to sweat up the hills and coast down them.
Thus you remember them as they actually are...

Brucey
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Re: Shimano XT 780 Hubs.

Postby Brucey » 22 Jan 2017, 11:56am

Gattonero wrote:
Well, to say that "alluminium has no fatigue limit" is, in fact, to confirm what I said: quick assumptions based on taboos, "alluminium always cracks" seems some people worse fear? I don't get it.
Alluminium alloys DO have a fatigue limit,


if you "don't get it", possibly you misunderstand what is meant by 'fatigue limit'. This is engineering parlance. Steel, Ti, and a few other metals have the property whereby they can withstand cyclic loads indefinitely provided the stresses don't exceed a certain fraction of the yield stress. Aluminium does not exhibit the same feature when tested in the same way. There are very many engineering applications where aluminium would be used were it not for this feature.

In this specific case of Shimano XT780 hubs, ironically we are talking about a freehub that gives problems, and is made out of steel :mrgreen: while the alluminium axle is only suspected to be culprit until proven wrong: if had "not fatigue limit" how comes that cracks or other catastrophic failures are unheard for those axles??


the answer to this little riddle is not very challenging; if something else breaks first, you won't get to find other weaknesses.... as it happens the shimano design (unlike the campag etc arrangement) has a low stress in the axle because of the bearing placement, and the fatigue life of the axle might be acceptable. It is just that at this rate, we are never going to find out because the freewheel bodies keep failing.... :lol:

Regarding cheap steel freehubs, yes most of them they do fail, imploding pawls and springs due to failure of the bearings.

as another poster has asked, pictures please..... 'implosion' has a specific meaning that cannot apply to freehubs...

Again, this magic steel material does rust just as some (not all!) alluminium alloys do corrode. It's a shame that your fine education tends to give a biased opinion on this.


actually all aluminium alloys will corrode, just in different circumstances. The fact that steel bearings corrode is arguably irrelevant; they only get to do this if they are not lubricated correctly, and (to date) no-one has manufactured a bearing for use on a bicycle where wear is absent without adequate lubrication. The best that has been done is to slow the wear down by using (expensive) corrosion resistant materials. If water (salty water!) can get it, dirt and muck are not far behind.

With adequate lubrication, all kinds of parts that you might dismiss as 'prone to failure' can have a long and happy service life. I think it is absolutely shameful that bicycle parts of all kinds are not fitted with ports that allow lubricant to be added easily; where these are fitted and used, all kinds of parts that might otherwise suffer (unless they are regularly stripped) will often last for decades.

I also find it incredibly depressing that the seals and lubricants used on bicycle parts (usually at best) just ape automotive practice; these designs only work reliably if the lubricant becomes somewhat fluid in service. This way there is a film of lubricant at the seal lips, without which the seals both won't work and will quickly wear. The greases that manufacturers put into hubs, pedals etc never become sufficiently fluid in service (speeds are lower and so are temperatures) and the lubricants don't have enough corrosion inhibitors in them. The net result is that the seals will fail and soon after so will the component.

cheers
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Gattonero
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Re: Shimano XT 780 Hubs.

Postby Gattonero » 28 Jan 2017, 10:10am

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
Image

Here it is, broken
Image

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?
It is by riding a bicycle that you learn the contours of a country best,
since you have to sweat up the hills and coast down them.
Thus you remember them as they actually are...

Brucey
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Re: Shimano XT 780 Hubs.

Postby Brucey » 28 Jan 2017, 12:13pm

re aluminium; 'no fatigue limit' is the prevailing view amongst engineers and metallurgists. Not the same thing as 'no fatigue life'. You could do worse than to look up 'fatigue limit'.

Your photos are very helpful. I think they show a pretty 'normal' (for this type of failure) pattern, where one or more of the pawl ears has broken off.

FWIW it is normal for one spring to work two pawls; that is how nearly all shimano (and other) freehub bodies as well as numerous other freewheels work.

Also, it is normal for there to be a small gap in the 1/8" ball bearings. Very occasionally it is possible to add another, but it is also possible for the balls to bind against one another if you overdo it; the spacing may change slightly when the bearing is assembled and the balls are contacting at the required angle. A small gap is safest.

I don't think it is an accident that the pawls with left-side ears are prone to breaking, and I think that the reason they fail is a design problem:

When applying high torque in the low gear sprockets, the outer part of the freehub body is likely to skew slightly, by an amount that will vary with the free play in the freehub bearings. This will also cause the pawls to be very unevenly loaded. Specifically the pawls with left side ears will see a huge load right at the corner. I think this is what breaks the ears off.

I think that if the pawls were made just slightly differently, i.e. with a relief grind near the corners, it would no longer be possible to apply such a large force where the pawls are most vulnerable, and that then they wouldn't break.

It isn't clear to me if the lack of reported failures in the 'road' hubs that are built similarly is because these hubs are built differently, or if it is simply that they perhaps don't see such high loads. I suspect it may be the loading issue, because if anything the skewing (load for load) ought to be worse in the high gears, given the way the bearings and pawls are offset to the left within the freehub body; thus, bar the loading issue, the pawls with right side ears ought perhaps to break more easily, yet it seems that left side ears more often break first.

cheers
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