Author Topic: Learning opportunity: busted rear hub  (Read 645 times)

Learning opportunity: busted rear hub
« on: December 25, 2020, 08:06:38 pm »
(More precisely: the the freehub body dust cap and the driveside cone.)

I'm working on a bike for a local charity and, in the time-honoured project creep of such things, "sorting the shifting", turned into "cleaning the drivechain", turned into "servicing the rear hub", turned into "hmmm, that's not right".

Something was stopping the cassette removal tool from seating properly - I reckon is was the freehub body dustcap causing that:





Then on further inspection the bottom edge of the driveside cone is damaged, as are some of the threads:



The 8 non-driveside bearings are dull, but could still reasonably be described as silvery.
The 9 driveside bearings are gunmetal grey.
The grease in the middle of the hub (away from the bearings) looked reasonably fresh (the bike could recently have passed through the hands of one of the other volunteers).

I did have one bearing make a bid for freedom when I took the axle out of the hub, but I'm pretty sure it was just the one.

The charity will almost certainly be looking to just replace the wheel with something they'll have hanging around, but I've not come across failed hubs before, so I'm up for a but of learning - is this sort of damage in line with missing a bearing in one side, or do you think there's something else been going on?

Re: Learning opportunity: busted rear hub
« Reply #1 on: December 25, 2020, 08:43:42 pm »
You said there are 9 balls on DS and 8 on NDS, so the badly pitted cone on DS can not be the result of a missing ball!

There is one trick that once worked for me at resuscitating a desperately pitted hub:

-pack the races with polishing compound,
-reassemble everything and slightly over tighten the nuts,
-turn the wheel by hand for some time
-disassemble and clean up everything,
-pack with fresh grease, and fit new balls, preferably slightly oversized ones.

A


Re: Learning opportunity: busted rear hub
« Reply #2 on: December 25, 2020, 11:13:08 pm »
that cone is toast and when they are that bad there is a fair chance that the cup is too.  So new cone(s) required, and the RH seal can usually be replaced by this (better) part

https://www.sjscycles.co.uk/hub-spares/shimano-fhm495a-rear-right-seal-ring-y3cr08000/

cheers

Paul

  • L'enfer, c'est les autos.
Re: Learning opportunity: busted rear hub
« Reply #3 on: December 25, 2020, 11:44:25 pm »
That’s interesting damage. The shiny bits suggest to me that things that should have been moving weren’t, and things that shouldn’t have been touching were, but I can’t quite figure it out.

Could it have been so badly assembled that the cup/cone were not in contact with the bearings?
What's so funny about peace, love and understanding?

Re: Learning opportunity: busted rear hub
« Reply #4 on: December 25, 2020, 11:58:56 pm »
That’s interesting damage. The shiny bits suggest to me that things that should have been moving weren’t, and things that shouldn’t have been touching were, but I can’t quite figure it out.

Could it have been so badly assembled that the cup/cone were not in contact with the bearings?

If the bearing gets slack enough the spindle rubs on the inside of the hollow bolt that secures the freehub; I think that is what has happened here

cheers

Re: Learning opportunity: busted rear hub
« Reply #5 on: December 26, 2020, 11:23:49 am »

Just went to have a closer look at the cups and there was the rogue bearing, sat in the non drive side cup, happy as you like.  ???

Anyway, yes, I'm not tempted to put the cones back into service, and I think I'll probably choose my battles and try and get them to buy a new chain for the bike before anything else. Let's hope there's an equivalent wheel knocking around that we can swap in.

Thanks for the tip about the dust cover upgrade, Brucey. Looking at the state of the drive side cup, it would probably warrant a whole new freehub body, as well as replacement cones and bearings. Cheap compared to the cost of a new wheel, but expensive in terms of what the charity usually spends on parts.

The French Tandem's suggestion of polishing compound seems very gentle and reasonable after finding a blog post last night where some people were describing the use of an angle grinder to smooth out pitted cones!

I've not seen the tapered end of a cone fractured off like this one before - that's about 2mm gone off the end and there's another section similar to that, maybe 50% of the circumference in total. Is that a typical failure mode? I've tried googling to see if others have posted similar, but it's one of those where I'm not really sure what words to use to describe it.

Could it have been so badly assembled that the cup/cone were not in contact with the bearings?

I couldn't possible comment. (But a volunteer-powered effort with varying skill levels and negligible quality control...)


If the bearing gets slack enough the spindle rubs on the inside of the hollow bolt that secures the freehub; I think that is what has happened here

Good call - I took the freehub body off and the worn area on the axle threads is an exact match for the length of the bolt, which is worn on the splines.

I guess if the cones weren't tightened in on the bearings enough that might have allowed for the bearings to sit down on the thinner end of the cone where it wasn't strong enough to take the force. The bike would have been heavily loaded with it's most recent user, as well.

Both the headset and bottom bracket are loose - looking forward to more learning opportunities there!




Re: Learning opportunity: busted rear hub
« Reply #6 on: December 26, 2020, 12:59:56 pm »

I've not seen the tapered end of a cone fractured off like this one before - that's about 2mm gone off the end and there's another section similar to that, maybe 50% of the circumference in total. Is that a typical failure mode?

Not at all unusual when someone is determined to ride a hub to destruction....

If the bearing gets slack enough the spindle rubs on the inside of the hollow bolt that secures the freehub; I think that is what has happened here

Quote
Good call - I took the freehub body off and the worn area on the axle threads is an exact match for the length of the bolt, which is worn on the splines.

Note that the shimano axle will have started life with truncated threads  (where it passes through the hollow bolt) anyway. Most of what you see is how the axle was made. However there is also a patch that looks like it has worn, near the cone.

Once the hub  bearing goes  a little slack (for whatever reason) the (varying) chain tension means that the balls in the RH side of the hub won't just roll around the cup and cone; they will scuff too and it is the scuffing that starts the 'death spiral' moving ever faster. 

cheers

Re: Learning opportunity: busted rear hub
« Reply #7 on: December 26, 2020, 02:33:59 pm »
Yeah, there are a couple of those worn patches along that area of axle. Noted about the truncated threads though - thanks.