It's a cheapo, mass produced MTB with all entry level running gear that is used exclusively on the road. The QR wheel spindle snapped on him one day.

The shop replaced it with a solid one under warranty which has now also snapped (bike is out of warranty now). He asked me to have a look at it. As far as I can make out, the wheel is in square (it's snugly into both dropouts). What I did notice is that he had it on the smallest chainring and on the smallest rear sprocket. When I asked him, he says he does that quite a lot.

It has an 7 speed rear hub and it isn't exactly compact so that makes the chainline pretty far out.

My theory is that stomping on the pedal with the chain in that position must be putting a fair bit of twisting force on the back end of the bike. Reckon this could be the reason for the snapped axles?

I've told him to avoid the big-big and small-small gear combinations in any case.

I don't think that is possible. He must be jumping curbs or the axels are incredibly cheap and brittle. In 50+ years of riding MTB, Road, Track, and cyclocross I have never broken an axel...bent a few on my MTB but, never broke anything on anyother bike. My wt has varied from 200 to 240 lbs and I am very rough on the MTB and crooss bikes. Ordinary riding shouldn't cause the axel to break even if he is Conan the Barbarian
Where is it breaking? in the center of the hub or on the dropouts?

From a purely mechanical theory aspect, if some tolerance is loose enough that something can move relative to the QR, then it is very possible to stress fracture the QR.

For example, if the sleeve that the QR goes though in the wheel bearing was too big and the QR could move around in it, then that movement repeatedly will eventually break the QR.

Not saying that is the problem, but you need to look at the fit up of everything and make sure it is tight and aligned correctly when the QR is tensioned. If it is, and I am betting it is not, then the post above is correct - you should not be able to snap a QR shaft and not break or bend something else along the way.

To be honest, I'd have thought any real abuse would have knocked hell out of the wheel itself which seems straight, it's not the highest quality item. He's a young lad and was hooked by a bike that looked good rather than one that was well made (or appropriate for what he's using it for).

The original axle was almost certainly a cheaply made one. This latest solid one looks and feels like a better quality item though (in so far as you can tell, 2-piece track nuts and suchlike).

I'll help him fit a new axle. I suspect his bearing adjustment may leave something to be desired too.

Oh, and it snapped more or less in the middle of the axle both times. Drop the wheel out of the frame and you can pull each end out of the hub.

People unclear on the question: me.

You say that "the QR wheel spindle snapped". I assume you mean the rod that runs through the hollow axle and has a cam on one end and a nut on the other? "The shop replaced it with a solid one".. The QR spindle (rod) again?

But those aren't axles. So "breaking rear axles" and "snapped axles" makes no sense.

Where are whatever-they-are breaking?

Beyond that, I'll refrain from more output until I get input with the correct terminology.

The combination he uses would tend to put the highest possible bending force on the axle which can come from chain tension.

Axles usually fail due to fatigue, that is, the metal gets brittle from being repeatedly stressed and relaxed. So his habit of smallest ring and smallest sprocket, curb hopping, and so on are what do it. It adds up long enough, then breaks.

no, not metal fatigue.
while cross chaining does add some twist, it's not as strong as hopping curbs or hitting obstacles hard.

https://en.wikipedia.org/wiki/Tensile_strength
it most likely snapped or bent from stress exceeding tensile strength, which is yield strength and ultimate strength.


if the bike has has a freewheel type wheel, that could be the cause. the bearings are placed too far in on multi speed freewheel wheels, which get more leverage to snap or bend the axle.

Ditto. Visit the park tool site, they have an excellent glossary on bike components. I am not sure what is breaking either.

Or post a couple of good pics of the broken parts.

Of course it's a freewheel hub. Based on his bike description what else would it be?

Quasi-technical word salad aside, of course it's fatigue or else it would break right away. Pepple cyclocrossed and MTB'd on freewheel hubs for years, downhilling on solid frame bikes.

Those hubs were a problem, though. They started changing to freehubs in the late 70s and they are universal on all but the cheapest bikes now. All you can do is not cross chain, don't curb hop, and replace the axle as soon as it shows signs of bending. Live with it, many did for a long time. View the rear axle as a replaceable part like a tire or brake pad. And don't ride around on a bent one or the pitting from the cones will destroy the wheel's cups, necessitating a new rear wheel. (bent axles cause bearing misalignment.)

Well, I have always (correctly) called the part the wheel rotates on a wheel spindle. It seems to cause confusion so I will henceforth referr to it (equally correctly) as an axle.

The first one was hollow and fitted with a quick release device. That axle broke between the two bearing cones. The QR skewer bent but did not break.

It was replaced with a solid axle attached to the frame with two nuts instead of a QR device. That also broke in the same place.

Perhaps the rear frame dropouts are misaligned. When the wheel is tightened against the misaligned dropouts, the dropouts cause a small bend and strees on the axle.

Does the rider weigh 400 lbs?

Freehub or freewheel? If freewheel, how many speeds?

And yes, check that the dropouts are parallel.

+ 1,234,512

Get a freehub wheel. We usually recommend this anytime someone brings a bike in for broken axle on a freewheel. Try as we might to tell them that even though it costs more now than just a new axle, it will save them money and headaches in the long run. Typically after the 4th to 5th broken axle + repair labour, they buy the freehub wheel. Ends up costing them twice as much cumulatively than if they just bought the freehub wheel in the 1st place.

So... tell your friend to buy a freehub wheel and get the dropouts aligned while he's at it. Problem solved.

This was my guess, too. I'd have a shop check the alignment of the dropouts before doing anything else. It sounds like the likely culprit. If it's not, then we can proceed with the diagnosis for other possibilities. But that's where I'd start. So to the LBS with the proper tools*, and then proceed. Unless you/friend have the proper tool and know how to deploy it.

* Here is one brand of the tool needed:

https://www.biketoolsetc.com/index.cg...item_id=CU-011

Something to check. I doubt any of the local bike shops have such a tool. They're pretty useless.

There is however a precision engineering guy up the road who I'm sure will be able to check the alignment using his jig setup (designed for building motorcycle frames from scratch in) and a good old fashioned plumbline and callipers.

Might even make a new axle out of something funky too. I saw he had some dural (super strong, aircraft grade aluminium) billet in the other day, good chance of a long enough bit being left over from whatever he's doing with it.

Find out by asking them. If they truly don't, they should get out of the bicycle-repair business. This is an, albeit rather expensive, common tool that any bike-shop should have. Without such - any dropout straightening job would be: a. Not even attempted. b: Done with a wrench using brute force. Phooey.

Please ask and post? Thanks.