Snap! Guess my rear axle is gone...

annirak

Über Member
Location
Cambridge, UK
I have a Trek FX 7.0. I started hearing a rubbing noise when I pedalled hard last week. It was consistent; only when I was on the downstroke, but it changed pitch based on speed and nothing else. I gave up trying to track it down, eventually, and took it to the LBS.

They say it's a broken rear axle, which fits. The mechanic showed me the problem by wobbling the two sides of the wheel--they do appear disconnected.

Is this a common problem for people riding with panniers? Or did Trek just cheap out on the FX 7.0's axles as well as the rims, hubs, cassette, and fork?
 

fossyant

Ride It Like You Stole It!
Location
South Manchester
Cheaper Hubs can do this.
 

Smurfy

Naturist Smurf
I would've thought that a bike with a broken axle would be almost unrideable. As for the cause, in my opinion breakage is often the result of poorly adjusted cones.
 

adamhearn

Über Member
Freewheels have axles that are prone to bending and even snapping (due to location of bearings to dropout, state of roads, etc.). Myself, I've snapped one and bent another. Strangely the QR skewer did a damn good job of holding it all together! Tom improve realibilty I have swapped the wheel with a freehub.
 
Location
Loch side.
I cannot see how a broken axle is caused by poorly adjusted cones. That one needs some explanation.

I don't know what type of hub is on that bike but rear hubs commonly use threaded 10mm steel axles or oversize (typically 15mm) aluminium axles. The former breaks from a stress riser in the threads of the axle in the unsupported section between the freewheel's inboard bearing and the hub's outboard bearing. Aluminium axles break in the same place but usually at the edge of either one of the two bearings where the bearing can concentrate stresses and create a stress riser. .
The way a stress riser is introduced is by way of chain tension on the hub which bends the axle forward underneath the chain, especially in the higher gears where large torque is produced. The continuous cyclical bending and straightening with each pedal stroke causes the crack to start and propagate.

Often people report a bent axle but upon investigation the axle is not bent, but has a half-through crack in it and it only bends when pedal forces are applied. A
"bent: axle, when removed is perfectly straight and a quick test is to plug the one end, insert the axle in a glass of water and blow on the open end. Bubbles reveal the crack.

A broken axle can also be caused by a frame with non-parallel dropouts. This causes the axle to bend when the QR or axle bolts are tightened. The mechanism however is the same as if it was caused by chain tension.

This can happen to cheap or expensive hubs. High cost does not exclude poor engineering, like found on some high-end stupid-light weight hubs such as Tune.

Maintenance mistakes can also cause breaks in aluminium hubs. If the axle is scored during removal and installation and the score is in that vulnerable area, it will also eventually break.
 
Location
Loch side.
Freewheels have axles that are prone to bending and even snapping (due to location of bearings to dropout, state of roads, etc.). Myself, I've snapped one and bent another. Strangely the QR skewer did a damn good job of holding it all together! Tom improve realibilty I have swapped the wheel with a freehub.
You are right. Also, an 8-speed freewheel is the worse. It has a large overhang and a large unsupported length of axle. It is said that the unreliability of the 8-speed freewheel led to the development of the freehub.
 

gbb

Legendary Member
Location
Peterborough
My 7100FX had Bontrager wheels on it which didn't last that long, I didn't rate them. Fitted a pair of budget solid axled wheels (£35 pair IIRC)...no problem since...which means ironically, the budget, cheap wheels are better than the Bontragers it was originally fitted with.
 
OP
annirak

annirak

Über Member
Location
Cambridge, UK
Sadly, the whole drivetrain is 7-speed, so I don't think a freehub is in the cards. Anyway, the LBS will have ordered the replacement wheel by now, so it's probably too late to change any of that.

Interestingly, the first thing which bothered me about this bike is the freewheel. I wanted higher gearing, but couldn't get it because the smallest gear on the cassette was already 14-tooth, and you can't get much smaller on a freewheel.

The QR skewers did an admirable job of holding everything together, though. I rode something like 18 miles on that broken axle.
 
Often people report a bent axle but upon investigation the axle is not bent, but has a half-through crack in it and it only bends when pedal forces are applied. A
"bent: axle, when removed is perfectly straight and a quick test is to plug the one end, insert the axle in a glass of water and blow on the open end. Bubbles reveal the crack.
Cracking tip of the day. Never heard it before but will be sure to use it :thumbsup:
 

adamhearn

Über Member
Sadly, the whole drivetrain is 7-speed, so I don't think a freehub is in the cards.
You can get a 7 speed cassette; fit with a spacer, adjust the rear mech if needed and the bike will be good to go. I'd speak to the LBS to see if they can supply one (they'll be up selling you a cassette and possibly a new chain to boot!).
 

TheDoctor

I've seen things you people wouldn't believe
Moderator
Location
Stevenage
I remember breaking the odd axle back when 5 / 6 speed freewheels roamed the earth. One of the advantages of a freehub is that the axle is supported at the ends, rather than at one end and near the middle.
I'd definitely go down the freehub, spacer and 7 speed cassette route.
 

Smurfy

Naturist Smurf
I cannot see how a broken axle is caused by poorly adjusted cones. That one needs some explanation.
Because loose cones permit bending of the axle just inboard of the loose cone. When the cones are properly adjusted, this bending is resisted by the cones bearing against the balls, and the balls bearing onto the hub cups. The bending is then resisted by the hub body, which although usually made of a weaker material than the axle, has a much larger second moment of area than the axle, and so easily resists the bending.
 

evo456

Über Member
Had this happen, like other bits its a consumable and will perish at some point, especially when subject to frequent stress. around £20 at a bike shop will get you back on the road along with a hub/bearing service, unless the hub is really worn enjoy the cheap biking until you really need to buy a new wheel. Keep an eye out on brake block alignment with the rim, usually tell tale signs of bent axle, worn hubs are rims coming out of true.
 
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