Back wheel locking up under effort?

[DWiggy]

Recently changed my chain as old one was between .5 -.75 limit, gears changing okay and bike seems to be working fine with no undesirable noises.
Apart from when I drop it into one of the rear small cogs get out of the saddle exerting a large sprint effort, there suddenly without warning a large crack noise from the rear and the back locks up lifting the wheel off the ground, it catches me unaware but manage to keep control....so far.

Anyone have any ideas? someone mentioned it might be the rear free hub failing (Mavic Aksium wheels)
I dont think its the cassette as its happening in gears I rarely use and thought chain slip (if that's what it might be) would happen in the frequently used gears which it doesn't?

Any ideas would be appreciated

 

[mjr]

It sounds like the wheel could be moving, hitting the frame, locking up and then moving back when you relax - the small cogs are furthest out and pushing it hard then will be trying to twist the wheel more than other gears. Hopefully the crack is just the wheel losing its grip on the dropouts, but if it's only the right dropout coming free, it could be bending the stays, so check carefully, especially around the left side dropout.

Possible causes include simply failing to tighten the nuts or QR enough (my mistake when I had this), bearings with too much play (or something in them may be shifting under extreme load which may be new hub time or at least new bearings time) or it may be something more serious.

Edit: that said, I don't understand how this would lift the back wheel up. I'd expect it to skid.

 

[Ajax Bay]

You may not have accurately seated your axle in the dropouts, tightened the QR in the normal way, which held it there till a big effort. The axle slipped out, allowing the wheel to 'twist' and the tyre foul the chainstay, and you may well have broken your rear axle.
This has happened to me, though the symptoms were different - a crack and wondering why I wasn't accelerating even though I was pushing hard. A 'wtf' followed by a 'Victor Meldrew' moment. Replacing the axle in the dropouts allowed me to get home (the QR held everything in place) but the QR ended up bent. Thankfully I had a spare axle and QR from a defunct wheel and was able to swap it out and ride again that day.
Take the wheel off, take the QR out and see whether there's play in the axle. Look at the QR skewer and see if it's still true. (Edits)

 

[DWiggy]

Just checked the following

• Quick release tight
• No play in wheel bearings
• No visible cracks (Ali frame)
• Cassette does have some play side to side about 2mm (Might see if I can tighten when I get back)

 

[Yellow Saddle]

Your symptoms are confused. You say the rear wheel locks up and lifts off the ground.

Firstly, when it locks up, does it remain locked? How do you unlock it?
A rear wheel lifting off the ground from braking is symptomatic of a front wheel locking up, not rear. I therefore don't understand. Answering question 1 would help.


My guess is that it is chain skate, when a chain skates over the top of a cassette sprocket under power. This makes a crack noise and the crank moves a couple of degrees until you lose power and the chain then falls back in place and pretends nothing is happening. The rear wheel lifting may just be how you envisage the problem because when the chain suddenly loses grip your body falls forward.

Don't exclude wear on the smaller sprockets to show up when you fit a new chain. My guess is that the problem will go away when you replace the sprockets in question.

 

[Crackle]

Your symptoms are confused. You say the rear wheel locks up and lifts off the ground.

Firstly, when it locks up, does it remain locked? How do you unlock it?
A rear wheel lifting off the ground from a locked rear wheel is symptomatic of a front wheel locking up, not rear. I therefore don't understand. Answering question 1 would help.


My guess is that it is chain skate, when a chain skates over the top of a cassette sprocket under power. This makes a crack noise and the crank moves a couple of degrees until you lose power and the chain then falls back in place and pretends nothing is happening. The rear wheel lifting may just be how you envisage the problem because when the chain suddenly loses grip your body falls forward.

Don't exclude wear on the smaller sprockets to show up when you fit a new chain. My guess is that the problem will go away when you replace the sprockets in question.

I'd go with that. I've had exactly that after replacing a chain a long time ago before I knew much about bike maintenance. It's a crack and a lurch before it catches again. It's the only thing you've changed.

Also don't do what I did and keep riding it until it stopped cracking! (I was about 16 at the time)

 

[DWiggy]

Your symptoms are confused. You say the rear wheel locks up and lifts off the ground.

Firstly, when it locks up, does it remain locked? How do you unlock it?
A rear wheel lifting off the ground from a locked rear wheel is symptomatic of a front wheel locking up, not rear. I therefore don't understand. Answering question 1 would help.


My guess is that it is chain skate, when a chain skates over the top of a cassette sprocket under power. This makes a crack noise and the crank moves a couple of degrees until you lose power and the chain then falls back in place and pretends nothing is happening. The rear wheel lifting may just be how you envisage the problem because when the chain suddenly loses grip your body falls forward.

Don't exclude wear on the smaller sprockets to show up when you fit a new chain. My guess is that the problem will go away when you replace the sprockets in question.

Thanks for you help (And others as well) I think the back wheel lifting might be a symptom of how I'm riding the bike at the time it happens (standing and leaning quite far forward exerting a lot of power pulling on the peddles) the resulting crack/jolt tips me off balance and it does feel like the wheel locks but I don't notice any skidding.

I think you might be correct re cassette, looks like I didn't catch the chain wear fast enough.

 

[Ihatehills]

There shouldn't be any side to side play in the cassette should there?

 

[Ajax Bay]

suddenly . . . the back locks up

Or does it? And lifting the back wheel while "out of the saddle exerting a large sprint effort" if a skate occurred is quite difficult to do, even by accident. Easy to lift a rear wheel deliberately, but not, I think, while sprinting.
I don't doubt that you have described the symptoms you perceived but as ^^^ maybe the skate, if that it was, threw your weight forward (because the pedal suddenly gave no resistance) and that seemed like a rear wheel lifting, to you.

 

[DWiggy]

Or does it? And lifting the back wheel while "out of the saddle exerting a large sprint effort" if a skate occurred is quite difficult to do, even by accident. Easy to lift a rear wheel deliberately, but not, I think, while sprinting.
I don't doubt that you have described the symptoms you perceived but as ^^^ maybe the skate, if that it was, threw your weight forward (because the pedal suddenly gave no resistance) and that seemed like a rear wheel lifting, to you.

You could be correct, it happens so quickly it certainly does feel like it lifts but skating would probably describe it better, I think it could be the shifting cables sticking, gave it the once over last night and it seems to be down sticking when down shifting

 

[Ajax Bay]

I think it could be the shifting cables sticking, gave it the once over last night and it seems to be down sticking when down shifting

How long since you changed your outers? You can live with it but it's annoying.

 

[DWiggy]

Gave the bike the usual clean and once over on Sunday, think I found my issue, it seems to have been the jockey wheels! the bottom one that is suppose to have no side to side play had more play that the top one, I also found that when in small front large back combo the chain was rubbing on the cage quite badly.
Luckily I had a set of replacement ceramic bearing ones from a while back, swapped them out and now no rubbing on cage also indexing is spot on unlike before and the resistance on the cranks is massively reduced!

Hopefully problem solved