Broken bike

[Maker]

I think I've mentioned before that I have a jump bike with a habit of snapping chains. As far as I can tell, the reason it keeps snapping is because the front sprocket is worn out - The gaps between the teeth are too deep so when the chain is under tension, it forces itself deeper between the teeth and stretches itself. Does that sound like a reasonable explanation for the snapping to you?
Anyway, a friend said he has a sprocket he'd give me, I told him I'll bring my old one into school the next day to check they're the same. There was a flaw in that plan though, the crank is rusted to buggery. First I tried pulling it my hand, then by hammering it, then by hammering it harder, nothing worked. I had a light bulb moment and I cut an old sprocket to fit under the crank arm( opposite the existing sprocket) and allow me to get my gear pullers on it but the sprocket bent. I found a thicker sprocket (about 6 mm steel) and did the same thing but it snapped it. Now I've tried a proper crank extractor and all that's done is ruin the internal thread for the crank bolt.

So, how do I get this stupid thing off? Preferably without cutting or drilling as they're £100 to replace.

Speaking of broken bikes, where's brokenbiker gone?

 

[Pete.]

If the bike has rear suspension then you can snap the chain when jumping by not having enough slack in the chain.

 

[Maker]

It's a hardtail and I don't have the chain very tight (It can be pulled to the chainstay) but such small sprockets and 24" wheels puts loads of tension on the chain. Usually when it breaks I'm pulling up onto a curb or riding slowly on soft ground which is probably when I exert the most force on the pedals.

 

[Pete.]

Could be the small front sprocket then. Low number tooth engagement and tighter link angle make not a long-lasting chain. I once trashed a 11,500lb motorbike chain in 16 miles with that combination, and a turbo.

 

[graffian]

Any markings on the cranks?
Back of the crank might have the part number and from that it should be easy to find the pcd, or you could even
make some measurements.

 

[hotrodder]

http://sheldonbrown.com/chains.html does a fairly good job of explaining how chains 'stretch' and what happens as things wear. While the mismatch in effective pitch of a new chain and worn sprockets locally increases stress on the chain it's typically more common for the chain to slip when any significant pressure is applied to the pedals if the chain is a bit slack or is tensioned by derailleurs/sprung idlers

The vast majority of broken chains i've seen were the result of them not being fitted properly. Especially Shimano chains. Common ones are people pushing pins too far out and then using brute force to push the thing back in creating large burrs and/or ovalising the holes in the chains side plates. Shimanos approach is to provide 'special' joining pins which are single use only and a different colour for idenitification... They take care of alignment but not using them or reusing them/splitting a chain on the same link twice pretty much guarentees a chain that'll break easily. The way the Shimano chains and pins work takes care of stiff links when rivetting, other chains like Sachs/SRAM need a second operation with the chain tool relieve the stiff link and overdoing this results in chains that are more prone to failing. That's probably part of the reasoning behind their powerlink

What are we talking about chainset wise? Aluminium cranks on a square taper bottom bracket, an octalink V1, V2 splined type or something else?
Is the extraction thread in the crank properly FUBARed? If it's still possible to screw a crank extractor into it with care and tighten it properly without it stripping again then some plusgas followed by warming the crank up to 100ish celsius usually does the trick
While it usually looks impossible sometimes a chainring that's mounted behind the crank spider can be gotten off without having to remove the chainset. Beyond that you're into making something to slot behind for a puller to bear against, personally i wouldn't be tempted to use the existing the chainring mounts as if the crank is properly stuck on there all you'll do is bend the spider

 

[Gareth J]

If it's worn out, sounds like you got to get it apart anyway. Chains/sprockets should really be replace together unless you stretch the chain locally early on. Pulling wheelies, crashing dodgily or something.

Get some heat into it, paint stripping gun, butane torch, kettle of hot water.

Time to replace the bearings too?

Good luck!

 

[chunkolini]

Dont use shimano chains, they are a stupid idea, any other make you just push the pin out and back in again to refit.
if any of the drive train is worn ie front or rear sprocket or chain replace the whole caboodle or it will never work right as all the parts wear to fit each other.
If the crank is really stuck soak it witha releasing agent for a day or two and then ride it around for a while with no bolt holding the crank on and it will hopefully work itself loose.

 

[Maker]

The vast majority of broken chains i've seen were the result of them not being fitted properly. Especially Shimano chains. Common ones are people pushing pins too far out and then using brute force to push the thing back in creating large burrs and/or ovalising the holes in the chains side plates. Shimanos approach is to provide 'special' joining pins which are single use only and a different colour for idenitification... They take care of alignment but not using them or reusing them/splitting a chain on the same link twice pretty much guarentees a chain that'll break easily. The way the Shimano chains and pins work takes care of stiff links when rivetting, other chains like Sachs/SRAM need a second operation with the chain tool relieve the stiff link and overdoing this results in chains that are more prone to failing. That's probably part of the reasoning behind their powerlink

Whenever the chain broke, it wasn't that a pin came out and separated the chain, the actual link snapped between the pins.

What are we talking about chainset wise? Aluminium cranks on a square taper bottom bracket, an octalink V1, V2 splined type or something else?
Is the extraction thread in the crank properly FUBARed? If it's still possible to screw a crank extractor into it with care and tighten it properly without it stripping again then some plusgas followed by warming the crank up to 100ish celsius usually does the trick
While it usually looks impossible sometimes a chainring that's mounted behind the crank spider can be gotten off without having to remove the chainset. Beyond that you're into making something to slot behind for a puller to bear against, personally i wouldn't be tempted to use the existing the chainring mounts as if the crank is properly stuck on there all you'll do is bend the spider

It's a steel crank on a steel 48 spline axle. The extraction thread on the crank is fine, it's the bolt hole in the axle that's knackered, the end of the crank puller is only a little bit bigger than the bolt so it pressed its way into the hole and pulled the threads off for the first few mm, leaving a great big burr inside. I'll try heat today hopefully. There isn't a spider on it, it's only a 28 tooth sprocket so it fits directly on. There's no way it's coming off with the crank arms on. It's a 2 piece crank so the crank arm on the sprocket side is permanently attached to the axle. It's the other crank arm that's now stuck so if I make some kind of puller ring, it won't be in the way of anything.

 

[AndyStobbs]

Get a pencil grinder and some 3mm diamond burrs and that'll sort the thread out, you'll have to sacrifice the first few threads. My rod puller is the same, so I screwed a bolt into the crank for the puller end to bare against.

Its amazing how tight stuff on humble little pushbikes can be. You need some heat on the job or grind a slit in the rod. It wont be the end of the world if you touch the crank a little.

 

[GrahamGKD]

If the extraction tool is forcing it's way into the thread on the end of the axle can you make something to block the hole such as a thick disk of steel - like a washer but without the hole in it? Another option might be a cut off piece of studding to fit the thread in the axle with a slot in the end so you can screw it out afterwards. Or maybe best of all a grub screw as that would be hardened and have a hex socket for removal afterwards?

Edit - I was assuming a bolt won't work as that would take up too much space to allow the extractor to fit in properly?

 

[Maker]

I put an offcut of round bar inside he puller so it couldn't get down the thread but even using a big bar on the allen key I couldn't get it off

 

[chunkolini]

Chuck it in the canal.

 

[chunkolini]

Can you blag another crank from anywhere?
Just cut the sucker off with a 1mm cutting disc, make several runs until you are most of the way through to the spindle then get creative with violence and knuckle juice.

 

[Maker]

Not a bad idea Chunko, I've got a most of a Carrera Vulcan that I pinched from a local recycling centre, the cranks on that are still in good nick.

 

[GrahamGKD]

I put an offcut of round bar inside he puller so it couldn't get down the thread but even using a big bar on the allen key I couldn't get it off

Oh, it is tight then. Is it a tapered spline or parallel - if it's tapered it would be worth tapping the end of the extractor with it under tension, or maybe you have already tried that?

 

[Rocketman]

If it's worn out, sounds like you got to get it apart anyway. Chains/sprockets should really be replace together unless you stretch the chain locally early on. Pulling wheelies, crashing dodgily or something.

Get some heat into it, paint stripping gun, butane torch, kettle of hot water.

Time to replace the bearings too?

Good luck!

Not quite true if you replace the chains before they stretch too much you should get a few chains before you need to replace the rear sprockets/block.

The front chain rings can wear significantly before they need replacing and will have little effect on a new chain, it's the rear sprockets/block if to worn ruin a new chain.

 

[Maker]

I've got an impact wrench but I don't have a compressor and only 3 impact sockets.

The cranks on that Carrera are a square taper and the sprockets are held on with rivets disguised as bolts so they're not going to be of any use. I wonder if I could use citric acid to dissolve the rust?

 

[Rocketman]

Maker there is something fundamentally wrong going on chains really shouldn't break that easy.

 

[Maker]

I've broke 2 new chains but when they broke, I swapped out the damaged links so I could get around until I get a new chain. Then I forgot and kept riding on a chain that had been stressed to breaking point until it did it again. I knew it was stupid when I was "fixing" it but I ignored myself and carried on, a night in A&E didn't teach me that lesson either, luckily the second time it happened I was fine, I just had to walk a few miles home.

I might take it to a cycle shop and see if they can fix it for me. Or put a new chain on it and sell it to some moron who'll have it stolen before he rides it long enough to find the fault.