Broken Crank Arm - Chainring Connection

Question

I was just riding my bike like normal the other day and the crank arm started slipping. Upon further inspection, I discovered the 'bolt' that connects the crank arm on the drive side to the chainring was sheared off.

What is this missing part called? How can I fix this?

I have no idea where to start with this repair... Can anyone help me? I'd love some advice on all my options from cheap to overhaul. Plus some expert terminology for all relevant parts so I'm not just saying stuff like a dummy :)

Answer 1

What you have is a 'one-piece' or 'Ashtabula" crank. Info can be found on this Sheldon Brown Page. They are called one-piece because the crank arms and axle are made from a single bar of steel.

The chainrings are not fixed to the crank axle and can rotate on it, instead there is a 'drive pin' that sticks out of the right hand crank into the chainring spider. That's what has broken off your crank. From the picture, I think it was press fitted into the crank and worked loose.

I did some quick googling and found that replacement arms are available and inexpensive. Your chainrings do not look worn so you will be able to reuse them (although they could use a good cleaning 😉).

If you are not experienced with bike repairs a local bike repair shop will be able to handle replacing the crank easily.

Update:

Park Tool has a good video on disassembly and servicing the bearings and re-assembly.

Answer 2

There are two major types of one-piece crank sets, differentiated by their axle threading. The more common size uses 24 threads per inch, but some bikes, particularly older U.S. made Schwinn and Mongoose models, use 28 threads per inch. No part of the bearing assembly is interchangeable between threadings.

-Sheldon Brown

First step is figure out which one you have and how long you want your new crank arm. If you don't have an old mongoose or schwinn it sounds like you probably have the 24 tpi. If you measure your crank, the size will be from the center of the bb spindle to the center of the pedal spindle. They seem to be about $15-25 online.

Minimal repair: Just get a replacement crank arm and do the repair yourself. I'd recommend getting some fresh grease too, so you'd probably end up spending $20-30 in parts and could probably do it yourself in an afternoon even if you've never done it before. From Sheldon Brown:

One-piece cranks are the easiest type to service, and require no special tools. All you need is a large adjustable wrench and a screwdriver.

Fresh Bearings repair: As long as you're replacing the crank arm, you might find it worth while to just spend an extra $10 for a new bottom bracket/bearings, especially if your bottom bracket is missing bearings or the surfaces they run against are deeply pitted. You also might want to spend another $10 or so for some new pedals if yours are loose or hard to spin (keep in mind most bike pedals don't fit one-piece cranks, so you'd have to be careful to get the right thread count/diameter). If the cups are stuck in the bottom bracket shell, however, this may require more tools.

Upgraded Components Repair: You can also get a $20 converter that will allow you to install a more modern cartridge bottom bracket with any regular square-taper crankset. You would need some more tools or to get a bike shop to do the work for you. You would also need about $25 for a crankset and $20 for a bottom bracket, $15 for new pedals (different standard). So something like this would be fairly involved and could easily be over $100 just for the parts.

Answer 3

One piece cranks have been around a long time. They are cheap and very durable if you buy a good quality chrome moly crank. The advice about servicing or replacing your bottom bracket bearings, cones and cups is a good one. The cups can be removed using a brass drift or other metal rod to drive them out of the frame with hammer blows. The new cups are driven into the frame with a rubber hammer. Good luck with your repair.