Author Topic: Rear cog on BMX  (Read 556 times)

Rear cog on BMX
« on: April 29, 2020, 12:01:02 pm »
Disclaimer I'm probably using incorrect terminology but have pretty much no BMX knowledge.


A guy I work with has chucked out his kids old and rusty bikes. My neighbours kid needs a bike so I've rescued a BMX. The brakes I can sort and going to byass giro system as will only be for cycling during lock down/about a bit so doesn't need it

The other issue is the single rear cog is seized. Before I cut the chain off I rode it round and it locked the rear wheel when I stopped pedaling after trying to chuck me off. I'm guessing these are pretty much a replacement item rather then servicing it. It's rusty as anything. How do you get them off? Also are the threads pretty standard as there was another BMX type bike as well and it can freewheel. Well it could if the tyres weren't falling off but can I swap the cog from this?

LittleWheelsandBig

  • Whimsy Rider
Re: Rear cog on BMX
« Reply #1 on: April 29, 2020, 12:34:05 pm »
It depends whether it is a screw-on freewheel (generally smallest 15t, occasionally 14t) or a much smaller freehub cog (usually <14t). If they are both freewheels, almost all are standard BSC thread (min. 15t - 16t) but a few are mini threaded (14t or 15t).
Wheel meet again, don't know where, don't know when...

Re: Rear cog on BMX
« Reply #2 on: April 29, 2020, 01:14:58 pm »
Thanks. I had a bit of time at lunch so took the wheel out and there appeared to be nothing to undo the cog by, Google having suggested a tool but as it was seized solid clamped it in a vice and it unscrewed. The one on the donar bike I managed to unscrew using a pin punch and a hammer so that's now on the bike I'm trying to fix up.

Re: Rear cog on BMX
« Reply #3 on: April 29, 2020, 01:16:26 pm »
 a photo of the wheel would clarify which it is.  Most (but not all) standard BMX screw-on freewheels use a large four-dog remover, e.g. this shimano SF-MX30



but as LWaB says there are lots of other possibilities

[edit; including screw-on freewheels that you have to take apart (or otherwise butcher) before you can unscrew them]

cheers


Re: Rear cog on BMX
« Reply #4 on: April 29, 2020, 01:32:35 pm »
Bruce I think the one in your image is probably much better quality then the either of the ones I removed. There is nothing I can see to fit any tools. If the whole bike cost much more then £100 I'd be surprised.

Torslanda

  • Professional Gobshite
  • Just a tart for retro kit . . .
    • John's Bikes
Re: Rear cog on BMX
« Reply #5 on: April 29, 2020, 10:55:31 pm »
Put the freewheel in a vice so the flange is between the jaws and the teeth rest on top of the vice.

Crush it. Turn the wheel anticlockwise. If the freewheel won't unscrew, crush it more.

Replacement (usually 16 or 18 tooth) should be relatively inexpensive from your LBS.
VELOMANCER

Well that's the more blunt way of putting it but as usual he's dead right.

Torslanda

  • Professional Gobshite
  • Just a tart for retro kit . . .
    • John's Bikes
Re: Rear cog on BMX
« Reply #6 on: September 29, 2020, 10:23:50 am »
Recently had to reove a freewheel from a wheel that was growing stuff. Green stuff.

How rusty? I used a chain whip...
VELOMANCER

Well that's the more blunt way of putting it but as usual he's dead right.

Re: Rear cog on BMX
« Reply #7 on: September 29, 2020, 08:57:50 pm »
It is possible on some cheap (chinese?) single freewheels to screw them on back to front so that the removal dogs are on the inside and invisible. In this case the pinholes for removing the cone will also be invisible. Destructive removal as outlined above can be used!
Moral of the story, make sure you screw on a new one with the removal dogs on the outside!

Re: Rear cog on BMX
« Reply #8 on: September 29, 2020, 09:08:25 pm »
It is possible on some cheap (chinese?) single freewheels to screw them on back to front so that the removal dogs are on the inside and invisible. In this case the pinholes for removing the cone will also be invisible. Destructive removal as outlined above can be used!
Moral of the story, make sure you screw on a new one with the removal dogs on the outside!

actually you don't really have a choice, not if you are stickler for wanting to ride the bike forwards, anyway.... ;)

cheers