I'm converting and old Bridgestone to a fixed utility bike for me and the boys. I've been riding for years with cog and no lock ring, but somehow the idea of high school kids riding that way seems less safe. In any case I found a website with a cog I'd like and their lock rings are "left threaded" . Please ''splain me" how the left threaded ring goes on.

When you pedal forward the cog is in a sense tightening against the hub. When you pedal in reverse, the force acts to loosen the cog from the hub which is threaded on in normal "righty-tighty, lefty-loosey" fashion (sorry, I don't know the technical terms).

Lock-rings are reverse threaded to prevent the cog from loosening and unthreading itself from the hub. When you apply backwards pressure to the cog it tends to want to loosen and come off the threads towards the lockring. When a lock-ring is snugly tightened against the cog it [lock-ring] is actually tightened [toward the cog] by the cogs pressure as it is threaded in the opposite direction. Basically pressure that wants to loosen the cog, tightens the lockring.

Ok, I'm obviously not a technical writter so I'll draw a diagram.

what I don't understand is why the screwing the left threaded lock ring onto the hub isn't like trying to screw a pedal onto the wrong crank arm.

what I don't understand is why the screwing the left threaded lock ring onto the hub isn't like trying to screw a pedal onto the wrong crank arm.

Well that's two strikes for me. The threads on the lockring portion of the hub are reversed as well. If your hub was intended for a track cog it will have two sets of threads, the diameter of the lockring is smaller than the cog so there will be a step up from the lockring threads to the cog threads. If this is not the case and your hub only has one set of threads all the way from the spoke flange to the outside edge, then you have a hub that was intended for a single speed freewheel only.

RUSA1586 (is that you, David?): Asterisk splained it right. If you're using the hub that came with that old Bridgestone, it's a freewheel hub, and doesn't have the smaller left-thread portion that a true fixed-gear hub has. So you can't use the track lockring.

With some combinations of cog and freewheel hub, the cog threads on far enough to expose some threads, and in that case you can use a lockring from an old-fashioned bottom bracket as a lockring (it's exactly the same thread), but it won't have the reverse thread. It's not as secure as the track lockring, but it does add something. I probably have one of those bb lockrings around if you want one.

Personally, I've never had any problem without a lockring. I clean the threads up well and use plenty of blue loctite, and crank it down well. I can still loosen it with a chainwhip if necessary.

Have you really talked your boys into trying the perpetual-pedaling thing? Cool.

I've been riding with no lock ring or Loctite at all and will continue to do so. I do picture the boys doing stunts that I don't try. I once foolishly mentioned skidding and chirping and I could see their senses of wonder and curiosity were piqued.

It's reassuring to learn that what I thought was impossible was not what was happening.

I would implore you to do something to hold that cog on. Someday, you'll need to backpedal down a hill and it's going to spin off, your chain will come off, and maybe jam, causing your wheel to lock up at a high rate of speed. I once saw an idiot mechanich do just this on a club ride, and he destroyed about $2k worth of bikes due to his folly.

A track hub with a loc ring might be a good idea.... I used to run a "suicide" set up with loc tite untill I spun it off and had to cruise a steep hill into a small town's main street at full speed!! Doing a cutty on a downhill bike around a turn is one thing, but on a 700c dam near impossible!

The local club wont even let fixie riders out unless they have a front brake and a propper track hub.

With a front brake, even in an emergency, I only need to slow the back wheel, not completely stop it. The more you slow it the better, to be sure, but riding down a steep grade on a bike lane coming off a bridge I'm able to slow down considerably without using the brake at all and if the cog loosens I don't feel it. I've been riding like this for 2 and a half years and have yet to skid or lock up the back wheel and it doesn't feel "suicidal" to me.