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Fixed-gear blowup...what happened?(5 posts)

Fixed-gear blowup...what happened?GTDave
Jul 3, 2002 4:23 AM
So I just put together a new fixie (early '80's Trek) using a rear wheel that I had built up from an old Sachs-Huret hub with a Surly 16 tooth track cog and a lock-ring from a bottom bracket. I have used this wheel for about six months (maybe 500-600 miles) with no problems. I re packed the bearings before I put it on the new bike.

On the shakedown cruise for the new bike, everything was going fine about 10 miles in. I then descended a big hill. I was going about 32-34 mph and the chain came off!!!! When I got it stopped (whew!) and un-puckered the saddle, I found that the lock ring had stripped the threads on the hub, and that the cog had backed off enough that the chain was able to drop.

So what happened? Near as I can figure, I was resisting the pedals and the force of that action caused the cog to back against the lockring and just stripped it. I will be buying a "real" track hub soon and I want to make sure that I don't ruin it too, what steps will I need to take to make sure I don't
re: Fixed-gear blowup...what happened?madstork
Jul 3, 2002 5:09 AM
GTDave,

I'm no expert on fixies, as I'm currently getting rear wheel parts to build up my first one. Go to this site for information that may explain what went wrong:

http://www.sheldonbrown.com/fixed-conversion.html

Maybe at your speed there was enough force to strip out the lock ring if it was holding tight against the cog.

Madstork
Hub condition or thread discrepancy is my guess.Quack
Jul 3, 2002 7:10 AM
Wow, chain off at 30+ and I'm guessing no brakes, good save.

I would bet that the old hub may have had lockring threads that were not up to the task or that the lock ring didn't match perfectly. Not to say that you couldn't strip out a hub with your leg power on a descent, but not too likely. I think a new hub and lockring will insure that it doesn't happen again. Chock it up to old parts and a little bad luck.

Enjoy!
re: Fixed-gear blowup...what happened?off roadie
Jul 3, 2002 7:50 AM
A lock-ring works on a very different principle than takling a 1x1 hub and putting atrack cog and a BB lockring on it.

A true lockring goes on a seperate set of "backwords" threads, so when the track cog tries to spin off, it actually TIGHTENS the lock ring.

A BB lockring, on the sam threads as the track cog, is essentially just a jam-nut, and it can tigthen way up againt the track cog (stipping the thread) and then start to slip.

This is why Surly does not warrenty the 1x1 for such use.

That said, I also use a 1x1 with a track cog. I put loc-tight on the threads, hammered the cog home with some hard climbing, and then had a shop tighten the BB lockring down with the bigest wrench they had. So far, so good. My bike is a MTB, with 36/16 gearing, so I doubt I'd ever get over 30, and even so, the torque on the track cog isn't quite as high when I'm "back pedaling". The taller your gearing, the more force the cog takes when you resit forward motion. I also have some hefty strong MTB brakes...
Not all parts are created equalStraightblock
Jul 3, 2002 3:19 PM
I'm using an old school Record high flange road hub with a SunTour track cog, and there's not enough threads on the hub to put a lockring over the cog without stripping the last few threads. Yet I've used the same cog on an old Normandy hub & there was plenty of room for a lockring.

Keep in mind that, assuming you used a steel lockring, the aluminum hub is much softer and you may have buggered up the last few threads when you tightend the lockring. Or in a past life someone may have crossthreaded a freewheel on the hub. Even if the outer 2 or 3 threads were damaged, the remaining threads would still hold a freewheel or cog, but not a lockring.

When you get it back together, be sure the cog is torqued tight before you install the lockring. Use a chain whip, or just ride it hard for a few miles, then put on the lockring. That way you're sure the cog won't tighten more while riding and allow the lockring to come loose.