Hoooowww???
 

Today I discovered that the thread on both my rear hub and freewheel were stripped. I went to my LBS and he told me he's never seen this before. The last time I removed the freewheel was to lightly grease the thread on the hub. There also wasn't the slightest problem when putting the freewheel back on. How the heck does this happen?

The wheel is an unknown 36H 27 inch double wall and the freewheel is a 7 speed Sunrace 13-25. I bought the wheel late september and the freewheel late october.

Threads can strip through excessive torque tightening them, which is basically how a traditional thread-on freewheel transfers drive. What size chainrings have you been running? I'm surprised you managed to strip the threads on both the freewheel and the hub, though, as the threads in the freewheel body are generally steel, while those on the hub tend to be aluminium, which means they aren't as strong and would tend to strip long before the ones on the freewheel.

Cross threading ? that buggers them up.

torque, pedaling, after tightened on the hub, is typically transferred to the shoulder on the hub.

52/42 Chainring

How did you realize the threads were stripped? Or the LBS found it when he removed the freewheel?

I'd be interested to know how much is transferred by friction at the hub shoulder and how much is transferred by the threads, I suspect you may be correct in assuming that most of it goes through the shoulder. It's unlikely to be torque anyway in this case, though, as the lowest gear the OP has is 42/25, and there are plenty of old/cheap MTBs fitted with freewheels using much, much lower gearing than that, not to mention tandems.

Assuming it failed while riding, I'm stumped.

I'd love to see a pic of the freewheel threads. Are you sure that the hub didn't just transfer all of its thread material to the freewheel threads, making them look smoothed-over?

Rode half a mile, stopped at a light, tried to pedal and the crank slipped and the freewheel was slighty loose and wobbly. I might've rode about 200 miles after the last time I put on the freewheel.

Airburst - 52/42

( If anyone happened to be curious the bike is an 86' Schwinn World Sport )


I didn't completely clean the thread in the freewheel so maybe. might be a while for pictures, I put it back on, no access to tools.

The highest gear doesn't matter, the smaller the chainring, the lower the gear, and therefore the more the torque on the freewheel threads. When I said 25, I was referring to your largest rear sprocket, which will give the lowest gear available when combined with your small chainring. That was the number I was giving.

I'd also suspect ThermionicScott is right, the freewheel threads are probably just full of the remains of the hub threads

Wasn't catching on quick enough, my mistake. This problem doesn't have me thinking straight. When I get the chance I'll clean it out well, I hope its actually fine.

I was considering looking for a new hub but I guess the cost of that and to rebuild the wheel may outweigh the price of a new wheel....?

It probably will, unless you can get a new hub very cheap and/or know how to lace wheels.

You have my curiosity up. Could you post some pics? I have messed with many old wheels and never saw even a bit of an issue, even with those that required way to much torque to remove.

-G

The same amount, according to Newton's third Law of Motion. Actually it doesn't depend on friction at all. The threads ramp in converting torque to thrust against the stop, (with the same thrust applied across the threads). What drives the hub is a wedging action which jams the thread at the bottom.. It's analogous to trying to push a marble down through a funnel which ends up too narrow for it to pass.

In any case it's virtually impossible for normal humans to strip freewheel threads through torque. The most likely scenarios are that a prior owner cross threaded a freewheel damaging the hub, then corrected and mounted the freewheel, but the damage was done. Also depending on the vintage of the wheel it's possible that you have a French thread hub. An English freewheel feels like a fit, but isn't and rides the tips of the threads which don't hold when torqued.

Now might be a good time to upgrade to a cassette wheel, and then you wouldn't have to worry about this type of problem again. :thumb:

Wouldn't a certain amount of torque be transmitted by direct friction though? In the same way that once a bolt is tight, the torque required to start tightening it again is greater than the torque required to to actually tighten it once it starts moving, due to friction?

Once the freewheel body butts up against the shoulder/stop on the hub, no further rotation is possible. The torque on the cogs ends up pushing the body laterally against the threads. It's like kicking a door-stop wedge under a closed door, once it stops moving, you can kick it as hard as you want, it's not moving until the door breaks. It's not the friction of the wedge on the ground that stops it (such as when the door is open), but the strength of the wedge and door materials. Compute the entire contact surface-area of the two thread-faces and multiply by the modulus of aluminium and you'd need a tremendous amount of torque on freewheel-body to strip the threads. Beyond what a human can generate.

In this case, it's most likely due to either mismatched French/English threading or the hub-threads were previously damaged from cross-threading.

Go back to my example of the marble in a funnel. What keeps it from going farther once it reaches the too narrow area, is the support from the walls. The ball pushes out on the walls of the funnel, the funnel pushes back (Newton's third law) Granted it's angular, but it's the same as how a tripod works.

Granted there's probably a tiny amount of friction keeping the freewheel from rotating that last fractional bit and wedging itself tighter, but it's the wedge action that does the work.

BTW- there's always a bit of friction at work. When you stand you're supported by the floor pushing up against your feet. But if there weren't any friction, at the least imbalance your feet would shoot out from under you and you'd fall on your rear end. So when I say there's no friction involved, I mean that it's not a meaningful factor.

This is what happened, the freewheel ended up being okay. I just finished rebuilding the wheel, my first time doing so, around a new hub.