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Bicycle Mechanics Broken bottom bracket? Tacoed wheel? If you're having problems with your bicycle, or just need help fixing a flat, drop in here for the latest on bicycle mechanics & bicycle maintenance.

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Old 07-07-03, 09:15 PM   #1
mekki
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More Nexus Problems

You guys were awesome with the last problem, so here's a trickier one for you. First, a little background:

The bike is a pedal-assist electric and weights about 40 lbs w/o the battery. I had it on the roof of the car, and like a dolt, tried to drive into garage. D'oh! The rear wheel had a four inch flat spot, the rear rack has a big ding, and the basket hanger on the front (yeah, its got a basket..sue me ) got nicely squished, but otherwise she looks pretty good.

Ever since the crash, I can't keep the chain tensioned. The drive side axle nuts keep getting seriously buggered, like when they finally give out and the chain falls off, I unscrew the nut and end up with a few full threads embedded in the axle. Ouch.

Surprisingly, the axle seems to be holding up OK and the threads show very minimal wear (a .5 mm by 2 mm patch is slightly silver where the rest of the axle is black). I've actually tried another wheel in there (I had a spare Nexus'd wheel) and the same thing happens.

On the advice of a mech, I checked the dropout alignment: Looks perfect

Can anyone think of a reason why this is happening?
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Old 07-07-03, 10:07 PM   #2
mechBgon
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Sure you're using the correct threading and diameter nuts?
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Old 07-08-03, 12:42 AM   #3
mekki
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Yup, checked it a few times - 3/8", 26tpi
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Old 07-08-03, 10:36 PM   #4
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Grab some chain tensioners at the lbs and call it a day. They should be able to take care of your problem so you will not have to worry about it anymore. Maybe the "crash" affected the dropouts in such a way that the nuts refuse to "bite" into them properly anymore. Serrated washers might also help but they will seriously tear up the paint on the dropout.

-Ryan
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Old 07-09-03, 02:55 AM   #5
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Is your axle bent? You have to take it out and roll it over a sheet of glass or mirror to see.

edit>> You probably cant remove the axle as simply as with a "normal" style hub, so its worth pursuing the other options first.

Last edited by MichaelW; 07-09-03 at 10:03 AM.
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Old 07-09-03, 07:36 AM   #6
hayneda
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One a hub geared bike, the inside axle nut's knarled finish keeps the gear mechanism from rotating against the dropout. On a derailer bike, it only has to keep the axle from slipping forward. If the inside of the dropout surface is all buggered up, the mechanical advantage of the hub gear mechanism is causing the nut to rotate with respect to the dropout. This is what's letting it move forward--as it rotates every once in a while, it allows it to slip forward slightly.

Dave
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Old 07-09-03, 05:22 PM   #7
mekki
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Quote:
Originally posted by hayneda
This is what's letting it move forward--as it rotates every once in a while, it allows it to slip forward slightly.

Dave
Bingo! I did an A->B with an identical bike and this is the problem: The dropouts have spread about 2 or 3 mm and the non-turn washers are turning a little bit The torque blows out the nuts and the whole thing falls apart.

The dropouts are aluminum and the bike guy wouldn't touch 'em to bend them back. I also took the bike to a local frame builder who said "Meh. Why not beef up the non-turn washer?" so that's the plan. I will also look into the chain tensioner (thanks BikerRyan!).

Now: Can anyone suggest a ghetto way of beefing up the washer to take up the slop? Here's a pic in case anyone is unclear:


Thanks for all the great tips, advice, and help! You guys have saved me a ton of headache by helping me drill down to the cause of this.
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