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Old 01-13-02, 03:03 PM   #1
mike
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Crank arm repair - will this work? Am I truly a genius?

This idea just struck me this morning and I rushed to put it to work.

I am asking my pro-wrench friends at BF.C if this will actually work.

As fate would have it, I have a mountain bike in which the chainring side crank is loose. No amount of tightening would fix it.

On inspection, I found that the square hole of the aluminum crank arm was a bit worn. "Alas, I'll have to get a new crank arm", I thought.

Then, the brilliant idea came over me today to try to shim the spindle/crank with aluminum foil.

I cut a thin strip of aluminum foil and wrapped it around the axle/spindle. Then, I put the crank arm on and tightened it down.

The initial ride around the block went well.

What do you think; will this work? Could I possibly be a genious, or has this been thought of before?
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Old 01-13-02, 03:12 PM   #2
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hmmmm, it seems like it could work, and it seems like the alu foil wouldent do anything at the same time. Hard
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Old 01-13-02, 04:03 PM   #3
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Did you use regular aluminum foil or the aluminum from a soda can, or something alone that line? I'd say if you would use the aluminum from a soda can, it might last for a little while. But, since it's a moving part, it probably wouldn't take that long to get wrecked...
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Old 01-13-02, 04:06 PM   #4
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What do you think; will this work? Could I possibly be a genious, or has this been thought of before?



1. for a while

2. yes

3. yes
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Old 01-13-02, 06:10 PM   #5
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Mechanically it may work OK, and it's a good emergency fix, but is the crank mounted evenly?
Check how the chainrings run against the derailleur. If there is any wobble, then that will be magnified at the pedals, and do BAD THINGS to your knees.
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Old 01-13-02, 07:09 PM   #6
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Quote:
Originally posted by MichaelW
Mechanically it may work OK, and it's a good emergency fix, but is the crank mounted evenly?
Check how the chainrings run against the derailleur. If there is any wobble, then that will be magnified at the pedals, and do BAD THINGS to your knees.
Hey, MichaelW, you struck a nerve with your response.

How can you adjust crank wobble? I usually just put the crank on and tighten it down. I never noticed any run-out when checking the chainring in relationship to the front derailure, but maybe I should be more careful. Maybe I have been lucky. Once I did notice this, but thought it to be a bent spider and then trued the spider.

Coincidently, I have noticed some harsh knee pain lately (although I only rode this particular bike a couple of days). You have me spooked.
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Old 01-13-02, 07:26 PM   #7
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Of course you're a genius, but this not news, it's a fact that we have established from many previous posts.

So, what causes crank wobble in the first place? Riding with the crank too loose?
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Old 01-13-02, 07:34 PM   #8
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Quote:
Originally posted by roadbuzz
Of course you're a genius, but this not news, it's a fact that we have established from many previous posts.

So, what causes crank wobble in the first place? Riding with the crank too loose?
Hee hee, thanks, roadbuzz.

Well, a loose crank will sure cause the crank to wobble. That I can attest to from experience. I suppose a bent crank arm, bent axle/spindle would do it too. A bent chainring or crank spider could also do it.

MichaelW really knows his stuff, though. I am looking forward to his response on what I/we may be missing. Maybe he is suggesting that my shim approach could cause the crank to seat on the spindle/axle in a way that is not centered.
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Old 01-13-02, 09:08 PM   #9
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using a strip of of alu as a shim is an old trick mechanics use temporarily for quick solutions.

greg lemond's mechanic used it once during the tour when greg's seat-post had become loose in the seat tube. the mechanic cut up a coke can as a shim.

could be a bit iffy with cranks though as michaelw say due to knee problems......give it a bash (try) see what happens...but a new crank might be more advantageous....

cabledonut.
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Old 01-14-02, 02:02 AM   #10
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Quite some time ago, a friend of mine had a set of the original Cook Bros. cranks and shimming them with coke can was a requirement for getting them tight. As long as you're careful and get it wrapped pretty even, I wouldn't worry too much. The worst that could happen is that you would further ruin a ruined crankarm. In the end, a new arm is your best bet but if money's tight, using coke can is a pretty solid solution.
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Old 01-14-02, 04:53 AM   #11
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Im not a great fan of the crank/spindle assembly.
Althought he taper angle seems standard, the max/min thickness of the two parts is variable due to manufacturing tolerance.
Ive had cranks which dont fit on deep enough, and they are vulnerable to missalignment and damage.
I dont think this is such a problem with high quality parts from the same manufacturer, but mix n match with care.

If shims seem to be needed as standard, then look around for another make of bottom bracket spindle.

The new splined systems solve the problem of missalignment, but have a whole new and exiting set of problems.
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Old 01-14-02, 08:53 AM   #12
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Beer can aluminum shims are a time-honored fix for loose handlebars (e.g. "Zen and the Art of Motorcycle Maintenance"). However, there is so much pressure at the crank-spindle junction that your shim will rapidly be extruded until it is so thin that the crank will again wobble. (The shim material is alot softer than the crank's dural.) Been there ... done that, in the early 1970s with a first-generation Sugino Mighty Compe crank. I even tried shimming with steel washers, with the same results.
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Old 01-14-02, 12:15 PM   #13
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Thanks for all the great input, guys.

MichaelW, I checked for wobble - I'm OK on that.

JohnE, You may be correct about extruding the shim. What I was thinking is that the shim is aluminum and so is the crank, the metals would be compatible - unless the crank is some kind of aluminum alloy. Again, I used thin aluminum foil rather than aluminum sheeting from a cola can. My thought was that the foil might conform better and fill the loose spots.

I rode to work in the snow today with good results. The crank arms are still holding tight.

I'll keep you posted.
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Old 01-14-02, 02:29 PM   #14
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Let's see how it goes, Mike. My objection is that the crank is made of dural, which is a very hard, tough aluminum alloy, whereas I am sure the foil is not.
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Old 01-14-02, 05:12 PM   #15
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OK, Fellows, I will put on 10 miles in the snow on the way home from work while running errands.

I'll let you know how it goes.

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Old 01-14-02, 08:02 PM   #16
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I'm betting that his jury-rigged crank will last about a month. Then, he'll finally replace the crank.
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Old 01-14-02, 11:15 PM   #17
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So far so good, Fellers.

Today was a good test. It was a snow day and I put on about 20 miles bashing the snow piles, jumping snow covered curbs, and having a ball.

Of course, the snow adds a lot of extra load to the pedals.

Ah, who knows. Maybe it won't last but a month as D*Alex says, but for now, it is working.
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Old 01-15-02, 02:50 AM   #18
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Don't hold your breath! Sorry to say, your fix probably won't last long. Nice thought though, try brass shims next time they'll lacst longer but, they're iffy too.

Ride Gently
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Old 01-15-02, 10:17 AM   #19
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I have Shimano Altus componants on my POS hybrid, and I believe the entire drivetrain is made from old aluminum cans-a time honored Taiwanese solution to that pesky recycling problem.
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Old 01-15-02, 10:42 AM   #20
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Mike, I am all for alterna-repairs. If it works, do it. So, lets say in a month it does give out. Put some more foil in there and you are good for another month. Worse case scenerio? You might just have to replace it...someday.
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Old 01-15-02, 11:12 AM   #21
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And someday is not now!
So this will give you the time to save some money for a new one !!!
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Old 01-15-02, 09:27 PM   #22
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Hey, guys. It's been a while since he set out. Do you think he's still out there?
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Old 01-16-02, 03:08 AM   #23
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Yup, Guys. What is this - day 2 or 3 since the aluminum foil shim repair?

Well, I have been booshing through snow piles and otherwise thoroughly abusing the machine and the crank is still tight.

The report thus far is still...:thumbup:
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Old 01-16-02, 11:06 AM   #24
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I used tin foil to shim my disk brake caliper in to place 3 months ago because I couldnt find a washer small enough. It's still holding up fine, however I don't think it is subject to as much stress as a crankarm.
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Old 01-16-02, 12:54 PM   #25
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Mike,
I think you are on to something. I'm just not sure aluminum foil is the material of choice for a long term solution. I would check at an industrial supply store or order from someplace like MSC to get some very thin stainless steel shim material. My guess is that it would be much more durable than the foil or aluminum can stock and might just salvage that crank for the longer term. Aluminum foil is just so squishy I wouldn't have much faith in it.
Good luck,
Raymond
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