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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 11-30-11, 11:54 PM   #1
ramseyrt
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Loose Crankshaft Bolt

Hello,

The left side crank bolt on my bicycle keeps coming off. When that comes off, my crank and pedal comes off; a real disappointment when I'm miles from home. So, I had it tig welded in place. The bolt no longer came off but the crank still began to wiggle some and squeak a lot. So, I had the tig spots ground off so I could replace and tighten the bolt again. The bolt has continued to come loose and fall off. Yesterday, I tried using some plumbing tape to secure the bolt; it came off again.

I'm at a total loss as to how to fix my problem. I'm willing to spend money but I'm stationed in Afghanistan; there are no bicycle shops here. I can buy parts online but I'm not smart on bicycle maintenance either. I'm not sure what I need to buy, if anything. I have access to tools so I could probably replace my crank, bottom bracket, and front chain ring assembly if needed.

Any suggestions on things I should try? If parts are recommended, what do I need to measure on my current bike to ensure I get the right replacement parts?

Thanks!

Rob
Bagram Air Field
Afghanistan
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Old 12-01-11, 06:17 AM   #2
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sadly the cost of fixing this is worth more than the bike. you are going to need a new crank and bb unless you can find a junk bike to pull something off of
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Old 12-01-11, 09:41 AM   #3
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The bolt was never torqued adequately to begin with and the loose bolt allowed the crankarm itself to be damaged. Welding the bolt in place wasn't the cure. By now, the crank is damaged to the point it must be replaced as it will never remain tight.
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Old 12-01-11, 10:50 AM   #4
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The fix is relatively easy, replace the crank. What happened is that you rode for a while with the arm loose. this allowed the arm to work back and froth a bit, so the corners of the spindle were able to dig into the corners of the square hole in the arm. That hole is now no longer square with flat sides. The sides are arched inward from the corners allowing the spindle to rock back and forth no matter how tight you tighten the bolt. Also the back and forth motion will continue to rock the bolt loose as you ride.

Don't worry about the spindle being damaged because it's hardened steel and can take tons of abuse. Mounting a new arm on it tightly, and keeping it tight are all that's needed. Also learn to be aware of loose cranks, which make a once per revolution clicking sound. New arms often loosen a bit, but if you attend to it promptly you won't damage anything, and usually after be retightened once, they don't come loose again, ever.
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Old 12-01-11, 10:55 AM   #5
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^^ This. Also the tightening torque need for crankarms is alot higher than you think and a lot higher than most new mechanics will use. Get them REALLY tight.
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Old 12-01-11, 11:27 AM   #6
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^^ This. Also the tightening torque need for crankarms is alot higher than you think and a lot higher than most new mechanics will use. Get them REALLY tight.
Except they can pretty easily be overtightened so the crank is again damaged and refuses to sty on.
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Old 12-01-11, 12:43 PM   #7
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I bought a bike where the same thing was happening. The axle is steel and the crank arm aluminium, so I think your axle will be ok, but you need to search Ebay for a replacement crank arm. With any luck you'll find a matching left arm.
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Old 12-01-11, 01:43 PM   #8
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You can purchase just the left side crank arm. Online search should show you various options. What you'll need to measure is the center crank hole to pedal hole distance and determine if 165mm, 170mm, 172.5mm or 175mm. Usually 170mm is the most common. Just a left arm crank alone for JIS tapered square spindle runs around $10 before shipping. Search for example "Vuelta crank arms" and you should find a silver finish lefty for that price. There must be black ones out there too. 14mm socket usually to put on the crank, and torque to 25 - 30 ft-lbs. For maintenance tips, see the pages posted by "Sheldon Brown."

I recommend that you do a survey of multiple bikes and their riders/operators on the base and ask if they have issues and need parts. Get an inventory list of parts and tools and then either try to contact whomever is the quartermaster/logistics guy in your unit and have them requisition parts, usually in units of 35 lbs - 50lbs (which is what they tell family trying to ship care packages overseas to APO/FPO addresses). It may take a bit longer to get to you personally, but it will keep more bikes in service on the base. If it's a no-go to get DoD to pay for bike parts, come back here and PM me with the "list." It's $12.95 for a USPS priority box for 12 x 12 x 5 inch box - if it fits, it ships. Will need name, rank, unit, address overseas, APO/FPO zip.

Things to put on the list:
- repair parts: tubes, tires, patches
- wear/tear parts: brake shoes (specify type - vbrake, disc, caliper, cantilever), cables, levers, brake noodles,
cable housing,
- small tools: freewheel remover (spec type), cassette remover, BB wrenches, headset tools, etc.
- parts/accessories: water bottle cages, LED lamps, LED tail blinkers, machine screws and nuts, bearings, head sets (spec type), etc.
- gruppo-parts: hubs, front/rear derailleurs, chains, brakesets, BBs, shifters, etc.

Having information on makes/modesl of bikes you're trying to service will be helpful.
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Old 12-01-11, 03:58 PM   #9
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Except they can pretty easily be overtightened so the crank is again damaged and refuses to sty on.
It's possible to overtighten crank arms but unless the mechanic uses a long breaker bar on the socket or hex bit, that's not a common problem. Leaving them too loose is far more often the cause of crank damage.
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Old 12-01-11, 06:19 PM   #10
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i find that with a 7inch std 3/8 ratchet and tightening by feel i fall short of 30 ft lb by a 1/4-1/2 turn or so. i am a strong young guy as well. can do do quite a few pull ups and dips at 190lb weight.
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Old 12-01-11, 08:48 PM   #11
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It's possible to overtighten crank arms but unless the mechanic uses a long breaker bar...
...or if someone in the military is told "it'll come off unless you get it REALLY tight"...
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Old 12-02-11, 10:38 PM   #12
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Loose Bolt

Hello,

Thanks for all the replies. You guys have provided a lot of feedback on getting the nut on tight. When I initially had problems with the nut comming off, I did tighten it as much as possible. After tightning the nut on like that twice, I began seeing serious wear on the nut's threads. That's when I had the nut tig welded to the post.

When I got the tig welds grinded off and took a look at the bolt's threads, they were totally shot. I was lucky in finding another bolt. It seems the bolt metal is a lot softer than the post metal. Given my past experience, cranking this bolt on again is going to result in stripped threads and a loose bolt and crank again.

I'd like to see about replacing my bottom braket. Looking on Amazon, I can buy one for cheap. My problem is that there are several sizes available. I looked around on my bike and bike manual but I haven't found any specifications marked for any parts on the bicycle. What do I need to measure to buy a replacement part?

If I replace the bottom bracket, should I replace the front chain ring and cranks? Someone told me I should try to replace all of those parts as a set to ensure compatability.

Thanks,

Rob

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Old 12-03-11, 12:05 PM   #13
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Tightening the bolt after the first time the crank has fallen off is closing the barn door after the horse has left. Insufficient torque when the crank was first installed allowed the crank to wiggle and damaged the flat faces of the taper. After then, no amount of tightening will keep the crank on, as you found. A new crank arm will probably stay on with less torque than what you used after the original fell off.

A lot of people reccomend using a torque wrench to install cranks, and while I have no better strategy to add, I have not had good success doing that... I do it by feel.
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Old 12-03-11, 12:19 PM   #14
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The first and most important thing to replace is the damaged crank arm, which cannot be saved, nor will it ever stay tight no matter how hard you tighten it. Unless you replace the crank every dollar you spend elsewhere is a wasted dollar. Unless there's a reason, like bearing problems, leave the bottom bracket alone for now. If you want to replace the crank mounting bolt (or nut) go ahead, they're cheap enough anyway.

Once you get the new arm, mount it dry and tight. You don't need a torque wrench, you'll feel the wrench torque ramp up as you need the bottom, and can judge when it's OK. then listen and feel for any sign of looseness in the cranks, and deal with it immediately so you don't end up in the same place again.
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Old 12-03-11, 12:38 PM   #15
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My advice is to get and use a torque wrench and find out the correct torque number for the new crank. You can slide by without one, but it's hard to judge without some practice. Over-tightening is possible too.
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Old 12-03-11, 07:57 PM   #16
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Originally Posted by ramseyrt View Post
Hello,

Thanks for all the replies. You guys have provided a lot of feedback on getting the nut on tight. When I initially had problems with the nut comming off, I did tighten it as much as possible. After tightning the nut on like that twice, I began seeing serious wear on the nut's threads. That's when I had the nut tig welded to the post.

When I got the tig welds grinded off and took a look at the bolt's threads, they were totally shot. I was lucky in finding another bolt. It seems the bolt metal is a lot softer than the post metal. Given my past experience, cranking this bolt on again is going to result in stripped threads and a loose bolt and crank again.

I'd like to see about replacing my bottom braket. Looking on Amazon, I can buy one for cheap. My problem is that there are several sizes available. I looked around on my bike and bike manual but I haven't found any specifications marked for any parts on the bicycle. What do I need to measure to buy a replacement part?

If I replace the bottom bracket, should I replace the front chain ring and cranks? Someone told me I should try to replace all of those parts as a set to ensure compatability.

Thanks,

Rob

There are two primary measurements for most bottom brackets - the shell width and the spindle length.

THe shell width is the width of the bottom bracket shell of the frame into which the bottom bracket cups screw. FOr most bikes this is either 68mm or 73mm. THere are a few other bottom bracket standards available but they are not commonly used on entry level mountain bikes like yours. The spindle length is the length in mm of the existing spindle from the end of the flats to the end of the flats (total length not counting the threaded studs). This should be somewhere between 107 and 127mm.

So to find a replacement you need to select a cartridge with the same shell width (68 or 73 mm) and a similar spindle length. And you need a new left crank arm. Just about any let crankarm will work as a replacement. The one thing you need to be aware of is that the square hole is sometimes oriented differently by forty five degrees in different crankarms, so make sure your arm and the replacement arm have the same orientation.
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