My nephew has a mountain bike with a square taper bottom bracket that the non drive side crank keeps coming off. Its the second time it came off. What could cause this? Is it something he is doing? How do I fix this?

Chances are it came loose and the inside taper in the crank is damaged, take the crank off and check for a ridge inside it, it'll stop the crank seating properly on the spindle. You may be able to use a triangular file to reshape the inside of the taper so it's smooth.

Once the arm is damaged it will likely be toast. If he did not notice the initial warning signs it may be too late to save it. Good luck on the fix, though.

As mentioned above, if it has been ridden while loose, chances are the square broach in the arm has been damaged. You can try fixing it, as [MENTION=410984]Trevtassie[/MENTION] suggests above, but in my experience this is seldom successful. A replacement non-drive side arm shouldn't be too costly, though. Unless you're a stickler for looks, it doesn't have to exactly match the old arm. Just try to get the same length as the original.

If there's no play on the NDS crank after tightening the bolt, then try using blue loctite on the bolt threads. If there is play, then the crank and BB should be replaced. You can only replace the NDS crank. But sometimes its difficult to get the same make/model as a single crank. I ended up replacing the crankset and BB the last time I had a similar problem. You could also replace with a cheaper used crankset to save some money, worse case. In my case, I found a good deal on a Campy Centaur crankset and just went for that (used to have Chorus).

Locktite won't do much good IMO. As the problem isn't the retaining nut or bolt but the damaged arm's fit on the axle. It's rare that a ridden while loose arm can be successfully re tightened and stay put long term. Andy

If you do replace that arm, be sure to re-tighten it after the first couple of uses. Also, might as well check other side before it loosens. I know there's a lot of debate about greasing the tapers or not. I found that whatever grease happend to be an your fingers from working on the bike was just about enough to coat them.

Do not retighten a tapered BB. It should be torqued to spec on installation on a greased spindle.

Installing Cranks by Jobst Brandt

My slightly greasy fingers will stay out of this aspect of "proper" retaining bolt maintenance Andy (who feels that never checking one's work is a mistake, regardless of what you do during the follow up check)

Looks like there are decently priced replacements for the cranks, I will talk to him about replacing it. He probably needs a new chain while hes at it this is I think a 2009 or earlier bike i can almost assure it has the original chain. thanks for the information.

Also, there's "squares" and there's "diamonds". Be sure that the one you buy matches the one you're replacing. Otherwise your son is going to develop a real funny peddling style.

30 lb/ft. If you don't have a torque wrench imagine a 30 lb. weight hanging on the end of a foot long wrench. If you're using a 6 in Allen key, you probably aren't getting 30 lb/ft and you may get to do the job again. Too much torque, by the way, is bad too. It will crack your new crank arm at one of the corners of the square hole.

This bolded warning is so often stated that one would think it's a fairly common place problem. But after touching many thousands of bikes over a bunch of decades I'm good with saying that cracking at the tapered square axle hole happens, maybe, one time to hundreds of damaged tapers. So for most practical installs, of tapered square arms, I consider lack of tightness to be the problem to be avoided, not overtightening, during the initial install.

Now repeated retightening can wear the arm's taper, as in drawing up the arm on the axle and in time having the axle end's becoming flush with the arm's face that the retaining bolt sits on. If this happens then the arm will no longer be able to be further tightened (without grinding off the axle end a bit). I do see this periodically, just yesterday as example. Having the taper interfaces greased will allow this drawing up to happen more easily.

What I was taught 45 years ago is what I have followed on both customer bikes as well as my own. The arm and axle tapers are dry but for a finger/skin oil wiping. the retaining bolt is tightened to about the 30 ft pounds often quoted (I don't use a torque wrench but have checked with one many times over the years and confirmed the aprox. 30Ft/lbs goal many times). On my own bikes, and after a hundred miles or so, I recheck and usually get a fraction of a rotation of the retaining bolt more tightness (like 2 to 10 degrees of rotation), again using the aprox. same torque as the initial install.l If the bolt saw the upper end of additional rotation I might revisit the torque again a hundred miles later. That's it until the annual going over my bike (which might be around 1000 miles, I have a few bikes that see similar miles per year). On customer bikes I suggest (written on the service ticket if one was generated, verbally if no ticket was done) they bring the bike back after a week or three for a free check and possible re torquing. I suspect very few will take us up on this re checking (the exception often being those who spent $ to replace their damaged arm). On new bike sales the assembly bolt tightening is the same as an initial install and the bike's 90 day free service is when we check the bolt's torque again. This set of procedures have served me and my customers very well with extremely few cases of arm loosening, and those have usually been after a long period on no follow up checks (as in never until the rider feels a loose arm). Of course there are situations when I advice more careful install and follow up checks. Riders with a history of arm issues or those who are extremely heavy do crop up every few years.

As with any mechanical process others will have their opinions and procedures that they have found to work. The above is what I do. Andy

That's one opinion. It is, however, not exactly in harmony with the general and best practices of the trade.

The practical problem with the general recommendation to grease tapers is that everyone has grease sitting around, but relatively few people have torque wrenches. So tapers get greased but not torqued to a value. And greased tapers make overtightening much more easy, so you end up with the OP's result.

Most manufacturer spec is to use a dry spindle. The major bicycle mechanic's institutes also recommend dry installation. So does the JIS manual: "Affirm that there is no adhesion of foreign matters and the like at the inner wall of corner hole of right crank and right corner face part of bracket axle..." JIS recommended torque is 30 Nm.

I disagree strongly with the general recommendation to grease spindle tapers because of the above, and for other reasons I won't go into depth on here, because they have been elsewhere discussed to death. Suffice it to say that torque values given by manufacturers are always for dry bolts and tapers unless specifically noted otherwise. Greasing either or both requires the torque to be decreased by as much as 25%.

I DO agree strongly with Brandt's recommendation that correct torque is far more important than whether the spindle is dry or greased. If you have a torque wrench, feel free to use a tiny amount of lube if you want. But take into account the effect that has on the torque value you should apply to the bolt. I also agree with him that too much torque will not crack your crankarm unless it has been done repeatedly, causing the bore to become oversized. (Which, may I point out, is not exactly in harmony with his recommendation to grease the taper, which increases the chance of overtightening even when using a torque wrench.)

Much of the practice in the trade is based on superstition and not engineering. Ask a machinist about how tapered bits are put in a drill press.

The new Shimano cranks I've fitted have come with grease in the taper?

+1

As Ghrumpy notes, "to grease or not to grease" has implications on the optimal torque. Huge machine tools use clean, ungreased ROUND tapers to hold cutting tools, and in many cases these machines are putting 25 or more horsepower into the tool - far more than legs put into a crank arm.

With my old Campy crank arms, I used ungreased taper, ungreased bolt and it worked fine and held the things on for many years. I think greasing the bolt, or using medium loctite, would be better. But again, the optimal torque should be different depending upon whether you have lube on the taper and/or the bolt.

Here's a large shell facing mill that has what is known as a Morse Taper (MT 7, for those interested):
https://www.algebra-inc.com/namedpic/...rbor-15679.jpg

This idea seems to be thrown out whenever someone has a disagreement. But it does not advance the argument; it is fallacious and incorrect. Fallacious in that it attempts to cast doubt on the value of any of the practices of the trade, regardless of their actual basis. Incorrect in that most of the engineering and servicing of bicycles and their parts is not unique nor specific to the bicycle, and it has been in constant refinement since the 1800s.
I would say that some of the incorrect practices in the trade result from the fact that relatively few bike mechanics are professionally trained, and even fewer home bike mechanics are. Such untrained mechanics, especially home ones, will usually do "what works" for as long as it does, and they are often quite proud of that. If that's what you mean by "superstition" then there might be some truth to your statement. They don't have that much influence on the professional trade, however. A significant amount of my time at the bench has been spent fixing mistakes made by home mechanics, and trying to explain to them the cause and effect of their problem.

Since the 1800s, there have been institutes and organizations that test and/or collate the experience of thousands of mechanics and engineers, and millions of bicycles, and develop recommendations for best practices. Some of those practices are likely based on practical considerations rather than textbook engineering, because the engineering gets tested to failure in the real world and often needs a workaround to perform as designed, or until it is redesigned. Practical considerations are not, however, "superstition." They are collective wisdom. Not that that wisdom is infallible, but to call it superstition does not do justice to history or to the efforts of those who have studied and tested these things.

With all respect due to Jobst Brandt, his way of understanding and doing things is occasionally in tension or even disagreement with other engineers, experts, and the collective wisdom. And he was not above using fallacious rhetoric to persuade or belittle others who disagreed with his conclusions. Often he was right, sometimes he was wrong, sometimes his way was just different. His voice, however loud it may be in the internet age, is not and should never be assumed to be the final word on any subject. To believe that it is, is what I'd call superstition.

I have, and I have some actual experience with them as well. As [MENTION=419049]WizardOfBoz[/MENTION] says, round tapers are used clean and dry, for the simple reason that lubrication decreases friction, and friction is what keeps a taper fit from moving. That is true whether the taper fit is round, square, or some other shape. Crank tapers are square because it's the simplest shape that will orient the two cranks 180° from each other on the ends of the axle.

Red Herring
tapered drill bits are steel (you remove them with a steel wedge and a hammer)
nowadays bike cranks are aluminum , their softness allows the stretching from overzealous tightening
a greased/oiled taper facilitates..

And irreparable damage riding them once the fixing bolt, or often, the nut, falls off or is let be loose..

fietsbob, On my old Paramount I used a scrupulously clean spindle and taper socket, and an unlubed screw. As I said above, I think I'd use grease on the screw and leave the taper really, really clean.

Is that the approach you'd recommend for Al cranks on steel or Ti spindles?

I will say this is the second bike he had this happen to lol not sure if hes just special or doing something in particular. First time was on a mongoose so I just figured cheap bike components.

I had a crank that kept loosening on me and driving me nuts. I ended up torquing the hell out of the crank bolt with a beastly breaker bar and it has been riding fine ever since.

Classic case of more is NOT better. You "solved" one problem but have almost certainly caused yourself another. Chances are excellent that if you ever have to remove the crankarm in question, you will not be able to reinstall it without either bottoming out the bolt, or bottoming out the crank on the taper. Might even crack it.

Better would have been to the clean the taper inside and out, and measure the torque instead of using a breaker bar. Most crank bolt wrenches are only about 6"/150mm long, which is sufficient to apply the recommended torque to the bolt without undue effort.

Since no one seems to believe Jobst Brandt and prefer anecdotal advice here's mine. I have two bikes with square taper BB and forged cranks. The road bike has 1254,529 miles on the crank arms When I have removed them to repair Phil BBs, or replace them with shimano's I grease the tapers and use a torque wrench set at 30 ftlbs. No problems with the cranks. The crankset is a TA Zepher triple.
My touring bike has 75862 miles on it and the only time I had crank problems was when I first built up the bike I bought a Sampson crankset. When I installed it the right side cracked at the BB. It was replaced and the new one did the same thing. They were not forged. They were cnc"ed from a block of 7000 series Aluminum. I replaced them with first with a Shimano mountain crank then with a Richie Logic set. They have been off and on as necessary with NO problems. I always grease the tapers on my and my friends bikes.

I have violated rule number 2. Sorry.
Bob, sorry about the bit thing. I haven't done that kind of work in over forty years and misremembered the facts.