Wariat

Hi all,

I'm a former BMX kid trying to get back in the saddle after a while away from cycling of any kind. With my BMX days long gone, I hit up a local bike shop to find a mid-range general purpose mountain bike. Decided on picking up a Giant Revel 3 for about $350, and have mostly loved it ever since.

My problem is that the non-drive side crank arm keeps getting loose. Specifically, the bolt holding it in place keeps loosening and falling off. Sorry for starting a new thread about it, but I couldn't find one that specifically talked about my crankset and the methods I used to try and fix it.

So far, I've taken it back to the bike shop I bought it from twice. The first time, the tech replaced the arm for me with a spare he had from the same model. The second time, he ordered another crankset and swapped out both arms for me and along with the newer bolt. After riding it around, I noticed the arm was loose again the second time I rode it.

Before this happened, I picked up a torque wrench plus some Loctite (both blue 242 and red) just in case. The bike shop tech mentioned to torque the bolt to 30 lbs./ft. So I applied the blue Loctite to the bolt, screwed it in, torqued it to about 32 lbs./ft., then let it set over 24 hours or so (didn't touch the bike until noon the next day). Riding around today as usual, I noticed it starting to loosen toward the last quarter of my ride and, out of desperation, here I am.

I was thinking of applying the red Loctite next, but with the words "PERMANENT" written huge on the package gave me pause. I don't know if I'm doing something wrong in tightening the bolt or if there's some kind of super secret trick that only cycling experts might know.

According to the bike shop tech and my after action inspection, the spindle seems fine, and the crank arm is brand new. Ever since the first replacement I've been riding around with an allen wrench and have caught the bolt before it gets way too loose and does some serious damage to the crank arm, so the parts themselves seem fine. I just can't get it to stay on. I'd be super appreciative for any suggestions since I mostly love the bike and having actual gears is a blast.

My bike is a Giant Revel 3 with a SR Suntour XCE, 28/38/48 crankset. Here is a direct link to the crankset I believe I'm using:

Details*- SR SUNTOUR Cycling

And here is a direct link to the Giant Revel 3 specs if you'd like to know more:

Revel 3 (2015) | Giant Bicycles | United States

I've been using the bike on mostly paved, slightly rough surfaces. Think sidewalks, slightly cracked asphalt, the usual urban and suburban stuff. Occasionally I'll take it over some grass or a small sandbank, but nothing hardcore. The most stressful thing I think I do with the bike is to pop up and off of curbs and sidewalks.

One important detail is that I'm a pretty huge dude: I weigh in at 350lbs. I noticed the weight limit in the user manual mentioning that the bike can more or less handle up to 300lbs., so this might be the root of my problem. That said, I've never had this problem before with my old BMX cranks, normally the pedals would give up with those every once in a while, but I've never had a crank fail as this one has.

I've also been considering picking up a brand new crankset, specifically one with two bolts holding the crank arm to the spindle. I'm not even sure if they make these for mountain bikes, but my old DK General Lee had a great SNAFU one and that crank was boss.

Thanks for reading this far if you have. Any suggestions or miracle cures would be fantastic, I'd appreciate anything you can shoot my way. Likewise, if anybody has suggestions re: new cranksets for this kind of bike or a dude like me, I'd also really appreciate the advice.

Also some pictures of the culprit:

bikeman715

If you do use the red Loctite , then you will need heat to remove it , which could hurm the arm in the removed . At this point I would change out the BB or at lease have the treads of it chase out with a tap , sound like there might be something wrong with the treads where the bolt not holding .

dsbrantjr

I would suggest that you not do anything "permanent" to the bike while you are working with the shop on a warranty issue, lest you open yourself up to finger-pointing. They seem cooperative, I would not do anything to jeopardize that relationship, or to void the warranty.

I agree that there might be a problem with the bottom bracket which is not obvious to visual inspection; perhaps it is machined improperly. Since a new crankset and crank arm have both failed perhaps a BB replacement is in order. Hopefully the new crankset has not gotten damaged as well.

Wariat

Thanks for the responses.

I'm a bit wary of the bike shop mostly because I just found out on my second visit that they're in the process of selling off the business. They've been pretty cooperative but I'm hesitant to go back for a third time. Plus, after the new parts came in, it took over a week for them to put it all together. I don't mean to be ungrateful since it's free service, but it sucks having to wait that long for something that apparently I could have done myself.

That said, changing out the bottom bracket seems like a good idea, and chances are the bike shop will do it for free. Just to be clear (since I'm an amateur), the bottom bracket is what the bolt holding the crank arm screws into? And the idea is that the bracket/spindle itself might have a bad thread, not the bolt itself?

Also does anybody know what the chances are that the red loctite will fix the problem? Is it just really strong duct tape on a problem that could get a lot worse down the line? Or could it be a semi-permanent solution?

Ideally, I'd rather not deal with the bike shop anymore but if I need a new bottom bracket, I don't see how I can't.

dsbrantjr

If you apply red Loctite not much short of a torch or heavy-duty heat gun is going to get the parts loose again. What will you do if the crank arm loosen up again and you can't get the bolt loose, or tighten it either? I would suggest exhausting all of your options at the shop before resorting to that.

You might consider contacting Suntour to see if they have any suggestions; maybe there was a bad manufacturing run or something else they could help with. If there is a defective part they have supplied they share responsibility with the shop to make it right.

Wariat

Great point, I'll give the bike shop a call and see what they say. Will update the thread if I find a solution.

AnkleWork

You should not need thread locker in any case. Is this a square taper crank/bb? Something in your blurry pictures does not look right. Sharp, well lit pics from two or three angles might make a difference.

cale

You have probably rounded the flats in the crank arm. You probably did it twice. You're plenty heavy to cause a crank arm to pull loose, that is if the pedals don't break first. Sound familiar?

Here's what you can do.

a) Buy a better crankset and bottom bracket recognizing that for you, standard cranks don't apply.

b) Learn to lay on the power more gently. Start up in a low gear (big cog in back, middle ring in front) and ease your way up to speed.

c) Stop jumping off curbs. Haha

Wariat

Yup, it's square.

Definitely could be the case, and another reason why I'm a bit wary of that bike shop. The tech seemed earnest enough but he said the BB looked fine, so I trusted his judgment.

This is my big worry, that the crankset, BB, everything is working normally, but I'm just too beefy for the setup. Everything works great when it works: there's absolutely no wobbling, the crank feels firm, and I don't think I jerk the pedals. But somehow the bolt keeps loosening.

In terms of looking for a better crankset/bb, anybody have any suggestions? Specifically for something heavy duty that can handle a big boy load?

I'm just going to contact the bike shop and lay it out for them, see what they suggest. At the minimum, I'll change out the BB. But if I'm bringing it in to them, it would be nice to just throw some extra cash at the problem for a beefier crankset. Only problem is that I'm totally unfamiliar with cranksets for mountain bikes/hybrids. Any info you guys can throw my way would be much appreciated.

cale

The bottom bracket generally doesn't break. The cranks are typically made of a softer metal than the bottom bracket which contains the hardened steel axle. The relatively soft crank arm metal is deformed by the hard steel axle. It really not a big deal. I rounded out plenty of crank arms in my youth. Mostly because the cheap aluminium cranks of the 70's were really badly made. Unfortunately, the crank you referenced reminds me of those old cranks.

I would have to do some research to find a suitable replacement but in general you could move up to a 2-piece crankset (the crankset contains the axle and runs on a "bottom bracket" composed of two external bearings and a sleeve in the middle).

Homebrew01

What's that sticking out of the crank arm ??

Anyway, you are not too beefy, and your riding styles does not matter. The problem is with your crank. The BB axle is steel, and the arm is probably Aluminum, so the BB axle will not get damaged.

1 thought is if the crank arm is going on too far, due to a mismatch, or rounded opening, then it can never stay tight because the crank arm slides too far onto the axle, beyond the reach of the the bolt & washer.

When the crank comes loose, is the bolt still tight ?? Or is it also now loose and easy to turn ??

cale

No one said anyone was too anything, Homey. But riding style matters. Really matters. That is the blood.

Homebrew01

Riding style doesn't make a crank bolt come loose.

cale

Starting up in too high a gear, a riding style choice, can cause a cyclist to round out the crank arm by applying too much leverage against the hard steel axle.

Yup it can, it can, and nothing you can say can make it otherwise Mr. Walk All Over Others On My Way to Show I'm Right.

Homebrew01

World champions have used square taper cranks for decades without problems.

cale

World champions have riding style in spades.

Wariat

The thing sticking out of the crank arm seems to be this heavy duty plastic washer. It appeared after the second time I took the bike to the shop. Should've asked more questions about it when I saw it, but the tech told me it's "just some plastic" so I took his word on it. The original bolt had a regular metal washer.

Anytime the crank gets to the point of being loose, I can feel it start to wobble when I pedal. Before that, the crank feels fine and has no wobble or strange behavior. As soon as I feel the slight wobble I stop the bike and check the bolt. While the bolt is still more or less finger tight in the crank, it's not wrench-tight. I carry around an allen wrench to retighten the bolt when I feel the wobble, and there's a fair amount of rotation when I retighten it. Not enough to do by hand, but enough to need an allen wrench and to put your shoulder into.

That's the impression I've got from the research I've done, and the bike shop tech also mentioned this. But I think what previous posters are saying is that while the outside of the BB might not be warped or messed up, the interior thread -- the part that the bolt screws into -- might be stripped or machined incorrectly. It feels like a long shot, but it's definitely something to check out.

Are steel crank arms a thing? I wouldn't mind a heavier overall weight if the crank can handle some extra punishment.

Wariat

Forgot to ask: is there much of a difference in quality in terms of two-piece versus three-piece cranksets for mountain bikes? Only reason I ask is that the stock two-piece (I think it might have even been one-piece?) crank on my old BMX bike was pretty cheap despite being steel. I eventually moved up to a forged three piece crank that was great and incredibly sturdy.

AnkleWork

Time for a new mechanic.

As for a steel one-piece crank: your frame probably won't fit one.

Two-piece vs. three-piece is not inherently an issue of quality but you may find that a two-piece crank is better suited to your use because of its very different attachment method.

Wariat

Just started looking at two-piece cranks and the way the crank arm bolts onto the spindle looks way better. When I bring my bike into the shop, I think I'll just ask for an upgrade.

AnkleWork

If you end up getting stuck with the original crank, don't give up on it. It won't hurt to try some other measures (which don't involve thread locker).

Wariat

Any suggestions? The only idea I have right now is to replace the BB since the inside thread might have been machined wrong.

cale

You probably moved to quality cranks. That's what's up here. Anything is possible but generally speaking what happens is that the left arm is more flexible (it doesn't have the additional structural design/material that contributes to stiffness on the drive-side). This slight flexibility does act on the retaining bolt which holds the crank to the axle, it stretches the bolt minutely and at the same time it acts, like a good lever, to move material around the square taper of the axle. All this is very slight and generally is not significant enough to loosen the crank arm. However, provided enough force and enough bolt stretch/movement, the crank will work continuously to enlarge the hole into which the axle is stuck.

The 2-piece design offers a few advantages. The entire structure, cranks/axle/bearings, is much stiffer because the drive-side crank has the axle spindle permanently affixed and this spindle is of a significantly larger diameter (eg stiffer) than the crank style you currently have. There are some measurements that I am not able to determine so, I'm afraid, I can't make more than general suggestions.

catonec 

You probably dont have to replace your bottom bracket (crank bearings and spindle). I would chase the threads, replace the bolts (if you havent already), and use the blue locktight. I have to use threadlock, and I have an expensive crankset so buying a new/better one as suggested above, most likely wont help.

AnkleWork

?!? How would that cause the retaining bolt to loosen? IMO you will not solve this by jumping to conclusions (no matter how deeply time honored that is in this forum).

Not to dissert at length, but the square taper joint is unique in machine design. It has dynamic factors not found in the usual bolted joint. It depends on a careful balance of elastic and plastic deformation in the arm, spindle, and bolt. You may have upset that balance and finding a new balance point could solve the problem.