chronic loose cranks, just replaced whole thing recently
 

Hi, i own a 7 year old fuji supreme hybrid bike. for the first 5 years everything was well, but 2 years ago, my left crank arm started falling off. I went to multiple bike repair shops, none could make it last more than 3 months. The last place i visited replaced both cranks, the bolt, the threaded rod that links both cranks and the front gear set, the new set is different as instead of a square hole, it is more like a pattern with many teeth to prevent widening the hole if it gets loose again, and the bolt is a 7 mm alem (hexagonal hollow). He even used blue locktight as well, in april or may, and i've been going much much easier on acceleration and started downshifting more on complete stops, but now it's loosened again last week. I pedaled using only the right side as soon as i noticed, and took it to be re-tightened, now a week again it's loose, which probably means the hole is deformed AGAIN.

I'm tired of being robbed by bike stores that look like they don't know what they're doing. The bike when new last me 5 years without crank problems even though i weight 230lbs, never, ever let the rear shift go out of the smallest gear, and pedaled with my full strenght more often than not. Now even when replacing the whole thing and paying attention to be easy on my bike it keeps falling off.

I've been thinking of getting an electric impact drill and new cranks, would that help? Or be damageful to my bike?

edit: i don't care what tool i have to buy, i just don't want to have to worry about this problem ever again, even if it means soldering the damn thing shut

What brand of crank was the first one and the new one?

What kind of BB is it? By that I mean is a square taper or and ISIS?

Is the 7mm bolt aluminium or steel? If AL get a compatible steel one instead, and use a torque wrench set to factory specs for that unit, I'm not sure but I think it's 35 foot pounds which means you may have to slip a pipe on the end of the wrench for added leverage, but check that torque rating just don't take my word for it. And use Blue Loctite on the threads. And do not exceed the factory recommended torque setting or you could ruin the BB.

Either you didn't tightened it up correctly to begin with, or there was inadequate lubrication of the bolt and spindle interface.

Did you replace the washer between the bolt head and crank with a new locking washer?

After you put it together, try wiggling them side to side laterally to see if you feel any looseness in two different positions: 12:00 and 6:00, then 3:00 and 9:00.

If this problem continues your bottom bracket may be shot because the crank holes are rounded out and the crankset is shot.

Read this too see if this helps any with further info: http://www.sheldonbrown.com/brandt/i...ng-cranks.html

Do not ride the bike with a loose crank arm or you will damage the inside locking taper on the crank. If you rode it loose with the new crank arm then you more then likely damaged the new one.

Soldering won't help but welding would...but please don't do that in less you don't care about the bike.

I am surprised that the LBS can't fix this problem, it's actually fairly common problem with newer bikes, not saying it happens to every crank, just saying it does happen with enough frequency that the LBS should be familiar with the problem. If they can't then take the bike somewhere else and never go back to that LBS, because it is a basic repair thus what else are they good for? tire repairs only?

Cranks get loose. Both square taper, and spline drive cranks. That's just a fact of life. Usually they only to be retightend once or twice in their lifetimes.

But the key is to be attentive to them and retighten immediately.

The ability for a crank to stay tight and carry high torque loads to the spindle depends on a tight rigid fit. Riding a loose crank distorts the inside of the mounting area and over time (not a very long time) enlarges it to the point that the crank will never be tight again.

Really? why? is this something unique to newer designed cranks?

The reason I ask such silly questions is because I have a bike with over 150,000 miles on using Suntour Superbe crankset and bottom bracket and it has never come loose. Funny thing about, all 8 of my 80's era bikes cranks have never come loose either! And those either have Suntour or Shimano or Sakae cranks. My only newer bike, a 07 Mercian uses a Campy Athena crank and bottom bracket and it too hasn't come loose...but in all fairness that bike only has 2,500 miles on it, so I guess time will tell.

I'm not telling you this because I doubt you, I'm mentioned all of this because if your right then the newer technology is not as good as the 80's in regard to the cranks. I've only had one crank loosen up in my lifetime, and that was on a 71 Puch that had the cotter pins.

Maybe I should have said "any crank can get loose"

You're preaching to the choir with regard to square taper cranks. All my cranks are square taper Campy, Spb Pro and XC Pro, and like you I've never had any problems, though I may have had to snug one up once. Nor have I ever had to replace a BB component in over 100,000 miles.

But ask any dealer and he'll confirm that cranks do loosen, especially lower cost cranks. While the various spline drive versions are claimed to superior in this regard my experience and observations don't bear this out.

The point is that if a loose crank is dealt with promptly, it'll be OK, if ignored it only gets worse. I also should add that there's no excuse for riding a loose crank since they announce themselves very plainly with a once or twice per pedal revolution click or creak.

When I worked retail folks would come in with cranks beat beyond salvage. I'd ask them if didn't they hear or feel the movement, and invariably they say yes, it's been going on for months.

From a strict engineering perspective, most of the crank/bb interface designs are surprisingly poor. The challenge of that junction is that it has to carry torque in both directions, with no real way of setting up a decent preload. Old square taper get some preload if mounted correctly, but octalink and its likes relies almost entirely on the initial fit of the parts.
And once parts have the slightest bit of movement compared to each other it's pretty much downhill from there.

SP, it sounds like you had a square taper style bottom bracket (the bearings and spindle mounted in the bike that the crank arms bolt on to), and now have a spline style bottom bracket. As the others have mentioned, either should last a long time if properly installed and tightened. Unfortunately, you have to depend on the bike mechanic to do this properly, and finding a good bike shop near you can be hard. If you don't mind posting your area, we might be able to suggest some places; Yelp can be a good resource, too.

In any case, once you get the current BB and cranks repaired or replaced, I'd *strongly* suggest taking the bike in for a checkup periodically (within a couple of weeks initially, and maybe every 6 months after that) so that a bike mechanic can look over the bike and make sure the crankarm bolts are still tight, the BB is secure, and so forth. Bikes are not a fix and forget type of item; they've got bearings and lots of moving parts, and need periodic maintenance.

The irony here is that the bad, old fashioned, antiquated and obsolete square taper system has done the job amazingly reliably for 4 decades, while the better, newer designs have more problems with staying tight.

The issue of torque reversal isn't big on freewheel bikes, since the crank only drives in one direction, and the reverse torque is only what's needed to lift the leg. On fixed gear bikes, the reverse torque can be very high, yet the square taper handles it with ease.

While I believe that a tapered interface is inherently superior, it's an academic issue unless one wants to enter into crank production. For the rest of us, we can choose any color as long as it's black.

the old design was indeed square taper, and i think this one is octolink. I haven't seen any kind of washer when i removed the BB, that might be part of the problem. If i buy a washer and re-tighten it myself, where do i put the washer? between the bike and the BB?

I don't have the original brand of the stuff in my bike, the bb was replaced once, the left crank, 4-5 times. I'm currently checking to see what the brand is for what i have right now. but honestly, if it comes back again, i'll just repolace the whole BB/crank combo with whatever you guys recommend. I don't mind paying for the tools and the poarts if it solves my problem. If i can get away with just a slight tightening every year it wouldnt be that bad, but right now it's a lose the bolt/crank every few months situation, which is unacceptable.

If anyone knows reliable shops in longueuil, canada, i'd be happy to go there. Otherwise, does anyone have a good online store for both the parts, and the torque measuring ratchet?

Thanks for the reply, that was very interesting to say the least. All my bikes do have the square taper design and I guess from what you said that's why I haven't had any problems.

While seated, yes. But unless you have the same foot forward everytime you go light or coast standing, the cranks will see a fairly serious reversal of torque. I'm thinking coming off a curb can be just as bad, if not worse, than fixie back pressure.

The center bolt that holds the crank against the bb axle, should have a washer between the head of the bolt and the crank arm. Sometimes a special bolt, whose head flares out at the base to a sort of integrated washer is used, in which case a separete washer isn't needed.

I agree, and actually use this to test for loose cranks. Standing and jumping a bit with the cranks horizontal, then reversing the position and repeating while listening for the telltale creak or click.

In any case the square taper system has proven it's ability to handle both this and fixed wheel service very reliably for years, so whilt it might be improved, it's well adequate to the task (if executed well). I suspect that what started to give the design a bit of a bad reputation was poor execution and it's use on ever cheaper bikes.

In the original iterations, there were quality forged cranks, on machined spindles with broad flats and small corners, over time we saw lower cost cranks of softer aluminum, on forged to net shape spindles with ever narrower flats, and large rounded corners. This pushed the concept beyond it's limits, and combined with the fact that the cranks were ridden by casual cyclists not attuned to their bikes led to large increases in failures.

Unfortunately, in opting to go to splines (improvement if executed well), they also chose to eliminate the taper, which IMO is the major flaw of current designs.

Like anything engineered, it takes good execution of good design to make things work. One without the other doesn't cut it.

Exactly. With a design consisting of only parallel planes of interface, and a very modest assembly force needed, there's no way to preload the interface. W/o preload, wobble, and subsequent wear of interface surfaces is just waiting to happen.

the shimano outboard is awesome

but the same can't be said for GXP and ISIS and ultratorque