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Gap between crankshaft and bottom bracket bearings. Does this look right?

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Gap between crankshaft and bottom bracket bearings. Does this look right?

Old 09-23-12, 02:13 PM
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diverguy
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Gap between crankshaft and bottom bracket bearings. Does this look right?

Three year old Cannondale Synapse 5, FSA triple chainset with a MegaExo bottom bracket. The LBS said the changed the bottom bracket bearings about two months ago. A clicking noise returned about a month later. Pulled the chainset and found A) a 1.5 mm gap between the inner bearing race and the shaft and B) the right bearing is making a growling sound in certain spots when turned by hand. Honestly, I'm not so sure they put in the right bearings. Does this look normal?


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Old 09-23-12, 02:30 PM
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No, for the bearing (or ANY bearing) to work properly, the axle must be supported with a tight fit. Otherwise it will flex, possibly leading to fatigue and failure (snapping under load).

You need to replace 2 items

1- the bearing (and possibly it's mate on the other side
2- the mechanic who considered this OK.

BTW- it's possible that the bearing is correct, but a shim of some kind was omitted (I don't stay up with every BB or crankset), but the effect is the same. Also check the other side, since riding this way would have stressed the other bearing causing excess load and wear.
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Old 09-23-12, 02:44 PM
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Originally Posted by FBinNY View Post
No, for the bearing (or ANY bearing) to work properly, the axle must be supported with a tight fit. Otherwise it will flex, possibly leading to fatigue and failure (snapping under load).

You need to replace 2 items

1- the bearing (and possibly it's mate on the other side
2- the mechanic who considered this OK.

BTW- it's possible that the bearing is correct, but a shim of some kind was omitted (I don't stay up with every BB or crankset), but the effect is the same. Also check the other side, since riding this way would have stressed the other bearing causing excess load and wear.
Thanks for the reply and insight. There is a sort of shim. Kind of. I thought they were a weather seal or something and didn't think to include them in the first photo. They're plastic, obviously worn, and certainly not sturdy enough to support any kind of load.



The bearing on the other side is the one that is growling and has a rough spot when you turn it by hand.

I'm going to A) Order a new MegaExo bottom bracket and B) Dump the mechanic. It's not the first time he has unwittingly screwed up but I've learned enough now to maintain it on my own.
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Old 09-23-12, 04:16 PM
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My guess is your LBS screwed up on the left side as MegaExo has a different bearing on the left than the right; the MegaExo has a tapered spindle that is 24mm on the drive side and 22mm on the left. Due to the fact that you have been riding this way it no doubt has trashed the right side bearing.
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Old 09-23-12, 04:43 PM
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Originally Posted by onespeedbiker View Post
My guess is your LBS screwed up on the left side as MegaExo has a different bearing on the left than the right; the MegaExo has a tapered spindle that is 24mm on the drive side and 22mm on the left. Due to the fact that you have been riding this way it no doubt has trashed the right side bearing.
+1, the right bearing is shot because the the axle deflected off line, and ran the bearing misaligned, causing excess wear at the sides of the tracks. Ironically the left side bearing is probably in perfect condition because it wasn't ever really loaded.

BTW- the asymmetrical bearings explain how the mistake happened, but not why it happened. The why relates to poor mechanic training. The mechanic knew how to assemble a BB following a sequence of steps, and probably using a torque wrench. When all was OK he could turn out reasonable work, but because he didn't understand how BBs work he didn't spot what should have been an obvious problem.

This is an example of what is becoming altogether too common, and what I call GPS training. Mechanics can do the work, but don't really understand how things work. I call it GPS knowledge by analogy to how people navigate today. Imagine two people from Chicago to Portland. One knows US geography and the other doesn't. They both plug Portland into the GPS and start out on I-90 as instructed. The difference is that one driver seeing the "Welcome to Indiana" will quickly realize that Portland Oregon is the other way, but the other may not realize the error until he's in Maine.
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Old 09-24-12, 10:24 AM
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Originally Posted by FBinNY View Post
+1, the right bearing is shot because the the axle deflected off line, and ran the bearing misaligned, causing excess wear at the sides of the tracks. Ironically the left side bearing is probably in perfect condition because it wasn't ever really loaded.
Perhaps I can understand this for angular contact cone bearings, but these are radial cartridge bearings that require no axial preload. The crank spindle is held in alignment by it's close fit to the inside of the inner races and won't wobble or deflect, unless it bends. I would think the spindle would need a lot more overhand than that to bend under normal pedaling loads.
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Old 09-24-12, 10:45 AM
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Originally Posted by Looigi View Post
Perhaps I can understand this for angular contact cone bearings, but these are radial cartridge bearings that require no axial preload. The crank spindle is held in alignment by it's close fit to the inside of the inner races and won't wobble or deflect, unless it bends. I would think the spindle would need a lot more overhand than that to bend under normal pedaling loads.
The types of radial bearings typically used on bikes are not designed for high thrust loads (which is why they shouldn't be preloaded). The shape of the grooves will provide location and tolerate light thrust, but no more. They're also very unforgiving of misalignment which is why they're always used in pairs. If the axle isn't supported at the other end, it deflects creating misalignment and high thrust at opposite sides top and bottom. There's also the risk of damage to the spindle, but that depends on the spindle's construction.

Since the OP says the right bearing is now rough, we can take that at face value regardless of the cause, though I suspect that side wear is the issue. I have mixed feelings about the spindle. If when he removes the spindle, the OP finds scoring or wear inboard of the right bearing, I think he'd be within his rights to ask the dealer to replace the crank. At the very least, he should contact the maker for an opinion on that matter. If the spindle looks OK then it probably is since either way the greatest stress (normally) is outboard of the bearing, so it may not make a difference.
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Old 09-24-12, 04:46 PM
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Originally Posted by FBinNY View Post
If when he removes the spindle, the OP finds scoring or wear inboard of the right bearing, I think he'd be within his rights to ask the dealer to replace the crank. At the very least, he should contact the maker for an opinion on that matter. If the spindle looks OK then it probably is since either way the greatest stress (normally) is outboard of the bearing, so it may not make a difference.
The spindle seems to be okay. I looked at it with a magnifying glass and couldn't find any indication of improper wear. I'm going to speculate that it's probably okay.
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