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Could chain ring wobble be due to BB even if BB seems ok?

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Could chain ring wobble be due to BB even if BB seems ok?

Old 10-30-18, 11:27 PM
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el forestero
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Could chain ring wobble be due to BB even if BB seems ok?

I have a 3-yr-old low-end hybrid bike from the Taiwanese brand Samata that I bought new on sale for $200 from an LBS in Asia. It wasn't maintained well until recently and has been taken for lots of 25 to 150 km rides, mostly on-road, in a wet, humid country. It probably has at least 5,000 km on it.

As I began learning to maintain the bike over the last year, I noticed that the chain rings wobbled a bit. A few weeks ago I serviced the BB shell for the first time and cleaned up and reinstalled the old sealed-cartridge square-tapered BB. This fixed some serious creaking I'd been hearing for a couple months. Didn't know enough at the time to think of checking whether the chain rings were still wobbling after servicing the BB.

The other day I installed a new Shimano FC-TY501 crankset, bought from Japan's branch of amazon.com, and found the new chain rings also wobble enough that it's hard to properly set up the FD. I thought the problem might be the BB, but I don't feel any but the slightest play in the BB when the crankset is removed, definitely nothing in proportion to the wobble in the crankset. The BB also turns very freely, so doesn't seem to be worn out in this respect either. On the other hand, maybe a tiny amount of play in the BB translates to a lot of wobble in the chain rings? I don't know.

I wonder how likely it is that a new low-end Shimano crankset like the one I just installed was damaged prior to sale or manufactured in such a way that it wobbles. Is it way more likely the BB needs to be replaced? Would an experienced mechanic check whether the crankset is the cause of the wobbling at this point, and if so, how? Or would they assume it's the BB and replace it and see if that fixes the wobble? Or could it be that the crankset isn't attaching perfectly straight onto the BB axle for some reason?

BTW, it appears I overtightened the BB when I reinstalled it and I'm having trouble getting it off, so will have to say for now that I don't know the BB model.
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Old 10-31-18, 09:04 AM
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Shimano's TY series is among their lowest cost production. So of course the tolerance they manufacture to will be less (or that any ring flatness will be less). You could always reinstall the old crank arm (only need to do the RH side) and see what it's wobble is like to start to establish your guide lines.

Stamped steel rings, assembled with rivets and having other elements (like a chain guard or shifting aids) have always tended to be less flat (or when spun, more wobbly) then the AL machined rings that attach with bolts. Whether the BB is also contributing to a wobble is really a different aspect that also needs attention. But without more investigation we can't say how much wobble comes from what part.

Did you lube up the BB shell and the BB unit's threads before installation? Andy
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Old 11-02-18, 06:39 AM
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Thanks for the info, Andy. Very helpful. Will try reinstalling the old chain rings to compare wobble with the new ones.

I greased the BB shell and the BB unit's threads before reinstallation. But the BB threads were a bit rusty, and I suspect this, in addition to my overtightening during the reinstall, is causing the sticking. Going to buy a bigger wrench so I have more leverage than I do with my current tool.

Hate to keep throwing good money after bad on such a cheap bike, but it's what I have, so will probably need to buy a new BB due to the rust, even if the BB is not the cause of the wobble.
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Old 11-02-18, 10:52 AM
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The tools you buy to service this bike will likely be useful on other bikes, so that's no waste of $. Shimano's range of BB fixed cup tightness (in the shell) is 35 to 50 foot pounds aprox. (depending on BB type and cup material). That's a fair amount of pressure and more then many home mechanics expect. Here's a torque chart to help you. https://www.parktool.com/blog/repair...s-and-concepts Andy
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Old 11-02-18, 11:17 AM
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After checking that nothing is amiss in the mechanical installation, your issue is not uncommon.

Something as simple as carelessly strapping the bike to a bike carrier in such a manner that a strap pulls across the rings could cause a bend. I did that once and all was well until it wasn't. I bent the ring back, and that got me until I decided to swap for different rings altogether. I was much happier with 50/34 than 46/36.

I had an FSA Vera crankset and the problem was one of the arms of the spider was out of plane with the other 4. A block of wood & a large hammer got it passable but it had a habit of dropping the chain under high load. In the end, a full crankset replacement was the solution but I got many thousands of miles out of it that way.

More recently on my commuter, I noticed an unexpected/unusual sound at the crank, but it was related to crank position in some gears but not others. The problem was a chainring bolt worked itself loose. Allowing extra flex in the big ring where the ring wasn't secured and the chain to rub the back side of the bolt (nut side) when in the small ring.

Good luck.

Last edited by base2; 11-02-18 at 11:20 AM.
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Old 11-02-18, 04:25 PM
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Thanks for the helpful info, folks!
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Old 11-08-18, 05:15 AM
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It turns out my torque wrench calibration was way off. Tightening with it in one direction read way too low torque and the other read way too high. I definitely tightened one side of the BB far above recommended torque and the other far below. No wonder things were rolling around. Just cleaned and regreased everything up and reinstalled in a good torque range, and now the drivetrain's much less wobbly.
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Old 11-08-18, 09:49 AM
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Still wobbling
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Old 11-10-18, 03:26 AM
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yes, it's still wobbling despite the improvement since reinstalling the BB. it does appear I have not-very-flat chainrings.
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