Bottom bracket becoming a little wobbly.

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Bottom bracket becoming a little wobbly.

Postby cdysthe » Sat May 20, 2017 5:00 pm

Hi,

In installed a cartridge bottom bracket on my Mongoose Stat last fall. I have been riding the bike since. When I did a little spring cleaning and check-up there were a little wobble when I pulled on the cranks. I took the cranks off and the cartridge could be tightened quite a bit. It was not loose by any means, but I could tighten and the wobble was gone. Is it normal for bottom brackets to become looser over time, or should they stay in place and something is not right?
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Re: Bottom bracket becoming a little wobbly.

Postby NYMXer » Sun May 21, 2017 5:30 am

It happens sometimes, things loosen up with wear or screws vibrate loose, etc.
Just keep an eye on it but it sounds like it's not an issue.
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Re: Bottom bracket becoming a little wobbly.

Postby dddd » Sun May 21, 2017 10:19 am

NY is correct, as there is a bedding-in of all contact surfaces from the cyclic forces applied.

Usually a single re-tightening then stays tight for a long time, this is why shops give a free check-up to check everything is staying tight.

Don't forget to check spokes, they tend to loosen more now from the disc brake torqueing on the hubs!

And headsets also very often loosen a bit over the first one or two rides, but much easier to re-tighten than the bottom bracket.

One thing when I tighten a bottom bracket cup, I reach the needed amount of torque and then hold the torque for a few seconds as the cup sometimes creeps along until fully tight. There are a lot of threads in there and they may need a little time to settle in under the force of the wrench.

Crankarms are notorious for loosening, so after the first torqueing, I'll stand and bounce on the pedals a few times, reversing the cranks 180-degrees each time, then re-torque the bolts.

Pedals should always be torqued to a solid value, then with the wrench force sustained for a few more seconds. Even a slightly-loose fit at the pedal threads can be noisy and eventually will eat away at the softer aluminum of the crankarm.

Always grease the threads first, as well as the other (cup) mating surfaces.
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Re: Bottom bracket becoming a little wobbly.

Postby cdysthe » Sun May 21, 2017 12:09 pm

Thanks! Went over all of it and ready for summer and fall. Seems like things stay for about 6 months and the it loosens here and there. I see you say: "grease the threads". All threads, and doesn't that make it more likely to loosen? This checklist is awesome! :)
cdysthe
 
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Re: Bottom bracket becoming a little wobbly.

Postby dddd » Mon May 22, 2017 9:21 am

The grease is needed to allow smooth tightening AND to prevent corrosion that causes frozen-together parts that cannot be removed non-destructively.

The parts stay tight as long as there is sufficient tension force along the threading, so the slanted thread surfacess deform elastically very slightly and a wedging action results, to provide grab.
This tension force is the result of full and proper tightening/torqueing, keeps things from loosening.

Steel parts threaded into aluminum results in electrical activity which accelerates corrosion, so grease is mandatory. Same with aluminum seatpost or handlebar stem in a steel frame, without grease these parts can become severely stuck in place from a quite-compacted and sticky layer of hard corrosion.

Mainly the grease prevents water from getting in between the parts and causing the electrolytic corrosion when the two part's metals are different.

Steel-on-steel needs grease as well, since galling (cold-welding) of same-metal threads can immediately result and leave you with torn-up or frozen threads.

Lessons learned the hard way over the years, so you won't need a torch to take things apart later!
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