Diagnose Bottom Bracket?
 

How do you determine if you should replace the BB bearings or the whole bottom bracket?

I guess it would start with, how does it spin? Second would be disassembled, cleaned, and inspected. If the rolling surfaces are smooth the bracket is fine, bearing are probably good, inspect the bearings sometimes they can chip. When a bearing breaks sometimes they destroy the races or spindle but not always.

Many people just replace them every time they service. If the grease is good at service I keep, if the grease is contaminated I'll inspect closer.

Sometimes the brackets just wear out, usually one will see pitting on the races, but not always, worn bracket could have some slop side to side at the power position, but not at the 90' from.

Sometimes if the cups are borderline I'll replace caged bearings for loose.

Knowing what brand and model/type of BB you have will help find an answer.

Technically the bearings are the complete system that allows good rotation. So a "bearing" has an inner surface (cone) that the rolling elements travel on, an outer surface doing the same (cup) and those rolling elements (the balls). Most all loose ball BBs have the cones being a part of the axle. As the axle cones have less surface that the balls contact, compared to the cups, the axle ball tracks (cones) tend to break down (become pitted) before the cups do. Often the balls will look fine but the axle not so.

With a cartridge BB the factory assembly often has no prevision to take the unit apart to access any one part separate of the whole. So if your BB bearings are worn out or otherwise bad the usual is to replace the BB as a complete unit.

While with a loose ball BB one can replace the individual parts the ability to source the cups and axle is getting less and less with the cartridge design having pretty much taken over the supply chain availability. The cost to replace loose ball BB parts will not be too much less than replacing with a cartridge unit. Andy

Some cartridge bearing bottom brackets permit bearing removal/replacement.
Better ones are designed for that very level of serviceability.
Others don’t have enough space inside to get a puller in and the bearing out.

Barry

If it's a cartrige bearing or external cups I'd probably just replace the entire BB. If old style separate spindle, bearings and cups, then I'd replace whichever needed it.

However....

Never have I ever had to replace a BB for wear. I have replaced them for other reasons.

And I suspect you have never worked in a bike shop doing repairs. Just yesterday I replaced a BB on a customer's 20ish year old Trek road bike because the axle spun roughly. This was a cartridge unit (a Thun). Until the crank arms were removed it was easy to not notice the wear. Andy

I have, back in the days when I was commuting up to 9000 km per year in all kinds of weather conditions

I probably have never kept a bike long enough to come close to wearing out a BB and I certainly don't ride a lot during any year. Lucky to get 3000 miles a year. I imagine many that put serious mileage on a bike do wear out BB's. However this is something that is done after 40,000 to 50,000 miles (64,000 to 80,000 km) On a bike with a decent brand BB if it wears out earlier, then IMO, there are some other mitigating circumstances that probably don't involve riding the bike.

For some that's just worrying about them when they are not showing any signs of giving an issue. Why worry about a $9 to $20 part?

First you tell us what kind of bottom bracket you have. You can take the chain off and tell by spinning the crank if the bearings are worn, but how you service it depends on what you have. Don't you guys realize there are more than one kind of...pretty much anything? Hubs, headsets, bbs. You have to provide some information to get some back.

How about 16-25,000 miles? Those were on bikes that were ridden in all kinds of weather (commuting and touring). Both cartridge BBs, both "frozen" into frame, both took a month of penetrating oil soaks to get the old BB out. Lucky for me they wore out two months apart, so I had the first fixed before the second was kaput.

FWIW, I installed the new BBs with plumber's teflon tape instead of grease. Still have my fingers crossed that they'll be easier to get out than those installed with thin layers of greased and then ridden in the rain to wash that grease out.

+1 Teflon tape over the threads is pretty much a guarantee that the cups/cartridge will remove without any drama.

I've replaced a few BB's due to wear.
I notice by an annoying click under load.
Swap out the pedals first since it's easier, if the click doesn't go away, then BB swap.
Usually 10k miles for me but a lot of dirt in involved.

pdlamb
HillRider

Thanks for the Teflon tape advice.
Never thought of that.

(also, if it's the older ball and cup design, I try to overhaul it, if it's a cassette, swap the whole thing)
((my last swap was for a bike with Power Spline))
((($15, whoot!)))

A small rant or eye opening- Often we reply with advice that is based on what we would have done when the bike was bought or initially assembled. for BBs this is stuff like plenty of grease, good bearing preloading, well machined BB shell, corrosion resistant cup to shell prep. But the VAST majority of bike out in the real world don't have this attention to details when assembled or sold. The cost pressures to get the product out and the benefits of a best prepped bike are lost in the real world for, maybe, 9o%. That left over 10% sold through your LBS has only slightly more attention to out of box assembly, and believe me when i say that nearly no shops take apart already assembled aspects of a new bike.

In my world I see all types of issues that if had been better assembled, initially better addressed or after the miles racked up were serviced by a skilled wrench would never grow as quickly or be as much as a surprise as they seem to be by all the posts we read here. So, while our advice is more often good stuff we need to understand that very few bikes meet our personal standards. Disappointing, yes, but more and more cost of doing business rules over the right stuff. Andy

Thank you Mr. 66 for your helpful tips.

Thanks Andrew for sharing your knowledge.

Iride01, what reasons HAVE you replaced a BB, if never for wear?

Thanks Andrew for taking the time to share those thoughts.

Thank you Mr. 66, Crankycrank, Andrew R Stewart, Barry2, Iride01, alcjphil, pdlamb, HillRider and CrimsonEclipse for your helpful replies.

There's never enough time to do it right, but there's always enough time to do it over.

Plus just fixing the bikes that get ridden enough to uncover the shortcomings in initial bike assembly keeps the LBS' mechanics in business!

Just curious, though. What fraction of the vanishingly small fraction of bikes that are built up from (custom) frames by a shop would you guess are built right from top to bottom? In addition to BB threads, I'm thinking about headset bearings greased, wheel bearings greased, even spoke threads properly treated (by whatever means you consider "right"). Do the bike shop wrenches doing such jobs typically grease the derailer mounting bolts and bottle cage mounting bolts? Or should the new owner plan on stripping and rebuilding any bike when new instead of riding the nice shiny new bike?

Because I was putting new components on the bikes and the new cranksets did not use the same type BB. If I stayed with the same ancient BB standards then my crankset and other component choices for those bike will have been severely limited.

What about it? Are you saying every BB will wear out at 16 - 25,000 miles?

I'm not by any means suggesting the OP's BB isn't worn out. However the OP didn't give any indications that their BB was worn out in their first post. So if there wasn't anything suspicious for them to even relate about their BB, then I will assume that it's not worn out until the OP gives some clues that it is.

I don't think I could really say. But I will suggest that the focus on what is the tiniest portion of bikes sold, those which are built up by a LBS from a frame and a pile of parts, is not very productive. Yes, the LBS assembly increases the feedback to those who did the work (the shop wrenches) and that is a good thing. But generally most shops will take some pride in their assembly and high end sales. Do know that the customer for that high end bike is vastly different (most all the time) from the customers who buy factory assembled bikes. This difference often results in the bike's after purchase life being maintained better and less miss treated (as in the high end bike rider usually knows more and is willing to spend the $ to keep their expensive bike in good working order).

But all this is conjecture on my part. I know of no after the sale studies that focused on this question, at any price point. I suspect some of the bigger brands and some industry players (not necessarily bike brands) have tried to seek answers to some of this topic.

Back in the 1980s and when I had my shop we sold Terry Precision Bikes For Women. The hand built frames (made just 5 miles from my desk) came as a frame, fork in one box and the parts in another (wheels were already built by a guy I had taught just a few years prior). Terry got a lot of negative feedback from their dealers because of the time sink to assemble the bikes (and remember that a Terry bike sale took more time to transact than the usual production branded one did. If the customer brought along her "expert biker" male the time to sell was even longer:) ). As Terry started off shore sourcing the bikes still required a frame up assembly. This changed quickly as Terry grew and could contract the assembly over seas too. I liked the bare frame build up method as I did want to do it with my standards but recognized the time and cost savings preassembled bikes had. Andy