I used to be just lately servicing an outdated classic mountain bike from the 90s and was struck by how clean the backside bracket bearings have been after virtually 40 years. With fashionable backside brackets, I’d depend myself very fortunate to search out bearings performing so properly after even 40 months.
How have we bought right here? And why does it appear inconceivable for producers to get backside brackets proper nowadays? I believe to know the place it has gone fallacious, we have to take a look at the evolution of the underside bracket.
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The unique ‘fashionable bicycles’ used cup and cone backside brackets [for an overview of all styles, check out this guide to bottom bracket standards]. On this system, the axle had ball races manufactured into it, these would information the bearings, after which the cups that threaded into the body can be adjusted to take away side-to-side play.
These steadily developed into what we used to name cartridge backside brackets. These all-in-one items had the bearings already fitted to the axle and preloaded, so all you wanted to do was screw the entire unit in from one facet and the alternative cup then simply offered radial help. These cheap items would set a shopper again as little as £15, and the bearings would typically last more than the remainder of the groupset. The important thing factor right here is that the bearings and axle have been each made by the identical producer, and, the bearing was preloaded and supported from each side. The system might function at its optimum, no matter body high quality.
I’m not within the behavior of reminiscent of the ‘good outdated days’, with out acknowledging the pitfalls. Sq. taper axles did – if over-tightened – have an annoying tendency to flare the crank arm taper making the match free and subsequently creaky. I bear in mind on a number of events having so as to add small strips of ‘shim inventory’ (learn, Coke can, in non-mechanic converse) to the taper to attempt to eke out the lifetime of a mistreated crank. Nonetheless, the methods, on the entire, labored properly.
Then, the battle for decrease total system weights got here to backside brackets. The biking trade moved to two-piece cranks, the place the axle was now a part of the crank and never the underside bracket; it was all downhill from there, in my e-book.
The problem two-piece cranks created is that there’s now the chance for this complete entity to encompass elements made by varied totally different producers, all with their very own tolerances, eg, permitted variations from the specification.
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Creaks – which fashionable backside brackets are famend for – happen when there’s undesirable motion between two elements, and, that is virtually inevitable when tolerances don’t align. Add to that the potential for misalignment that we now have by separating the 2 fundamental bearings into unconnected separate cups, and, you’ve bought a recipe for bother. Even with a superbly confronted and reamed backside bracket shell – whereby materials has been eliminated to intensify the possibility of an ideal match – it solely takes a fraction of a level of distinction in alignment to start out creating issues.
Press-fit methods – the ‘go to’ of the 2010s, whereby the cups are pressed into the body, with no cup – appear to take the brunt of the flack, with a lot of the trade now hightailing it again to threaded choices. However truly, if you concentrate on it, all bearings are press-fit, some are simply pushed right into a threaded housing first.
Press-fit backside brackets, similar to this BB386 EVO variant, get pleasure from a blended popularity nevertheless it’s typically the standard of the body that is accountable
(Picture credit score: Future/Simon Fellows)
There’s nothing essentially fallacious with a press-fit backside bracket system if what you’re urgent it into is an ideal gap within the body, and there’s the issue. One of many causes that threaded backside brackets have a barely higher popularity is as a result of typically it’s a single tube that’s threaded which virtually ensures a greater concentricity within the bearings. Now that quite a lot of producers are transferring again to threaded shells of their carbon frames, the place all sides is just not related, I actually hope they’ve spent the cash making the moulds that are used to create the frames precisely, in any other case, we’re no higher off.
Not solely did the outdated one-piece cartridge backside brackets of outdated guarantee a properly toleranced placement of the bearings, to forestall misalignment, however additionally they hardly ever wanted re-greasing. It is because they tended to return with full contact seals in every bearing. A few of the very nice ones made by firms like SKF – one of many largest producers of bearings – would additionally pack them with marine grease. Full contact seals and thick grease forestall the ingress of water and muck, so the bearing can run easily.
Sadly, the bike trade in its final knowledge determined that this wasn’t adequate, because it elevated drivetrain resistance, and thus had a detrimental influence upon the marginal beneficial properties sought by the uppermost echelons of our sport. In response, thinner greases and non-contact seals began to look, all within the identify of searching for efficiency over longevity and reliability. It nonetheless makes me shudder after I see a house mechanic spin the cranks in a ‘Dream Construct’ video, cranks that whiz at excessive velocity aren’t a fascinating function, they’re simply an invite for crud to get into the bearings.
What’s the answer? I’m actually not advocating that we transfer again to sq. taper axles, nonetheless, having a backside bracket unit the place each bearings are pre-fitted right into a toleranced shell makes quite a lot of sense, guaranteeing the bearings are concentric and parallel, and there are a couple of examples of those available on the market now. One of many merchandise I discovered attention-grabbing on the 2024 Eurobike tradeshow was the brand new backside bracket unit from a small firm known as Bikone, a sponsor of the UCI WorldTour group, UAE Crew Emirates.
The Bikone system mounts each bearings right into a single housing, but additionally gives an inside preload by having a extremely toleranced tube between each bearing interior races. This prevents extreme side-to-side loading, which ensures the bearings run fantastically easily and shouldn’t put on out wherever practically as rapidly as the choice. On the discount worth of just about £400, this new innovation arguably shares traits with the SKF-Thun cartridge BBs and Profile Racings BMX cranks from the 80s. I suppose that’s progress.
