Bottom Bracket Guide25 July 2018
There are a lot of bottom bracket standards out there these days, they are all touting various benefits to their different specifications and features. Here we give you the low down on what's really what and why. No matter what the marketers would have you believe this simple maxim holds true "bigger is better". The further apart your bearings are the more stable they are and the larger and greater the number of balls in your bearings the longer they are likely to last. You have to factor in seal design and drag to the equation and also what the balls and races in your bearings are made from. There's a lot to think about.
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Ceramic or steel balls and races?
Ceramic bearings are harder, they run faster with less drag and they aren't prone to pitting via rusting. They are sure expensive though with a couple of replacement bearings likely to cost you upwards of £50. Hybrid bearings use ceramic balls on hardened steel races, they are faster than steel bearings, but we have doubts about the combination of super hard ceramic balls and moderately hard steel races. They aren't too expensive though so for the money can offer enhanced performance. Steel bearings come in least expensive, with the longest track record and are the best understood. They aren't the fastest, they aren't the longest lasting but if you aren't looking for the ultimate in low drag then the lifespan/cost equation can be very favourable. In short, if you're riding in wet and muddy conditions year round then keep it real with steel. If you're building up a summer race bike or TT/Tri bike then full ceramic bearings could be the ticket to faster times. The middle ground is the hybrid ceramic bearing that can give you a taste of the high life for modest outlay. The decision is yours.
Bearing sealing, which is best?
The quality of sealing varies between brands and designs but, in short, there are two main types. RS provides you with a single lip seal on one side of the bearing and a close running edge on the other. 2RS includes a moulded lip on both sides of the bearing and retains grease best at the expense of some added drag. There are other variations on this theme but in general we'd go for the 2RS every time.
Press-fit or threaded cups?
This is the big question and there's a lot of hype surrounding the different styles. We're big fans of keeping things simple unless there are good reasons for the added complexity. We choose our BB designs based on the criteria of service life and fitness for purpose.
A threaded BB requires a full-length aluminium insert that runs the width of the entire BB shell. It requires the frame to be faced on both external edges of the shell and it then requires two separate threads to be machined into the interior of the shell. Though these threads must be concentric for the smooth running of your BB they can't be cut at the same time- without removing the cutting tool, so concentricity cannot be guaranteed, at least not without care and attention to detail. The pros of this design type, however, are significant. They don't creak, the bearings can be positioned outboard to give the stiffest possible BB platform, the external cups allow for multiple levels of sealing and aren't prone to contamination from inside the frame, (where do you think all that rear tyre sprays ends up once it hits your seat post?) There's also limited fit options. There's only two shell widths, 68m and 73mm and only English or Italian thread options. We like threaded BB's for these reason, we also like them because the choice in replacement parts is huge and every shop sells them so you never need wait for a replacement. Down sides do exist though, that full-width BB shell does add about 100 grams to the weight of your frame.
Press fit BB's come in many different shapes and sizes. There are two BB shell diameter options, 41mm and 46mm and then there's the 68mm or 78mm shell width options. Add into that all the BB options and there are literally thousands of choices- BB Right, BB30, PF30, PF386 Evo, BB90/BB95, BB86/92. You can see why cyclists find it a bit of a minefield. As long as you know the internal diameter of your BB shell, and know what cranks you are trying to fit then you can work out what BB you need.
There are many reasons why manufacturers introduced press fit BB's. You can use a plain internal BB sleeve. There's no need to accurately face both outside edges of the BB shell and no need to machine two threads into the ends of the BB shell. So it saves the manufacturer significant cost and time. It also allows them to use a lighter BB shell, which saves you weight. The downsides are well understood, however. There can be a lack of precision in the fit of the parts and being squashed together rather than threaded into place means the interface between BB shell, bearing cups and BB axle might not be all it could. Reports of cranks coming loose and consistent creaking have made the press and bike forums ever since PF BB's were introduced. In our experience, these can be remedied by the careful selection of the right parts and by ensuring that they are fitting exactly to specification.
We like both styles of BB and use each where the benefits make the most difference to the ride and feel of the bike. We aren't wedded to one concept in preference over the other because the truth is that both of them are equally good, they just have different strengths and weaknesses.
Planet X, On-One bottom bracket chart.
|Pro Carbon Track||X|
|456 Carbon Evo||X|
Have fun and ride safe.
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