Nothing gets your heart a flutter like bottom brackets. That may well be a lie. This again is an old blog post I wrote for a now defunct bike company. They made race BMX frames and I feel the reasons for running a threaded bottom bracket on a race BMX frame is the same as for why you should on a single speed bike.
Bottom brackets are a boring but really important part of your bike. They can also be infuriating to the point where by murder seems less of a crime, especially if said person was the guy who invented BB30.
This is just a quick run down on the various systems and will hopefully explain why we went threaded.
So far at the minute we’ll ignore BBright and BB386 Evo, as one is propriety to a road brand and the other is not seen on BMX.
So the threaded bottom bracket. This is a threaded shell and is usually found in 68mm or 73mm shell length. In BMX we tend to use the 68mm version but the main difference between that and 73 is the number of spacers you would need. The next main difference in bearings is are they for square-taper or outboard. Square-taper is found on cheaper or kid’s bikes. The spindle is a mere 17mm compared to the 24mm spindle you get on cranks that run on outboard bearings. The outboard system led the way for many years and rightfully so. It is easy to set up and gives great life to bearings. Some manufacturers of high-end bikes refuse to move from this, particularly Santa Cruz and it is worth noting that Bradley Wiggins asked for this system on his Pinarello.
The BB30 was released by Cannondale in 2006. BB30’s bearings press directly into the bike frame, the next difference is that the shell and spindle diameters that increase in size but not the shell width. They did this to achieve a “lighter and stiffer crank” and to have more room in the bottom bracket area to attach larger down tubes, seat tubes, and chain stays; and very low bottom bracket weight, as there are no cups for the bearings to sit into. It also thanks to 68mm shell width can allow a narrow Q-factor.
PF30 was a bit of a help for frame manufacturers as it is the same as BB30 but the bearings come in little plastic cups. This allows frames to be made cheaper as they do not have to be as well finished as the BB30. The plastic cups will take up some of the tolerance issues. Takes you back to the old American bottom bracket system, doesn’t it?
In 2007 Trek got in on the act and brought out the BB90/BB95 on their new Madone. They went with a similar idea to Cannondale and went with bearings that pressed into the frame. But rather than increase the spindle width they increased the shell width to 90mm on road bikes and 95mm on mountain bikes. Which the same as BB30 gives room for bigger seat tubes, down tubes and chain stays. Allowing a stiffer bike again. You could, therefore, use the same chainset as a conventional bottom bracket as the shell width comes to around the same as 68mm with bearings included or 73mm with the bearings included.
We then come to PF86/92 which is the same idea as PF30. The advantages are similar to those for BB90/95, a wider BB shell that doesn’t need a new crank width, plus lighter weight compared to threaded aluminum cups.
So why do I prefer the conventional Hollowtech style? Warranties. How many BB30 bearings have I warrantied in my job? More than a lot. How many Hollowtech bearings have I warrantied, 1 in 5 years. Want to ride your bike and not have to worry about your bottom bracket?
Bearings on BB30 they seem to have a really short life, this may apply more to road bikes than BMX but I’ve seen them fail on one ride, I once killed a set riding a Scottish Winter round, not great if you are paying £40 for bearings for every race.
Creaks, they will do your head in and appear far more frequently in press fit bottom brackets. According to Chris Cocalis of Pivot Bikes and creator of BB92, (which is Shimano’s version of the press fit bottom bracket and is their competition to the above systems. It works in the same way as BB30 but still designed for 24mm cranks) if a bottom bracket shell is out by 0.2mm you will have creaks. Given that when I worked in a shop we would have new bikes arrive in where once you removed the cranks the bearings would slide/fall out this rule is not being stuck. Perhaps as it is mostly a cheaper way to make frames but it needs higher tolerances than a lot of companies are willing to work to. This is a nightmare when you work in a shop.
It is also worth noting that a 30mm spindle would not be a huge amount stiffer than a 24mm to make it worth while. If you talk to an engineer about this, most state that to make it worthwhile on a weight/stiffness tip you would need to be using a 40mm spindle.
Now Mid and Spanish are the two main sizes found on freestyle BMX frames, why then did I not use them, especially since FBM were the originator of the Mid size and they made my frames. Well, the main reason is if you use Mid or Spanish then we are pretty much going to be using a BMX freestyle crank and for a lot of people buying my frame that would also be a crank change as well as a frame change. There is also the fact that it is a lot harder to get a 44t chainring that fits on without a spider and most of the compatible Mid/Spanish cranksets don’t come with the ability to run a spider. So threaded just gave us more options and therefore gives you more options. I’ve also yet to see a BMXer fit a Mid/Spanish bearing without just hitting it with a hammer, I’d quite like to use a system to make people fit things correctly.