This is a cross posed from Pinkbike: http://www.pinkbike.com/news/poll-press-fit-bottom-brackets.html. The formatting has gone a little to rat sh*t, but might be interesting to some.
The Specialzed Carbon Demo's threadless bottom bracket shell is an aluminum insert which doubles to house the swingarm bearings - both are press-fit types.
There is an on-going battle between bike makers, retailers and customers over press-fit vs threaded type bottom brackets, with a large number of riders and bike shops opting for threaded cups. Frame and bike makers, however, have been systematically switching to press-fit types and if the trend continues, it will only be one or three years before all production mountain bikes will be converted to threadless bottom brackets. Before all you Knights of the Royal Order of the Threaded Bottom Bracket draw your swords, please understand that technically, ALL bottom brackets are press-fit designs. The only physical difference is that the conventional BB's bearing is pressed into a cup, while Press-fit types. like the popular BB30, have their bearings pressed directly into the frame. After they are pressed or tightened into a functional bottom bracket, both systems operate in exactly the same way.
Niner specifies threads for its WFO 9 AM/enduro chassis and for all the frames in its lineup. Frame sales are a large part of the business and Niner insists that its bikes are cross-compatible among component makers and also user friendly.
Arguments in favor of threads are numerous, but only three are viable: Threaded cups can be serviced with simple wrenches that almost everyone owns; threaded cups allow component makers to have complete control over every moving part of the crankset from pedal to pedal; and finally, threaded type bottom brackets seldom creak, while at present, press-fit bottom brackets often creak.
There is a mechanical reason why seatposts, headsets and bottom brackets creak when they are stressed. When a smaller diameter tube is being held inside a larger-diameter tube, there is a significant difference between the stiffness of the larger and smaller members. When a bending moment is applied to the smaller tube, the smaller-diameter member flexes more than the larger-diameter one which is holding it. The smaller tube elongates slightly as it flexes and it slips inside the larger one. That microscopic movement causes the creaking noise. The slip can be exaggerated when a bearing is pressed between the two members, such as a bottom bracket, because flex in the bottom bracket axle is multiplied by the distance from the axle to the outer diameter of the bearing.
Because bicycles have a number of places where this situation can occur, and because cyclists in general are enflamed by any recurring squeak or rattle, designers have spent many sleepless nights figuring out ways to silence seatposts, hubs, pedals, headsets and yes, bottom brackets too.
Once assembled, the external-cup, 73-millimeter threaded bottom bracket (top) and the 92-millimeter press-fit type (below) are functionally the same and use the same size bearings. The Press-fit, design, however, offers more bearing support, it has fewer and less complicated parts, and it gives frame makers room to widen frame tubes or get creative with suspension components.
Those naive enough to believe that threaded bottom brackets emerged from the cycling industry's womb creak free would be dead wrong. One hundred years passed before bike makes got a handle on that one. Headsets creaked like baby toads until mountain bike designers ditched the threads and adopted oversized steerers.
There are strategies in place to silence bottom brackets. Some thread-in cups have Teflon liners which allow the bearings to rock silently. Some are glued in tightly with thread-locking material. Some makers, like Chris King, rely on precise manufacturing tolerances and insider secrets to ensure silence. There is also a theory that aluminum external threaded cups flex just enough to prevent slip between the shaft and the bearings and thus naturally prevent creaking at the source. The point is moot, however, because over time, natural selection has eliminated all the brands that made noisy thread-in bottom brackets.
Proponents of threads either ignore or forget that even a perfectly faced and threaded frame can require the skills of a safe cracker to get the cups started without cross-threading them.
It is easy to mess up the fine threads of an aluminum bottom bracket cup. Acros makes a tool which keeps the cup and frame aligned to avoid cross-threading expensive parts.
Anyone who has cross-threaded a cup the night before a race or ride can attest that it is game-over unless luck is on your side AND you have a sharp set of bottom bracket taps in your tool box.
Park Tool's press-fit bearing removal and replacement tools are simple to use and they get the job done in minutes - however, removing a threaded external cup with a spline wrench is a much more civilized process.
A large number of press-fit supporters come from the manufacturing sector because threading and facing frames is costly and bolloxed threads can be a significant manufacturing and warranty issue. Pressed-in bearings are simpler to install and replace, and press-in BBs are less costly. More important, though, is that carbon construction can produce a beautifully precise press-in interface, but it does not lend itself well to threads, so typically, a threaded-aluminum insert is bonded into the BB shell. Press-fit BB shells are made the full width of the bearings (92mm wide instead of 73mm) which also gives frame designers room to widen the frame tubes and make the BB area much more rigid. A number of home and shop tools now exist that make removing and replacing press-fit BB bearings a three-minute operation, and bearings can be purchased from any number of sources, including on-line industrial suppliers in any country. It would seem then, that the last real barrier to press-in is noise.
A bit of an exaggeration, but the bottom bracket area of Pivot's carbon Les Fat illustrates how much wider and more rigid the frame can be made when press-in type bearings are used.
One can extrapolate that it is only a matter of time when bike and component makers will silence press-fit bottom brackets. Shimano has made great strides by encasing the bearing in a press-in nylon cup (other BB makers are now using similar cups). The thermoplastic has just enough give to absorb any bearing movement, and also, it can overcome a slight amount of misalignment or out-of-roundness in the frame. Nylon also does not transmit sound well, so the cups act as a silencer, which is a key feature, especially when one considers how carbon frames tend to amplify sounds like musical instruments. Eliminating creaky press-fits can't be easy, otherwise it would have been done long ago, but "difficult" is not "impossible," .
