The Bottom Bracket (BB) is located where the downtube, seattube, and chainstay meet. It is a tube that bearings sit in, and allow the crank spindle to rotate. There are four main different types of bottom brackets: American, Euro, Spanish, and Mid. Lesser used varieties include the Yorkshire, ISIS, and the Press-Fit Euro.
The American has been the traditional bottom bracket on BMX bikes for many years. However, it is far from standard; there have been a wide variety of inner diameters and widths over the years, and there has never been a standard size for it. American BBs work by pressing a pair of steel cups into the frame, with either a cup-press or - more generally - by pounding them in with either a rubber mallet or a hammer and a 2x4. The bearings and spacer are then placed into the cups, and the spindle is slid in. Grease is recommended, but not required for installing the cups, but is necessary on the spindle.
The Yorkshire - designed by "G-Sport" George French - is not at all widely used, but is similar to the American. It has a standard width and diameter; basically, it a 16g steel tube of 2" diameter cut to a length of 66mm. It uses plastic cups, which allow it to accept American-sized bearings.
The Euro, although lighter and lower profile than the American, is far less durable; rather than using a pair of large bearings, it uses four smaller ones set in a pair of steel or aluminum cups. These are installed by threading one cup in, inserting the spacer and sliding the spindle in, and then threading the other cup in. Of course, the threads should be greased beforehand on the threads and spindle.
ISIS - a mountain bike standard - is rarely used on BMX bikes; simply put, it is a large-diamater of the Euro.
The Mid - designed by FBM - uses R12 bearings, the standard bearings for the American on cranksets that uses 19mm (3/4") spindles; for cranksets with 22mm spindles, custom-sized bearings and spacers must be bought separately. To install a Mid, grease the bearings, spindle, and the inside of the bottom bracket up until the lip, push in one bearing (depending on how tight the constraints are, you may be able to do this with your hand, but in some cases a rubber mallet is necessary), insert the spacer, slide the spindle in, and then press in the other bearing.
The Spanish - designed by Fly - uses bearings smaller than the American/Mid/Yorkshire uses, but larger than those on the Euro. The installation method is the same as that of the Mid.
The Press-Fit Euro is rarely used, and follows the Mid/Spanish method of directly pressing the bearings into the frame; however, instead of using larger, more durable bearings, it simply uses regular Euro bearings, although exactly which bearings (as different cups use bearings of different sizes) varies from manufacturer to manufacturer.
Note: Aside from American-Yorkshire-Mid, and the Euro-PFEuro, bearings are not interchangable. MacNeil produced plastic cups for the Spanish that allowed them to use normal Euro bearings, but these were not popular and did not fare well on the market, so they are no longer available. Fly also produces a Euro-Spanish conversion kit that holds Spanish bearings outboard of the bottom bracket; this has similarly proved unpopular.
For more information on the advantages and disadvantages of the major bottom bracket types, please refer to George French's tech column in Ride UK
#79, which can be accessed online here
. Please note that this article is older, and is not entirely up to date.