American Custom Bicycles in Steel, Titanium and Ti/carbon mix

Bottom Brackets

(68mm BSA, BB30, PF30, T47, 73mm BSA, 100mm BSA, and Eccentric)

Seven offers a wide array of bottom bracket shells in our frames. Our standard, for all frames, is 68mm English (BSA), but as with every feature of your new bike, we will recommend the standards and technologies we believe will give you the best ride experience.

BB30 or PressFit 30 Bottom Brackets

These are terms for two standards of oversized bottom bracket shells that accept 30mm crank spindles and direct-fit bearings, or bearings housed in cups that are pressed into the frame’s bottom bracket shell. By eliminating the use of external cups, this standard aims to reduce both overall bike weight and the q-factor of the pedals. The larger bottom bracket shell also accommodates a 30mm spindle, instead of the more common 24mm spindle, increasing drivetrain stiffness. The 30mm standard requires the use of a completely different bottom bracket from than a 68mm BSA shell, so there are no parts that are compatible with both standards.

T47 Bottom Brackets

T47 is an emergent standard that mimics the frame dimensions of PressFit 30 shells (68mm road/73mm mountain), but the shell is threaded, which reduces movement, and thus noise, produced by some PressFit shells. The larger overall shell makes configuring bikes with internal wiring much easier, and together with over-sized chainstays can yield a dramatically stiffer drivetrain.

73mm BSA

These wider shells are now the default for Seven’s mountain bikes. Initially developed for “plus size” mountain bikes that require greater clearance for large tires, the 73mm standard lines up well with new crankset offerings from all the major players.

100mm BSA

The standard for fat bikes. A 100mm shell gives that over-size clearance necessary for running 4” and 5” tires.

Eccentric Bottom Brackets

An Eccentric Bottom Bracket (EBB) is a device used to add tension to a bike chain or belt. A traditional bottom bracket threads into a cartridge in the EBB, which allows tension to be increased or decreased by swiveling the EBB clockwise or counterclockwise. When the appropriate tension is achieved, the EBB is held in place by expander bolts inside the unit. We like EBBS to tension belts and chains for belt-drive bikes with internally geared hubs and the timing belt on tandems, because an EBB adjusts chain tension at the bottom bracket, allowing the frame to have traditional horizontal drop outs.