Road Bottom Brackets – The Basics
When it comes to the construction of the crank, a road bike is not that different from other bicycles. Both inner bearings, also called the bottom bracket, sitting in the bottom bracket shell of the road bike frame, which in turn is bearing the cranksets. And even more than with other types of bikes, the bottom brackets on a road bike are subject to powerful mechanical loads due to the intensive level of sporting use. And there are also dirt and water which will ensure that the bearings of a road bike will wear out over time. So, it’s no coincidence that road bike bottom brackets are a classic wearing part, that must be regularly replaced, as well. Which inner bearing you need depends on the diameter of the crankshaft (spindle), the prescribed distance of the two bearings to each other (shell width) and the bearing seat of the frame, i.e. the outer diameter of the bearings. The selected road bottom bracket must therefore fit into your frame and also on the crank that is mounted.
Bottom Brackets for Road Bikes – The Most Common Versions at a Glance:
- Italian thread: This former standard is still to be found on old steel frames. With Italian thread, both road bottom brackets have a right-hand thread measuring 36 x 24 millimetres, the shell width is 70 millimetres.
- BSA (Hollowtech II / GXP / M24): Widely implemented with Shimano’s Hollowtech II cranks, this is the basis for many modern standard road bottom brackets. Other manufacturers use the term GXP or M24. The spindle diameter can be either 24 mm in diameter on its full length (Hollowtech II / M24) or 24 tapered to 22 mm at the end. The bearings are housed inside the bottom bracket cups, while sitting outside the frame shell, that have a left-hand thread on the right and a right-hand thread on the left, labelled as 1.37” x 24 TPI. Typically the shell width is 68 millimetres on a road bike.
- Pressfit/BB92/GXP Pressfit: This bottom bracket version is mainly based on crank spindles with a diameter of 24 millimetres (e.g. Hollowtech II). However, the bearings are housed in plastic shells directly mounted in the bb frame shell. The shell width is 86.5 millimetres on a road bike, the outer diameter of the bearings is 41 mm.
- BB30: An open standard with an axle diameter of 30 millimetres. The bearings, without a shell or cup, and with an outer diameter of 42 mm, sit directly in the bottom bracket shell (shell width on the road bike: 68 millimetres) of the frame, which requires particularly lower manufacturing tolerances. The bearings’ outer diameter is 42 mm on the BB30 standard.
- PF30/Pressfit T 30: This bottom bracket standard is a combination of the BB30 spindle with a diameter of 30 millimetres. However, these types of road bottom brackets are not pressed directly into the frame, while sitting in a plastic shell – so that manufacturing tolerances of the frame can be better compensated. The outer diameter is 46 mm.
- Special cases – Trek BB90/Specialized OSBB/Cervélo BBRight: These frame manufacturers deviate slightly from the common standards but can usually be adapted with spacers or adaptors. For these special cases, the manufacturer’s specifications must be followed to the letter.
Road Bike Bottom Brackets – Can I Change the Bearings by Myself?
Depending on which bottom bracket standard has been used in your road bike, it can be easy or difficult to change the bearings. Our recommendation is to always follow the assembly instructions for the crankset which requires a new road bottom bracket. The manufacturer’s instructions will also tell you whether you need any special tools and which one.
BTW: At BIKE24 we offer the right bicycle tool for you to change your bearings.