Bearing science: Which bottom brackets/inner bearings are there on the bike?
Kai Hilbertz
· 23.02.2015
Some people still refer to the bearing in the bottom bracket shell as the "bottom bracket", while experts speak of the "bottom bracket". With the exception of special designs such as Pinion, every bicycle has either an internal or external bottom bracket, and tandems have two. The development of the inconspicuous bottom bracket has had a significant influence on the development of the bicycle, especially in the last 20 years.
In the 1930s, the French manufacturer Stronglight launched the first bottom bracket with a square cone onto the market, to which first steel and then aluminium cranks with a correspondingly square-shaped mount were attached. The system was vastly superior to the wedge mounting that had been common up to that point and became established worldwide. Today, many bicycles are still sold with square bottom brackets.
With the development of mountain bikes in the 1980s and 90s, the loads on frames and components became ever greater. In 1996, Shimano introduced the XTR BB-M950 bottom bracket, the first bearing with an octagonal mount under the name "Octalink". Because the shaft diameter increased from approx. 16-17 mm to almost 22 mm at the same time, the new bottom bracket was stiffer than the classic system. Other companies countered with "ISIS-Drive". The decagonal mount became the open standard.
Because the inner diameter of a standard BSA bottom bracket shell is less than 34 mm, there was still a problem with the multi-tooth bearings. The larger shaft meant that the size of the bearings had to be reduced. This reduced the longevity.
To achieve greater rigidity and a longer service life, the developers could either enlarge the bottom bracket shell or move the bearings outwards. In the meantime, both have happened, and in many variants.
Cannondale made the start in 2000 by enlarging the BB30 bottom bracket housing so that a 30 mm aluminium shaft could be used. However, this initially remained a special solution developed by the company itself. Three years later, Shimano introduced its "Hollowtech II" model. The shaft, now with a diameter of 24 mm, formed a single unit with the right-hand crank arm. The bottom bracket consisted of just two bearing shells, which were now screwed into the outside of the housing rather than inside it.
This technology caught on, and today most trekking bikes are fitted with bolted, external bottom brackets. But the developers continue to tinker.
BB30 also became the open standard in 2006, and since then new bottom bracket standards have emerged regularly. Most road bikes and many mountain bikes are now fitted with "press-fit" bearings without threads, which are simply pressed into the frame. Some of these will certainly also be used on trekking bikes in the future.
To help you choose the right bearing in the event of a defect, we provide purchase recommendations and explain the most important differences in detail.
A comparison of the bottom bracket types on the bike:
Bearing + crank with square taper (square)
Square bottom brackets and cranks with corresponding mounts are no longer standard, but are still used on new bikes. Available with axle widths of 102-131 mm, with steel and titanium shafts.
Multi-tooth bearing + crank, here "Power Spline"
Shimano's Octalink (8-cornered) and ISIS-Drive (10-cornered) were the top dogs, but there were also Power Spline (12-cornered) and Howitzer (10-cornered). These have since been replaced by external (multi-tooth) bottom brackets.
External threaded bearing, Shimano HTII
Shimano launched the first Hollowtech II bearings and cranks on the market in 2003. Today, external bottom brackets with 24 or 22/24mm shafts are the standard for touring and trekking bikes from various manufacturers.
The full-length article from issue 1/2015 is available as a free PDF download.
