Firstly, what is a bottom bracket? A bottom bracket is the central bearing that the crank arms run through allowing you to pedal smoothly. A worn bottom bracket will often feel loose and may 'knock' or creak under pressure from the cranks. There are many different types of bottom bracket on the market today and in recent years the numbers of common "standards" has increased.
This used to be a fairly standard fitting part on a bike with most bikes using a square tapered axle that came in a few different lengths. As crank and frame materials and technologies developing over the years there is now no such thing as a standard bottom bracket.
The square taper bottom bracket was the norm for a long time. This part had open bearing cups that threaded onto each side of the frame and the bearing play was taken up with the tightness of the left had cup. This fit was upgraded when Shimano released the sealed bottom bracket. This still used a square taper axle as all pedal arms or cranks had this square interface which was attached with either a 14mm socket bolt or an 8mm allen key bolt. The quality and fit of the sealed bearing lengthened the life of this component by years.
Shimano then released the Octalink hollowtech bottom bracket on their XTR and Dura Ace level componentry. A true race level component that had a circular splined interface. The axle reduced issues with the crank working itself loose and flexed far less than the square taper axle and the sealed bearings still meant that this components life was log. The extra stiffness of the circular axle made pedal power more efficient therefore producing a far more efficient ride. Similar style from other brands were also released with slightly different spline arrangements, these were ISIS drive and Power Spline.
A few years after the Hollowtech 2 range revolutionised the bottom bracket. Previously all bottom brackets sat within the frame. The Hollowtech 2 bearings sat within their own cups outside of the frame. The axle was now attached to the crank arm and crank bolts as we knew them were gone. The axle was now wider, this offered great stiffness and could now be made from ally rather than steel. Weight reduction in this area had been difficult before this point. The bearings were now preloaded by use of a cap that pulled the crank arm without the axle onto the spline and 2 m4 bolts then clamped this crank arm onto the axle. Crank arms were all to be made of alloy with this fitting type therefore reducing the weight of the crank and again improving stiffness. Other brands like FSA, Race Face and Truvativ all released similar products known as X-Type and Giga Type. These are all still found on new bikes today although this system has also been superseded.
So the bearings had been moved outside of the frame but frame manufacturers figured that greater stiffness could be achieved if the bearings were kept the same but put back inside the frame. With some combined design skills from the bike brands and the componentry brands the Press Fit bottom bracket was born. Instead of the bottom bracket cups being screwed into the frame they are pressed in to the frame. This allowed them to increase the size of the cups once again offering great stiffness in the bottom bracket and the frame. The crank arm still has the axle/spindle affixed to it however this has grown in diameter again which has increased the stiffness in this component too. Once again there are a few different styles within this type fit depending on what brand you have on your bike as no brand wants to pay another patent fees.
The current pressed-bearing standards and their developers are as follows:
- BB30 (Cannondale)
- PF30 (SRAM)
- BB90 & BB95 (Trek)
- BB86 & BB92 (Shimano)
- BB79 (Cervelo's BBRight)
- BB386EVO (FSA and BH bicycles)
To confuse things further all of the above systems including square taper are still used today and all have different axle length options. For square taper and octalink types this is dependent on the crank interface and how many chain rings are used. With press fit the axle length can vary due to the width of the frame where the press fit bottom bracket is fitted.
If you have just a frame all can be quite confusing but the manufacturer specifications list what is suitable on new frames and older frames will fit more than 1 of the other options so there is a little bit of choice depending on what brand you prefer to use.