The term "bottom bracket" is usually shortened to "BB" and "Mountain Bike" to "MTB". There are so many BB standards and the MTB created more. The bottom bracket is a point on the bike where engineers have tried to solve all of the issues. Square tapers were pushed to their design limit with creaking issues, from both the bearings and the crank arms. BB's have slowly evolved over time to address these issues and make things lighter. At OLSEN HQ we have tried to be consistant across the range and have developed our frames around 2 approaches:
T47 x 92 (or 86.5) INTERNAL standard: T47 is a new bottom bracket standard. We have chosen this standard as we can get a 92mm wide bottom bracket. Currently popular with custom frame builders and starting to be adopted by larger frame manufacturers. T47 is based on PF92, allowing the chainstays to be wider, providing more tyre clearance.