Why alpha sizing?
1. It’s easier to understand
“Medium” is intuitive. “17.5” is not. Alpha sizing makes it easier for you to know what Trek mountain bike will work for you. A lot of other brands also use alpha sizing, and if you ride a medium in a different brand, you’ll probably ride a medium Trek as well.
2. Frame sizes are measured differently today
Trek frame size used to be measured by the length of the seat tube. But modern mountain bike geometry is based on reach, not seat tube length. Alpha sizing is simpler and helps you find the right fit by the measurement that matters most.