If you're looking to add a power meter to your mountain bike pick up a set of these crank arms along with a Quarq DZero DUB XX1 spider and you'll have one of the most reliable and accurate units on the market. They share the same lightweight and stiff construction as SRAM's renowned XX1 DUB SL cranks but come equipped with an 8-bolt direct mount interface that pairs perfectly with the aforementioned spider. Together they're capable of measuring both left and right side power output to within +/- 1. 5% accuracy.