Power meters aren't just for roadies anymore. The Quarq SRAM XX1 Power Meter Crank Arms are specifically designed for BB30 systems with a CNC machined aluminum spider that houses the power meter. Five strain gauges, placed strategically throughout the spider, measure the Newtons applied to the pedals. With a claimed accuracy of 1. 5%, the SRAM XX1 Power Meter Crank Arms effectively and accurately take your training to the next level. The Quarq Power Meter communicates with your head unit via ANT wireless. That means there's a good chance that it's compatible with your existing system. The Power Meter also features Quarq's proprietary Power Balance technology, which measures and displays the power that you're generating in both crank arms separately, so you can determine which leg you're getting more power from, and thereby tweak your training program to even things out. Another nice feature with the Power Meter is the tool free battery replacement, so you can swap the batteries and go riding, rather than pulling your cranks off the bike and mailing them in for a replacement. One ongoing issue with power meters is their susceptibility to fluctuating temperatures, where rapidly rising or falling mercury necessitates one or more mid event zeroings. The XX1 Power Meter now addresses this with a dynamic temperature compensation that includes over 10,000 data points. It figures out the atmospheric details for you with minimal drift, so you can focus on turning the cranks, not babying them.