The RPM SPEED arrives ready to fit to the hub of either wheel as per your preference. To attach, hold the mount in place on your selected hub and gently stretch the strap portion of the rubber shim-strap around the hub and secure using the bottom two hooks on the mount. When installed correctly, the Wahoo RPM SPEED should not be in contact with any other parts of your bike.
(https://support.wahoofitness.com/hc/en-us/articles/218757787-How-do-I-mount-the-RPM-Speed-)