Blackburn makes the Trackblock 3 because stacking old college textbooks or two by fours under your front wheel is just not pro. The Trackblock 3 levels and stabilizes your front wheel as you do a workout on your indoor trainer. It has three slots that are unique to different wheel diameters -- 700c, 650c, and 26in wheels. You'll be able to thrash yourself in front of the TV on those long winter nights, as the injection molded construction of the Trackblock 3 makes it virtually indestructible. Optimized for use with the Blackburn Trakstand Mag series of trainers to keep your bike level; use with other trainers to raise bar height and simulate climbs