What's the difference between the Topaz 2 and the Topaz T3AIR? The Topaz 2 has been optimized for use on a Trunnion mounted bike. These bikes typically have a lower leverage ratio which the HV can is tuned around. If you have a trunnion mounted bike with a lower leverage, (starting at 2.4 and lower) use the 2. If it has a higher leverage 2.4 and up, use the Topaz 3.