The axle adapter kit is ideal for bikes with 12mm thru-axle given the diameter of the 5mm drive trainer. The thru-axle adapter is required for coupling the rear triangle with the thru-axle support on the Tacx® FLUX and NEO 1st generation trainers with 5 mm quick release ab origin system. For the new Tacx® FLUX and NEO 2T models, the adapter kit is always included in the box.