rainer Caps allow thru-axle bikes to be used with most wind, magnetic or fluid cycling trainers. Cold weather and short days are never far from a Minnesotan&';s mind so Wolf Tooth engineers created Trainer Caps to reduce barriers to winter workouts. Non-drive side axle caps are joined by drive-side plugs that can be installed or removed without tools to make the prospect of hopping on the trainer a little more bearable. Compatibility: Wolf Tooth Rear Axles (not compatible with rear axles from other manufacturers) Plug has O-ring retention no tool required