SURLY EXCELLENT ADVENTURE AXLE

The Excellent Adventure Thru-Axle upgrades your bike’s rear end for hauling cargo trailers, should you feel like taking Rufus along on your errands. 
Designed for use with slotted horizontal or vertical single-position dropouts, the Excellent Adventure Axle is compatible only with 12 x 148mm hubs.
Compatible Surly models include Karate Monkey, Krampus, Grappler, Ogre, Lowside, ECR, and Troll. Once again, the 12 x 148mm hub requirement cannot be overstated. 

Excellent Adventure Axle Instructions