The Snowjob.

Via: the kneeslider

"Sold by Advanced Recreation Equipment of Mountain View, California in 1973. The conversion is a twin track assembly that your motorcycle bolts into after removing the wheels and it looks like the chain is used to drive the tracks. The motorcycle tilts within the tracks which would give the front ski the ability to carve through the turns. From the coupon in the magazine ad, you fill in the blank and indicate what kind of motorcycle you have and they would make one to order."