MAKE Magazine illustrator Tim Lillis created this “Tricks of the Trade” video to show how you can lock two bicycles up with a single U lock.
The video illustration is wonderful, but Tim lives in SAN FRANCISCO CALIFORNIA, a Kryptonite Lock Top 10 USA bike theft city. All four of those wheels will be stripped while Tim and his friend grab a bite to eat.
Better would be to find a thinner pole and try to loop the lock through both rear tires inside the triangle. I’ve managed to do that with road wheels. Your rear tires are the expensive ones and easier to remove (there’s no wheel retention “lawyer lip” in the rear dropout). For extra security, carry the front wheels with you as you go grab a bite to eat.
When I go up to San Francisco, I generally bring two bike locks with me — a mini U lock and a thick cable lock. They’re individually defeatable by experienced bike thieves, but the theory is that the thief moves on to an easier target and so far that theory has worked for me.