While quick-release hubs certainly make it easier for cyclists to remove and reinstall their wheels when doing things like fixing flats, they also make it very easy to steal those same wheels. As a result, riders typically have to remove the front wheel when locking up their bike, or run a secondary cable lock from it to the main U-lock. Quick Caps, however, are designed to make doing so unnecessary – they're little padlocks for the quick-release levers.
Created by British engineer Curtis Dorrington, each Quick Cap consists of a weatherproof anodized marine-grade aluminum locking body, and a hardened steel shackle. To use them, you just slip the body over the bike's quick-release lever, put the shackle around the cylindrical base of that release, then slide the shackle into the body and lock them together.
Once in place, the locked Quick Cap will make it impossible to pull open the lever ... until it's unlocked with the user's key. Should thieves instead try simply unscrewing the lever from the hub axle, a protruding section on the underside of the locking body will catch against the fork/drop-out, keeping the lever from rotating.
According to Dorrington, each Quick Cap weighs 51 grams, and it takes approximately 900 Nm (664 ft lb) of force to break them. He's currently raising production funds, on Kickstarter. A pledge of £10 (about US$16) will get you one, with £19 ($31) required for a pair – assuming the funding goal is met. You can see a product demo in the pitch video below.
Other recent approaches to making quick-release wheels theft-proof include the Sphyke C3N system, which replaces the levers with mini combination locks; infiniti3D, which replaces them with a key lock; and Pinhead, which only unlocks with the use of a product-specific tool.