[w00t]

Bicycle spokes are designed to increase the strength of the rim without the weight of a solid wheel, and they do pretty well, with this test showing that a 1400-2600N force is needed to compress the rim 1mm on a tensioned-spoke wheel.

http://www.rouesartisanales.com/article-23159755.html

According to the NHTSA, all cars driven in the US will compress no more than 10mm in a 5mph crash. Given a 1000kg car, that means a 8000N force can crush the bumper up to 10mm. If we assume that the crush-resistance of a bumper is linear, which it probably isn't, that means 800N would be needed for 1mm of compression, making a to-spec bumper nearly half as strong as a bike wheel.