Hot Wheels motorcycles:
http://addictedtostyrene.blogspot.com/2006/09/some-older-warhammer-40k-projects.html
They actually work pretty well - and if you head on over to a toy store (or big box with a toy section) you can generally find a few racks with a bunch of different options.
Most the Hot Wheels/Matchbox bikes are around the right size for orks and other larger 28mm figures. You can mix things up with different parts for kits and detail them out a bit - then give them a suitable paint job. Although the wheels are smaller than other
40K bikes, they are still pretty chunky.