[mikeash]

Home Depot has decent swivel wheels for maybe eight bucks retail. The wheel that rotates doesn't have to be active.

[bnegreve]

Hum, I think a free swivel wheel would only work for very cheap designs. If you're not convinced, you can search for 3 wheels vehicles on google image [1]: none of the vehicles there have the 2 wheels + 1 free rotating wheel design that you recommend.

https://www.google.com/search?q=3wheels&biw=1673&bih=1124&so...

Edit: actually, some baby carts have it .. alright

http://www.aliexpress.com/store/product/EMS-Free-shipping-Go...

[mikeash]

I think that's more of a speed thing than a cost thing. At higher speeds you want to control all the wheels to ensure you don't go sliding off into a utility pole or something.

[bnegreve]

Yes, by "cheap" I mean "low quality with low requirements".