Rims are around 6"? inches. Typical Goped/Razer type wheels. I aired up the front tire fine - to 45 psi (was around 25 psi) - but cant access the back valve-stem (due to the chain/sprocket and frame). Do I need a "valve extender"? - if so which type? (seems there are many types -with different threads and maybe for different vehicles-types also. Nothing is simple anymore ;-(. I'm ignorant about such things, but would sure appreciate any help on the matter. ............. still need to join the 20 mph club ;-/.