BIke wheel sizes

dhutch

Slightly odd request maybe.
- Im trying to work out the gearing for a bike wheeled electric kart project.

The driven wheels are 20" with 2.25" bmx type tyres, and i need the circumference. Or at lease what those numbers related to in terms of diameter.
- 2.25 is the nominal diameter/width of the tyre i beleave. But is 20" the nominal diameter of the wheel, or the wheel with tyre?
- If the former, would the overall diameter would be 20+2.25+2.25=24.5" ? from which i could work our cicumerfence/rollout.

Unfortunatly i dont have access to the wheels to messure roll out.
- However there is to be some scope to make changes to the gear ratio if its not dead right.

Daniel

robbarker

This page might help:

http://www.sheldonbrown.com/tire-sizing.html

I doubt you'll be able to find the external diameter of the tyres unless you go to the tyre manufacturers site - this may have to be approximated until you physically measure them..

Please maintain a dignified silence when you click on the link - Sheldon Brown is a cycling legend who, sadly, passed away a few days ago. :-(

secretsam

So: my 27" wheels are 8mm bigger than a modern 622 (old fashioned 700c) wheel

i.e. I'd need a drop of 4mm in brake height were I to change wheel sizes