# Meaning of "gear of 20 inches"...

Discussion in 'Bicycle Mechanics and Repairs' started by Maz, 1 Jun 2008.

1. ### MazGuru

If a bike is described as having a "bottom gear of 20 inches", what does it mean?
Thank you.

One full revolution of the pedals will move the bike forward 20"

everything you ever wanted to know on the sheldon brown site

http://www.sheldonbrown.com/

3. ### longersVeteran

This will explain it far better than I ever could.

4. OP

### MazGuru

Ah, thanks.
Here's me thinking it was something to do with chain length travelled.

5. ### Wobbly JohnVeteran

Not exactly - one turn of the pedals will move you forwards by the distance travelled in one turn of a 20" diameter wheel (about 63")

6. ### Rhythm ThiefVeteran

Measuring gears in inches dates from the time when people rode Ordinaries and the only way to change the gearing was by increasing the size of the wheel. A 25" diameter wheel would take you further than a 20" diameter wheel, f'rinstance.
I thought it was based on how far one revolution of the pedals would take you, but Wobbly John has sown the seeds of doubt in my mind now.

7. ### Dave5NÜber Member

It is much more helpful to talk about gear development, as that is independent of wheelsize.

BC gear retrictions work on metres development. It's the minimum distance the bike can travel with one whole turn of the pedals/chainset.

8. ### hubgearfreakÜber Member

he's about a third of the way there though

maz, having a 20" gear means you have an extremely low bottom gear

9. OP

### MazGuru

Ah. Okey doke. Thanks, everyone.

10. ### The Young Soul RebelNew Member

You mean maximum, of course

Location:
NE England
I'm afraid it does not mean it will move the bike 20"/rev. It means the drive is as if you were pedalling a wheel of 20" with the cranks directly attached.

I.E. 20 x pi inches

The measurement was started when Saftey Bicycles started to replace 'Ordinary' Penny Farthing bikes. It was an attempt to equate the feel of the gearing with something the old 'Wheelmen' would recognise.

13. ### hubgearfreakÜber Member

gear inches is also independent of wheel size.

gear development does tell you how far you'll travel for one turn of the pedals. that's exactly what it is. ie. gear inches x 3.14

external tyre diameter is only part of the equation (unless you're an ordinary man). totally irrelevant on it's own without reference to front and rear sprocket and possibly (for perverts like me) internal hub ratios

14. ### Over The HillGuru

After however many years since school days you all now have a use for Pi in converting wheel size to distance travelled.

Those maths lessons were not a waste of time after all