Gearing

Welshkev

I'm sure this has been covered, but doesn't seem to be working for me at the mo.

Could someone please, in simple terms, explain gearing ratio's to me?

I hear people talking about 53/39 etc - as I understand it, this is the number of 'teeth' on each cog?

How much of a difference does changing it make? How do you know what to choose if buying a new bike? Do many people have different cassette's and do they change them often? Where would a 'compact' fit in to all this?

Sorry if these seem like stupid questions, just struggling to get my head around all the guides online - just want it put simply to me!

quozl

53/39 means that there's 53 teeth on the big ring and 39 teeth on the small ring in the front. It's a double rather than a triple as there's 2 rings rather than 3.

50/34 is a typical compact double. It gives a wider spread of gears than a double as there's a bigger difference between the two chain-rings.

In the back it'll vary from 11 at the smallest to 34 for the biggest (which needs a mountain bike derailleur to handle the necessarily long chain).

If you're turning 34 in front (small chainring of a compact) and you're in the 34 on the back then for each rotation of the pedals that you do, you'll turn the back wheel once.

If you're turning 34 in the front and 17 on the back, then for each rotation of the pedals the back wheel will turn twice (as it takes 17+17 to match the 34 up the front).

It's a simple as that.

A 34 in the front and a 28 in the back will let even fairly unfit people get up most hills.
A 50 in the front and a 11 in the back will let you do 52 km/h at 90 revolutions of the pedals per minute.

People can swap in wider cassettes if they're going to do really hilly terrain. How much that actually happens unless they're going to the alps, I dunno. I've swapped a wider cassette to a narrower one as it gives smaller gaps between the individual gears and I didn't need the really easy gears.

Lumen

http://www.stevehoggbikefitting.com/cycling/2011/05/compact-cranks-vs-standard-cranks-gearing-implications/

rp

see also http://sheldonbrown.com/gears/