Gearing Calculator

[Young Un]

How do you work out your gear inches? I can vaguely remeber the formula/ calculations but I can't for the life of me think what it was exactly.

thanks

 

[betty swollocks]

good old Sheldon RIP

 

[Crackle]

Wheel diameter in inches *(front cog/rear cog)

i.e. 700c wheel is

26.28*(30/12) = 65.7

You can also add in your crank length but if it's between 165 and 175 it's hardly worth it.

 

[Young Un]

thanks!!

 

[Greenbank]

You can also add in your crank length but if it's between 165 and 175 it's hardly worth it.

Crank length has no effect on gear inches or meters development only Sheldon's self-invented "gain ratio" measurement that no-one uses. Tyre width (and therefore height) does have a small effect though.

Gear Inches are often quoted because the makers of the new fangled chain driven "safety bicycles" wanted to show how they were superior to the existing ordinary/penny-farthing bicycles.

The gear size in inches is the equivalent wheel diameter of an ordinary that goes the same distance for one revolution of the cranks.

The maximum size of the wheel on an ordinary is limited by your inside leg measurement. Too big a wheel and you simply won't be able to keep your feet on the pedals at the bottom of the pedal stroke, an even bigger gear will mean you wouldn't even be able to reach the pedals at all.

I ride 46x17 on 25mm tyres which works out at 71.4". I wouldn't be to ride an ordinary with this gear size because I'd need an inside leg measurement of at least 35.7" plus the length of the cranks. Being a 5'9" short-arse means my inside leg is only about 32".

The safety bicycle manufacturers would happily quote their machines had a 85" gear which couldn't be matched by an ordinary.

 

[Crackle]

That's very informative that Greenbank. I knew it related to Ordinary's but wasn't sure how.

Gain ratios eh. Ahh, that makes some more sense now. I have come across a few people who use gain ratios but personally I've never got my head around them.

 

[jimboalee]

Starley's Rover was 40 x 20 using a 26" wheel. 52", which was the most popular size Ordinary.
I set up my Sturmey 3 speed bike so middle gear is 52". 44 x 22 to a 26" wheel. 3rd gear is 69". The bike weighs 32 lb, so the suitable Fixie gear is half way between 2nd and 3rd = 61".

In 1937, Claude Butler launched the 'Tour de Engleterre', the first production 531 bike. It had a Simplex Champion du Monde Pro' 3 speed derailleur. CB would gear the bike to the customer's spec. My uncle John rode in the RAF team, and his spec was 48 chainring, with the middle cog 24. This was 52". Climbing cog was 28, because the bike weighed 22 lb. Speed cog was 16, giving 78".
For a bike of 22 lb, the fixie gear I would fit will be 65".

 

[peejay78]

http://software.bareknucklebrigade.com/rabbit.applet.html

 

[EasyTom]

And another table (plus other info) here:
http://www.fixedwheel.co.uk/tech.htm