Half decent drive train length calculator

I just went from a 20 sprocket to an 18 and the position of my hub on the dropout position didn't change much at all. This got me thinking whether the received forum wisdom on this topic can be right, and, having thought about it, I reckon it's not.

This led to a some geometric doodling and a bit of algebra. The geometrical modelling isn't perfect, but it should be fairly accurate.

To cut a long story short, I've developed a little Excel based calculator (attached) that allows you to find the BB to rear hub centre distance for any given chain length and chainring / sprocket combo. More importantly, you can use it to see how much further forward or backward your hub is going to travel in your dropouts when you change gearing - and hence whether you'll need to mess with your chain.

1 Attachment

• DriveTrain.xls.zip

1/8 inch per tooth is the answer

The answer is 42

1/8 inch per tooth is the answer

Roughly, and mostly, yes.

just buy a chain.
if it's too long, cut it.
if its too short, add links.

innit ?

is that me being a Neanderthal ?

Fantastic work.

For those with little room for adjustment in their rear drop-outs (track ends) this could be really useful, and save some trial and error.

I for one thankyou for your efforts.

just buy a chain.
if it's too long, cut it.
if its too short, add links.

innit ?

is that me being a Neanderthal ?

It's an approach that works for most people. I run a double fixed hub so I wanted to see what the biggest gearing difference was that I could get on my two sprockets, without having to split the chain.

Fantastic work.

For those with little room for adjustment in their rear drop-outs (track ends) this could be really useful, and save some trial and error.

I for one thankyou for your efforts.

Cheers mate, much appreciated.

1/8 inch per tooth is the answer
not necessarily, more room to move with smallerfront chainring.

not necessarily, more room to move with smallerfront chainring.

As the calculator demonstrates...

handy, thanks heaps!

Does anyone know of an online calculator or a way of working out what cogs you can run either side of a hub using the same chain?

for example on my hack bike i'm running 32t chainring and 20t cog for polo, but want a decent gear inch on the other side for going for the papers etc. so could i run a 13/14t on the other side using the same chain?

is there some kind of equation that takes into account cog size, chainring size and drop-out lenth that would save me buying a load of cogs and having to try them all out?

any help would be great...

With the same length chain, adding a tooth to a sprocket moves the axle approximately half a link forwards.

edit:
dho - that should be a quarter of a link, 1/8"

This might help in some way:
http://www.londonfgss.com/thread15171.html

thanks

Just found this, which may be handy :) [no idea if posted elsewhere

http://www.machinehead-software.co.uk/bike/chain_length/chain_length_calculator.html#download]

Stumbling across this thread makes me realize how much i fu**ing love this forum..

Hi I'd like to set up a fixie with two chain rings and a rear hub with two fixed sprocket. I wanna achieve a high and low gearing. Was wondering how you can calculate which gearings will work with the same chain length. I know this can be done I met someone a while back who had done it but can't remember what ratios he used. Also Shelton brown mentions it but they aren't the ideal gearings I want. Cheers

:)

What would be your idea gearing? 30' and 100'?

You need the same tooth difference at the front and the back to keep the chain length the same.
the bigger the difference, the bigger the difference...

you could do a 16-46 and 20-42 setup, check sheldon brown's gear calculator. your high gear should be about 70GI (for road) and your path/cyclocross gear should be about 50GI.
If it's for polo, you'd want something at about 30GI.