Cog to Chainring. Any Rules???

Hi,
I’m currently running a 48-18 -(74 gear inches) setup and am looking to lower the gear ratio a little.
Whats the best way to get it down to the high 60’s/low 70’s without swapping the chainring?

Are there any ratios which are a no-no?

Any help is appreciated. Thanks.

Check out Sheldon’s advice on skid patches, which will give your tyres a longer life if you skid… http://www.sheldonbrown.com/fixed.html#skid

On my bike, running 700x23 tyres, 48-18 gives you 72 GI, not 74.

So to get to 68 GI you only need to go up to a 19t sprocket. Which will mean you will have 19 skid patches… and so no tyre wear problems.

Going up to 20t sprocket will give you 5 skid patches… not so good, at 21t skid patches won’t be a problem again but the GI will be low - 61.

Sounds like you need a 19 tooth sprocket.

Here’s a handy ratio/skidpatch chart i found somewhere. I’ve picked up a bunch of second hand cogs and chainrings around the place and having some different options to play with is really handy, especially if you can get a fixed/fixed hub.

In theory 48/19 is one of the best ratios you can get for skidding. Either that or 49/19. Just under 70 gear inches, and odd numbers are always good. A high skidpatch count is obviously good for your tyres, but also helps your chain/chainring/sprocket wear, in the long run at least.

And i guess if you’re running 27" tyres, those ratios will all be a bit higher, which might explain your 74 inches rather than 72.

cool, sounds like 19t is the way to go.

I’m running 700x23’s so i guess i used a dodgy gi calculator!

Thanks for your help.

i really like this:
http://software.bareknucklebrigade.com/rabbit.applet.html

That rabbit calculator is great, can’t believe someone coded all that, even with a diagram for the skidpatches as spokes on a wheel…

I just discovered that the Denny Trimble chart I’ve been using above must be intended for 27" tyres, as it gives results 2 inches more than what the Rabbit one does. Duh. Americans!

This means all this time I’ve thought I’ve had a higher gear than I really have… just thought I had awesome legs of steel…

No matter how many skid patches i have, I always seem to kill tyres in the same 3 inches. I run 49x17 and supposedly have 17 skid patches, but I’m not so sure…

If your chainring tooth count goes down by the same amount as your sprocket goes up, the rear axle will stay where it is. But a 1t difference at the back shouldn’t be a problem. The part of the sprocket that is in contact with the chain is currently 9t, it will go up to 9 1/2 t. So your rear will move roughly 1/4t toward the cranks. I think. (Anyone?)