Gear Ratio Help Needed

[Euro Braker]

I thought, as christmas is coming I think I need some new cranks. I came to the conclusion that the Echo 6061s are better than the t-bird cranks on it at the moment, then I looked at a sprocket for it. I am running a rear freewheel (18tooth) so a front fixed cog will be nessercary.

My bike is a mod with a 20tooth front and an 18 freewheel. I found a website that told me that ratio is 1.6.

So, what is a gear ratio for? I have no clue at all, and I searched tf. To me it means a random number and I cant work out how you can work out good sprocket sizes for your current setup. Whats the best sprocket to have? Can you fit those tiny 12 tooth on the Echo 6061 Cranks as 18:12 seems popular with mod riders?

Thanks in advance

[dave85]

Ratio is given by the teeth on the front cog divided by the teeth on the rear one, so for yours it is 20/18, which is about 1.1

The higher the ratio, the harder it is to pedal and the further you go with one rev. of the cranks. Most mods run with 18:12, which is 1.5, quite a lot harder than your current gear.

You can get 20t screw on cogs which will fit screwed cranks like the echos, thats the best option unless you're moving the freewheel to the front. (By the way,12t cogs will fit on the same threads as freewheels, but a 12:18 ratio wouldnt be much use at all)

[RobinJI]

18:12 is indeed popular, but the other way around, with the freewheel on the cranks and the fixed gear on the hub. If I was you I would take the freewheel off your hub and put it on the cranks, then get a 12t fixed gear for the hub.

Don't use that website again, it was very wrong, if you have a 20t ring and an 18t freewheel, then your gear ratio is 1.11 not 1.6

The way you work it out is the number of teeth on the front ring, divided by the teeth on the rear ring. so in your case, 20/18=1.11111111

18:12 is 18/12=1.5 this is a really good ratio for mods in most peoples opinions, and pretty much everyone uses it.

[Euro Braker]

So I take I need to swap my freewheel around or have a fairly large front sprocket.

So how come mod and stock have different prefered ratios?

[the judge]

So how come mod and stock have different prefered ratios?

Loads of different factors really: crank length, wheel size etc...

[RobinJI]

Ah yeah, this is due to the wheel size.

The gear ratio is purely a measurement of how much the wheel turns compared to how far you have turned the cranks, as in with a ratio of 1.5 for each 1 turn of the cranks the rear wheel will turn 1.5 times.

To work out how far this actually makes the bike travel you need to take into account the wheel size.

the distance the bike will travel per turn of the wheel is equal to the circumference of the wheel.

This is worked out by the diameter of the wheel x Pi , with a 20" wheel, the circumference is 20x3.141592654=62.83"

So with one turn of the cranks, the wheel will move:

(front sprocket teeth/rear sprocket teeth)x(Pi x wheel diameter)=distance traveled.

with a 20" wheel and 18:12 gearing this is:

(18/12)x(3.141592654x20)=94.24" traveled for one turn of the cranks.

with a 26" wheel and 20:17 gearing this is:

(20/17)x(3.141592654x26)=96.1" traveled for one turn of the cranks.

If they used the same gear ratio then they would tavel different distances for each crank turn, so the geering would be different, even if the ratio was the same

Edited November 19, 2006 by Dont you Just Hate it When...