ratios, how is it said

[noddy]

hey all mechanically minded people.

i have a debate going (annother one!) with my friend as to how you present the gear ratios seen on bikes...
IE 1:2 or 2:1
to me it seems that it should be driver gear then driven gear. therefore one turn of cranks=two turns of wheel (1:2)

my friend thinks opposite. (2:1)

how is it said...

[Superman]

2:1

2 sprocket teeth to every 1 drive tooth.

[Renegade]

I've mostly herd the 2:1 ratio which goes for shimano? And the 1:1 actuation ratio for sram?

So in other words its 2:1?

[tu plang]

well it is the driver gear, then the driven gear. however its the ratio of the driver gear radius (and/or teeth number, they are directly proportional) to the ratio of the driven gear radius).

[fattyandthepiemakers]

On another related topic...

When scaling drawing (lets say... AutoCAD) if the real size of the object is too big for the sheet and is needed to be scaled down to half the size... The ratio would be 1:2?

...Not that I'm in heated debate or anything like that...

[warrior]

Its driver than driven

[FuTAnT]

You are correct, and as other people have stated it's 1:2. N1:N2 is your ratio where N1 = front chainring tooth number and N2 = rear cluster teeth number (for whatever gear you're in).

Edit: Fixed the ratio, I meant : not /. If you want to normalise the ratio then it becomes N2/N1 and is expressed as 1:N2/N1. I hope that makes sense.

[StormFire]

ratios are always expressed the same way, whether it be part of the dB scale such as a measure of attenuation, or whatever else you can think of that uses a ratio, where by it's always a ratio of the output over the input.

To put it in to a bike sense, your input is the cranks, and the rear wheel is the output, so a 2:1 ratio means that the rear wheel spins twice for every time that a crank spins.

And for that autocad example, a 1:2 ratio would mean that your output would be half the size of the original (or half scale).

[Tomas]

On another related topic...

When scaling drawing (lets say... AutoCAD) if the real size of the object is too big for the sheet and is needed to be scaled down to half the size... The ratio would be 1:2?

...Not that I'm in heated debate or anything like that...

Correct. One unit on the page represents two in real life....

1:10 000 = Maps

10:1 = small things.

[FuTAnT]

Correct. One unit on the page represents two in real life....

1:10 000 = Maps

10:1 = small things.

And just to confuse things, a 1:10,000 map would be regarded as a large scale map, while a 1:250,000 is a small scale map.

It makes sense, but can be confusing at the same time if you just think of the numbers instead of what level of detail each offers.