why is there 3 disc sizes available 180 182 and 185 mm

Who does 182mm discs?

Hope do 183mm as well, or are they one and the same, and one of us wrong?

Why are there: 140, 145, 160, 164, 165, 170, 180, 183, 185, 190, 200, 203, 205, 210, 220, 9"(I assume 229mm) 230mm?
There must be more, I know only of those

I hear that 186.5 rotors are the next big thing.

Yup, I've often wondered this too. Suspect some sizes started in inches in Americanadia, and others in metric Europe.

singlespeedstu – but that's to only one decimal place…!

Thats only 203mm and hayes 9" (I assume 229mm?) isn't it, theres no 152 or 178mm (6 and 7") I don't think, or is there?

A quick inch to mm comparison shows no obvious relationship:

Inch mm
1 25
2 51
3 76
4 102
5 127
6 152
7 178
8 203
9 229
10 254
11 279
12 305

Well, you have the metrics to imperial conversion. Then you have different rotor sizes (hope mini were 165, 185, 205 and mono mini were 160, 180, 200).

To the previous sizes you can add 130, 150 and 170 from the hope XC 4 too ;).

Anything to do with hope making 185 and hen some manufactures saying the fork was only okay with a 180?