Hi,
i am new to biking sport and i have recently got a bike with disc brakes. the rotor size is 160mm, but i have noticed that what is writen on the caliper is R:160 F:180
so what is F:180 stand for?

Many thanks in advance.

R:160 = Rear 160mm
F:180 = Front 180mm

Is it written on the actual brake caliper or on a brake adapter like in the following photo ?

180 Front/160 Rear adapter

Is the brake 51mm IS (International Standard) or 74mm PM (Post Mount) ?

Brake Mount Standards

IS mounts on forks are 160mm and IS mounts on frames are 140mm
IS brake calipers come in different sizes, front 200mm/rear 180mm, front 180mm/rear 160mm and front 160mm/140mm
The 20mm size difference between front and rear on the brake caliper is because of the 20mm difference between the frame and fork IS mounts.

IS brake calipers (the left caliper is a 160mm front/140mm rear and the right right caliper is a 180mm front/160mm rear)

The 180 Front/160 Rear PM to IS adapter in the first photo has different sizes on it because of the 20mm difference between frame and fork IS mounts.

With PM to IS adapters: 160mm front = 140 rear, 180mm front = 160mm rear, 200mm front = 180mm rear, 220mm front = 200mm rear

many thanks for the quick reply.
the F:180 R:160 are written on the actual mount bike from the back side, but i have no idea if the brake 51mm IS (International Standard) or 74mm PM (Post Mount); however, please find below a snapshot from the brake caliper, it might answer your question

https://docs.google.com/file/d/0B0zI...it?usp=sharing
https://docs.google.com/file/d/0B0zI...it?usp=sharing

i doubt the F stand for FRONT and R for REAR since it is written on both front and back calipers while my both rotors are 160.

The frame has IS brake mounts.
The brake caliper definitely isn't post mount.
The brake caliper doesn't look to be IS mount either.
The brake looks like it's on a special adapter to attach it on the IS frame mounts.
The brake looks very cheap and was probably mass produced in China for a few $

The phillips head screw attaching it looks dodgy.

If "F:180 R:160" is on the adapter that's connects the brake caliper to the IS frame mounts then that means if you put the brake caliper with that adapter on the front you will need to use a 180mm rotor, when it's mounted on the rear you use a 160mm rotor.

When I say adapter I mean the part that looks similar to this.

Does the front have F:160 R:140 on it ?

Well, you're wrong. It's industry standard to do that. That stamped info helps define the application for those adapters for the builders/assemblers.

BTW, you need to turn your QR handle so it points toward the back -- pointing toward the front like it is, it can catch on something if you ride through 'close quarters', and pop the QR open. Not a good thing for the ride, the bike, or the wheel.