I would like to upgrade brakes on my bike but I have a dilemma / question - currently, on my front wheel there is 180 mm rotor and on back wheel there is 160 mm rotor. Is there any reason why bike manufacturer did this (is it assumed that bike doesn't need that much stopping power on rear wheel?). Would I gain anything by installing same sizes rotors (180 mm) on both wheels and is there any risk to bikes frame?
Advantages of smaller discs
- Less prone to damage
- Less prone to warping
Advantages of larger discs
- Dissipate more heat
- More stopping torque
The calliper and pad choices are more important than the disk as far as performance. Bigger discs increase performance cheaper than better callipers. At some point, bigger discs are needed to get enough performance, but mostly disc size choice is about the cost vs weight trade off.
In your case, I expect the current 180F/160R is a nice balance of cost and performance. Better callipers and 160 on the front would save grams but either loose performance or cost $$$. 180 on the rear (if it fits) wont help much with braking power, so the cost to change is pointless
Given the front and back wheels are not interchangeable, there is no disadvantage of different size discs. As such, its probably a lot easier (and cheaper) for you to keep your existing discs
This is a common set up, as the majority of your stopping power is assumed to come from the front wheel, and many frames, especially older frames, had less clearance on the rear triangle of the frame, so often the 180mm rotor wouldn't fit.
You can change it if you like, but there is little necessity to do so.
If you choose to change it, you will only need a rotor, and the appropriate adaptor for you brake caliper type.