How do to determine if an adapter is needed for disc brakes and rotors? I have an old older (not too old) bike that already has disc brakes, but I would like to replace them with a totally new set and new rotors.

How can I determine if a given replacement (caliper and rotor) is plausible my bike, and if so if an adapter will be needed or not, and if it is, what to get?

For the particulars, I have a 2000 GT Zaskar and one brake setup I am considering for example Hope v4 with 203mm front rotors and 183mm rear and would like to know if this would even fit.


 

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Just to enrich this answer, there are the two systems nowadays for mounting the brakes on the fork and frame; these are IS or postmount:
http://www.pricepoint.com/TechPages/TECH_Page_Disk_Mount_Types.htm

Normally you can find adapters to use one or the other.

Regarding the disk-hub mounting, there are also two options, 
- 6-bolt (the name is self-explainable, it is the standard in the market):
- or centerlock (Shimano proprietary), which are fixed by a lock ring, similar to the one used to fix the cogset to the rear hub, but on the disk side:
http://www.pricepoint.com/TechPages/TECH_Page_Rotor_Types.htm

You have adapters to mount 6-bolt rotors into centerlock hubs, but the opposite is not possible.

Just one final consideration, if you are running a 26" wheel, current brakes allow a huge braking power with relatively small rotor. Nearly nobody would use more than 160mm on 26", unless you are doing freeride or DH.

Regards.