I decided to upgrade my old s-works hardtail from v-brakes to discs (therefore also triggering an upgrade of wheels & forks)… the one thing that I'm not sure of is how do I choose the right size rotors? Looking at stock xc bikes they range from 185 front / 160 rear to 160 front/140 rear – some even at 205 front/185 rear.

What are the variables that I need to consider correlating to rotor size, besides stopping power? On the one hand, I'd buy them as small as possible as they'd be slightly lighter and still better than v's. On the other hand I'd rather have better brakes than shave a few grams.

It might also be useful to be able to interchange wheels with my Enduro which runs 205/185 but in my mind that might be a little large for the hardtail?

The hardtail will be mainly used for marathons and long xc races / multi-day

Some forks, particularly older ones are not warrantied for over 180mm rotors. Similarly some frames will not like having a honking great disc on the back.

I'd just go 160/160 to be honest, or stick a 140 on the back if you're light and worried about weight.

Go for 180f & 160r unless you want a very light race bike or powerful DH/AM/FR brakes.

Maybe depends how heavy you are, but 160/160 or 185/160 would probably be fine.

The only other thing to consider apart from stopping power is bigger rotors will fade less on long descents.

Bigger rotors shed heat better.

IMO 160 / 160 is fine for UK XC use. Go bigger if you are heavy or ride alps or drag your brakes a lot.

I weigh 75kg – and for the alps / anything big I use my spec enduro. New forks are SID Team (great price from CRC) so it sounds like 160/160 would be ok. thanks for the advice!

Personally I'd go for 180 Front, 160 Rear; Speaking from experience having used 160f/160r on my HT for a couple of months I went back to the larger front rotor as I didn't feel the smaller one was up to speed on the steeper descents that a 120mm forked HT allows.

But he's on a 100mm forked lightweight XC hardtail, with a bigger bike for bigger stuff. I'd get 160s, they'll be more than enough for normal XC stuff.

I'm pretty heavy but have found 160/160 perfectly adequate.

Ditto most of the above, 160/160 is fine for 90% of riding/riders in this country.

I run 185/160 because I tend to brake realy late, realy hard, but didn't want the weight of a 203mm disk (they'r thicker, and have more arms).

160 front and back. Although I've a 180 front and 160 rear, I'm not certain I've got that much extra TBH.