Disc Size Advice

I'm going to be building up my first ever disc equipped bike soon.

In preparation I bought a 160 and a 180 rotor, planning (motorcycle style) to have the bigger rotor on the front. I am now advised by my son that he would have used 160 / 160 and that a 180 will be overkill on a lightweight hardtail (Dave Lloyd CATS WISKAS).

Your views please: Do I get a second 160 rotor? Or do I stick to my original plan?

If you are setup for the 180 already then its no harm in sticking with it.

I run a 203 180 combo on a lightweight HT, you can never have too much braking IMO. I'm not a weight weenie so the extra few grams on the rotors and adapters don't bother me but being able to haul anchor at any speed down any hill makes me happy .

COOL!

In that case 180 & 160 it is.

Kid's eh? What do they know?

I'm running a 180/160 on my single speed, I do ride it hard though ,as you already brought it,use it, otherwise a 160/160 would be fine with a few grams saved.

That combo will be fine. Having a 203mm on a 9mm hub is not recommended as it can apparently generate enough torque under braking to rip the wheel out the dropouts. (Although I have this set-up atm on my FSR, waiting for a 20mm front wheel)

disc brake rotor sizing depends on rider weight and terrain.

bike manufacturers tend to fit 180/160 for L hardtail and FS frames due to the weight of riders on those frames, and probably the same for M but typically 160/160 for M. For downhill 203 on the front is common.

Also the type of rotor makes a difference - I am heavy rider but try to weightweenie my bikes so all of then are on 160/160 - I find this good enough for the sort of riding I do, namely XC. I have 2 bikes with 160/140 but find the 140 too weak - will be replacing them with 160.

Lightweight discs achieve that by being 0.2 mm thinner plus have metal removed from the braking surface - e.g. KCNC razor - I find they are less grippy than more solid disc e,g, Hope SAW or Avid XX.

Without knowing rider weight and disc rotor models, hard to say but I would be surprised if 160;160 is not good enough.
BTW if you have post callipers and post mount fork, the difference between fitting 180 or 160 on the front is an adapter (spaces the calliper futrher out) and longer M6 bolts.

If you've bought the rotor etc, I'd leave it as it is.

I tend to ride with bigger rotors because I like lots of braking power. My Chameleon used to have 203mm front and rear (but that was mainly because the brakes were off my DH bike that I split and sold. I was too lazy / tight to buy new rotors and mounts).

Modern brakes are pretty powerful so you can get away with smaller rotors.

I'd prefer my brakes to be more than enough for the job, rather than "good enough".

There will always come a situation where you're going to need good, reliable, powerful brakes! A larger rotor means better cooling properties (so less chance of fade) which in turn leads to generally better control over braking and the bike in general.

I always used to fit the biggest rotors I could get away with on all my bikes, whether they were for XC, Enduro or DH duties. Admittedly, you don't need a 203 rotor on the rear, but I ran my Cove Stiffee with a 203/203 Mono M4 combo. There is something very reassuring about knowing that you have enough braking power for any situation, and the small weight penalty was well worth it.

I ran a 238/203 combo on my Intense M1 - Hope had to send me a custom size rotor for the front brake as I was running an early set of 2003 Monster Ts which had a really odd mount size! I remember sticking the 203 rotor on and it not even coming close to the pads!