rosefarts

New bike, got all the stuff to put together minus the rotors.

Sram Rival flat mount calipers with Rival levers.

Rear end is 140mm and front is 140 to 160 if I flip the adapter. So I have options.

If it matters, I'm a 140lb rider and will be riding 700x40 tires.

My hunch is that I'd be happiest with 160 up front and 140 in the back.

Not a hug deal since this is something I can change if I don't like. Curious what others think?

tyrion

Bigger rotor in front makes sense to me. I have 180 front 160 rear.

Steve B.

Yes. Larger rotor in front makes sense. I run 180 F, 160 R on a hardtail.

msu2001la

Unless you are doing a lot of aggressive downhilling where you'd potentially overheat your brakes, a 140mm rotor is going to be fine, especially since you are only 140lbs. This is the standard rotor size for road bikes and overheating them is not a common occurrence.

I think the biggest reason gravel/CX bikes come with 160mm is because it looks more "off road" and differentiates the bikes more from a traditional road bike.

unterhausen

I use 160 front and rear. But I'm much heavier. You don't list a location, so it's hard to judge if you are going to be a lot of long downhills. We have some fairly long downhills around here. It's nice to be able to drag the rear brake without worrying about fading.

dsaul

I use 160 front and rear, but I could probably get away with a 140 rear. If you ride anywhere with long downhill technical sections, you will be glad for larger rotors.

dabac

All things being equal, I prefer to run a bigger rotor up front. Mostly b/c it pleases my sense of engineering aesthetics.
My last attempt at setting a bike up 140/160 didn’t end well. Braking power was OK, but the 140 rotor had so many cutouts that it gave an unpleasant pulsating feeling. So I switched them for an 160/180 matched set of rotors with a more conventional pattern. And a smoother feel when braking.
My main winter bike uses 160/160. It came like that and I can’t find sufficient reason for a makeover.

rosefarts

Southern Colorado. Biggest descents around are probably 15 miles. I've done all this on Canti's too.

The theme here seems to be bigger. So 160 up front for sure.

DarKris

+1 for 180F 160R. Esp. at my weight it's very beneficial

Marylander

Not all forks are rated for 180mm front rotors although I wonder how heavy you have to be before you start to stress things. That said, I have 160/140 combo on my disc bike and no significant descents so it's not particularly important for me.

vespasianus

Yes, bigger rotors in the front and at 140 lbs, you will be fine with a 160F/140R combo.

fourfa

Couple of references to 180mm fronts here - if anyone has successfully set up 180s on flat-mount forks, I'd love to know how you did it. I'm on the verge of swapping to a post-mount 4-pot XT caliper on 180 front with a stack of adapters, but it's not elegant. FWIW I'm over 200lbs and my local gravel descents (really MTB trails) are rather steep and I'm finishing descents with hand cramps on my 2-pot hydraulic 160F/160R.

If y'all are actually talking about mountain bikes here on the gravel sub, it would be polite to say so

Rolla

At 140 pounds, you can't really get this wrong. Once installed, you'll likely never think about it again.

Elvo

I'd go with the biggest you can - 160/160 unless weight or cost is a major concern.

dwmckee

Either option will work but since you are so light the smaller rear rotor will help give you a little more rear modulation to avoid skidding the rear wheel on loose stuff.

rosefarts

Probably drop bar mountain bike, or some kind of heavyweight application like a Flanimal or something like that.

I don't know of svelte cross or gravel bike, one that will allow you to ride without a beard, that has 180s.

My Swiss Cross V2 certainly doesn't.

xseal

I'm a motorcycle guy - rear brakes are control devices (mostly) and fronts are for stopping. All my bikes have larger rotors on the front (gravel = 160/140, XC and Fat = 180/160 and enduro = 203/180). I also have them set up with the front brake on the right and rear on the left. Works for me.

rosefarts

I lived in Bermuda for 2 years, scooters only for expats. Had a Honda Trail 90 for a while too, super fun.

I have swapped levers before but couldn't get used to it. Probably because I'd been riding bikes pretty seriously since I was 14, them motors were just funny.

ft3safety

I may have missed it but what type of riding will you be doing? Steep long descents? Racing climbs where every gram counts? The 160/140 sounds okay with me but if you can fit 160/160 it might be better... just in case you drop your bike and somehow bend only your front rotor. Then you can swap them out and ride with just a front brake to get home... That is if you're using 6 bolt and not centerlock.

shoota 

I weigh much more than you had no problem with 140/140. I'd think this would be more than fine for you, especially if you're hydro.

jp911

This.

Cars are similar if you look at brake rotor size, caliper size, and number of pistons. Of course, you can't control front-rear distribution like you can on a bicycle or motorcycle, but under braking weight distribution is biased to the front...even in a well-balanced vehicle, so the front brakes are working harder to stop/slow. Race cars have brake balance adjusters that allow for changes to front-rear distribution based on track surface, weather conditions, or even tweaks for a specific turn. Too much rear brake can be diabolical in a car, just like it can with a bicycle.