I just upgraded an old mountain bike with XT hydraulic disc brakes. The rotor on the back is 160mm. The rotor on the front is 180 mm. There is a rather different feel to the back brake vs. the front brake. I just assumed it was because one of these wasn't bedded in as well as the other one. My kid suggested it was instead a property of the different disc diameters.

Which is a more likely explanation, and if it is due to the difference in disc diameter, which brake would you expect to appear more powerful?

My bike (mechanical calipers) came with 160/160, and I quickly noted that there was more lever effort required to get the front to "feel" the same as the rear brake, and the side-effect of the front pads wearing out more than twice as fast as the rear. So I switched the front to a 180mm, and now the braking feels a lot more balanced-- and the pad wear is a bit more evened out.

The front brake will likely feel more powerful, but that's the brake that does 90% of the work anyway.

Yeah, so in this case, the front brake feels less powerful, which was the opposite of my expectations as well.

On other bikes (road and mountain), I have two equally-sized 160mm rotors, and the front "feels" more powerful, and indeed provides most of the stopping power.

I'll switch the pads around, and see what happens...

180mm should stop harder and resist brake fade better given all else equal.

The front brake should provide the larger portion of braking power all things being equal. That's normal.

If the lever is spongy feeling you probably just need to bleed them. They should come already bled but poop happens. Luckily shimano brakes are super easy to bleed yourself. Plenty of youtube videos out there showing you how to do it.

The farther out a brake track is the more torque (braking power) you'll get for the same pad pressure. Move the identical brake twice as far from the axle and you'll get double the braking force. It's a simple question of leverage, and explains why the extreme case (rim brakes) can operate with such low pad pressures.

Though I'm not a disc brake fan for various reasons, this is one often overlooked advantage. Brakes can be individualized for various riders, especially those with weaker hand strength simply by using larger rotors (if the mounting system allows it).

OP is saying the front feels WORSE people.

Pads might be contaminated or worn, or you might need a bleed. If lever feels spongy or compresses all the way to the grip, then I'd lean toward bleed. If the lever is firm when you squeeze it, then it could be contaminated pads.

Everything is brand new, and pre-bled (and not squishy, leaky, etc). More likely the pads aren't yet bedded in properly (or could have got contaminated, but I saw no evidence of this).

On my road bike, I usually just bed them in by riding the bike a bit while applying the brake, and then the rest happens naturally. I've never used any of the more extreme, wacky methods.

Oh, that's interesting. Pad contamination is not always visible though.

The system is pretty simple, right? You can see whether the pads are being squeezed against the rotor. If that part is working, then the problem is either pad or rotor. A rotor you can hit with brake cleaner, and pads I just replace. I don't use any of the whacky methods for decontaminating.

But the bike is now. So maybe like you say, just ride it for a while and see whether things even out.

The bike is almost 10 years old. The XT brakes are pre-bled drop-in replacements for some Juicy Avids I suffered with for all these years. I also replaced the rotors with IceTechs. These are already a major improvement, but I was surprised the back brake seemed to have more immediate stopping power. I do a lot of my own disc brake work on my road bike, which has what are essentially XT brake calipers, icetech discs, and a different but functionally equivalent (ultegra) lever. So I am familiar with what can go wrong. I was just wondering if there was any inherent difference in feel with a 180 rotor vs. 160 mm rotor. (If anything, I would have expected it to feel like it had greater stopping power). I'll swap the pads around and see if that makes a difference.

Oh, yeah. That's a great combination. Love the Ice Tech rotors. I have to admit that I'm deeply-shallow and mainly was attracted by their looks. LOL! But hey, it's all good.

I ride down (and up) extensive, > 10% grades where I live, so I blow through a lot of brake pads and rotors on my road bike. They work even better than they look.

I'd expect the front to be more effective in slowing the bike down, but the rear to lock up the rear tire quicker. The rear will not stop the bike as quickly though it will lock up fast. The front will not lock up as fast but will and should stop the bike far more effectively than just using the rear. I run a 200mm in front and a 160 at the back. The front always slows the bike down a lot more quickly. For me, a bigger-than-160mm rotor in the back would just be overkill with no benefit.

Consider Physics?, because of the Applicable Newtonian law , the rear wheel is Un weighted as your mass wants to remain in motion and your mass is shifted to the front Wheel axis , as you use friction to try to slow Your self Down.

The front wheel should feel stronger. In a perfectly bled system, with equal rotors. The back brake has a longer hose, which usually makes the rear feel slightly spongier than the front.

It's much easier to lock a rear wheel, but the front should feel significantly stronger.

I just dropped in 2 deore xt brakes on my hybrid a couple months ago both with 160mm rotors. I didn't get Icetech rotors as I'm not worried about heat and to save a few grams I went with the stronger rotor, in case I crash. I think it's psychological - the rear locks out easily while the front doesn't. Therefore you feel the front is not holding its own.

No.

Weight transfer forward during braking means the font is doing more work, so in an equal F-R setup it would feel weaker.

This is why cars have larger discs and calipers in front than in the rear...