Building up a Colnago G3X gravel bike, and first time using disc brakes. Are these appropriate? Other recommendations? Also, at 5’7, 120# do I need 140/160mm or 160/160? Thanks!
I’d go with 160 mm front and rear, even if you are on the lighter side. XTR rotors are a great option for Shimano groupsets. I have a pair on my mountain bike. Do you have a particular groupset in mind?
1 Like
Yes. I have the Shimano GRX groupset in 2x11, it just didn’t come w/ rotors. found a chart that recommended RTMT800 or SMRT800. Not at all confusing 
1 Like
For gravel riding 160/160 is fine. Anything bigger is more downhill orientated.
140 rear is better here. The smaller rotor is lockup control on application and control on sustained braking. For sustained braking the size rear rotor doesn’t really matter because to transfer the weight to the front. On dry dirt with a big soft tire, you often have more traction than asphalt, so you can get away with more brakes, but the smaller brake saves your rear tire and let’s you control the rear wheel more. I’m much heavier and could do a 140.
Shimano rotors are the best…hopefully you can find some in stock.
I’ve got 160mm on my road bike front and rear. No complaints here.You quickly get used to using one fingertip for braking.
I live on a steep hill so I always give my brakes a good test at the start of every ride (road or MTB). It’s a good way to remind my brain which bike I’m on and how hard I can brake without losing traction.
Availability of the SM-RT900’s was limited so I ended up throwing on a spare RTMT800 I had laying around. Works well with the 105 brakes.
There are a few different types of lockrings for the centrelock type rotors that depend on your hubs and there’s also the 6 bolt type. It’s worth making sure you’ve got the right tools and any optional fittings if you need them.
1 Like
In my experience it isn’t.
I’ve had different combinations of rotors on different road and mountain bikes, and you’ll just get used to it. 160 mm rotors have better braking action, because they provide full braking force more quickly. But modulation isn’t any worse. I’d always err on the side of larger rotors.
Rotor type has a larger influence in my experience than size when it comes to modulation. My previous road bike had non-Ice Tech Shimano rotors, and they were significantly worse than my mountain bikes XTR Ice Tech Rotors (with XT brakes). My new road bike’s SRAM brakes also are a significant upgrade (rotor size was the same for both).
But even more significant is that modulation is engineered by the respective group set manufacturer. I really like my mountain bike’s XT brakes, those puppies are bomb proof. But I don’t like the brake lever feel of Shimano’s hydraulic drop bar STI levers. If you look through reviews, you see the claim that this was a deliberate decision. In the mountain bike world, you see people preferring one brand over another because they prefer the balance the manufacturer strikes between bite and modulation. I vastly prefer the feel of my SRAM Force eTap AXS brakes to the Shimano RS-505 (105-equivalent) that I had before.
2 Likes
There is actually an option to go bigger than 160 mm rotors. the shimano page shows that its possible to use 180 mm rotors. I cant find out which flat mount adapter this is.
[SHIMANO DEORE XT FLAT MOUNT Disc Brake Caliper | SHIMANO BIKE-EU]
Personal opinion, when it comes to brakes bigger is better. Have yet to hear someone complaining “I got too much brakes on a bike”.
1 Like
Yup. Two years ago I had a very nice rental mountain bike with 4-pot Hope disc brakes. Those were niiice. No matter what (little) I threw at them, they were completely unfazed and consistent. On a mountain bike, I’d go for 180 mm front and rear if it were up to me. On a road bike 160 mm front and rear should be the default, with an option to go 180 mm at least in the front for heavier or sportier riders.
1 Like
Thanks for the input everyone. Found some Dura Ace RT900, 160 f/r. Just need my handlebars and wheelset and the build begins!
2 Likes