force10

I'm trying to decide what rotors to put on an upgraded set of wheels. The SM-RT70 rotors (105) were stock. Can anyone tell me the advantage to the MT-800 rotors (Ultegra)? 160mm for either.

I see a small weight advantage to MT800's. Is there anything else I am missing? Also, would using the upgraded rotors affect switching between wheelsets (not planning much of this)?

Thanks.

Iride01 

MT-800 is Deore XT isn't it? SM-RT800 is Ultegra. SM-RT70 is neither 105 or Ultegra.

The better more expensive rotors will likely handle heat dissipation better. They won't necessarily last longer before needing replacement. Don't just automattically buy them for heat dissipation if you haven't had any issues with heating. Though if you want the best of everything and don't care if you need it, then that is a valid reason too!

msu2001la

The MT800 rotor is XT, not Ultegra. The Ultegra rotor is RT800.

The main difference between these and the RT70 rotor you have is weight, and cooling ability. Both the MT800 and RT800 have additional cooling fins in the center of the rotor that help dissipate heat. There's no issue with running different rotors on different wheelsets, but you might need to add shims to avoid having to re-align your calipers each time you swap.

I currently swap two wheelsets on my CX bike. One set has MT800 rotors (centerlock) and the other set has RT56 (6 bolt). With shims they both work fine, no adjusting needed.

Edit: I should add there's very little difference in braking power/feel between the rotors, but the XT rotors do feel a little more "grabby". This also might be because they are on much lighter carbon wheels, whereas my RT56 rotors are on heavier alloy wheels.

force10

Thanks. I see I was confused - thinking the grx groups were using road rotors. This is for a gravel bike.

In any event, the two of you have answered my question. The difference is heat dissipation and weight.

KJ43

The GRX groupsets do use road rotors. The GRX 810 is basically Ultegra level and mine came with the SM-RT800 rotors. I've seen the groupset listed at some online shops with the XT rotors though.

msu2001la

What is the difference between XT (MT800) and Ultegra (RT800)? The Ultegra rotor oddly weighs 20 grams more (in 160mm size they are listed as 128g and 108g). The ultegra version might be a tiny bit more aero and has slightly larger cooling fins?

Ultegra RT800 rotor:

The MT800 rotor:

cxwrench

Slightly more surface area=slightly better cooling.

force10

That explains my confusion.

jonathanf2

I had an incredibly hard time aligning my rear MT-800 rotor using GRX400 hydraulic brake calipers. Enough so, that I ended up going back to cheaper/heavier non-ice Shimano SM-RT10 rotors. The front GRX caliper aligned fine with MT-800 rotors, but on my rear I tried every technique and was still getting rotor rub. I even swapped out the brake pads, reset the calipers multiple times and tried centering the caliper. Also the rotors appeared to be fairly true, but the MT-800 appears a tad thicker, enough to cause rubbing. Once I put the SM-RT10 rotor back on, there was no issues whatsoever. It sucks, because I'm using the MT-800 rotors on my road bike which are using TRP HY/RD cable hydraulic calipers and the braking is better. There's less squeaking, there's a nice weight savings (180g vs 108g) and visually they look nice. The difference between the calipers though is that I can adjust the TRP pad spacing, while I can't adjust spacing on my GRX400 calipers.

My opinion is that if you're using them on mechanical or cable actuated hydraulic brake calipers, they're highly recommended. If you're planning on using full hydraulic brakes, I'd definitely consider other style rotors.

msu2001la

I'm running the MT800 rotor with GRX 400 hydro brake calipers and have no issues with rubbing or noise.

jonathanf2

Like I said, my front was fine but the rear was giving me a headache. I might give it another try later on, but it got to the point I just wanted to ride my bike without worrying about any issues.

msu2001la

My GRX equipped Cannondale SuperX came stock with MTB rotors. (ST-56, which Shimano lists as part of the Deore M6000).

I don't think it really matters if you run "road" or "MTB" rotors with these groupsets. Either will work.

msu2001la

You could try installing shims on the rotors to see if that helps. It's possible that the new rotor sits just slightly inboard or outboard compared to the old one, and is just beyond the range that re-centering the caliper will clear.

These are .25mm each, so you can install 1-4 of them on each hub and move your rotors in/out by tiny amounts to get perfect alignment: https://novemberbicycles.com/product...shim-pack-of-4

jonathanf2

Thanks for this! I definitely like the MT-800 rotors. I might try swapping the front/rear rotors around and see if that makes a difference before trying the shims.

Camilo

I believe it's generally thought that road riding has a greater chance of long downhills with longer steady braking requiring more heat dissipation. My guess is that like a lot of things on good bikes - handlebars, forks, stems, wheels, brakes, whatever - the features that are important to mega-strong pro and amateur riders just don't make that much difference to those of us who don't tax their equipment like they do. Stiffness? I don't tax it at all. Same probably goes for heat dissipation.

That said, I use MTB rotors on our gravel bikes. The're not Shimano GRX, but Sram AXS, but same difference. I think that a full AXS gravel bike would normally be outfitted with high-ish end Sram road rotors. I put the bikes together and, not really knowing much about rotors (first disk bike), I went with what I thought was a good bang-for-buck price point where I found a discounted high end Sram MTB rotor that was quite a bit cheaper than the similar grade road rotor that wasn't discounted - but would probably have been supplied had I gotten a full built AXS bike. Looks more or less like the Shimano MTB rotor above - Steel ring with aluminum "spokes" but not heavily finned like the Shimano road rotors.

If we in fact ever seem to have heat dissipation issues, I'll "research" which rotors have the best rep for heat dissipation regardless of brand and go with them. We do a lot of road riding with our gravel bikes and being old and more cautious than in years past, I do tend to brake more every year! But zero issues at about 2,000 miles of combined use on two bikes so far.