Rotor

Pretty much guaranteed to get opposing answers to this one 😀 

I've experienced glazing, squealing and cooked brakes with RT66 rotors on Zee brakes (admittedly with UberBike Race Matrix pads). Whereas I've had none of those issues using RT86 rotors with XT 4-pots with the fancy pads with cooling fins. Unsure if it's the pads or rotor making the difference but I suspect it's a bit of both.

Hhhmmm that's interesting as I currently use the Uber bike Race matrix pads!

Normally I'd just buy the XT rotors but they're three times the price of the SLX rotors at the moment.

The SLX ones are plain steel wherreas the XT ones are the aluminum cored icetech rotors on an aluminium spider.

Hard to say if there's a real difference as it's hard to be objective about upgrades like that but I do think the RT86 feel more consistent, like you're getting the same initial sharp bite each time and it doesn't fade after a few seconds because the rotor is staying cooler. But that could just be in my head.

Another consideration is that XTR rotors (RT-MT905) are 'only' an extra £8-10 more than an XT at the moment. Perhaps easier to get your head around paying the premium over SLX when it's XTR 🤣 

I inherited a pair of XTR 203s on a new-to-me bike and they're immense. So they should be at that price though. Additional cooling material on the inside of the braking surface - not sure what difference it makes in reality but it's got to help a bit.

Aaarrggghh I'd have to get a matching pair of XTR rotors then 🤣 

I've just gone from RT66 to RT86 (203f/180r for both) and notice absolutely no difference other than the RT86 look cool.

IMO- if you have decent brakes working properly, and you're not doing anything really unusual, or absolutely enormous, then you don't need fancy rotors or pads. And if you don't have good brakes or they're not working properly then no amount of fancy disc will fix that, the fix is to sort the brakes themselves. 

Basic, reasonable quality steel is usually the smart way to go, the cheapest Shimanos are only recommended for resin pads, more aggressive pads destroy them pretty quick

Posted by: Northwind

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IMO- if you have decent brakes working properly, and you're not doing anything really unusual, or absolutely enormous, then you don't need fancy rotors or pads. And if you don't have good brakes or they're not working properly then no amount of fancy disc will fix that, the fix is to sort the brakes themselves. 

I used to think that too but after experiencing XT/XTR rotors and Shimano's finned pads (NO4C metal), I think they do make a difference. The 4-pot XTs I've got now are out-performing the Zees I had previously, the difference being I used UberBike Race Matrix pads and SLX rotors with the Zees (which I thought were great at the time).

Maybe my 'not doing anything really unusual' is a bit different, but last night's ride - which is a regular for me - didn't phase my brakes at all, whereas in the past they'd be honking and generally being a PITA a quarter of the way down.

I would have thought there’s very little difference unless they’re ’resin only ‘ rotors 

you don't need fancy rotors or pads

I'd disagree.  Rotors don't seem to get the same credit as pads or even brakes. But what information is out there shows they do make a considerable difference. e.g. SRAM state that HS2 is 7% more powerful than centerline.  And I found RT66 rotors more powerful than SRAM when I swapped, and if Shimano can say XTR is better than XT, is better than RT66 then they're probably better again.

The cooling performance probably depends on how and where you ride.  

Slow and techy - heat never builds up

Fast and hard braking - plenty of time and airflow for cooling down

Smooth but steep - that's the killer. Everyone comfort brakes / drags them at some point. No one want's to be doing silly speeds into a blind bend even if they can technically ride it; outside of a race on a taped course there's either self preservation or just the risk of hitting someone coming the other way. 

Finned pads probably make more of a difference though. On the road bike (the only palace i actually have them) I can still can feel the levers being pushed back as the caliper warms up under heavy braking, but it doesn't seem to fade despite the rotors turning funny colors.

https://enduro-mtb.com/en/best-mtb-disc-brake-can-buy/

There's a table of stats from their lab testing, including max rotor temperature. Appears to show XTR markedly cooler than SLX setup - 182degC vs 241degC. How those lab results translate into real world use though I don't know.

All MTB brake rotors are made of steel – at least on the friction surface. The thickness of rotors can vary depending on the models and manufacturers, and is primarily linked to heat dissipation. More material enables better heat dissipation, preventing the sensitive calliper from overheating – at least in theory. In our lab test, however, we weren’t able to confirm this theory. Shimano, on the other hand, use a sandwich design with an aluminium core on their XTR ICE-TECH rotors, which is intended to improve heat dissipation, while at the same time reducing weight. Our lab results confirmed that sandwich structure makes perfect sense, with the Shimano XTR brakes reading the lowest temperatures – an effect that is particularly noticeable with sintered pads.

XT MT800 rotors are a little bit cheaper:

https://www.merlincycles.com/shimano-xt-mt800-ice-tec-centrelock-rotor-180696.html

Ice-tech rotors are absolutely worth it. So consistent. I'd even argue that going 180mm front and rear in ice-tech (because they are often available heavily discounted in that size) might be better than 'standard' rotors in 203/180.

Well I managed to an RT86 203mm rotor for £25 +p&p from Merlin which is much cheaper than anywhere else.

Looking forward to trying it out and my old 180mm front rotor will now be going on the back so it's a good upgrade all round 🙂