SMRT76 rotors give excellent performance using the 6-bolt mounting system
Review from MBR February 2010
Even though these new rotors are XT level, they're being introduced as an XTR-grade option to sit above the current stainless steel (XT) SM-RT75 rotors.
They are a two-piece construction, in the form of an adapted CentreLock spider, which features an aluminium carrier and a stainless steel braking surface. The 160 and 180mm rotors both use a five-arm design; the 203 is six arm. This explains why the difference in weight between the older rotor and the new one is only 3g, or so Shimano claims. We've weighed both and there's only a 1g difference, but it's not a big deal because that's not why you are buying them.
The two-part design is stiffer and helps dissipate heat much more efficiently and, if they're anything like the XTR CentreLock rotors, they're less likely to go out of true and run straighter for longer. Also, since Shimano sells its disc brakes and rotors separately you can now buy dedicated rotors for your wheels rather than the other way around.