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  1. #1
    love my Simonds 519
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    XTR M965 group: SM-RT96 to SM-RT77 rotor change over worth a shot?

    Hi, I've an older full squish with an XTR M965 group where the discs are prone to squealing when wet (they're fine when it's dry or damp; rain, snow, and stream crossings all provoke squeal until the brakes dry out). This seems to be a rotor or pad resonance, though the rear brake infrequently hits a frame resonance as well in the right conditions (that seems to be coupled through the mount and not the hydro line). Tried all the basic stuff a while back when I picked up the bike used without any great luck. I've been living with the squeal as it hasn't been that big a deal---most of my wet riding is on a different bike---but it's getting old. So I've lately I've been casting about for other fix possibilities.

    I've some shims on the way to try some alignment tweakery and need to pick up some anti-squeal paste. However, I happened to notice Shimano's WH-M965 manual contains a note the SM-RT96 rotor may produce unwanted noise and that the SM-RT77 rotor is recommended (fifth bullet in the upper left corner). My bike is not fancy enough to have the M965 XTR wheels but it does have BR-M965 (the brake), HB-M965 and FH-M965 (front and rear hubs), along with SM-RT96 rotors. So, while my wheels are the usual three cross lace up, the braking arrangement seems likely to be identical to that of the WH-M965.

    NOS SM-RT77s eBay for $35ish so I'm tempted to give Shimano's guidance a try. Anybody see a reason why this would be a bad idea or wouldn't work? So far as I know all Centerlock rotors are interchangeable so I'm assuming the XT SM-RT77 rotors will fit on my hubs.

  2. #2
    love my Simonds 519
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    Having looked at this a bit more I can see a couple reasons not to go with a SM-RT77. The SM-RT77 pattern trickled down to the SM-RT63 and then the SM-RT53. So, if one doesn't mind the 30 gram weight difference, there's no particular motivation to pay double for an SM-RT77. Or, for the same budget, try the non-IceTech SM-RT79 on the idea it'd probably perform better and run quieter (IceTech isn't that much more expensive but putting a cooler running rotor on a bike with of history of minimal fade and lots of needing to have the pads dried off to shut up doesn't exactly seem like a wise choice). Couple midway options in the SM-RT64 and SM-RT78 too.

    A little investigation confirms the RT79 uses the same 14mm track width as the RT96. So I think I'll try 79s as this morning's application of anti-squeal paste made things quieter but the brakes are still annoyingly loud when wet.
    Last edited by twest820; 08-20-2013 at 10:26 PM.

  3. #3
    love my Simonds 519
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    Follow up forks who may be interested: RT79 on the front killed the rotor resonance. Haven't tried it on the back but may get a second one for the rear at some point. The front brake still squeals due to fork resonance (the pads are stronger than the crown; under hard braking with a really wet rotor the axle judders visibly back and forth as the Fox F100X winds up and then pushes through the pad grip) but it's harder to excite the fork resonance and it's lower pitched and less annoying when it starts up. Beside the changes in punch pattern something that probably helps is the CenterLock splines are a much snugger fit to the hub on the RT79 than the RT96.

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