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Shimano XT M8120 brake

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Shimano XT M8120 brake

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flag hsmithm3
Posted: Aug 26, 2021 at 10:54 Quote
Hi
I recently installed a new set of XT M8120 4 piston brakes (F/R) including rotors/pads. While the rear brake feels great, I can't get the front brake to feel very powerful. It almost feels like they haven't bed in correctly... I cant lock up the front if I want. I am coming Sram Guide R brakes with 200mm front and 180mm rear which the front felt much more powerful. My new XT's are 180mm front and rear. I'm not sure if moving to a smaller front rotor has really reduced braking power (my assumption is that the larger front rotor would just dissipate heat better on longer runs, not diminish the overall braking power when not overheated). I have done 2 lever bleeds and swapped the pads from front to rear. Same result. Any ideas? The rotors look fine - both the rear and front don't look glazed.
flag eshew
Posted: Aug 26, 2021 at 11:21 Quote
It seams like you've tried plenty of solutions. Those brakes should absolutely lock up the front no problem.

I'd guess there's some sort of contamination on the rotor/pads...although swapping should have resolved that.

A quick fix I use often when braking strength issues pop up is to torch the pads (uninstalled) with a propane torch for 45 seconds each. Then lightly sand the pads after they cool, and clean the rotors.

99% of the time after a minute of breaking in it's back to 100%

Good luck with the solution!
flag redman733
Posted: Aug 26, 2021 at 17:40 Quote
and scuff up the rotor with 100 emery...scuff them good too, they get a glaze on them that takes a little elbow grease to get it off.
Also place a piece of emery on a flat surface and rub the pads back and forth to clean them up too.

IF you didnt contaminate anything this will make them good as new.
IF you contaminated your pads?? you can try what eshew said, but I never had good luck cleaning brake pads so i'd just get a new set of pads on amazon or ebay. They are dirt cheap there.
flag rete
O+
Posted: Aug 26, 2021 at 21:56 Quote
Re larger rotor, the added leverage from the longer torque arm generates a significant increase in brake torque. It's a simple braking torque arm calculation you can do on the back of a napkin - center of axle to center of pad.

A 200mm rotor will give you approximately 12% more braking power compared to a 180mm rotor, everything else being equal. And it dissipates heat better. For 50g weight saved it's not worth it...
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