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Old 09-09-10, 01:16 PM   #1
sram
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Bent rotors

Hi, I just bought two 160mm 8 each shimano RT61 rotors (http://www.chainreactioncycles.com/M...px?ModelID=815) to replace my tektros (13 each) (http://www.chainreactioncycles.com/M...?ModelID=34829) that got bent in the car. I kept bending them with a crescent wrench, but somehow they kept returning to their warped state. Anyway, I just got the new shimanos, bolted them on, went to dial in the pads and about 40% of the front and 30% of the rear rotors are slightly bent. Any reason why, or am I just being fussy? I like sharp brakes, and when I run the pads close enough to the rotor to get the desired stopping power the above (rough) %s are rubbing. If it helps, I'm using tektro aquila and avid bb7 mech calipers. Thanks.
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Old 09-09-10, 01:59 PM   #2
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theres a tool essentially a slot cut in a piece of steel , put the wheel in the truing stand, and you need a reference point
like the one at the rim, and just tweak the edge of the disc by bending it a trifle , and re checking it.

Be clever and impress your friends and a magnetic base and a dial indicator
will let you judge your handiwork to the nearest thousandth.
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Old 09-09-10, 03:56 PM   #3
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Be clever and impress your friends and a magnetic base and a dial indicator
will let you judge your handiwork to the nearest thousandth.

I got a cheap a magnetic base and a dial indicator for only $21..great for truing wheels
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Old 09-09-10, 05:27 PM   #4
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My LBS suggested a wire tie around the seat or chain stay snipped to just touch the disc - poor man's dial indicator

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Old 09-09-10, 08:48 PM   #5
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Start by flattening it so it sits as flat as you can get on a piece of plate glass. A window will do as a vertical table but a much nicer option is a chunk of 1/2 inch glass with the sharp edges beveled by the glass shop. I got a 12 inch square piece for $10 or $12 a few years back. Show up at the shop on a Friday near quiting with a 6 pack in hand and you'll likely get it for the cost of the beer.

As for tools there is more than a big crescent wrench. You do what is needed. Longer dish bends may require some serious flexing over a soft rounded edge of a table or piece of wood. Kinks produced by things like the adjustable wrench or from a vise are to be strongly avoided. Kinks or sharp crease lines when found tell you that you're holding an expensive and high tech piece of scrap metal since there's nothing you can to do correct a kink in the metal with home shop tools. So work carefully.

But in the end bending it to flatten the disk IS the accepted manner for repair. And keep in mind that it is spring tempered stainless so you need to bend it to the yield point and just a hair past that. Bend it short of that point and it'll just spring back to the shape it started with. Go easy and try it in stages until you notice that it's moved.

Last edited by BCRider; 09-09-10 at 08:53 PM.
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Old 09-11-10, 10:51 AM   #6
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Ok, thanks for all the tips. But before I do anything, should the rotors even be bent? They went brand-new from the box to the bike, and when I adjusted the pads around half of the rotor was rubbing on the pads. I thought new rotors were supposed to be 100% straight, so should I try and return them? They've been used for about 2 miles on XC.

Oh, and what about holding a sharpie so it sits just off the disc, to mark the bends?

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Old 09-11-10, 12:11 PM   #7
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Most Mechanical Disc calipers push the disc into a fixed pad, which is adjusted to be very close to the Disc.

Hydraulic brakes mostly have 2 pistons to pinch the disc between them .
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Old 09-11-10, 12:16 PM   #8
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new rotors are not true, you must true them
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Old 09-11-10, 12:22 PM   #9
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And even if they were true when you bolt them to a hub the seats on the hub may be out by enough to flex the rotor. Or even just the bolts biting into the spokes of the rotor could distort it enough that it would no longer be flat enough for our needs. It also depends on how fussy we are about this sort of thing. By the sounds of it you're more fussy than some.

Oddly enough the need for truly flat rotors is more with the cheaper one sided calipers. On those to get best performance the rotor must run VERY close to the fixed pad. Pretty much just barely skimming it in fact. This really shows any runout on the rotor in the blink of an eye.
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Old 09-11-10, 01:13 PM   #10
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Well, after a few hours and one slight bend, I've got the brakes sharp enough for me. I would love hydro brakes on this bike but don't have the money, and it's only a Specialized P3 dirtjump bike, so it's not that heavy. Saying that, I do use it for DH, but I don't wanna spend much money on it, as I'm saving for a Kona or something that I can win races on.

Thanks a million guys.
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