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Viewing Rotor in Caliper - Rear Brake

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Viewing Rotor in Caliper - Rear Brake

Old 02-18-24, 03:58 PM
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Viewing Rotor in Caliper - Rear Brake

What is the easiest way to see the rotor spin through the rear calliper: standing behind the bike, or in front of the bike; and, should the bike be tilted front wheel up, down or level?

I could easily see the front wheel, regardless if Iím standing in front of the bike or behind the bike with the bike on the stand, or just by holding up front wheel and spinning it. Without even trying, I could see the space between the pad and the calliper on both sides.

Iíve used the white paper and light both in front and behind the rear calliper but itís tricky compared to the front. At least for me. Thanks for any help you can offer.
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Old 02-18-24, 04:20 PM
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Generally; front wheel low, looking through the back of the caliper where the rotor enters as the wheel spins. White paper on the floor. You'll be looking in line with the pads, tangent to the rotor.
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Old 02-18-24, 04:54 PM
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Originally Posted by Kontact
Generally; front wheel low, looking through the back of the caliper where the rotor enters as the wheel spins. White paper on the floor. You'll be looking in line with the pads, tangent to the rotor.
Thanks Kontact. That worked. I wedged a piece of thick white paper behind the bottle cage and pointed a bike light at it while spinning the wheel to check for the rub. Found and corrected. And I replaced the chain. Good mech day for me
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Old 02-18-24, 05:15 PM
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I use a white bucket... Because I happened to have it nearby and then found I can move it around into the right position with my foot
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Old 02-18-24, 05:58 PM
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I'll shine a flashlight through the rotor and pad gaps. Andy
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Old 02-18-24, 06:01 PM
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Why do you need to see it?
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Old 02-18-24, 06:04 PM
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Originally Posted by Rick
Why do you need to see it?
I think he inferred that the rotor was out of true

/markp
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Old 02-18-24, 06:24 PM
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Originally Posted by Rick
Why do you need to see it?
Because that is the correct way of centering the pads and making them square to the rotor. Holding the brake lever closed does not work reliably.
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Old 02-18-24, 11:54 PM
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I think he inferred that the rotor was out of true
I have never considered Doing all that to see if the rotor is straight.

Because that is the correct way of centering the pads and making them square to the rotor. Holding the brake lever closed does not work reliably.​​​​​​​
I straighten my rotors in my truing stand using a dial indicator that reads in .001". I put the wheel in and do what you say is not reliable. It works great for me.
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Old 02-19-24, 01:00 AM
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Originally Posted by Rick
I have never considered Doing all that to see if the rotor is straight.



I straighten my rotors in my truing stand using a dial indicator that reads in .001". I put the wheel in and do what you say is not reliable. It works great for me.
+1. Depending on whether you use a plunge-style or lever-style of indicator, I'll bet you could rig a small fixture to clamp to the seatstay or chainstay, to fix in the field, although my trunk bag full of basic tools in already too heavy. Plus you'd also need the straightening tool. Or if you lack a truing stand, I have none, I always just true in the bike with something clamped to the seatstay or chainstay. Come to think of it, I ought to dig my indicator out of storage and use for truing wheels. Yes I do true them that close, take longer than the LBS, but then they stay in true forever, unless I break a spoke or bonk the rim.

Last edited by Duragrouch; 02-19-24 at 01:04 AM.
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Old 02-19-24, 07:37 AM
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Originally Posted by Rick
I straighten my rotors in my truing stand using a dial indicator that reads in .001". I put the wheel in and do what you say is not reliable. It works great for me.
Then you are either lucky or haven't noticed the problem. Once you work in a shop and adjust these things all day you see that this method doesn't produce reliable results because it hides the tendency of the calipers to twist as the mounting bolts are tightened, among other things.
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Old 02-19-24, 09:39 AM
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Then you are either lucky or haven't noticed the problem. Once you work in a shop and adjust these things all day you see that this method doesn't produce reliable results because it hides the tendency of the calipers to twist as the mounting bolts are tightened, among other things.
Would you describe to me how you tighten the caliper so it does not twist as you say.
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Old 02-19-24, 11:48 AM
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I have a follow up question (I have TRP Spyre on a Salsa Fargo).

These brakes have external pad adjustment on each side. Is there a good way to set these up:
- barrel adjustment on cable
- pad adjustment
​​​​​​- when tightening the caliper (1/4 turn alternating between bolts)
- putting a thin shim between pad and rotor when tightening
- note: rotorís are straight (now) and pistons moving well (cleaned the pads and calipers yesterday). Iím asking in general so when the work on this bike I know a good process to follow from step one.

Thanks (always learning!).
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Old 02-19-24, 03:28 PM
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I will try this when I change the pads
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Old 02-19-24, 07:16 PM
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Originally Posted by Rick
Would you describe to me how you tighten the caliper so it does not twist as you say.
I hold it and watch the gap as I tighten.
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Old 02-19-24, 09:36 PM
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I hold it and watch the gap as I tighten.
I will give that a try next time. Thanks
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