Brake Rotor Question

[GregT]

I recently had a new front brake rotor installed, as I was getting a high frequency pulse when I applied the brakes at high speed, a sign of a warped rotor. The new rotor has solved the high frequency pulse problem, but now when I slow down gently as I approach an intersection, holding a light but steady pressure on the front brake, it feels as if the rotor is really warped, with the wheel turning most of a rotation at one speed, but the rest of the rotation feels as if I've momentarily squeezed the brake lever harder, and then returned to the original pressure. This on-again, off-again braking continues until I come to a complete stop.

My local bike shop test rode the bike, and confirmed what I was experiencing. Upon inspection the new rotor doesn't look warped. The mechanic said it might be a "pedal" rotor that is designed to create this effect because it also increases braking power by giving the brake pads more to grab on to. I've Googled "pedal rotors" with no success. Has anyone else experienced this? It's not unsafe (I don't think), but it prevents me from bringing the bike to a slow stop in a steady manner.

[670cc]

Google “petal rotor” to learn about that rotor type. As I understand it, the wave style outer edge of the NC rotor is for styling and supposedly to aid cooling, but not to intentionally cause pulsing. It sounds like the shop is throwing out excuses to make you go away.

The NC rotor should not pulse. If it’s not warped, I’d suggest cleaning it well, in case it has contamination in one area.

I once slightly bent an NC rotor during a tire change, which of course caused a slight pulsing during braking. As for “looking” warped, I could not see it or measure it, but it was bent. I replaced it with a used rotor from eBay and problem solved, meaning no vibration or pulsing at any speed. My other NC does not pulse or vibrate either. What you describe is not normal. The shop needs to address this or get another rotor.

[ld_rider]

I've Googled "pedal rotors" with no success.

Tossing this out there...

Try petal rotor...like in a flower. The purpose is to allow cooling of the disk as well as a slightly less unsprung weight compared to a typical rotor.
If you have a dial indicator you can check for runout, not sure what the specifications call for.

Were the pads replaced at the same time the new rotor went on? Is possible the pads are somehow conformed to the old rotor's geometry/wear pattern and the shiny new rotor is upsetting them??

I'm wondering how the determination was made that the original rotor was at fault in the first place. Do you know what the lateral and axial runout of the rotor was when it was measured by the mechanic?

EDIT: I see 670 already gave you the correct spelling of petal..

[Junkie]

If they swapped the rotor and it's pulsing like that, they need to make it right.

[kalifornia]

warped rotor isnt the only possible sign. possibly be gunked up brake pistions. if you ride on salty roads or if its original fluid, your pistons wont appreciate you. there is a pin or two that runs through the pads and it really likes silicone grease . u have anti lock brakes?

[Junkie]

I don't see how sticky pistons could cause pulsing (unless they caused the rotor to overheat and warp)

[kalifornia]

two different rotors and still a problem. My guess is its not a rotor problem. a bent wheel could fool someone into thinking their rotor is warped, so could shot wheel bearings, so could a unbalanced wheel, so could loosened fasteners, head bearing. maybe start looking at other possible causes.

[TacomaJD]

two different rotors and still a problem. My guess is its not a rotor problem. a bent wheel could fool someone into thinking their rotor is warped, so could shot wheel bearings, so could a unbalanced wheel, so could loosened fasteners, head bearing. maybe start looking at other possible causes.

It's not like the same problem across two different rotors....it's two different problems across 2 different rotors.

NC rotors are fixed, which means they do not float like higher performance braking systems, which means they are easier to warp. Old rotor might have been warped, and in removing the wheel and removing the rotor, and installing a new rotor, jostling the wheel around then mounting it back on the bike, it could have been dropped over onto the rotor causing a brand new rotor to become warped. Even torqueing the rotor bolts down wrong might could cause some warping.

Also, don't call the guy that told you a petal rotor should create such an effect a "mechanic", it sounds like he needs to go back to school. LOL

But here is something that should add up if it is indeed warped. If it has a strong pulsing feel at lower speeds, it should definitely exist or even be amplified at higher speeds. So now it is silky smooth at high speeds?

[kalifornia]

yes JD tru dat, then of course it could be a bent wheel or something else causing a problem for both rotors could it not? I'de say thats more likely than the mechanic dropping a part then installing the damaged part. if the op can get the wheel off the ground and spin it, maybe then he can start sorting the problem. And I might as well add that since a rotor can be replaced with a simple wrench faster than it takes time to drive to a mechanic. I wonder how deep the op's diagnostic experience could be.

[TacomaJD]

yes JD tru dat, then of course it could be something else causing a problem for both rotors could it not? I'de say thats more likely than the mechanic dropping parts. if the op can get the wheel off the ground and spin it, maybe then he can start sorting the problem.

It would not make sense for one thing outside of the rotor itself to cause 1 problem with the initial rotor, then cause a different problem with the 2nd rotor. If you think mechanics (and I use that term loosely) don't fumble people's parts around, you trust them way too much. Remember, this dumbass told him that a petal rotor should have a pulsing sensation....