Brake Rotor Runout

Posted By: 70Cuda383

Brake Rotor Runout - 05/11/12 02:23 PM

Trying to finish up some fine tuning on my viper brake swap on my Dakota. it stops great, but I have an issue with an overly soft pedal that I have to pump, release, then apply again and it'll finally stop hard.

I already upgraded to a 1 1/8" bore master cylinder from an 03/04 dakota that was for a disc/disc application. the stock viper master cylinder is only 1 1/16" so it's not pedal stroke/volume that's my problem.

I have a rotor with a noticable/visible wobble when it's spun on the hub, but the local NAPA said "it didn't look any worse than any of the other rotors" and they turned it anyway and said I was good to go. but...it still has a visible wobble.

But, when applying the brakes, I have NO pulsation or vibration.

I'm thinking the wobble in the rotor is pushing the pistons/pads back into the caliper, and my first pedal pump is taken up by pushing them back out, then the second pump creates the clamping force for stopping. It's not a full, to the floor pump though, I can give it a 1/4 - 1/2 stroke, release and quickly apply again and its a nice firm pedal, smooth braking with no vibrations or pulsating.

so...time to get out a dial indicator, how much "runout" is acceptable on a rotor?

Posted By: Challenger 1

Re: Brake Rotor Runout - 05/11/12 02:26 PM

less than .003"....005" can be felt in the petal.

Posted By: 70Cuda383

Re: Brake Rotor Runout - 05/11/12 02:31 PM

Quote:

less than .003"....005" can be felt in the petal.

that's what I was thinking. but...again, I don't feel any vibrations or pulsating in the pedal when applying the brakes.

does a 4-piston fixed position caliper "absorb" that pulsation as the rotor passes through it? As the rotor pushes in on one side of pistons, the fluid it displaces simply transfers to the opposite side and pushes them out...and ultimately you don't feel anything in the pedal?

Posted By: DAYCLONA

Re: Brake Rotor Runout - 05/11/12 02:42 PM

I have rotors Blanchard/Surface ground rather than "turned", results are far superior in obtaining an almost zero runout condition

Posted By: 70Cuda383

Re: Brake Rotor Runout - 05/11/12 03:09 PM

Quote:

I have rotors Blanchard/Surface ground rather than "turned", results are far superior in obtaining an almost zero runout condition

so, where does one go to do that? and how much does it typically cost? brand new rotors are about $50 a piece

Posted By: 70Cuda383

Re: Brake Rotor Runout - 05/11/12 03:54 PM

is that the same machine that they use to surface flywheels with? I didn't realize you could do a brake rotor on one of them, with the way the "hat" sticks up a few inches past the contact surface of the rotor.

Posted By: Challenger 1

Re: Brake Rotor Runout - 05/11/12 03:58 PM

I've never done a rotor on my surface grinder. Can't afford to do much to them when they are 50 a piece new.

Posted By: bordin34

Re: Brake Rotor Runout - 05/11/12 03:58 PM

It sounds like you are experiencing knock back through a rotor with too much runout. If for some reason it cant be fixed they do make anti-knock back springs that you put in the calipers, but they are normally only used in racing applications.

Posted By: Ply72rr

Re: Brake Rotor Runout - 05/11/12 09:21 PM

You don't feel a pulsation yet but you will after a few thousand miles. Find a shop that has a on the car brake lathe,that will take all the wobble out and take care of your low pedal due to piston knock back.

Posted By: DAYCLONA

Re: Brake Rotor Runout - 05/11/12 10:06 PM

Quote:

Quote:

I have rotors Blanchard/Surface ground rather than "turned", results are far superior in obtaining an almost zero runout condition

so, where does one go to do that? and how much does it typically cost? brand new rotors are about $50 a piece

I've found a few muffler/brake shops that have the equipment designed just for Blanchard grinding rotors (same process used to cut flywheels) I have new and used rotors squared up this way, esp used, if there's any hard spots from being overheated a brake lathe can't cut that hard spot well, and you end up with a warped rotor...blanchard gring is done with a ceramic stone that grinds the surface, the rotor needs to have the races/bearings supplied when done,...price is currently $30 a pair, just ask your local shops if they turn or surface grind rotors to find a source

Mike

Posted By: 70Cuda383

Re: Brake Rotor Runout - 05/13/12 08:27 PM

little update...took some measurements.

the one on my driver side was showing .031 of wobble as I spun it, the passenger side was showing about .015 of wobble.

guess that's why I gotta pump the pedal twice after driving a mile without using the brakes, but if I use them quickly, like from stop sign to stop sign in the city, I have a nice firm pedal.

Just to make sure my hubs aren't the source, I took the one from the passenger side that showed .015 and put it on my driver side, and it showed .013.

that .002 less could mean that a tiny bit of wobble is coming from the hub, but it could also mean that the rotor seated differently on the hub, or that the dial indicator was not at the same exact point on the rotor, although I was trying to make sure I measured as closely to the edge as I could each time.

now, time to figure out the cheapest way to fix this!