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My rotor diagnosis (~long)

1406 Views 13 Replies 11 Participants Last post by  ctsv154
I have had a very noticeable steering wheel shake and lunging when I applied the brakes. So much that the wife has even commented on it. I really don't feel the pedal pulsing except maybe due to the steering wheel shaking it. A friend of mine who tracks his car and has a lot of personal experience with brake systems says that 'many people mistaken a good rotor with brake material deposits to a warped rotor. What happens is that under some conditions the brake pad will leave behind material on some portions of the rotor surface. This creates an irregular friction surface that gives the perception that the rotor is warped when the brakes are applied. Don't jump into buying new rotors just yet!' Conceptually this sounds like a good possibility. Could it be that simple and cause that much to be transfered to the steering wheel? It's real frigg'n annoying.

Thinking that my rotors were warped, I wanted to make sure before buying new ones. I retorqued the wheels that resulted in no change in drivability. The shake was still there - no change whatsover. I had no dial indicator so I jacked up the front of car and spun the front wheels to see how the rotor contacted the brake pad. Not an exact science but what I was looking/listening for was an uneven sound while the pads would sweep the rotor. My brake pads just barely touch the rotor and rotating the wheel gave me the impression that the rotor was perfectly straight and evenly spaced from the pad throughout the entire rotation of the rotor. There were no occillations in the sound while the rotor made a complete rotation. I took the car out for a ride again, pondering if it was now a suspension part that might be causing the problem. (The original rotors on my Cobra would roughly only touch 1/2 of the rotor. There was a portion that actually would stop the rotor in exactly the same spot everytime but on the opposite side it was free to move.)

I gave the front suspension a good [novice] lookover and found nothing that I thought could be a problem. The car is solid while going over bumps, waves or washboard type road surfaces with no noises, cluncks, squeaks, etc. - nutt'n. So, I unofficially ruled that out.

Later on, I was on a long trip in the pouring rain -the entire car is soaked. Everyone knows that wet brakes initially do not perform well as the water lubricates the brake surfaces. When I applied the brakes there was no shaking of the steering wheel or pulsing in the pedal during the first few seconds of applying the brakes. Noticing that the shake/pulse didn't occur, I applied this test a few more times while driving in the rain giving the rotors an opportunity to get splashed with water. Again, the brakes were real smooth and there was absolutely no shake in the steering wheel or pulses in the pedal for the first few seconds. This is unlike the behaviour when everything is dry.

I set myself up for a slow speed test to see if the lunging went away. While driving around town, I gave the rotors a chance to get wet again. Normally I would get a forward-to-rear lunging while going slow (~10-15mph) with just a little bit of brake pressure. At times, this would rock the driver and passenger(s) back-and-forth in unison while the brakes were applied and slowly coming to a stop. Later, I had the road to myself, still raining, I did a few hard stops from a good clip. This is how brakes are supposed to feel like! At this point I thought it was too good to be true so I went on my way. My conclusion is that the shaking was caused by the irregular friction surfaces that grabbed differently as the rotor passed by the pad. Since I didn't feel anything in the pedal I thought that maybe the rotor surface were still true.

Later in the week when it was dry again. I applied brakes for another simple test. What a difference! The shake and lunging are virtually gone!!! I took the car out on the highway and did a few real hard brake sessions with plenty of time to cool off the brakes without using them. There really aren't too many places or times one can do this around here without drawing too much attention. I don't need that. Results: What a friggn' difference!!! The shake and lunging is barely noticeable now!!! With a few more sessions maybe I could cure this this thing?!! Right now I can barely feel any of the brake anomalies I had witnessed before. Maybe my friend was on to something?!

As of now, my rotor purchases are on hold but I did manage to purchase a dial indicator that I haven't used yet. Not sure if I got lucky and 'fixed' what was causing the problem or some other anomaly is occuring here that I am unaware of. What I do know is that I no longer grind my teeth when I use my brakes. Maybe you won't need rotors either.
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Google brake pad burnishing or bedding. Your friend was right! :)

Since this was a used car and its hard not to get a ticket around was a cheap fix.
bedding those pads is an important step, no doot!
This started happening to me a few weeks before Summit. It wasn't bad, so I left it alone. After the first real track session,,,problem solved.
I think with street use, where there is a mixture of ~heavy and light braking, some brake material & rust can build up. The track would clean that up real quick. My first track session I remember my brakes smelled like a CNC machine or a lathe right after cutting a large peice of steel. The track 'resurfaced' those rotors perfectly.
I think this is exactly the problem I've been having. It rained today and my brakes seemed fine, but no doubt the problem will be back with the sun tomorrow. I was just online shopping new slotted sport rotors, but I'm going to try your solution first.

Fingers crossed ! Thanks much for your help !!!
Update: my rotors continue to operate smoothly - no "warp" feeling.
good deal JOE!!
You guys are correct. I've installed 100s of brake systems on Subies and we often get the same "my brakes are warped" thing. Being a race car driver, I kinda laugh and tell them, let me take it out and fix em for you. I take the car out and slam on the brakes (really I do 60-5, 60-5, 60-5 then let em cool) and the customer is amazed how it fixed their "warped" rotors!

The chance of anyone actually warping their rotors on these cars is slim to none. Wear them down to a large lip, yup. Embed them with pad material, yup. Actual warping, not likely. :)

VERY GOOD INFO - was bringing mine in for this tomorrow.

Going to do some agressive braking tonight to possibly remove some gunk down there.
I had the same thing happen to me. After having the rotors cut and installing new Hawk pads, I was briskly driving through the mountains when the car started shuttering under braking. After using the search function and I went out at midnight and did 4 60-5’s and two 80 – 5’s then drove around for about 15 minutes without touching the brakes.

It got a little better but not much. I ended up replacing all the rotors, driving around for a few days to clean them off. Then going out and bedding in the brakes. I also cycled through ATE brake fluid. I’ve autocrossed the car a few times and the brakes feel great.

The only thing I can think of is the guy who turned the rotors, didn’t do a very good job.
Wow, I installed GMPP Cross drilled rotors and fresh stock pads a few months back. This week after getting my car back the vibration in the steering wheel under braking was very noticable. Ill try giving them anoth good burnishing and see if that solves the problem!!!
Am I the only one that drives crazy enough not to have this problem? lol My front rotors have about 20k on them and they look like a flywheel. Very small cracks around the entire surface of the center of the rotor. Often I have blue rings around the rotor. Oh, I dont track my car. ; )

I will tell you performance friction makes some awesome brake pads. The stop like crazy and make very very little dust. I'm talking with stock pads I could wash the car and drive home and have break dust. With the new performance friction pads I can drive around for a week before they wheels really get that dusty look to them and they stop about 10 times better. No joke!
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