pman9003

I finished installing a STR-42/STR-21 BBK on my NB today. Kit came with the V8R rotors for the front and I used brand new plain centric sport rotors on the rear. After bleeding and checking for leaks I started driving it around the neighborhood slowly to functionally check everything. Everything was fine after few slow (under 25MPH) laps around the neighborhood so I drove a mile to a large empty parking lot to do a few more gentle stops from slightly higher speeds (~35-40MPH), again everything felt fine after a few laps around the lot. I started heading back home, got it up to about ~60mph feeling that all was fine and then applied some light brake and immeditely got a shudder I could feel in the pedal and the whole car. I was not able to feel it in the steering wheel, and based on the butt sensor I believe its coming from the left rear. I didn't have time to get it jacked up again to inspect everything today, but wanted to ask on here as I am baffled as to how I have developed a shudder in less than 10 miles of driving at low speeds.

-New V8R STR42/STR21 BBK
-New Centric Plain Rotors on rear
-New Stoptech 309 pads all around
-Rear Hubs are new OEM from Mazda, so no rust or dirt to throw things off
-Front Hub faces were cleaned of rust/dirt with a wire brush and brake cleaner
-Lugs are properly torqued, I checked this twice.
-Everything was installed using good torque wrench(s) to specs provided with kit
-Nothing unusal other than brake shudder above ~55-60mph, its cyclical and the frequency goes down 0 as the car slows to below ~45-50

Can a brand new rotor be "warped"? Could uneven pad deposits form that quickly? Has anyone else gotten a rotor that was bad out of the box?

These are the only things that initially come to mind given all the symptoms. I am looking for any other ideas/suggestions for things to check before I start throwing new rotors at the car.

matrussell122

iTrader: (1)

Take the wheels off. Put lug nuts on to hold rotors in plac . Then measure runout on the rotors then remove rotors and check the hubs.

pman9003

I'll have to order a dial indicator and do that. Thanks for the suggestion!

Just curious, if the issue was the hub runout wouldn't it cause some shaking or something at all times when driving? I have 205 BFG Rivals on 15X8s, so not a cushy wheel and tire combo.

matrussell122

iTrader: (1)

Also what kind of control arm bushings do you have or when was the last time they got replaced?

Does it do it all the time or just right as you step on it then how away?

pman9003

The bushing in the knuckle is a new factory bushing. The ones in the arms are original from the factory.

The shudder is speed dependent. Braking above ~50MPH results in a shudder, as the car slows down the shudder goes away. So if I apply the brakes at 60 the shudder is there until the speed drops to below ~50. At lower speeds there is no shudder under braking. If I am not braking everything feels completely normal, even at higher speeds hence my question about the hub.

I ordered a dial indicator, should be delivered on Wednesday. Until then I will get the wheel off and check the rotor with a micrometer.

matrussell122

iTrader: (1)

Are you feeling it in the brake pedal or steering wheel

pman9003

I feel it the shudder in the pedal, not the steering wheel. I am pretty sure its coming from left rear as I can also feel it in my seat.

LuckyKid

Sounds like bad bedding. Is the rotor splotchy?

andym

I have a similar issue with my brakes. How does one fix the bad bedding to re do it? Just brake hard for long enough or do I need to mechanically remove the layer and re-apply the layer?

sixshooter

iTrader: (12)

Use brake cleaner on the face of the rotors and try again. This time don't pussyfoot around and go right to the high-speed stops just like the instructions usually say for bedding in brakes. You have essentially smeared some transfer material on one portion of the rotor but not the whole rotor. So sometimes it grabs and sometimes it releases.

HarryB

Exactly what Six said. Although it would not hurt measuring runout.

pman9003

I think this is the issue. The outside face looks okay, but I have not had a chance to take the wheels off due to work and what not.

Yeah normally I would go straight to full on bedding, but the fact that this was a completely fresh BBK install I wanted to make sure things were okay at low speeds first. Also StopTech indicates pussyfooting is okay until you have a chance to do a proper bed in. I think my issue was I started to not pussyfoot it, but also didn't go to full on bedding.

Amazon delivers my dial indicator tomorrow, so I will be checking that.

Thanks for all the replies!

ryansmoneypit

iTrader: (6)

You have debris between the rotor and hub. take the rotor off and thoroughly clean the hub face and rotor with a wire brush and brake cleaner. Ive seen this many times and unfortunately, it usually ruins the rotors, it super-heats portions of the rotor and changes the grain structure. sooooometimes a simple resurface fixes, but not always.


I just did some really rough math. If you have rust or an eyelash under the rotor, lets call it .002", thats almost .009 at the rotor outer edge, a rough stop.

sixshooter

iTrader: (12)

I assumed he assembled everything correctly. I guess I shouldn't necessarily assume that.

Midtenn

iTrader: (15)

So make sure the rotor is fully seated on the hub. Many rotors don't have enough chamfer on the back side of the rotor to clear the shank on OE or ARP studs.

ryansmoneypit

iTrader: (6)

When you reach the point in miataing where this is a problem, you should have the experience and know how to fix it. get out a die grinder or hop on a lathe.

Midtenn

iTrader: (15)

I use a chamfer bit on a drill press myself. It's not something a lot of people know to look for from my experience.

ryansmoneypit

iTrader: (6)

Perfect.

pman9003

So I am working through troubshooting this issue. I started with the left rear as I believe that corner is the issue. At the outer edge of the friction surface of the rotor I am measuring a total lateral runout of .0015". Is that an acceptable amount for a sport size rear rotor?

EDIT:
Does anyone have recomendation for a dial indicator mount that works well on the car? The Noga magnetic arm I got works okay with the rotor on, but if I try to measure the runout of the hub I am seeing some inconsistent measurements.

pman9003

So I found the issue. The runout on the rotor and hub was fine and there was no debris between the rotor and hub, and everything was installed correctly. The issue was actually the rotor itself, there is a small bump, almost like a pimple on the inboard friction surface. Of course I couldn't see this until I took the rotor off and it gave it a good look over. You can see it as the shiny spot in the picture. You can actually feel it as a high spot when you run your finger over it. Guess you can get a bad rotor every now and then.