Weird brake dust deposits on disc after machining

[EuroSteve]

Hi All,

I recently got my front discs machined to remove some shudder I was getting under braking. The guy came out and used an on-car lathe and did what appeared to be a good job. The discs have never been machined before and he was careful to take just the minimum amount off (he did it in 2 passes per disc).

I've been gently bedding them in over the past 3 weeks, and the bedding in whine noise is just about gone (can still hear it when braking from 80+ km/h down), however i'm seeing a strange brake dust deposit pattern appear on the left side disc only.

Left side
http://img2.imageshack.us/img2/5254/13102009261.th.jpg (http://img2.imageshack.us/i/13102009261.jpg/)
http://img101.imageshack.us/img101/5876/13102009260.th.jpg (http://img101.imageshack.us/i/13102009260.jpg/)

Right side
http://img246.imageshack.us/img246/2803/13102009265.th.jpg (http://img246.imageshack.us/i/13102009265.jpg/)
http://img246.imageshack.us/img246/7608/13102009264.th.jpg (http://img246.imageshack.us/i/13102009264.jpg/)

Also, when travelling at 80+ km/h, I can hear a very faint "pulsating" type noise, as if maybe the brake pad is just rubbing and hitting the "high" spots on the disc? The sound wasn't there before having the discs machined.

Any ideas? I'm going to give the guy a call who did the machining, but I thought i'd get some unbiased opinions first.

Thanks,
Steve

[tknova]

Could maybe be some high spots on the brake pad?

Now the rotor surface has changed, the pad is now wearing in different locations prior to the rotors being machined.

[EuroSteve]

Could maybe be some high spots on the brake pad?

Now the rotor surface has changed, the pad is now wearing in different locations prior to the rotors being machined.

I was thinking that, but it's been 3 weeks now, I thought the pads would have bed back in by now?

Also, if it were high spots on the pads, wouldn't the deposit marks be circular around the disc, not perpendicular across the disc? Weird...

I might pull the wheel off this arvo and see if it's the same on the inside surface.

-Steve

[chrome]

The left disc appears to have a "wavy" surface, based on the pattern left behind by the pad deposits. Would appear that the disc had previously been severely warped and the machining did not remove sufficient material.

Perhaps best to call the machinist and let him have a look again at that disc.

Also, ensure your lug nuts are torqued down evenly to the correct value (80Nm) in a criss-cross pattern. Mis-torqued lug nuts contribute to disc warping issues.

[aaronng]

Check the left side by taking the wheel off and making sure that the locking screw is on the rotor and is tight.

[Terrano08]

If u ever get the discs machined and don' change the pads you should scuff the pads down as they are moulded to the shape/angle of the disc prior to machining. Especially if the disc was very rough. THe best way to do this is to get some sandpaper around '40' grit and put it on a flat bench and slide the pad back and fourth over the sandpaper until it is all new pad ( no shiny bits)

[EuroSteve]

Thanks for the tips guys! I've spoken with the machinist and he thinks that the disc may have suffered a rapid temperature change (or several) at some point and it's created hard spots. I'm pretty sure they are where the ribs are, so makes sense. So, it seems on-car lathes can't handle hard spots and instead just ride over them (so to speak).

Interestingly, the left disc had a lot more run out then the right when he first machined them. Also, I had a new set of Hawk Ceramics ready to go in, but there was still over half pad left on the OEM pads, it seemed a shame to waste em, so I left em in (on the advice of the machinist).

Anyway, he's offered to take the disc off and machine it on an "off-car" lathe. Apparently they can do a better job of handling the hard spots.

Will keep you guys posted! If another machine doesn't fix it, I guess i'm up for new rotors. Any recommendations?? ;-)

-Steve

PS. And before you mention it, the shudder was there before I bought the car. I don't drive through puddles (if I can avoid it) and never wash the car after a drive. :)

[aaronng]

When you get your new rotors, don't sit at the traffic lights with your foot hard on the brakes, especially after slowing down from 80km/h or higher.

[EuroSteve]

When you get your new rotors, don't sit at the traffic lights with your foot hard on the brakes, especially after slowing down from 80km/h or higher.

Ha! I am actually conscious of not doing exactly that! Was tought that lesson many years ago at a track day in my mates MX-5, "Don't sit with your foot on the brakes after a run, and don't put the hand brake on!" ;-) I don't have that problem racing karts, cos it goes straight on the stand after a run. :)

But thanks for the tip Aaron!

-Steve

[aaronng]

Ha! I am actually conscious of not doing exactly that! Was tought that lesson many years ago at a track day in my mates MX-5, "Don't sit with your foot on the brakes after a run, and don't put the hand brake on!" ;-) I don't have that problem racing karts, cos it goes straight on the stand after a run. :)

But thanks for the tip Aaron!

-Steve

The stock Euro brake pads are very bad when it comes to this. If you search, you'll find that many auto Euro owners end up with the brake shuddering problem! So I'm not surprised with your experience.

I currently use DBA blank rotors and aftermarket pads. I used to use Hawk Ceramic pads and they are good for street in that they don't deposit as much on the rotor as the OEM pads.

[HunterZero]

I am wondering if the ABS sensors might not be working quite right on that wheel, and the car has undergone hard braking a few times? Might be an idea to get the ABS wheel sensors checked while they have the wheel and rotor off.

- HZ

[aaronng]

If the ABS wheel sensor is malfunctioning where one sensor is reading differently to the other wheel, you should get an ABS check light appearing on your dashboard.

[HunterZero]

It might have a slight fault that's not being registered. It might still be working, but not in the correct tolerance. The sensors should be free of brake dust and metal filings.

- HZ