Deep groove in front rotors

[kolosy]

Prepping the car for the new season and remembered something. I ran a few sessions with the winmax r5 pads last season and then swapped in a set of ferodo ds1.11. After that the rotors developed a single deep groove near the outside perimeter. The car is at the cage builder right now so I can’t take a closeup or measure, but it was probably a mm or two deep and you can kind of see it on this picture.

It’s almost like something got caught under the pad, or it developed a hot spot. Obviously I need to pull the pads and take a closer look, but assuming there’s a deep groove near the perimeter, should I use standard minimum thickness rules or is this a different beast?

[M3SQRD]

The pad radial depth (PRD) doesn’t match the rotor radial depth (rotor annulus depth = rotor OD - rotor IR) and/or the PRD radial positioning may be offset from the rotor annulus depth which can result in pad overhang on the rotor IR or rotor OR (PRD = rotor annulus depth but the caliper bracket incorrectly aligns/centers the pad on the rotor) and/or the pad material may have been incorrectly placed on the pad backing plate (or in your case you’ve switched pads from different manufacturers).

All of the above can create unswept or overhang rings on the rotor IR or OR. It’s hard to tell from this picture but it looks like the first pad used the full rotor surface and the second pad PRD is shorter than the rotor annulus depth. It would be easy to identify the car if you had it sitting in your garage.

Here’s a picture showing the pad PRD was too short:

[kolosy]

Originally Posted by M3SQRD

The pad radial depth (PRD) doesn’t match the rotor radial depth (rotor annulus depth = rotor OD - rotor IR) and/or the PRD radial positioning may be offset from the rotor annulus depth which can result in pad overhang on the rotor IR or rotor OR (PRD = rotor annulus depth but the caliper bracket incorrectly aligns/centers the pad on the rotor) and/or the pad material may have been incorrectly placed on the pad backing plate (or in your case you’ve switched pads from different manufacturers).

All of the above can create unswept or overhang rings on the rotor IR or OR. It’s hard to tell from this picture but it looks like the first pad used the full rotor surface and the second pad PRD is shorter than the rotor annulus depth. It would be easy to identify the car if you had it sitting in your garage.

Here’s a picture showing the pad PRD was too short:

Ok, but why would that create a groove, instead of just a step down?

[M3SQRD]

Originally Posted by kolosy

Ok, but why would that create a groove, instead of just a step down?

I’m referencing to the lighter colored rotor surface at its OR - is this the groove you are referring to? Let’s agree on the groove before I go any farther.

[kolosy]

Originally Posted by M3SQRD

I’m referencing to the lighter colored rotor surface at its OR - is this the groove you are referring to? Let’s agree on the groove before I go any farther.

You can best see what I’m describing at the “4 o’clock” of the wheel. It’s a lighter colored ring surrounded by darker areas. It set maybe 3-5 mm in from the edge of the rotor.

[M3SQRD]

Originally Posted by kolosy

You can best see what I’m describing at the “4 o’clock” of the wheel. It’s a lighter colored ring surrounded by darker areas. It set maybe 3-5 mm in from the edge of the rotor.

Ok. That’s the one I was referring to.

Using the Winmax pad first, it looks like the full rotor surface was used. The Ferodo pad was used second and it did not use the full rotor swept area on the IR and OR. The Ferodo pad transfer layer is really dark. I’ve never used Ferodo race pads so is this dark pad transfer layer normal for the DS1.11 pad compound?

Based on the one picture, it looks like the outer light area is not uniform over a full 360 deg bad sweep. It’s not there from 4 pm to 5 pm it could extend back to 12 am) and it reappears from approximately 5 pm to 7pm (it could continue past 7 pm but the wheel blocks the view). I suspect the Peragon/Winmax R5/W4 and Ferodo DS1.11 have incompatible pad compounds.

Once the DS1.11 pad material (higher CoF than the R5/W4) is transferred onto the top of the incompatible R5/W4 pad material, the DS1.11 is acting like a brake lathe and created a step in the rotor OR which would feel like a lip or possibly a groove. You can verify this by checking whether there’s a similar lip on the rotor IR.

Changing topics. Have you been able to increase pad life? Last time we had discussed this, I recall you were going through a set of pads in one track day.

[kolosy]

Originally Posted by M3SQRD

Ok. That’s the one I was referring to.

Using the Winmax pad first, it looks like the full rotor surface was used. The Ferodo pad was used second and it did not use the full rotor swept area on the IR and OR. The Ferodo pad transfer layer is really dark. I’ve never used Ferodo race pads so is this dark pad transfer layer normal for the DS1.11 pad compound?

Based on the one picture, it looks like the outer light area is not uniform over a full 360 deg bad sweep. It’s not there from 4 pm to 5 pm it could extend back to 12 am) and it reappears from approximately 5 pm to 7pm (it could continue past 7 pm but the wheel blocks the view). I suspect the Peragon/Winmax R5/W4 and Ferodo DS1.11 have incompatible pad compounds.

Once the DS1.11 pad material (higher CoF than the R5/W4) is transferred onto the top of the incompatible R5/W4 pad material, the DS1.11 is acting like a brake lathe and created a step in the rotor OR which would feel like a lip or possibly a groove. You can verify this by checking whether there’s a similar lip on the rotor IR.

Changing topics. Have you been able to increase pad life? Last time we had discussed this, I recall you were going through a set of pads in one track day.

yeah, this is tough without the car here. i'll have to wait until it comes back and check then. i understand what you're saying, I just don't understand why it would create a single localized groove. like a V groove or notch into the rotor, with the same height on either side of it.

and yeah, i've gotten better pad life out of the Winmax and Ferodo pads. I also started playing with closing off the cooling ducts on certain tracks where the brakes could actually overcool on long straights (like Road America, specifically) and that's helped as well.

All that goes out the window when the car comes back 4-500lbs lighter though, so I'll have to play with all that again.

[DrFerry]

kolosy The deep grooves indicated in the image below are what would worry me. I once had a rotor snap in half coming under the bridge at Road Atlanta just before the downhill leading onto the front straight. The rotor failure was caused by crack propagation leading to fast fracture. The groves I'm pointing to butt into the diagonal vent slots and at this juncture it's possible cracks could begin due to fatigue.
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[kolosy]

Originally Posted by DrFerry

kolosy The deep grooves indicated in the image below are what would worry me. I once had a rotor snap in half coming under the bridge at Road Atlanta just before the downhill leading onto the front straight. The rotor failure was caused by crack propagation leading to fast fracture. The groves I'm pointing to butt into the diagonal vent slots and at this juncture it's possible cracks could begin due to fatigue.
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lol @ "new underwear location"

i don't remember those grooves being as deep as the one that concerns me, but will take a closer look once i get the car back.