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      07-08-2018, 02:02 PM   #1
ayao
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Pad radial depth and unswept rotor issues?

I recently bought some new pads Raybestos ST45 (R2600 fitment) for a front AP BBK (caliper CP5060).

In spite of being a Google zealot, I have ended up with a set of pads that have a radial depth of 51mm which will leave a 3mm band of unswept rotor adjacent to the rotor hat.

I understand the theoretical risks of uneven temperature zones between swept and unswept zones that could lead to premature rotor cracking, but I'm wondering if anyone has actually experienced any problems with this radial depth mismatch.

Thanks.

(And incidentally, if anyone ever is looking for Raybestos pads for the Essex AP CP5060 caliper, it appears that the R2602 fitment is more correct than the R2600.)
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      07-08-2018, 04:04 PM   #2
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I'll have to look back to see what I ran a little while back at laguna. Had to use their pads in a pinch for laguna. My rotors were already on their way out and cracked, but I wouldn't attribute that to the pads.

My knee jerk reaction is that 3mm isn't too bad. These rotors expand some. I don't know if there is a "rule of thumb" for how much unswept rotor is acceptable, but I'd start to question it starting at around 5mm, depending on the rotor and overall pad placement. Sounds kinda silly to think about 2mm like that...
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      07-08-2018, 04:34 PM   #3
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Yeah I figure there are also some manufacturer tolerances that will affect this but I guess the standard has been set in my mind by the Ferodos I have been running which look like they were designed for the setup and leave little to no unswept area. 3mm on calipers ends up being the height of the horizontal part of the J-hook so itís really not very much but the armchair HPDE warrior in me struggles with knowing itís not perfect. But if there is really no significant practical disadvantage like shortened rotor life then the $150 cost savings on pads might be justifiable.
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      07-08-2018, 06:33 PM   #4
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I would give the Essex guys a call and ask if that much will cause problems. They are obviously the US importer for AP Racing and can best speak to it.
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      07-09-2018, 03:51 PM   #5
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Yes, I will reach out to them. I suspect, however, that I will get a rehash of whatís on their website: ďLeaving a portion of the disc face unswept can create a temperature differential across the face of the disc, and doing so could lead to premature disc cracking.Ē
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      07-19-2018, 10:05 PM   #6
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Probably more of a concern with a one piece rotor. You're rotor isn't going to explode or anything sudden. I ran a crappy UUC BBK back in the day...did Ok, didn't crash and I'm still alive.

The issue you will run into is if you change pads which cover the entire rotor surface.
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      07-19-2018, 11:42 PM   #7
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What specific issue do you foresee if changing to pads that cover the entire rotor?

My current plan is running street pads (DS2500, which cover entire rotor) and swapping track pads (3mm less radial depth) for HPDE events.
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      07-19-2018, 11:54 PM   #8
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Quote:
Originally Posted by ayao View Post
What specific issue do you foresee if changing to pads that cover the entire rotor?

My current plan is running street pads (DS2500, which cover entire rotor) and swapping track pads (3mm less radial depth) for HPDE events.
You'll get odd wear on the pads.
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      07-20-2018, 02:34 PM   #9
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Figured as much!
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      07-24-2018, 08:40 AM   #10
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Quote:
Originally Posted by ayao View Post
What specific issue do you foresee if changing to pads that cover the entire rotor?

My current plan is running street pads (DS2500, which cover entire rotor) and swapping track pads (3mm less radial depth) for HPDE events.
The 3mm difference between the 54mm and 51mm radial depth pads isn't going to cause you any serious problems most likely. The calipers in our smaller CP8350 kits that we sell for some applications take pads ranging from 39mm to 50mm. I've seen guys run that entire range. The 39mm is definitely not something we'd recommend, but I haven't heard of any issues in terms of major disc cracking. I think a large part of that is that the AP Racing discs are extremely stout.

From the practical side, here's what's going to happen. When you run the shorter 51mm pads, they're going to wear down the face of the disc at their bottom edge. When you swap over to the taller 54mm pad, that pad is going to sit on top that lip that has formed on the disc face. They won't be sitting completely flush, and that little ridge will cut into the pads and give you a little bit of funky wear on the bottom edge. You may get a little crumbling on the bottom edge of the DS2500 as well, but that's ok as long as the bulk of the pad puck remains intact.

Given that the funky bit of wear will be on your mild sport pads when you're just tooling around at low speeds, I wouldn't expect any problems at all. In other words, you should be good to go. The situation is not ideal, but I wouldn't expect any issues.
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      07-24-2018, 11:21 AM   #11
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Thanks for the feedback Jeff - this is helpful to know. I can live with uneven pad wear but I was just more concerned that there would be disc issues, which sounds less likely.
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      07-24-2018, 01:44 PM   #12
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If you run the "smaller" pad for dozens of track days, wear down a lot of the disc, then switch to a bigger one, you'll have an issue with the pad dragging and wearing unusually.

If the wear difference (height?) is a few MM, that's when I would be leery. 1 or 2mm more rotor material around the outer edge will wear away without much of a problem, IME.
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      07-24-2018, 01:55 PM   #13
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Quote:
Originally Posted by dparm View Post
If you run the "smaller" pad for dozens of track days, wear down a lot of the disc, then switch to a bigger one, you'll have an issue with the pad dragging and wearing unusually.

If the wear difference (height?) is a few MM, that's when I would be leery. 1 or 2mm more rotor material around the outer edge will wear away without much of a problem, IME.
This was Porterfield's opinion as well - that if I was going to swap back to street pad in between occasional HPDE events then there wouldn't be much of an issue, versus running the track pads for a 24 hour event, creating a big height difference, then the uneven wear could pose issues for pad swaps.

In the end none of it probably matters and it just keeps Armchair HPDE Warriors (me) awake up at night for no justifiable reason.

On a side note, I really like the ST45 pads - very linear torque, reasonably easy to modulate, wear well and seem to treat rotors ok. I did light up the second square of my brake caliper temperature strips but that's about it. Hard to beat the pricing as well, even if fitment isn't quite as factory-minty-fresh as the Ferodos I was running before.
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