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Discussion Starter #1
Over the winter I installed front and rear EP uprights, and in the rear I installed Lotus APR 2-pot brake calipers (same as stock Elise front calipers) and Carbotech brake pads. The rear disc rotors are AP Racing 304mm. All works well, but the rear brake pads overhang the outside of the disc by 4.3mm at the center and about 3mm near the leading and trailing edges (see photo). The Carbotech pads are part # CT109L (AX6 and XP12, both compounds fit the same).

My first thought was that the disc rotor is undersized (it's the size recommended by EP), but the minimum radial clearance between the outer edge of the disc and the caliper is only 2.4mm. There are no spacers on the caliper. I could go to a larger disc (308mm) and use spacers to fine tune the caliper position, but I can't see leaving less than 1mm of radial clearance (to accommodate thermal expansion of the disc), which would still leave about 3mm of overhang.

I have zero experience with the "standard" front brake setup (APR 2-pots with 288mm discs), so I have a couple questions. Have you seen this overhang with these same brake pads, or with other brake pads, with the "standard" front brakes? How much radial clearance is there between the disc and the caliper with the "standard" front brakes?

I'm not sure this is actually a problem. I plan to chamfer the outer edge of the pads with a file or grinder, but it clearly isn't ideal, I've never seen brake pads overhang like this before.

Thanks in advance for your thoughts, -Ed
 

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You've got a lot of problems here, but none of them are the brake pads.

The OEM front calipers were designed for a 282mm disc. Putting them on a 304mm or even 308mm disc yields the problem you describe.

You need to get the correct calipers and discs designed for the EP Racing uprights you have. If you don't want to buy the correct brakes and insist on continuing down the "redneck engineering" route, that overhang will not cause any catastrophic failures... keep going as you are... but I wouldn't.
 

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Discussion Starter #3
...You need to get the correct calipers and discs designed for the EP Racing uprights you have...
Thank you for the reply, but apparently I should have provided more information. The uprights are the Elise Parts "Ultimate GT Race Uprights", you can see them here. They are, in fact, designed to take the 2-pot front calipers (see point #2 in their list on that page), what they don't mention on their product page is that the 304mm diameter disc is the best fit for this upright/caliper combination, but it is exactly what they recommended to me when I was ordering these uprights.

I believe the disc size discrepancy (304 vs 288) explains only part of the overhang (visually evidenced in the photo above by the larger diameter witness line from the disc versus the outer radius of the pad). But I'll make a couple more measurements and draw it up, and post the drawing when done. The critical points I need to measure are the locations where the caliper bridges the disc (the points closest to the hub center), which determine how close the perimeter of the disc can be to the caliper (and pads).

Later, -Ed
 
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Thank you for the reply, but apparently I should have provided more information. The uprights are the Elise Parts "Ultimate GT Race Uprights", you can see them here. They are, in fact, designed to take the 2-pot front calipers (see point #2 in their list on that page), what they don't mention on their product page is that the 304mm diameter disc is the best fit for this upright/caliper combination, but it is exactly what they recommended to me when I was ordering these uprights.

I believe the disc size discrepancy (304 vs 288) explains only part of the overhang (visually evidenced in the photo above by the larger diameter witness line from the disc versus the outer radius of the pad). But I'll make a couple more measurements and draw it up, and post the drawing when done. The critical points I need to measure are the locations where the caliper bridges the disc (the points closest to the hub center), which determine how close the perimeter of the disc can be to the caliper (and pads).

Later, -Ed
Just because a part will "fit" does not mean that it was designed for it. Brakes are a combination of calipers and discs. You need both. Not just the discs.

Sig
 

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Discussion Starter #5
Just because a part will "fit" does not mean that it was designed for it. Brakes are a combination of calipers and discs. You need both. Not just the discs.

Sig
I'm OK with "fits" and "works", even if the fitment is less than ideal. I've used this setup as-is on the track, and it worked well, definitely an improvement (better/less front-rear bias) over the stock rear brakes (288mm discs with Brembo sliding calipers, 308mm discs and APR 4-pot calipers up front). As I said before, it appears to me that these pads would overhang even when used with a 288mm disc (with the caliper properly located for that disc size), but I'll do a drawing to either prove me right or show that I'm wrong. The only additional measurement I need is the distance between the two points where the caliper bridges the disc (the points on those bridges that are closest to the hub center), I've already measured the distance between the disc outside edge and those points. But it would also be helpful to know that distance on the stock Elise front brake setup, because that would define for me the minimum radial clearance needed between the disc and the caliper (or, if you prefer, the ideal location of the caliper relative to the ideal diameter disc).

-Ed
 
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I'm OK with "fits" and "works", even if the fitment is less than ideal. I've used this setup as-is on the track, and it worked well, definitely an improvement (better/less front-rear bias) over the stock rear brakes (288mm discs with Brembo sliding calipers, 308mm discs and APR 4-pot calipers up front). As I said before, it appears to me that these pads would overhang even when used with a 288mm disc (with the caliper properly located for that disc size), but I'll do a drawing to either prove me right or show that I'm wrong. The only additional measurement I need is the distance between the two points where the caliper bridges the disc (the points on those bridges that are closest to the hub center), I've already measured the distance between the disc outside edge and those points. But it would also be helpful to know that distance on the stock Elise front brake setup, because that would define for me the minimum radial clearance needed between the disc and the caliper (or, if you prefer, the ideal location of the caliper relative to the ideal diameter disc).

-Ed
You are doing way too much work! Take a look at the part number (somethin like cp5119) stamper on the bottom of the caliper and download the customer drawing... like for the CP5119 https://www.apracing.com/drawings/cp5119-29cd - Sheet1.pdf
 

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Discussion Starter #7 (Edited)
You are doing way too much work! Take a look at the part number (somethin like cp5119) stamper on the bottom of the caliper and download the customer drawing... like for the CP5119 https://www.apracing.com/drawings/cp5119-29cd - Sheet1.pdf
Thanks for that, unfortunately it doesn't have the dimension I need. Their published drawings are adequate for designing or selecting mating parts (hub carriers/caliper mounting brackets, pads, discs), but they're not complete engineering drawings. I only need to make one measurement, so it's not a big deal (but I'm away from the car today).

-Ed
 

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I find it interesting that the link to the caliper above clearly lists the maximum rotor radius as 141mm, or a diameter of 282mm. Lotus exceeds this with the stock 288mm rotor.
 
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I find it interesting that the link to the caliper above clearly lists the maximum rotor radius as 141mm, or a diameter of 282mm. Lotus exceeds this with the stock 288mm rotor.
That caliper is just posted as an example of what AP Racing offers for the public. That caliper is for a 10mm solid disc and the Lotus discs are 25.4mm ventilated discs. I'm pretty sure Lotus has AP Racing build bespoke (proprietary) calipers for their cars...
 

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That caliper is just posted as an example of what AP Racing offers for the public. That caliper is for a 10mm solid disc and the Lotus discs are 25.4mm ventilated discs. I'm pretty sure Lotus has AP Racing build bespoke (proprietary) calipers for their cars...
My understanding, which could be wrong, is that the only proprietary thing about the "Lotus" AP calipers is the "Lotus" lettering instead of the AP lettering.

The calipers you listed, again I could be wrong, are for narrower discs but are otherwise dimensionally the same as the ones supplied to Lotus.
 

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My understanding, which could be wrong, is that the only proprietary thing about the "Lotus" AP calipers is the "Lotus" lettering instead of the AP lettering.

The calipers you listed, again I could be wrong, are for narrower discs but are otherwise dimensionally the same as the ones supplied to Lotus.
OT, but my understanding is different. None of the calipers AP offers off the shelf match the lotus spec caliper. The CP5317 is the closest, but the piston diameters are smaller than lotus spec. The CP5119 has larger pistons, but the rotors it works with are different. I *think* the lotus spec 2 pot is mostly a CP5317 with a larger bore piston, but don't quote me on it. The CP5111 is the lotus partnumber, and AP won't sell it to just anyone.
 

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OT, but my understanding is different. None of the calipers AP offers off the shelf match the lotus spec caliper. The CP5317 is the closest, but the piston diameters are smaller than lotus spec. The CP5119 has larger pistons, but the rotors it works with are different. I *think* the lotus spec 2 pot is mostly a CP5317 with a larger bore piston, but don't quote me on it. The CP5111 is the lotus partnumber, and AP won't sell it to just anyone.
Sorry, I should have been a little more specific. I believe, again I could be wrong, the bodies of the calipers are the same with differences in bore diameter and more or less material where the two caliper halves bolt together for different thickness discs. The piston bore diameter and width for rotors does not affect what the OP was asking about.

Although, looking again at the CP5316/CP5317, the bridge between caliper halves may be different too, but that information does not align with the information from the OP.
 
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