What is the REAL explanation for the Coupe rotor sizes?

[Alphamacaroon]

One comment that constantly comes up when people see my Coupe— "Did you know the rotors on your rear are bigger than your front?" As if to subtly say "Stupid kit builder, you must have put them on backwards" ;)

It's at this point where I say something like "Yeah, that's the way it's supposed to be. The car has more weight distribution towards the rear end, so you have to put bigger rotors there. Blah, blah, blah." But I cringe every time I say this, because I don't really understand WHY this is true other than hearing a few people say something about it on the Internet. And after some time searching I really haven't found a great explanation as to the physics behind it.

Can someone offer a real technical explanation as to why the Coupe has bigger rotors on the rear vs. the front? Related questions:

1. The moment I step on the brakes, the car transfers weight to the front tires, so is the weight distribution still rear biased under braking?
2. Is it less about rotor size and more related to the pistons, pad size, etc.?
3. Is it just this way with the standard brakes? Is the Wilwood setup different than the standard brakes? Are the front brakes larger or equal with the Wilwoods?
4. Rear engine Porsches are about as tail heavy as it gets— do they generally get larger brakes on the rear? If not, then why?

I'd love to explain the reasoning with a bit more confidence, so any help would be greatly appreciated!

[wallace18]

It is only if you have the IRS rear end. It is not an issue. I have done 427 MK4's with IRS and Standard FFR front brakes. As long as you have a good front bias adjustment there is no problem IMO. It is all in the setup. The IRS comes from a new Mustang IRS rear end and those rotors are larger. The fronts are from a 94 Mustang. I hope this helps you out.

[edwardb]

Strictly related to the brake combination chosen when using Mustang brakes. Not specifically intentional due to bias or whatever. The optional Wilwoods for IRS are 6 Piston / 12.88" diameter x 1" width rotors in the front. 4 Piston / 12.88" diameter x 0.81" width rear.

[Alphamacaroon]

Okay so there really is no benefit to it— it's just the strange combination of what stock brakes fit each configuration.

Yeah, and I wasn't saying it was an issue per se, just that it was a strange combo that seems counterintuitive, people comment on it, and wondering if there was a technical explanation for it. Seems that really there is no technical explanation other than "that's just the way it works out with the stock brakes". I guess I really need to upgrade to the willies if I don't want to continue answering that question :D

[michael everson]

Get a 1995 and later Mustang Cobra 13 inch front brake kit. Direct fit to the FFR spindles. I bought them through Rock Auto
Mike

[David Williamson]

a function of budgets and how much you want to spend. I have Wilwood's on the Coupe that I build without much concern to cost but on my Mkll I still have the 93 donor brakes. Often a car show I see people looking at the 9 inch drum back brakes, it was build on a budget and since it works fine I kept them.
David W

[Ron J]

Get a 1995 and later Mustang Cobra 13 inch front brake kit. Direct fit to the FFR spindles. I bought them through Rock Auto
Mike

Hello, you wouldn't happen to have made a note of the part numbers for the 1995 Mustang Cobra 13" brake kit? If so, can you provide them? I looked at the offering from RockAuto and couldn't discern which kit had the 13" rotors, Thanks ahead for your response.
Ron

[Paul2STL]

Ron, might be hard to find a kit but you can piece together what you need. I am using 04 Mach 1 calipers on my roadster with 13" rotors and 15 Mustang GT IRS calipers with 13" rotors. When you get the calipers get the mounting brackets with them as the Mustang GT brackets will not work. The 95-04 cobra calipers are the same as the 01-04 Mach 1s, the later cobra calipers have cobra embossed on them witch cost more. I just used the Mach 1 with a cobra sticker logo on them. Below is a link to the calipers and to rotors kit. I went on summit and got the rotors/pads kit. Look through my build blog that in my signature block and you will see some picks of my set up.

https://www.rockauto.com/en/moreinfo.php?pk=4384335&cc=1425013&jsn=939&jsn=939

https://www.summitracing.com/parts/pwr-k1304-26?utm_source=bing&utm_medium=cpc&utm_term=%2bpower+%2bstop+%2bk1304-26&utm_content=Power+Stop&utm_campaign=Part+Number+Ad+Groups

[Ron J]

Ron, might be hard to find a kit but you can piece together what you need. I am using 04 Mach 1 calipers on my roadster with 13" rotors and 15 Mustang GT IRS calipers with 13" rotors. When you get the calipers get the mounting brackets with them as the Mustang GT brackets will not work. The 95-04 cobra calipers are the same as the 01-04 Mach 1s, the later cobra calipers have cobra embossed on them witch cost more. I just used the Mach 1 with a cobra sticker logo on them. Below is a link to the calipers and to rotors kit. I went on summit and got the rotors/pads kit. Look through my build blog that in my signature block and you will see some picks of my set up.

https://www.rockauto.com/en/moreinfo.php?pk=4384335&cc=1425013&jsn=939&jsn=939

https://www.summitracing.com/parts/pwr-k1304-26?utm_source=bing&utm_medium=cpc&utm_term=%2bpower+%2bstop+%2bk1304-26&utm_content=Power+Stop&utm_campaign=Part+Number+Ad+Groups

Thank you for taking the time to respond. Just so I am clear, not wanting to sound like a complete ignoramus, can you clarify:
My Gen 2 Coupe was supplied with 11" front rotors, calipers and spindle. Am I understanding you correctly that the Rock Auto calipers you noted will bolt to the FFR supplied spindle and will provide enough clearance for the 13" rotor, caliper and the FFR 17" wheel?
Ron

[Paul2STL]

Thank you for taking the time to respond. Just so I am clear, not wanting to sound like a complete ignoramus, can you clarify:
My Gen 2 Coupe was supplied with 11" front rotors, calipers and spindle. Am I understanding you correctly that the Rock Auto calipers you noted will bolt to the FFR supplied spindle and will provide enough clearance for the 13" rotor, caliper and the FFR 17" wheel?
Ron
yes that is what I have on my roaster
https://thefactoryfiveforum.com/attachment.php?attachmentid=72870&d=1503859067

[Ron J]

thank you for the clarification, the pic really helps,
Ron