Lets talk brakes

[DB83]

So, there have been a few discussions on here, especially in the track forums area, about upgrading to the GT350 brakes and to be careful with the brake balance. That if you do the rears you have to do the fronts to keep the brake balance the same, so i did a bit of number crunching to try and start an educated discussion on the topic. I'm interested in replacing the rear callipers purely for weight. I was shocked when i replaced my brake lines at how heavy the rear callipers on the GT PP are, especially compared to the fronts that are so much bigger!
My plan/thought is buy the rear callipers only, and adapt them to work with the rear GT PP hub, and run a hydraulic handbrake Teed into the brake system. (Yes it means the handbrake will also actuate on the front brakes).
Anyway, on to the discussion.

First the numbers.

Front Callipers
GT350 uses 6 piston callipers with piston sizes of 34/36/38mm making a total piston area of 6119.82 mm2
GT PP uses 6 piston callipers with piston sizes 36/36/36mm making a total piston area of 6107.28 mm2
GT uses 4 piston callipers with piston sizes 46/46mm making a total piston area of 6647.6 mm2

Rear Callipers
GT350 uses 4 piston callipers with piston sizes 30/32mm making a total piston area of 3022.22 mm2
GT PP uses single piston callipers with piston size of 45mm making a total piston area of 1590.43 mm2
GT uses single piston callipers with piston size of 45mm making a total piston area of 1590.43 mm2

Front Rotors
GT350 uses 394mm rotors with a swept area of 78550 mm2
GT PP uses 380mm rotors with a swept area of ??????? Anyone help out here
GT uses 352mm rotors with a swept area of ?????? Anyone help out here

Rear Rotors
GT350 uses 380mm rotors with a swept area of 51400 mm2
GT PP uses 330mm rotors with a swept area of ?????? Anyone help out here
GT uses 320mm rotors with a swept area of ?????? Anyone help out here


So, from what i can see, as far as callipers go the fronts on the GT PP and GT350 seem to be very close, close enough to be negligible in my opinion. The swept area i suspect will be slightly higher for the GT350 than the GT PP due to the larger rotor, but this needs to be confirmed once i get the actual numbers for the GT PP and GT. If anyone can help here then please do!
The rears on the GT350 however do seem to have a much higher piston area and much larger rotor size, which would indicate a much higher rear brake bias, again, to be confirmed once we have the swept areas for the GT and GT PP.

So, you can buy this kit directly from ford for the fronts and rears, but in my mind it is still changing the brake bias heavily towards the rear compared to the GT PP. Am i missing something? They don't supply you with a new calibration, or a new master cylinder to change that so unless the front swept area is massively different to the GT PP fronts, i don't get it?

They way i see it is that, you could in theory do just the rear conversion, and maybe run a slightly higher friction pad in the front to compensate for not using the larger front kit. ???????

Anyway, i'm all ears, and please can we keep this discussion relevant and technical. I don't want this to turn into another argument over why bother changing the rears if you don't need too, etc... I want this to be a technical discussion in to the brake bias query between the different brakes.

Thanks!!

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[Dana Pants]

FYI, running the brake handle to the main hydraulics is illegal and arguably dangerous. A major purpose of the hand brake is to slow the car in case you lose fluid.

You can do a lot with pads for bias, but usually high friction pads are also for higher temps, so they may be unpredictable when cold vs hot. I’m sure if you spend hundreds and hundreds of dollars on pads you can find a random combination that works.
Keep in mind you have no way to really test the pad bias without doing autocross or similar.

I have increased my front bias compared to stock PP1 at the moment, and hard braking is still sketchy AF on track because the back gets so unweighted.

 

[DB83]

With the handbrake issue, that is a fair point, and probably look into a different solution with a mini calliper like they do with the GT500.

With the pads, decent companies like ferrodo, will publish they're mu factor for each pad material and there temperature working range, so it eliminates a lot of the guess work, you can calculate the bias change before you buy anything.

I'm lucky enough to live in an area that has an unused stretch of road about 4 km long with a huge straight, so i would be able to conduct multiple stops and measure the stopping distance, and safely feel if the car gets more or less sketchy.

 

[Dana Pants]

For the Hawk example (first pic I found), the cold stop area is a mess of intersecting lines, and this makes the Valid pad combinations much much less. Basically put HP+ on the front, “OEM” on the rear, and hope it’s enough.

edit: option B is to just send it and report back if ABS can manage it or not.

 

[DB83]

Very valid point for the cold stopping of pads. Something you would have to bear in mind when selecting pads and remember on that first cold morning start.
Also, I wouldn't be using the pads a drastically adjust the brake balance, it would be just to tweak it, so I would realistically only be looking at 1 or 2 compound differences anyway.
Another thing that would be important would be the mu characteristics of the pad. You wouldn't want a rear pad that locks up immediately from cold, when you have a front pad that won't do anything. You want them to have similar mu behaviour through the temperature curve.

 

[Brian@BMVK]

What performance shortfall do you currently have? Overall brake torque, bite, rotor temperature? What pads have you used? Have some, have you tried swapping to a two-piece rear rotor that improves cooling?

The GT PP brake setup is extremely capable even with the sliding rear caliper, and typically needs at least a 1 compound stagger to make sure the rear doesn't get into ABS early and make the car difficult to drive on entry. I have a worksheet with the hydraulic bias and total brake bias ratios (with the rotor swept area factored in). The GT PP has ~81% front, and the GT350 67%. This is most certainly complimented by different ABS programming.

 

[DB83]

Hi Brian. Currently ruining the GT PP front / rear (comes as standard in Europe) with stock pads, upgraded lines, Steeda 2 piece rotors front / rear, and Motul RBF600 brake fluid. The only time i had a problem with brake fade was on a mountain run with the club I'm in, where it was a lot of heavy braking but not a lot of high speed to get air through the discs to cool them down.
My biggest "issue" I suppose is just the weight of the rear callipers. They are really heavy especially when compared to the aluminium 4 pots, so feel there is decent improvement in unsprung weight to be had here.
Its interesting the GTPP has an 81% front bias when the GT350 has a 67%, this is a huge difference. Especially when you consider Ford will sell you the kit without any ABS calibration necessary. How is that possible? I'm sure there are other factors such as weight transfer and initial weight distribution, but i still find that is quite a large bias difference. Unless of course they use radically different pad compounds front to rear compared to the GT PP?

 

[Brian@BMVK]

Hi Brian. Currently ruining the GT PP front / rear (comes as standard in Europe) with stock pads, upgraded lines, Steeda 2 piece rotors front / rear, and Motul RBF600 brake fluid. The only time i had a problem with brake fade was on a mountain run with the club I'm in, where it was a lot of heavy braking but not a lot of high speed to get air through the discs to cool them down.
My biggest "issue" I suppose is just the weight of the rear callipers. They are really heavy especially when compared to the aluminium 4 pots, so feel there is decent improvement in unsprung weight to be had here.
Its interesting the GTPP has an 81% front bias when the GT350 has a 67%, this is a huge difference. Especially when you consider Ford will sell you the kit without any ABS calibration necessary. How is that possible? I'm sure there are other factors such as weight transfer and initial weight distribution, but i still find that is quite a large bias difference. Unless of course they use radically different pad compounds front to rear compared to the GT PP?

Yes it certainly is.

 

[DB83]

Hey Brian, are you able to share the swept area info for the GT and GTPP brakes?

 

[Norm Peterson]

The GT PP has ~81% front, and the GT350 67%. This is most certainly complimented by different ABS programming.

That sounds like too much to be compensating for with only ABS calibration, especially when the presumably harder-stopping GT350 is given the higher amount of rear brake bias.

PP vs GT350 - is there a difference in the relationship between front and rear mean pad radius, or differences in the front vs rear OE pad compounding going on here? Is the master cylinder the same, or are there any differences in the bores?


Norm

 

[racingandfishing]

Not a comparison to the Shelby brakes, but lots of good reading here:

https://www.vorshlag.com/forums/for...s-uses-limitaitons-and-upgrade-options?t=8519

Also note that they discovered the PP has a different master cylinder than the GT.

 

[Ewheels]

I appreciate that you are looking at this from a performance standpoint rather than an aesthetics one.
However, I still don't think this is a worthwhile upgrade. Performance gain per dollar is simply not worth it. I know some more hardcore GT350 track guys who upgrade to AP racing calipers but that is so they can run 18" wheels for slicks.
Unsprung weight savings is great but I just don't think it's worth the time and money. If you have that kind of money to spend, awesome, but I would think that money is much better spent somewhere else.

I would hate for you or anyone to spend thousands of dollars on this upgrade and to not physically notice a difference.

 

[Brian@BMVK]

Not a comparison to the Shelby brakes, but lots of good reading here:

https://www.vorshlag.com/forums/for...s-uses-limitaitons-and-upgrade-options?t=8519

Also note that they discovered the PP has a different master cylinder than the GT.

Which just changes the pedal force required. That's been debunked. The non PP GT setup has more total piston area than the GT PP. Vorshlag's issues are from running too aggressive of a rear pad.

 

[DB83]

I appreciate that you are looking at this from a performance standpoint rather than an aesthetics one.
However, I still don't think this is a worthwhile upgrade. Performance gain per dollar is simply not worth it. I know some more hardcore GT350 track guys who upgrade to AP racing calipers but that is so they can run 18" wheels for slicks.
Unsprung weight savings is great but I just don't think it's worth the time and money. If you have that kind of money to spend, awesome, but I would think that money is much better spent somewhere else.

I would hate for you or anyone to spend thousands of dollars on this upgrade and to not physically notice a difference.

I think your assuming i'm buying the whole rear kit. I've already got the GT350 rear callipers which i got for next to nothing, and am looking at adapting them myself. I have access to a machine shop where i can make my own parts. So from a cost point of view, not really a factor. I'm only looking at the science and maths behind it all.

 

[DB83]

Which just changes the pedal force required. That's been debunked. The non PP GT setup has more total piston area than the GT PP. Vorshlag's issues are from running too aggressive of a rear pad.

Is that confirmed to be the only difference between the two? I was looking at part numbers on the levittown website and interestingly the GT PP master cylinder is only used on the GT PP. Both the regular GT and GT350 use the the other master cylinder. Which in the case of the regular GT has even more front hydraulic bias according to the piston areas mentioned at the beginning.

None of this adds up!!

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