Brake Bias Question #KnowledgeTransferMonday

[Biggsy]

Just needed help to understand something about brake bias.

Over the years, several people have inquired about upgrading rear PP brakes to rear GT350 calipers (and associated parts) because they didnt like the look of the single piston rear. The response to those inquiries were that it would mess up the brake bias and shift a lot of it to the rear. It made sense to me until I was browsing AP brakes. The AP front brake system allows the use of OEM rear brakes for both the GT350 and PP. The AP kit uses the same front caliper for both cars. Initially I didnt think it made sense because of what I've read on brake bias. Then I figured the bias must be from the Master Cylinder only to find out that the GT350 and the GTs use the same MC according to Ford's site.

So given all this, why would a front PP caliper work with a GT350 rear caliper or where is the bias really coming from? Just trying to gain some knowledge on the topic. Thanks!

@Optimum Performance Tim this seems to be your area of expertise lol.

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[Optimum Performance]

The GT350 uses the base brake M/C, the PP GT uses a larger M/C. The ABS ultimately controls brake bias, if there is no rear grip, 30" rear brakes will do no more than the stock rear single piston brakes. It's why we typically recommend a staggered brake pad compound on most Mustangs. The GT500 seems to respond better with more rear brake pad in experienced hands but it's an outlier with its ridiculous brakes, weight balance, aero, and GRIP. There is a reason the Factory Ford Racing Mustangs (S197's) ran bone stock rear brakes, because they did not do much, they were more for turning the car, the front did all the stopping. Current rules and a modern chassis allows a lot more brake in the rear because the weight transfer is much less.

Curb appeal you can spend all the money you want, from a standpoint of dollars vs. Performance using the OEM stuff, not so much. You want Performance and Looks, Essex Rad-i-cal front and rear. More compact (wheel fitment), lighter, 25mm pads etc.

There has been a lot of talk about M/C but when you calculate piston areas between all the models vs. M/C sizes supplied by Ford it becomes obvious it doesn't have much influence in the S550 chassis.
The GT PP has the largest M/C with the least piston area, less than the base GT with a smaller M/C. The GT350 has the largest area but uses the smaller M/C.

For true track Mustangs switching to the smaller M/C would give more modulation on a GT PP rather than the on/off switch brake pedal they come equipped with but "normies" feel better when the pedal barely moves and puts them through the windshield.

When I refer to GTPP brakes, that includes all sub models, Mach 1 etc. They are all the same system.

 

[Biggsy]

The GT350 uses the base brake M/C, the PP GT uses a larger M/C. The ABS ultimately controls brake bias, if there is no rear grip, 30" rear brakes will do no more than the stock rear single piston brakes. It's why we typically recommend a staggered brake pad compound on most Mustangs. The GT500 seems to respond better with more rear brake pad in experienced hands but it's an outlier with its ridiculous brakes, weight balance, aero, and GRIP. There is a reason the Factory Ford Racing Mustangs (S197's) ran bone stock rear brakes, because they did not do much, they were more for turning the car, the front did all the stopping. Current rules and a modern chassis allows a lot more brake in the rear because the weight transfer is much less.

Curb appeal you can spend all the money you want, from a standpoint of dollars vs. Performance using the OEM stuff, not so much. You want Performance and Looks, Essex Rad-i-cal front and rear. More compact (wheel fitment), lighter, 25mm pads etc.

There has been a lot of talk about M/C but when you calculate piston areas between all the models vs. M/C sizes supplied by Ford it becomes obvious it doesn't have much influence in the S550 chassis.
The GT PP has the largest M/C with the least piston area, less than the base GT with a smaller M/C. The GT350 has the largest area but uses the smaller M/C.

For true track Mustangs switching to the smaller M/C would give more modulation on a GT PP rather than the on/off switch brake pedal they come equipped with but "normies" feel better when the pedal barely moves and puts them through the windshield.

When I refer to GTPP brakes, that includes all sub models, Mach 1 etc. They are all the same system.

Thank you very much for breaking that down for me. Everything you said makes sense. So in order to take advantage of 350 rear brake only, one would have to swap out the abs module from a GT350? (and deal with whatever compatibility issues regarding that)

My car is the only car I have experience with driving on track and I completely agree about lack of modulation. It was difficult to learn how to trail brake due to the on/off switch feel you mentioned. It made me feel as though I wasnt doing something right which could also be true lol.

 

[Optimum Performance]

Thank you very much for breaking that down for me. Everything you said makes sense. So in order to take advantage of 350 rear brake only, one would have to swap out the abs module from a GT350? (and deal with whatever compatibility issues regarding that)

My car is the only car I have experience with driving on track and I completely agree about lack of modulation. It was difficult to learn how to trail brake due to the on/off switch feel you mentioned. It made me feel as though I wasnt doing something right which could also be true lol.

Well no, the abs module does not matter for a brake swap, short of the cars built using a more aggessive strategy than the PP1. The abs is going to modulate based on wheel speed inputs.

Brake swap away, the car will work around all of it.

If the car still has a license plate no need to go any deeper.

 

[Biggsy]

Well no, the abs module does not matter for a brake swap, short of the cars built using a more aggessive strategy than the PP1. The abs is going to modulate based on wheel speed inputs.

Brake swap away, the car will work around all of it.

If the car still has a license plate no need to go any deeper.

Understood. Thanks Tim for taking the time to explain. Have been coming across different pieces of information and wanted to make better sense of it.

 

[TeeLew]

1. Current rules and a modern chassis allows a lot more brake in the rear because the weight transfer is much less.

2. There has been a lot of talk about M/C but when you calculate piston areas between all the models vs. M/C sizes supplied by Ford it becomes obvious it doesn't have much influence in the S550 chassis.

3. For true track Mustangs switching to the smaller M/C would give more modulation on a GT PP rather than the on/off switch brake pedal they come equipped with but "normies" feel better when the pedal barely moves and puts them through the windshield.

1. Please elaborate.
2. Completely agree.
3. No, you're backwards.

 

[Optimum Performance]

1. Please elaborate.
2. Completely agree.
3. No, you're backwards.

1. Which part? New cars have better rear grip vs previous generations
2. OK
3. How does a smaller piston requiring more travel to move the same amount of fluid (Base/GT350 MC) not give more modulation (more pedal movement) than a larger M/C bore?

 

[TeeLew]

1. Fair enough, but that's a different thing than reducing weight transfer. The S197 and S550 have the same wheelbase and roughly the same CG height. Weight transfer is the same for a given deceleration.

3. Because we don't modulate based on travel. It's not an air brake. We modulate based on the load in our braking foot. A larger master means that a greater force needs to be applied to the pedal to produce a given brake torque (all else equal). This also means that when at the limit of braking, the driver has a larger window of modulation (w.r.t. pedal force) between rolling and sliding.

A smaller master gives the light-switch effect, because it works in a smaller force window. The advantage is that you don't have to press as hard, if that's an issue. (It's usually not).

Let's look at 2 exaggerated hypothetical scenarios. It all kind of hinges on the require peak braking force.

Let's say we need to put 1000 PSI in the line to reach peak braking and track bumps or whatever mean we have to modulate the pedal 10% to avoid locking.

If it takes 100# of pedal pressure for maximum braking (big master), then we modulate with a 10# variation in pedal force.

If it takes 50# of pedal pressure for maximum braking (small master), the we will modulate with a 5# variation in pedal force.

This variation is equally important on initial application. We're trying to achieve that peak pressure as quickly as we can without turning into a smoke show or engaging Anti-lock. The smaller master means a more narrow window.

Put a bathroom scale against a wall and push on it with your foot to see how much more difficult it is to keep in that 5# window than the 10. Now consider you're doing all the other things involved with driving.

The only time where pedal travel (fluid displacement) is actually an issue is on the initial application. In that regard, the smaller master will be more prone to a long pedal and knock-back problems than the larger one. Once the pads have made contact with the disc, we aren't moving any fluid (neglecting line swell), we're just changing the line pressure.

The issue with the bigger masters is whether or not you can physically reach the necessary line pressure. If the driver has too big of a master, then they just won't be able to push hard enough (consistently) to get the necessary line pressure to reach lock-up or you've got to push the pedal so damned hard that it's all kind of the same.

I really don't think the difference in master sizes from Ford will go to either of these extremes, but the trends are what they are.

 

[TeeLew]

Well no, the abs module does not matter for a brake swap, short of the cars built using a more aggessive strategy than the PP1. The abs is going to modulate based on wheel speed inputs.

Brake swap away, the car will work around all of it.

If the car still has a license plate no need to go any deeper.

I got an Ebay salvage ABS module from a GT500 CFTP to try. I thought it might have a more aggressive ABS tuning. I installed it, but at the time, had the wrong ForScan cable, so I couldn't get it to work. I'm on BT now and have access to the rest of the car systems and need to try that swap, again. I honestly don't know if there's any difference, but I thought it might be an interesting test.

Has Optimum played with different versions of the ABS module or do you have any thoughts on the swap?

 

[Optimum Performance]

Uncharted territory. I'm not sure you need a different module, you just need the file from say a PP2/Mach 1. I'm not a Forscan guy, so let us know how it goes. Plugging in a module from a different vehicle likely is not going to be plug and play because the module will show its programmed for a different VIN.

I'm more of a drive the equipment you have rather than making the car even more talented than the driver, person.

 

[TeeLew]

Uncharted territory. I'm not sure you need a different module, you just need the file from say a PP2/Mach 1. I'm not a Forscan guy, so let us know how it goes. Plugging in a module from a different vehicle likely is not going to be plug and play because the module will show its programmed for a different VIN.

I'm more of a drive the equipment you have rather than making the car even more talented than the driver, person.

That's exactly the issue. I believe I can change the VIN in the brake controller, but I just couldn't do it on the first shot. It was $100, my time and a bunch of brake fluid. At the very least, I got the opportunity to bleed brakes with my son and daughter. "Pressure-open-down-closed" is something every kid should know.

I've bought parts from you, so improving the car and driver seems to be something we both work towards.

 

[Mountain376]

Just needed help to understand something about brake bias.

Over the years, several people have inquired about upgrading rear PP brakes to rear GT350 calipers (and associated parts) because they didnt like the look of the single piston rear. The response to those inquiries were that it would mess up the brake bias and shift a lot of it to the rear. It made sense to me until I was browsing AP brakes. The AP front brake system allows the use of OEM rear brakes for both the GT350 and PP. The AP kit uses the same front caliper for both cars. Initially I didnt think it made sense because of what I've read on brake bias. Then I figured the bias must be from the Master Cylinder only to find out that the GT350 and the GTs use the same MC according to Ford's site.

So given all this, why would a front PP caliper work with a GT350 rear caliper or where is the bias really coming from? Just trying to gain some knowledge on the topic. Thanks!

@Optimum Performance Tim this seems to be your area of expertise lol.

That's because, from the hydro-mechanical standpoint of the brake system, front vs. rear, they are pretty similar, actually, between the 6th Gen PP 6-piston brakes and the GT350. Yeah, the GT350 rears do produce more force than the single-piston PP brakes, but it isn't a crazy amount.

The largest contributor to brake force, front vs. rear, is going to be piston size/collective piston area. When the S550 came out, the main reason the brakes felt so much stronger on initial bite, compared to past Mustangs and even many other performance cars, was due to the complete change in piston size philosophy - Ford went to running pretty aggressively large piston sizes.

Rotor diameter is going to have an effect on brake force, certainly, but it does far more for thermal mass than brake force when considering piston sizes.

I wouldn't think someone swapping the GT350 rear brakes onto a GT with the front 6-piston PP brakes would cause anything terrible to the balance that the hydraulic controller/ABS wouldn't be able to regulate.

 

[Trace]

This is a 2-year old thread so I'm not guilty of hijacking it I hope. I admit up front I don't understand a lot of what has been written here. But I have a question that I believe is related:

I have a 2021 Mach 1 with the HP option. In my spare parts I have 2 new REAR calipers and rotors for a GT 350.

First, can I use these parts with the existing master cylinder and ABS module?

And second what parts do I need to make them work? I think at least rear axles, but I'm not sure about e-brakes. The Mustang has a manual e-brake but I think I read the e-brake setup is different from a Mach 1.

Thanks in advance.

 

[Grintch]

"Current rules and a modern chassis allows a lot more brake in the rear because the weight transfer is much less."

I completely don't understand that statement. What rules? If anything, modern chassis have more powerful brakes which causes MORE weight transfer. And as he says latter the rears don't do much, yeah, because of weight transfer to the front.

Doing things that move brake bias to the rear (like putting bigger rear brakes on) might put you in a ditch when a rear lockup causes a spin. If you have wider and/or sticker tires (especially up front) this just gets worse (why big brake kits sometimes do only the FRONT). Now maybe ABS will save you, and maybe the well away from stock balance will freak it out.

As a bare minimum I would use one of the online brake bias calculators to see how much you are changing it with the new parts. And if you insist on doing the change for looks, consider adding a new adjustable proportioning valve to re adjust the brake bias back to the front.

 

[Thomas]

This is a 2-year old thread so I'm not guilty of hijacking it I hope. I admit up front I don't understand a lot of what has been written here. But I have a question that I believe is related:

I have a 2021 Mach 1 with the HP option. In my spare parts I have 2 new REAR calipers and rotors for a GT 350.

First, can I use these parts with the existing master cylinder and ABS module?

And second what parts do I need to make them work? I think at least rear axles, but I'm not sure about e-brakes. The Mustang has a manual e-brake but I think I read the e-brake setup is different from a Mach 1.

Thanks in advance.

All other factors being equal, I think you would be better served simply not using them. You are increasing the effective rear pad radius going to a 380mm rotor from 330mm, and rear piston size is increasing ~50% which means that you are going to lockup the rears sooner with the additional brake torque. ABS will have to intervene earlier, and more often assuming the EBC system programming can handle the difference. Now...if you add 300lb of ballast to the trunk to get a 50/50 weight distribution, then you could take advantage of the performance. But....that's like cutting your head off to fix a toothache.

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