S550 : rear brake upgrade, ran the numbers and got a surprise

[steveespo]

Found the article about stock style abs I had read. Short version, it assumes you are on ice until it determines otherwise everytime you hit abs. So upgrading the abs system first then allows what you have to work better straight away. Then you can address wear rates etc.

Apex Automotor | Quick definition of "**Ice Mode**" in ABS, as this can be really scary or even cause an incident when it happens on track | Facebook

Quick definition of "**Ice Mode**" in ABS, as this can be really scary or even cause an incident when it happens on track. If you've experienced it, you'd know. *I posted this on another page, but...

www.facebook.com

Ford system is a bit more sophisticated than the GM systems, especially the earlier Gen Camaros and Corvettes. But the Ford system has g level limits where when the calculations on brake pressure, speed and deceleration rates go off the tables you get the brick pedal. It is real and Sandro certainly is exceeding the limits. As long as the car isn't rapidly slowing one or two tires more quickly than the others the system plays nice. But once hard ABS is called for it can get dicey. Or icy. I have found that a firm progressive pressure build vs a "spike" of the pedal will keep me out of ice mode at the typical corners it happens, Turn 1 VIR, Turn 7 at Sebring, Bus Stop at Watkins, and less frequently the Bus Stop at Daytona. If you can do it get the Bosch. It is a very adjustable system that can be tailored to specific tire and track conditions. Since you don't care about the Advance Track programming, disabling it and the ABS through tuning is not an issue. Can't wait to see the upgrades, it won't make the brake peak performance that much better but the modulation, consistency and lack of worrying about ice mode will be gone.

 

[Stan Whitney]

Another direction to think about. No ABS is better than bad ABS. You can convert to a proper dual master cylinder with adjustable bias. Once you get through the trial and error of cylinder sizing for your calipers, pad compounds for the pedal effort, you can fine tune the bias based on conditions and car setup with a knob on the dash. I did that on a C5 20yrs ago and that is how pro world challenge cars ran at the time.

Your car with race tires and lots of aero probably does not need much ABS except for bumpy tracks. You probably reduce brake pressure once you feel ABS kick in anyway because it is hurting not helping.

Then upgrade the caliper hardware, then add race ABS on top for the icing on the cake. Bosch ABS brings yaw control for stability which can be a big advantage, and the ability to drive to the apex while 'in ABS' mode.

 

[honeybadger]

The first time you smash the brake pedal and it lets you keep it down all the way to corner apex you’ll forget how much you paid.

Plus, the ability to dial it up or down depending on track conditions is amazing

 

[honeybadger]

Skip m4. M5 is a huge upgrade

 

[Dave_W]

You are at the point that you are asking the ABS and brake components to work with things they weren't calibrated for. Suspension, aero, power, weight have all changed.

THIS. Stickier tires and higher friction front pads (and front aero) create braking g-forces that far exceed what could be accomplished with street tires & pads. All the related physics changes happen, notably more weight transfer onto the front contact patches and off the rear contact patches.

I agree with others that the long-term fix at your level is probably a motorsports-grade ABS system.

However, if you want a quick fix, have you thought about putting a proportioning valve in the rear brake line? My thinking (which has a few ASSumptions) is that the increased overall braking force is unweighting the rear tires, making them easier to lock up or even just "drag" on the pavement at a slower rpm. I think the ABS detects the change in rear vs. front wheel speed and goes into ice mode. Are you logging the individual wheel speed sensors at a high resolution?

If you can keep the rear wheels rotating instead of sliding, the ABS might stay out of ice mode. What if you reduce the friction of the rear pads/rotors by changing pad compound and/or reducing the clamping force of the calipers? I'd consider pad compound "coarse" tuning and hydraulic pressure "fine" tuning, but I haven't used a proportioning valve myself.

Proportioning valves pass 100% pressure up to a "knee point" limit, and then they pass a percentage of the pressure. All, or at least the vast majority, of aftermarket proportioning valves adjust the knee point, not the pressure ratio split after the knee point is hit. Note in the graph below what changes is the amount of pressure before the line bends, not the angle after it starts the bend.

from https://shop.wilwood.com/blogs/news/how-does-a-proportioning-valve-work

 

[steveespo]

THIS. Stickier tires and higher friction front pads (and front aero) create braking g-forces that far exceed what could be accomplished with street tires & pads. All the related physics changes happen, notably more weight transfer onto the front contact patches and off the rear contact patches.

I agree with others that the long-term fix at your level is probably a motorsports-grade ABS system.

However, if you want a quick fix, have you thought about putting a proportioning valve in the rear brake line? My thinking (which has a few ASSumptions) is that the increased overall braking force is unweighting the rear tires, making them easier to lock up or even just "drag" on the pavement at a slower rpm. I think the ABS detects the change in rear vs. front wheel speed and goes into ice mode. Are you logging the individual wheel speed sensors at a high resolution?

If you can keep the rear wheels rotating instead of sliding, the ABS might stay out of ice mode. What if you reduce the friction of the rear pads/rotors by changing pad compound and/or reducing the clamping force of the calipers? I'd consider pad compound "coarse" tuning and hydraulic pressure "fine" tuning, but I haven't used a proportioning valve myself.

Proportioning valves pass 100% pressure up to a "knee point" limit, and then they pass a percentage of the pressure. All, or at least the vast majority, of aftermarket proportioning valves adjust the knee point, not the pressure ratio split after the knee point is hit. Note in the graph below what changes is the amount of pressure before the line bends, not the angle after it starts the bend.
View attachment 111406
from https://shop.wilwood.com/blogs/news/how-does-a-proportioning-valve-work

Friend of mine tried something similar to this with a S197 car, 15" six piston from the S550 and the prop valve from Wilwood. It was a disaster. The car saw the reduced pressure to the rear and the ABS kept adding rear brake bias until it locked all four wheels by the time we reached the Carousel at Road America on the out lap. So yes disabling the ABS would be the way to avoid that. But you will definitely need to change your driving style, if you get ice mode now, you likely will be locking up tires and flat spotting $600 hoosiers.

 

[TeeLew]

ps.AP RACING front kit is smaller than my oem .Why go below OEM rather than match it? Less piston area means less fluid displacement, so a firmer pedal and with far better pad volume and cooling you don't need more torque. I dont need the peak; I need concistency. and maintain the bias with their smaller rear.

View attachment 111241

There's not even a question in my head. AP all the way. The taper wear is impossible to cure on a sliding caliper, so that's really not an option. Wilwood makes fine stuff for hot-rods and circle tracks. I'm not trying to dump on them, but they're just not the same level as AP.

Yes, you'll give away a little rear stopping power theoretically. It will not remotely be an issue on track. It will just rattle the anti-lock a little less.

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I just read your later post. I'm surprized these calipers cause problems if you use them alone.

I would definitely listen to them. They know their stuff.

So, all four it is, I guess?

 

[flyhalf]

There's not even a question in my head. AP all the way. The taper wear is impossible to cure on a sliding caliper, so that's really not an option. Wilwood makes fine stuff for hot-rods and circle tracks. I'm not trying to dump on them, but they're just not the same level as AP.

Yes, you'll give away a little rear stopping power theoretically. It will not remotely be an issue on track. It will just rattle the anti-lock a little less.

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I just read your later post. I'm surprized these calipers cause problems if you use them alone.

I would definitely listen to them. They know their stuff.

So, all four it is, I guess?

But cannot be rear only tho. Unless also change ABS i guess

 

[TeeLew]

Short answer is I'm a big fan of AP.

 

[biggsy]

@flyhalf Said this on the FB post but you can give Pro System Brakes a call and see what Alcon options they have to offer. I know AP is the go to but Alcon is also available for our cars.

 

[Randomtask]

I was having the same issues with the rear single piston caliper - Just generating stupid heat and destroying pads. Front brembos have been great, the oem rear not so much.

Finally bit the bullet and went full AP. The kit, the delivery paperwork and quality is instantly noticeable. The calipers are a work of art. How do you say ! The ENP finish is spectacular. I know aesthetics aren’t #1 here, but it helps.

So as you say @flyhalf they don’t recommend just the rear kit for brake balance reasons.

Just did a first day on the AP’s in 100deg heat here in Texas. Biggest difference I would say I noticed straight away is the modulation and feel from the AP - definitely felt that trail braking into some sections. I ran the Ferodo 3.12 pads and was very bit as happy with them. Love the extra material on the pads too.

The 4 piston in the rear I think will take a little adapting to. The other thing to note is I had to mill out the rotor hats to fit my extended MSI studs in the front. There’s a little step to them that the AP hat wouldn’t clear. Another item is if you dislike pad rattle, get the anti rattle clips for the rear, they are super noisy cold.

Other than the $$$ I’d go AP.