BBK bias discussion

[Zazzn]

Ok, so looks like form the PM's and the activity going on in this forum not everyone agrees with what is being presented.

SO lets talk.

#1 Upgrading your front breaks ONLY with a NON-ABS car will throw out your bias depending on what you are putting on, how much bigger how much more clamping force.

#2 Even on a ABS car you will need to lock the front's to trigger the abs before you can get maximum breaking form the rear which will require you to press the breaks even harder when the abs are pumping for you.

All front break upgrades like ARZ's, Dowards will do the above. You will need to match the rears to the front. Close to stock specs to achieve the same balance as stock OR you need stronger rears and a proportioning valve to adjust the rear's so it doesn't lock up first.


With the help of the community lets crunch some numbers.

I need the front Piston size. I already have the stock rears for mk3, and the PBR calipers. We also need the MK4 rears's and we can also look at the mk4 fronts.

So lets get cracking and lets talk.. We need unmolested unbiased opinions here.

 

[IJ.]

As I said in the other thread Yes the brakes need to be balanced front/rear as a well setup Mk3 chassis puts a LOT of braking on the rear compared to most cars I've tweaked.

 

[Doward]

Just to end this once and for all - here's the results of my calculations way back when I started this endeavour.

Approximate Front / Rear Bias:

Bone Stock 1988 Toyota Supra Turbo:
~71% Front
~29% Rear

Bone Stock 1995 Toyota Supra Turbo:
~68% Front
~32% Rear

Doward BBK on Stock 1988 Toyota Supra:
~71% Front
~29% Rear

Doward BBK with 1995 Toyota Supra Turbo Rear Brakes:
~48.5% Front
~51.5% Rear

Doward Front BBK With Doward Rear 12.6" BBK:
~65% Front
~35% Rear

Nightpager Front BBK with Stock 1988 Rears:
~70% Front
~30% Rear

Well, what do these numbers mean?
Ok, first, IGNORE THE PERCENTAGES FOR A MOMENT.

Second, understand that 75% of 1000 is MUCH LARGER than 75% of 100. If you don't grasp this simple concept, you will not understand what brake biasing is describing.

Brake Bias 101:

First, understand that any braking system has a maximum amount of torque per input in the hydraulic system.

Second, understand that an automotive proportioning system's job is to 'even' the amount of torque being transferred from the wheels to the rotors (aka braking).

Third, understand that locking up the rears before the fronts can lead to incredibly fast accidents. NEVER have the system lock the rears before the fronts on the street!

Ok, here's the meat and potatoes: (if I mess up something, I'm sorry - it's 3:30am here, I'm dead tired, and honestly I'm getting sick of the rampant misinformation, If I screw the pooch on something, apologize to the pooch for me and get it some lube.)

Take a factory setup - like the MA70 Supra. The fronts and rears are capable of fairly even braking. Set the bias for 70/30, and all is happy.

That that same car, and put giant front brakes on it. My kit is designed to KEEP the stock bias - but now, your 70% of the total brakes is MUCH higher than stock, so the fronts lock up well before the rears.

Quick aside:
I did this ON PURPOSE, as I don't want anyone being stupid and locking up the rears before the fronts! Yes, built in a layer of idiot proofing in my brakes. With a WORKING ABS system, you stop SO MUCH FASTER than stock, that I know if the rears even thought of locking up, we'd have a lot of ass backwards MA70s here. Let's face it, many of you are in serious need of driver mod.

Now look at the MKIV setup - we move some of that massive braking to the rear of the vehicle. Oh, and raise your hand if you have a factory NON-ABS MKIV Turbo Supra. None that I am aware of rolled off the line. Toyota wasn't taking a chance with you slamming on the brakes in an area of questionable traction and having that rear end come around on you.

My rear BBK, matched with my front BBK, gives you a much more road-race style of braking. Still firmly toward the front, but with a lot more work going on back there.

I even threw in the MKIV TT rear brakes, as I did the calculations way back when I first built these kits - and Tekdeus actually proved my calculations correct.

There's a hell of a lot more than just percentages going on in the braking system. You've got to look at total braking power, and decide how much of what you have needs to go where. I'm tired, I'm going to freaking bed. It's 4am!

 

[TurboStreetCar]

Just out of curiosity are those percentages based upon piston diameters? or calculations based on rotor size and all? Interesting information.

 

[ToySup88]

I have the Doward fronts and MKIV rears (haven't installed yet though).

What happens to the percentages if you stagger the brake pades - go with Porterfield R4s on the front and R4Ss on the rears?

 

[tacoma_kyle]

Yeah, I have to second that. What are the numbers based off?

 

[Doward]

Total caliper piston area / pad contact area / rotor diameter / pad distance from center of rotor (levered torque generation)

The only thing I did not play with was pad co-efficient. If you use the same pads front and rear, the co-effecient of friction should be damn near identical and can therefore be treated like any constant.

More friction front or rear will move bias in that same respective direction (front or rear). Also, this is UNPROPORTIONED BIAS - I've never opened a MA70 proportioning valve to determine the final bias value.

 

[TurboStreetCar]

Nice i was wondering if the rotor diameter and leverage was taken into account. Good job Doward!

 

[Doward]

Well, I know I certainly wouldn't buy a brake kit built by someone who didn't take those items into account

 

[Zazzn]

So does anyone have the actual piston sizes....
Doward - you are the expert, can you explain why the front is so touchy vs the stock system if it's not the Bias, (as it's equal to stock) then it has to be the pad area, and piston pressure or something else.

I admit I'm no breaking expert, I don't care to learn all the calculations if I don't have to... But since putting on the kit I press 20% of the pedal I'd get 40% of threshold breaking on stock, now if I press 20% I get more like 65% of breaking threshold on the front only while the rears are still only giving say the stock 10-15%. Note I didn't lock up and I don't intend to lock up the wheels to trigger abs... If you road race you are thought to drive and feel the car and maximums so hitting abs is frowned upon.

 

[crisp]

Interesting topic, guys! I can't add any "expertise", but as I mentioned in another post of mine, with Andy's (ARZ) 4-pot Wilwood kit on FRONT ONLY (rear are LAME-O STOCKIES!:cry: ...for now...) I have found that the front braking is stellar, and the rear seams to "push" onto the front pads, working them "harder", especially when the rears get a bit "hot" and start to fade heavily. The Wilwood's STILL do their job, but as the rears FADE, the fronts seam to be "squirming" the rubber against the road, and I KNOW I can LOCK-UP the fronts in this configuration if I stab at them. (I actually WANT to be able to lock them... as I don't care for the ABS mitigation setting in "earlier" than what I would want...)

Very happy with this, but looking to improve the REAR as well, so they don't PUSH on the front end so hard. On a side-note, with the stockie rears being so "weak", they don't lock at all for me, but over-work the fronts, which I'm SURE improving the balance should help. To IJ's note, makes SENSE that the rears DO and SHOULD carry better pads to balance the decel on these cars. I'm going to attribute that to the WEIGHTINESS of this platform, whereas I've known other cars where JUST BBK's in the front and some "drag" on the rear were all you needed. I think the supra NEEDS better rears when the fronts get a fine upgrade... (so I gotta start saving my pennies!)


PS: You DON'T need a HUGE (bigger) master cylinder to make the Wilwoods work, if anyone was wondering...


My uneducated $.02



-crisp

 

[IJ.]

Chris: Ummm I never mentioned pads??

Zazzn: What is it you hope to achieve?

As stated YES to retain braking balance front to rear there needs to be an upgrade in rear brakes, it's just common sense and doesn't need an engineering degree to be understood.

Buy a set of rears end of issue.

ARZ does a rear set Doward is currently developing a set, it sounds like you believe you've been mislead in some way with your purchase?

 

[Doward]

Zazzn, it's piston area. I stated it before, it's simple hydraulics -

1" MC pushing on a single 2.37" piston vs a 1" MC pushing on 2 x 1.59" pistons, over a MUCH greater pad area, with increased leverage over stock (13" vs 11.885")

You've simply got too much front brake IN YOUR non-abs application - I suggest you go back to stock front brakes until the rear developments are complete.

 

[Doward]

Just to put this in perspective -

1997 Ford Cobra:

13.0" Front Brakes with 2x 1.59" pistons
11.6" Rear Brakes with 1x 1.5" piston

That's right, the '94-'04 Cobra uses the same size rear caliper, with a slightly larger rotor than the MA70 platform.

 

[tacoma_kyle]

Cool that you factored in everything. Did you just run some numbers to find proportions with reference to stock?

 

[Doward]

I ran a ton of numbers to come as close to stock as I could, knowing that I'd be upgrading the rears in the future

 

[crisp]

As I said in the other thread Yes the brakes need to be balanced front/rear as a well setup Mk3 chassis puts a LOT of braking on the rear compared to most cars I've tweaked.

Chris: Ummm I never mentioned pads??

My bad. Meant to indicate I agree/"sense" the need for a LOT of braking (as I mis-stated "pads" --> meaning "brake force") on the rear to "balance" and maximize the stopping effectiveness of these cars... mine STOPS/SLOWS okay with the Wilwood's, (surprisingly well!) but would greatly benefit from GOOD rears to be best.

Hope that was a LITTLE clearer!:biglaugh:




-crisp

 

[adampecush]

now if we could just figure out what the bias of the stock proportioning valve was...

 

[IJ.]

now if we could just figure out what the bias of the stock proportioning valve was...

Should be easy enough with a pair of 0>2000 psi gauges.

 

[Zazzn]

Chris: Ummm I never mentioned pads??

Zazzn: What is it you hope to achieve?

As stated YES to retain braking balance front to rear there needs to be an upgrade in rear brakes, it's just common sense and doesn't need an engineering degree to be understood.

Buy a set of rears end of issue.

ARZ does a rear set Doward is currently developing a set, it sounds like you believe you've been mislead in some way with your purchase?

IJ, I'm trying to restore the balance I had when stock... I guess I have to wait like Doward said... I guess it's one of those upgrades with unforeseen consequences. I didn't expect the results of what I got when I got the BBK. The situation is magnified because like 90% of all supra owners out there I run a staggered tire/rim setup.

Basically when I stopped form 200km I found the breaking threshold on the fronts but was sorely under working the back since the fronts where getting squirrel and the back didn't seem to be doing much as indicated by the huge nose dip when I got on the breaks.

Doward do you have a excel worksheet with these calculations in it?

Any chance we can see it?