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Brake bias on OEM cars

Hello folks,
Quick question about how OEM brake bias is set up on cars. I understand that old cars used to have the ‘proportioning valve’ that would help control the HYDRAULIC brake bias (which nowadays is replaced by EBD), but I would like to know the actual brake bias as in how much more front bias would a large 6 piston caliper have over a small 4 or 2 piston caliper. I understand that brake bias hydraulically generally stays the same (unless EBD/ABS is enabled, meaning 50/50 until a wheel locks up) but what I’m asking is basically how would you calculate a car’s true brake bias using caliper size and piston width, etc?


Why would you worry about this ? And what would you do with that information anyway ? Don’t you think the people who designed the vehicles would put the braking system at the best for most conditions.

Brake bias requirements are a rather significant calculation based on the car’s geometry and center of gravity height and front rear location. No car is 50/50, none. Not even a 911 which has the engine behind the rear axle.

Now the commanded bias these days, applied by the master cylinder, is designed to balance the car for light to medium braking. Under heavy braking the ABS is used to prevent the rear brakes from ever braking harder than the fronts. That is a federal requirement. No prop valve is needed.

The mechanical bias, the 3rd part of this, is the pressure area and type of calipers and the diameter of the rotors front to rear.

This probably creates more questions for you than it answers. There are books on the subject. One written by a friend of mine named James Walker Jr. Google it if you want a deep dive.

Basically you can’t calculate brake bias only with caliper features and dimensions.

Edit: The book is available on Amazon;


This is Car Talk. OP is talking about cars. Why are you upset about this? OP gives no indication that they are “worried” about this issue. Maybe they’re just curious and thought they’d ask a question of people who might be able to answer. What’s wrong with that?


Are you planning on performing some modifications to your car’s brake system?


Actually I’ve never seen a vehicle with 50/50 brakes. Front rotors and calipers are usually larger on the front of FWD vehicles while RWD trucks have a wide bias range. And early proportioning valves merely delayed the front brakes being actuated until a fixed threshold of pressure was met on the rear or pedal travel exceeded a pre set limit.

I recall Dodge incorporating a rear proportioning valve that was proportional depending on the loaded weight determined by a lever connected to the rear axle and the frame.

A critical but inconspicuous component in brake bias is the relative diameter of master cylinder bores and wheel/caliper piston diameter and count. A great deal of bias mathematics are involved with those relationships alone.

Doesn’t matter if it’s FWD or RWD…when you brake the vehicles weight shifts toward the front of the vehicle…thus the need for larger brakes.

Absolutely true!

Even in rear engine cars with significantly weight more static weight on the rear wheels! Porsche 911 has fantastic braking capability… but the front rotors are still larger because roughly 55-60% of the braking is done by the front wheels at maximum braking. The even loading of the tires makes for VERY short stopping distances.

A FWD car, with its heavy front weight bias, can easily get to 90% of the braking on the front axle since so much shifts forward under max braking. The FWD car will never beat the stopping distance of an equal weight and tire sized rear or mid engine car for stopping distances because the front tires have far more work to do…

All because tires aren’t linear. Increase the load on the tire by 20% and you’ll only get about 18% more grip. Increase it 40% and you’ll only get about 32% more grip.

Well for sure @Mustangmanand @MikeInNH are correct but then beyond family sedans and high performance sports cars there is a world of vehicles with their own particularities. I once had a customer with about a dozen “roach coaches” which were at the limits of their designed load with the heating and cooling systems and well beyond once the food and drinks were tossed on. I won’t bore anyone with the details but getting the trucks brakes up to the job was a learning experience for me and I feel sure a great many shops have dealt with problems as difficult as I faced then and worse.

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Pardon me for replying so late, I forgot to check up on this thread… whoops.

I understand that no car’s physical brake bias is 50/50, I believe what I would like to solidify now is what the hydraulic pressure is like on a ABS/EBD vehicle, whether or not its 50/50 or the brake pressure is biased toward the front. I’ve done some research on master cylinders, and from what I have read, they apply the same pressure to the back and front. Would this mean under normal braking, the hydraulic brake bias is 50/50 until a wheel locks up and the EBD/ABS intervenes? The only instance I’ve seen of a master cylinder affecting hydraulic brake bias is when it is a stepped bore tandem master cylinder, something most road cars don’t have. Again, beg my pardon for replying so late!

Then pressure applied front to rear is the same but the braking force is not the same. The braking force depends on the caliper piston diameter, the brske rotor effective radius and the friction coefficient of the brake pad. Big, multi piston front calipers on big disks provide much more braking torque than smaller piston rear calipers on small disks so the brakes are front biased.

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Alright, thank you for the clarification (literally a month later, lol)

In addition, some systems used a ‘brake proportioning valve’ to modify the pressure balance between front and rear. I don’t know if that (or the modern equivalent of it) is still used.

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From what I understand the brake proportioning valve is dead and has been dead since the invention of ABS, now under heavy braking the ABS/EBD module adjusts front to rear bias under heavy braking.

That is correct but there is a fail safe pressure relief valves embedded in the ABS to prevent rear bias if the ABS fails.

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As an example… My 4runner had very large 4 piston calipers in the front. The rear calipers and pads were half the size of the fronts and the caliper only had one piston.

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The auto shows I watch don’t seem to care about those kind of things. They just buy stuff and try to make it fit. I’d love to be able to answer that question but the star gate is open and will shut down in twenty minutes. Gotta go.

Yeah, I would watch those cars shows, what they did not show you was the twenty retakes before they demonstrate how easy it is.
One show, I think it was ‘Shade Tree Mechanics’ did show the problems, but not the cost.
Likewise I watch a boat improvement show, they showed all the work and problems, but no mention of the cost, epoxy resin and fiberglass is not cheap.

My 2006 and 2007 vehicles have both ABS and a proportioning valve. The purpose of the proportioning valve is not to reduce pressure to the rear brakes but to prevent the pressure from building up too fast.

With smaller caliper volume back there, the pressure would build up much more quickly than in the fronts and could override the ABS or at least lock the rear wheels before the ABS has time to react.

My 2014 does not have a proportioning valve as far as I can tell. The ABS block is considerably larger than on the older vehicles and it may have a proportioning valve built in, but I don’t know if that is true. The brake lines go from the MC and then one line per wheel from the ABS. The 2014 also has traction control that the older vehicles do not have.

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