Brake bias


I’m going to try to write a post which will explain brake bias. Well, at least it will try to. I’ve got some book-learning under my belt, but I’ve never been anywhere near the design side of brake systems.

There’s 4 elements that contribute to our brake bias.

  1. The brake bias valve. The curve of front brake hydraulic pressure to rear has a “knee” at about 355psi. See pic below. The knee is caused by the bias valve. At pressures below 355psi, both front and rear slaves get the same pressure. But above that, the rear only gets about 2/3rd the pressure of the front brakes, call it 66%.

  2. The rear slave is about 1/3rd smaller than front. Smaller piston means smaller surface for hydraulic pressure to put against so less force pushing pad against rotor. So 66% drops to 44%.

  3. Rear rotor is smaller diameter therefore pad has smaller lever arm to act on wheel. Not sure how much smaller, call it 10%. So now we’re at 40%.

  4. Rear pad is smaller than front pad. Not sure how much smaller, call it 10%. So now we’re in the ballpark of 36%. Note that there is some debate about this. Theoretically friction is independent of surface area, but in practice that’s not really true. So I would argue that the surface area bias remains a fair point.

The sum of that is brake pedal pressure results in rear tires only getting about 36% of the braking force of the front tires. Our rear brakes don’t do much stopping. I’d argue that mostly what they do is just keep the rear end from coming around.

Video showing the brake bias valve at work. Bottom line: The video perfectly shows the action of the brake bias valve. You can see the front/rear pressures climb up to about 300, then the front starts climbing faster than the rear.

Details. In 2015 I managed to convince myself that I had a braking problem. It was some braking problems at Sebring that drove this. My stopping distances just seemed way too long. So I put together a hydraulic pressure test rig.

This effort was much harder than I thought it would be. I had to work thru a lot of problems with hoses and fittings, and I had to have a means to bleed the hoses that the gauges were on. All of this took weeks of figuring out what the problem was and then figuring out a solution. But once I had everything working, I could put the test rig into any place in the hydraulic system and test the pressure at that point.

For several weeks I connected up to every place in the brake hydraulics, trying several MC’s, bias valves, and 4 different ABS unit. It took a lot of repetitive testing before I could be pretty sure that what the gauges were telling me was “real” as opposed to something was still screwed up with my testing and I was getting BS #'s.

For prob 6 wks I was up to my elbows in brake fluid almost every night. Every time I did anything I had to bleed the whole system so I got pretty efficient at it.

The video shows the front pressure hitting a peak of about 700psi and then not going higher. The rear pressure continues to build. That totally fit what was happening to me at Sebring so I felt vindicated. But with all of the swapping around, the behavior went away and I was never able to duplicate it. No combo of MC, bias valve, and ABS pump would bring back the Sebring behavior. So I never really figured out what the cause of the problem was.

I tried hard to reproduce the problem in the video and at Sebring in order to ID the component that was causing the reduced brake pressure to the front. But try as I might, I could not reproduce the symptom. I couldn’t get my shit to fail. That forced me to ask the hard question…“just how sure am I that there ever was a real problem?” So by then I was so full of second guessing myself that I just said “screw it. It’s working now and I’m tired of being soaked in brake fluid”.


Related to your video. I would assume this issue could be attributed to an issue with the master cylinder itself.

The only thing missing about your test is a stroke measurement how far you pushed the pedal in. If you had one this would make sense.
(edit: I looked second by second and tried to get the first frame of each second and record it.)


Assuming the pedal travel is constant to create the rear pressure, I extrapolated the front pressure during the rest of the travel using the average of the first few seconds where it didn’t plateau yet.


Considering that the travel of the master cylinder is unknown but continious and the rear pressure is correct, front portion of the master cylinder couldn’t build pressure between in a certain area.

Since cars are still on the road but still aging, I am starting to see more and more of these master cylinder issues related to pedal travel. My theory is that if a car has been used as a Daily driver, then turned into a race car, the master cylinder was used within a certain range for extended time. The moment you go out of that groove, the pedal pressure build up is un-predictable. Since we can’t expect everyone to have always changed their fluid on time, corrosion sets in and pits the surface making it difficult to seal.

Looking into it more: Here is an example I was able to find. It seems vehicles that can’t find a replacement have to deal with this issue more then us, where MCs are still available.


Those graphs are from data you pulled from watching the video and recording #'s each second?


Yes, that was my lunch break.


Dude! That is some serious studly shit. You sir are totally awesome.

If we were in the same region we’d drive everyone totally bat-shit.


Thanks! Just looking up MC’s and there is a Girling and ATE difference. (I have been pushing for a whole ATE system so far.) Just need to make sure what booster I have.


None of that matters. I kinda gave up an chasing after MC ideas. As you look closer you’ll find that things get fuzzy. A bewildering array of MC’s are speced as OEM replacement altho they’ve different designs, people who have not the first idea of fluid mechanics will swear this or that totally impossible assertion, boosters are the same even tho many insist they aren’t. It’s all kinda chaos. I think the percentage of bad info re. braking is higher than any other element of our ride. I found it all kind of demoralizing. How do you discuss an hydraulics issue with someone that doesn’t understand that hydraulic pressure and piston force are different. He and all his buddies are totally wrong but if you push to understand what they’re talking about you just make them mad at you.

Here’s what I think is true…

The MC cylinder front/rear size issue is a red herring. What people think is different front/rear piston sizes is actually a feature called “stepped bore” that prefills the slave in the first inch of pedal travel. It’s just a misunderstanding in interpreting the specs. At least that’s my theory.

Bigger MC’s require more foot pressure which results in more firewall deflection. It’s that deflection that makes our brakes soft. But a smaller MC means more pedal travel and a person could get in trouble there. Go to small and thin pads won’t make it to the rotor. Smaller MC piston means more brake feel because more pedal deflection is required to make the pads move. Of course, not everyone agrees with this, but I think the school that likes big MC’s is wrong.

There’s no difference between a Girling and an ATE booster. I’ve got a new ATE booster I’d be happy to sell you. They’re expensive.

What does make a difference:

The optimum front caliper is an ATE with metal bushings. ATE with rubber bushings allow flex because rubber. Girling calipers flex because they’re not stout enough.

Our flexy firewall makes a difference. That’s what causes our brake pedal softness, assuming we’ve ATE calipers with metal bushings. Push hard on a 911 pedal and it’s like pushing your foot on a boulder. You can calibrate braking by pedal pressure easily enough, but the pedal doesn’t actually move hardly at all once you’re past “take-up”. Our problem is that under hard braking our MC is pushed forward and rotated up. It’s very visible.

If I was going to build a car I’d find a reason to weld some extra sheetmetal on the firewall where the MC attaches. Call it a repair. That will make the firewall more stout so the MC won’t move so much when you press hard on the brakes.


Currenlty, Ate front with brass bushings(Made a huge difference) with only the mid rubber line in the back left in terms of brake lines.

One thing about our brakes I don’t understand is why don’t the pads have steel holder? See picture below.
image My 240sx 1989, has girling style calipers(no abs), and the brake feel is 100% better.

About bracing the master cylinder, speaking legality, washers would be allowed to be added? I pulled the dash out yesterday(for beautification) . I could take some pictures or film something. Because you are correct, pedal force=//= piston force exactly. Been through that with 300zx/ willwood and then stock calipers with my 240sx.


Re. Girling Brakes. I’m not familiar enough with them to understand the issue. All I know is ATE brakes.

Re. Washers. I dunno. IIRC the attachment point of the booster to the firewall has a fair amount of surface area (I’m trying to visualize it as I write this). I’m not sure that some washers are going to significantly spread the load.

One fix would be more boost, so the driver wouldn’t have to press so hard. Are new boosters more “boostie” than old boosters? Maybe the problem is we’re all using 30yr old tired-ass boosters . I do have a new booster stashed away, but I don’t know that I’ve ever done any hot laps in an E30 with a new booster.


Boosters are just large diaphragms I don’t see them do anything other then leak. If they leak, they won’t be as effective. I would rather have a larger master, so I don’t need to press as far. In the best world, the common tuners talk about using e32 brake systems on the web.
IX booster…


Ok I need to stop digging and go to bed.
E32 Brake booster
One more link…
E32 master cylinders- let’s clear up the confusion



I have not yet had the opportunity to participate in SPEC, so my perspective is slightly different, but I hope you will permit me to share what I know.

For reference, I run two enduro e30’s, both using Girling Masters and Girling calipers. I have no trouble with the girling components, and find we can out-break almost every car we run against. We do run ABS.
One car has the stock 1989 BMW Girling Booster and original Girling master.
One car has Porsche 944 Booster (also made by Girling), and a Rebuilt O-Reilly master.
I do not know how this would translate to Toyo’s.

  1. I do not see how a booster could “degrade” over time. It either holds pressure, or it doesnt. Its force of “assist” on the pedal is a factor of its diaphragm surface area and the vacuum pressure behind the diaphragm. Its just “psi” after that.
    When switching to the 944 booster, I was concerned about the change in brake feel between the two cars due to the smaller diameter booster. The stock on is approx 10 inches, and the new one is approx 9 inches. Fortunately, the e46 engine makes more vacuum due to a trick venturi part, so the levels of boost arent as far off as originally anticipated. The area is 64in2 vs 78in2, so say 9psi78in2 = 706lbs. 11psi64in2 = 700lbs

  2. If the booster “holds” pressure, which is easily tested with a tool like a mity-vac, than it should be considered good.
    If it doesnt hold pressure, it is likely to cause lots of problems. I had one 944 booster leak That caused a really tough idle and driveability problem to diagnose. It will also cause tune issues, as its essentially a large vacuum leak.

Ok, on to the original reason I wanted to post.

You can calculate the difference the brakes make front/rear based on known data.

What I don’t know for sure are the following:
Stock E30 “Pedal Ratio” - IE the distances between the pivot pin on the brake pedal and the master clevis, and the distance from the clevis to the center of the brake pedal "pad"
Stock E30 master “stepped bore” and how it affects line pressure. In your post above about stepped bores, you refer to something as a “slave” - is that the caliper that you are referring to as a slave? Is the stepped bore’s work done after that first inch, and then the rest of the travel is all a solid bore? I have never had a stock master (either girling or ate) apart, so I do not know. I believe from internet data that the stock bore is 22.2mm
Brake booster force application- Is it a additive force (aka pedal force + booster force = total force) or is at force multiplier (pedal force * some booster ratio equation = total force) or is it something else?
In my previous equations, I have been working under the impression that the line-pressure at the outlet of the master is around 1000psi, and that is approximately what I see on my data trace with a pressure sensor installed between the master’s “front” outlet and the abs “front” inlet.

What I do know: (and sorry, i converted to all inches in my calculations to stay with PSI)
Front Caliper Piston Diameter = 48mm = 1.89 inches
Front Caliper Piston Area = 2.8 in2
Front Pad Height = 1.76 in
Front Rotor Diameter = 10.24in
"Effective Radius" aka the point at which the braking force acts to stop the car = 4.24in
Effective radius = (Rotor Diameter - 1/2* pad height) /2

The front clamping force can be determined by the line pressures, (system force * caliper area), then reduce that by the friction of the pad (aka “mu”), then that can be converted to a torque with the force @ distance calculation.
With my above numbers, I get
Clamping force = 1000psi*2.8in2= 2800lbs
Mu = .4
Rotor force = 2450lbs
Wheel Torque = 792ftlbs

For the rear,
Rear Caliper Piston Diameter = 33mm = 1.3in
Rear Caliper Piston Area = 1.33in2
Rear Pad Height = 1.58 in
Rear Rotor Diameter = 10.16 in
Effective radius = 4.29 in

Clamping Force, assuming equal pressure (aka no knee valve) = 1326lbs
Rotor Force = 1061 lbs
Wheel Torque = 379 ft lbs

Per your chart above, which is new to me, at 1000psi inlet, the outlet will be about 660 psi.
Clamping Force drops to 875lbs
Rotor Force = 700 lbs
Wheel Torque = 250 ft lbs

Based on that, 250/792 = 31.5% rear bias.


Here is a brake pressure trace for reference.

This is me, braking into Turn1 @ VIR during a race.


That was a good effort . I went thru the #"s and they look good to me, except for neglecting the area of the brake pad itself. Eyeballing the front vs. rear pads, I’d say the rear pad is about 20% smaller. It would be simplistic to assert that brake torque is proportional to pad size, but it’s probably in the ballpark of proportional.

So maybe call it 210/792 or 26.5% rear bias.


Hey Ranger,

Friction does not change based on area of contact.


μ= “mu” is the coefficient of friction, which is a material property affected by both materials
N = Normal Force, which in this case is a property of the fluid pressure.

The reason behind this is because as the surface area increases, the distribution of force also increases, so each little “zone” has less pressure.



Understood, but I don’t think I said "friction is prop to area of contact. I said “brake torque” and I also threw in a sly caveat saying “in the ballpark of prop.”

Anyone that took HS physics understand that, theoretically, the force of friction is independent of surface area. But, as you probably know, theoretical friction is just an approximation. The real force of friction has to be figured out empirically.

I bet that if you cut off all but 5% of your brake pad area at all 4 corners, you’ll be down on brake torque. Among other things, that itty bit of pad is going to immediately overheat.