Bm57 Brake Bias / Proportioning Valve

[_Wing_]

so i'm looking at brake bias and what is available in the BM57 range. anybody got their GTR r33 and r34 service manuals handy- it'll save me and anybody else that is looking for info from downloading the two FSM's for one line of info.

Sooooooooo... can you goto the BRAKE section, at the end there is this section:

SERVICE DATA AND SPECIFICATIONS (SDS)

General Specifications

there, it will state the split point, can you paste up what the details are there?

for anybody interested i've looked up the Z32 TT and it is:

z32 tt: split point = 3,492kPa (498psi) x 0.4

[djr81]

Not that this is what you are after but the corresponding figure for the R32 Gt-R with the 1" bore M/C is 1961kPa (284psi) * 0.4.

[_Wing_]

s15 = bm50 = 2,942kPa (427psi) x 0.2

[_Wing_]

anybody wanna add more BMC sizes/splitpoints, we can use this as the BMC reference guide

[Legionnaire]

Epic thread bump, I know, but I'm doing brake upgrade currently, was educating myself about factory proportioning and found this great thread... not so much info though. Info on S13, S14, S15, Z32 is extremely easy to find, but I couldn't find any info whatsoever concerning R-chassis except the info posted by djr81. So any info about proportioning on R32, 33, 34 is appreciated, BM44, BM50, BM57. Please specify if the m/cyl is intended for ABS or not.

[KiwiRS4T]

Got no data but I put a BM57 from a GTR in my Stagea and used a Z32 TT repair kit to recondition it and all the bits looked the same. (Currently have 8 pot D2s in the front and GTR 2 pot rears).

[Legionnaire]

Thanks for info, what MY GTR is your m/cyl off of?

As far as I know reconditioning doesn't involve prop. valve rebuilding, 'cause there's basically nothing to rebuild. Proportioning used in 300ZX BMC's is very different to skylines though, they're more rear biased. E.g. 1990 Australian and European models, both with 15/16" and 1 1/16" BMC have 35kgf (498psi) x 0.4 bias, 94-96 models which had 1" BMC have it reduced to 25kgf (356psi) x 0.4, which is still higher than R32 skyline.

[KiwiRS4T]

I believe it was an R33 GTR - just used the m/c kept the Stagea booster. I'm not knowledgeable in this area - I presume the proportioning is elsewhere other than in the m/c?

[_Wing_]

Unfortunately the prop valve is built into the master cylinder

[Legionnaire]

Yep, I'll post a couple of pics later.

Hey _Wing_, did you find any additional info on your initial question, or you found a workaround?

[badhairdave]

The work around is a patrol/navara BMC with external prop valve.

It can be engineered in a road car provided the prop valve is mounted in the engine bay and inaccessible/locked once setup.

Not really the information you are after but it is a solution.

Edited March 5, 2012 by badhairdave

[Legionnaire]

Thanks for the tip, I agree, it is a solution, however all external prop valves that I'm aware of only Tilton publish thier reduction ratio - it's 0.33 (67% reduction); Wilwood is not so clear, they state "Maximum reduction of 57%", but don't clarify whether this value is constant of not, don't state knee point adjustment range and have no graphs to refer to. I might contact their tech department to find out.

Factory Nissan in-built prop valves on Z-, R-, and S-chassis (and many others) almost always use 0.4 ratio (60% reduction), except for S15, which uses much higher 80% reduction, but very different knee points for different chassis, with Silvias having the highest knee at 40kgf, and GTR32 having the lowest at 20kgf. Unfortunately I couldn't find full FSM for R33 and R34 in english (or any other language for that matter) that would contain such info, and driving an R34 I'm a bit concerned as to whether they used reduction ratio similar to S15 or kept original 60% reduction.

Porsches of different generations use at least 5 different prop valves, they are external to BMC and ABS, non-adjustable in-line type, all have 0.46 reduction ratio, have 5 different knee points (18, 33, 45, 55, 60 bar) and cost very reasonably. I might try one of those.

The idea behind the question is to figure out what brake bias is considered optimal by Nissan themselves, make some calculations and tables/graphs and to try to work from there.

[badhairdave]

my understanding is that all after market adjustable valves are adjusting the knee point.

So 1:1 pre knee and then fixed ratio post knee. by moving the knee point you effectively reduce the rear pressure.

Seems to work.

I'm positive i have seen a wilwood graph but buggered if i can find one now.

Generic one:

Edited March 6, 2012 by badhairdave

[Legionnaire]

Exactly, knee point is adjustable in a certain range, but reduction ratio after that point (steepness of slope on your graph) is fixed - in this case it's 66.6%, or to be more precise, 1:3.

This is Tilton graph by the way, with 1:3 reduction, or __x0.33 in Nissan terms.

The following info is for those who are interested in knowing how proportioning actually works and why it's needed, or can't grasp at all what we're on about here.

Car has to have its braking front biased, mainly because of weight transfer during deceleration. The weight gets transferred to the front, forcing tyres against the road and allowing more deceleration to be realized through increased grip. That's why front brakes are often stated to do most of the braking job, which is true up to a point. Of course this weight doesn't come from nowhere, it's removed from the rear axle, so rear tyres have lower grip under brakes and therefore less potential for stopping the car. Those are the basics, and are described very nicely on stoptech website.

How much weight is transferred exactly - that depends on a lot of things, car weight and CoG, suspension stiffness, tyre grip, etc. Car manufacturers know all these parameters and spend thousands of hours test driving their cars and optimizing brake bias. They draw several graphs in the process, e.g. for a fully loaded car and for a car with only driver inside. One of the good examples of such graphs is attached, pic no. 1. It's a great pic and it's very useful for further explaination. Notice that there is a range of desired f/r brake pressure ratios depending on car load. OEMs often target the lower curve, as it implies more front bias (more safety and stability), and try to tune factory fitted prop valves so that they closely approximate the lower curve.

How they tune brake bias exactly? Most modern cars, and almost all nissans manufactured after ~2002 are equipped with electronic brake force distribution, and therefore have no mechanical bias valves, they rely on electronics to get the job done. It's actually a good thing, because they can follow ideal curves much closer and can compensate for more input factors and external conditions. That's also why modern ABS and EBD usually work together and are controlled by a single ECU.

Our cars are much simpler than that though, so we have mechanical bias valves, which is much worse that EBD, but is still much better than no proportioning at all.

Most of mechanical prop valves have a drilled piston and a spring inside, see attached pix. On pics no. 2 and 3 the piston is black, on the fourth it's gold in color.

Brake fluid flow is not restricted when the piston is floating freely on the spring. But when line pressure is high enough, it overcomes spring resistance and forces piston against its seat. From this point on fluid travels only though the hole in the piston, so pressure after (downstream of) the piston rises slower than in front of it. Pressure at which piston shuts its circuit is usually called "knee point". Spring stiffness determines knee point (at which it changes rate of pressure increase), while hole diameter determines reduction ratio after knee point (determines rate of pressure increase).

This method of pressure reduction gives us only two straight lines to work with, and two straights are actually a pretty poor way to approximate a curve, but it's still approximates it better than only one straight line

Now to the numbers. Nissan FSMs usually have info about proportioning, often in BR section, subsection Service Data and Specifications. They look like a pair of numbers. First number is knee point in kPa (psi, kgf/sq.cm), second number is reduction ratio, or reduction multiplier, which is applied above knee pressure.

The calculations that determine line pressures after knee point are pretty simple and are applicable to any prop valve that uses same reduction principle. Let's take BNR32 as an example. It has 20 kgf/sq.cm or 284 psi x 0.4 prop valve. This means that up to 20 kg's line pressures in front and rear circuits are identical. Above that point rear line pressure rises at lower rate than front pressure, rear pressure rise is 0.4 times front pressure.

So if line pressure is 10 kg, then front pressure is 10 kg, rear is also 10 kg; if line presure is 40 kg, front circuit receives full 40 kg, rear pressure is [40-20] x 0.4 + 20 = 28 kg. All the pressure above knee point, 20 kg in this example, is reduced by some factor, 0.4 in this case.

Adjustable prop valves allow knee point adjustment, but reduction ratio remains unchanged. Some of the valves are continuously variable inside certain range, others have preset points as can be seen on the graph in badhairdave's post.

The problem with Tilton valve, which has 0.33 reduction ratio, is that if I adjust it to have factory knee point of 20 kg, I might end up with lower than factory rear pressure everywhere, so I'll either need larger rear brakes (pads with higher CoF, bigger pistons, bigger rotors), or softer suspension, or accept underperforming rear brakes and increased stopping distances. If I increase knee pressure to, say, 28 kg, I may get rear lock-up at light braking because of increased rear bias at low pressures, but still underperforming rears at heavy braking with line pressures above 95kg.

With higher than desirable reduction ratio the situation is reversed - underperforming rears at light stops and tail-happy car at heavy braking if knee point has been chosen incorrectly.

I know of readily available valves with several different ratios, but I would like to know factory ratio as I want to calculate bias vs. line pressure curve that is considered optimal for the car by Nissan, and want to engineer my future brake upgrade from there.

[KiwiRS4T]

Hmm not sure I understand all that but here's another question - I read somewhere else about someone testing their brakes with a brake bias meter of some sort - can a 4wd dyno give you this information? And is brake bias on our cars a constant regardless of speed, braking effort etc ( I have seen some cars have a proportioning valve which changes the bias according to the weight in the rear of the vehicle).

[Legionnaire]

A convenient way to measure brake bias would be as simple as a pair of manometers, one connected to the front and other to the rear brake circuit. This method won't tell you anything about brake torque though, so if you're after torque, then that would have to be some kind of dyno.

Bias on our cars, even with valve described above, is not constant, that's the idea behind proportioning. The harder you brake, the more the f/r pressure inequality and more front bias you get. To confirm this you can do several easy calculations.

Let's once again look at BNR32 prop valve, 20x0.4. Pressure in kgf/sq.cm, rear pressure is calculated as per formula above, [line pressure - knee pressure] * ratio + knee pressure

front pressure___rear pressure__bias front/rear

___10__________10__________50/50

___20__________20__________50/50

___30__________24__________56/44

___40__________28__________59/41

___50__________32__________61/39

___60__________36__________62.5/37.5

___70__________40__________64/36

___80__________44__________64.5/35.5

___90__________48__________65.2/34.8

__100__________52__________65.8/34.2

...

__150__________72__________67.6/32.4

...

and so on.

Some cars have proportioning valve that changes knee point according to weight, so they get different bias for different weight distribution, yes, but reduction ratio remains constant. Nissan 4x4's have variable knee and 0.18 reduction if I'm not mistaken.

Edited March 7, 2012 by Legionnaire