Brake Proportioning Valve Basics Theory of Operation

[MrCreosote]

I was under the impression that a BPV made the rear brake pressure proportional to front brake pressure and that the adjusting valve/spring determined this ratio.

WRONG:

The BPV has a fixed ratio and the adjuster/spring sets the transition pressure.

NOTE: one barely legible cutaway of a BPV revealed there are 2 bore diameters - I believe the arear ratio of those diameters establishes the proportion.)

Here is a test of a Willwood Variable BPV and here are the results:

So far, I haven't come close to finding a discussion and illustration of a BPV operation.

https://wilhelmraceworks.com/blog/brake-proportioning-valve

47005






Some Thought Doodles: (don't read following - no conclusion. If I figure it out, I'll unshadow it)

I know how you apply boost to the atmospheric side of the fuel pressure regulator to maintain FP ahead of BP in a blow through carb. The regulator spring adjusts the offset.

So starting with a traditional regulator, the spring setting would determine the max rear brake pressure - a zero slope after the transition.
So to get some "after transition slope" have to apply LESS "spring force."

Regulator Basics:

Spring pushes valve closed
Pout on side of diaphragm that closes valve (Pout gets big enough, balances spring, closes valve)



Consider the dual bore piston:

There will be a pressure ratio between the two piston faces that produces no movement.
This could be the mechanism which supplements the regulator spring - when the PR is achieved, there is only spring force on the diaphragm.
Which side pressures applied?: As more brake pedal applied, higher pressures, and greater "spring pressure"


The above would count for the parts in the BPV cutaway: dual bore piston, spring, and a valve.

PLAY WITH SOME NUMBERS:

@ TRANSITION:

120 psi on both sides of piston producing Ft resultant on dual bore piston. (BIG BORE dominates)
240in and 180out psi

[GTS Dean]

What I infer from the graph is that the adjustable valve, when set to a low crack pressure, will begin to operate the rear brakes earlier. This would be useful for getting some initial bite from the rear, as well as allowing more trail braking. The more you preload the spring, the less variance you have in the pressure split. It is a useful tool to be able to tailor the system performance if you can't optimize your piston area ratios very easily.

[MrCreosote]

What I infer from the graph is that the adjustable valve, when set to a low crack pressure, will begin to operate the rear brakes earlier. This would be useful for getting some initial bite from the rear, as well as allowing more trail braking. The more you preload the spring, the less variance you have in the pressure split. It is a useful tool to be able to tailor the system performance if you can't optimize your piston area ratios very easily.

What is interesting is that it seems all the Tilton and Willwood BPV's have the same proportion or slope after the crack pressure. I would have though a person would want to vary that slope.

What really opened my eyes was playing with those brake balance calculators, was the effect of CG and weight transfer. The more weight transfer, the less rear braking you want.

With that in mind, the balances are worked out for maximum braking which is not surprising, but when driving on ice, you end up with a ton of unused rear braking capacity since there is no weight transfer.

Further for all braking below maximum, you are not utilizing the rear braking capacity, putting the majority of the braking on the fronts.

Obviously, if racing, if you change your tires or go from dry to wet, you definitely have to adjust your brake bias - at the point of max weight transfer i.e., maximum braking.

I know ABS on ice is phenomenal - but I'm not sure just exactly how it works when you apply more and more pedal pressure up to and after the front ABS intervention - what happens to the rears? I see 3 possible scenarios


Nothing - once ABS intervenes the state remains constant.
More pressure is applied to rears until they lock and ABS intervenes.
BEFORE the ABS intervenes on the fronts, the ABS begins to add more braking until all 4 wheels lock at the same time and ABS intervenes.

Another issue is what happens when on ice, ABS intervening and you transition to dry pavement? (I always experimented with ABS on ice but haven't had an ABS car now for over 10 years.)

I was wondering what F1 does. Do they ever partially brake? If YES, do they have some "fly by wire" braking system that balances both brakes under all conditions? If there is any partial braking, they would increase front brake life. Of course is brake life is not an issue such sophistication would be unnecessary.

However, how would a car handle if under partial braking you had the same braking force in all 4 corners? OR if the braking force was proportional to the corner weights? Given the extreme driving skills of F1 drivers, I would think there might be an advantage in such balancing.

FYI, on ice, I learned a long time ago you can get more braking using your emergency brake in addition to the pedal. At the time I did not understand why this was so. Now I do.

Fascinating subject.

EDI: Oh, and then there is Down Force.

[ellowviper]

I think on my 02 Dakota 4x4 the reason it only has rear antilock is that there are too many brake bias variables to have full 4-wheel antilock (towing, unloaded bed, loaded bed, loaded towing, uphill/downhill, rain-sleet, snow-ice)...so they just took the easy way out and said " screw it...let the fronts do what the fronts will do and we can just modulate the rears."

[MrCreosote]

I think on my 02 Dakota 4x4 the reason it only has rear antilock is that there are too many brake bias variables to have full 4-wheel antilock (towing, unloaded bed, loaded bed, loaded towing, uphill/downhill, rain-sleet, snow-ice)...so they just took the easy way out and said " screw it...let the fronts do what the fronts will do and we can just modulate the rears."

That doesn't sound too good for ice. I marveled how I could go down an icy hill and be able to steer while on the ABS. If your truck has no front ABS, you will have to be skilled to get max braking and steering too.

I did a little F1 searching and basically the fronts are Old School while the rears are Fly-By-Wire. The rears involve the hybrid system and the controller has to decide how much hybrid braking to include.

I was thinking that would be necessary to get the full stopping capacity of the brakes (all 4 lockup same time) which would be a valuable as horsepower. What is interesting is that while a driver could adjust proportioning periodically for fuel load, he cannot for downforce. So I suspect there's probably an AI that is monitoring everything and continuously adjusting brake balance.