Why....?, Brake pressure regulator Options

[ClayPerrine]

I will say this....

I have Boxster brakes on my car. I took the proportioning valve off and replaced it with a T fitting. The Boxster brakes are already balanced for a mid engine car, and that valve is just unnecessary with the Boxster brakes.


Clay

[brant]

I highly recommend getting 2 friends in a parking lot... one on each side of the car.
drive through at a reasonable speed and lock up your brakes...
have your friends determine if the front or the rear are locking first in a full on brake situation....

this is a good first step to tuning a proportioning valve, and a very good test for a T fitting too.

[Superhawk996]

I will say this....

I have Boxster brakes on my car. I took the proportioning valve off and replaced it with a T fitting. The Boxster brakes are already balanced for a mid engine car, and that valve is just unnecessary with the Boxster brakes.


Clay

There is no longer a separate proportioning valve in modern cars. The functionality is now handled by an algorithm called Electronic Brakeforce Distribution (EBD) within the ABS control module that montitors wheel speed and and has a proportioning function built into the controls to limit rear brake pressures even in the event of wheel speed sensor failures.

Boxter brake system would not pass FMVSS regulations without this EBD functionality or a proportioning valve . . which you've removed. To assume that the Boxster brake hardware alone as-sized for a Boxter is sufficient to ensure balance and prevent rear wheel lock is a false premise.

It's nice that your happy, but, I can assure you that there are braking scenarios that can put you and others at risk of having a rear wheel lock event. OEM's don't give away parts that aren't needed.

Please don't take this as a personal attack. It's not. It's just that the advice to replace the proportioning valve with a T is not based in solid autmotive engineering and brake system design principles.

Just want others to have the full story of what has been removed from the proposed system.

Note: Early Boxter was a bit more convoluted with both a proportioning valve and ABS module.

[Superhawk996]

I highly recommend getting 2 friends in a parking lot... one on each side of the car.
drive through at a reasonable speed and lock up your brakes...
have your friends determine if the front or the rear are locking first in a full on brake situation....

this is a good first step to tuning a proportioning valve, and a very good test for a T fitting too.

(IMG:style_emoticons/default/agree.gif) . . . but that is only a good start.


Don't forget to do that on split mu surfaces so bring a water truck.

And do it with various load conditions like front trunk fully loaded w/o passenger and still having vehicle at GVW. Front trunk fully loaded increases brake capacity on front axle and simultaneously reduces load on the rear tires making rear wheel lock more likely.

Please include some study of brake pad temps (cold vs. hot)

Let's not forget various speeds up to and including highway speed. Please do on race track or other closed course.

And then you start to get some sense of all the scenarios went into determination of what the appropriate bias point is for a produciton car.

If anyone wants to look at the full test sequence for foundation brakes and proprotioning valves. It is contined within FMVSS 135:
https://www.nhtsa.gov/sites/nhtsa.gov/files...-135-01_tag.pdf

Since 2012 ESC has beome mandated equipment and brings along it's own set of test procedures under FMVSS 126. FMVSS 126 contains pass/fail criteria to categorize brake controls systems performance under more dynamic and more challenging conditions than FMVSS 135.

Here is the link to FMVSS 126.
https://www.nhtsa.gov/sites/nhtsa.gov/files...-126-02_tag.pdf

[brant]

I agree.. its just a start.

I tune my proportioning valve on the race car each and every weekend.

depending on the temps... the weather.. for sure if its damp... I fine tune my valve on the fly during a race. it has to be close to work like this... and fine tuning only

and in this car I know my fuel weight, no cargo.. and very few variables. yet still have to fine tune the car to minimize lock up and get the best ratio...

[914_teener]

I will say this....

I have Boxster brakes on my car. I took the proportioning valve off and replaced it with a T fitting. The Boxster brakes are already balanced for a mid engine car, and that valve is just unnecessary with the Boxster brakes.


Clay

There is no longer a separate proportioning valve in modern cars. The functionality is now handled by an algorithm called Electronic Brakeforce Distribution (EBD) within the ABS control module that montitors wheel speed and and has a proportioning function built into the controls to limit rear brake pressures even in the event of wheel speed sensor failures.

Boxter brake system would not pass FMVSS regulations without this EBD functionality or a proportioning valve . . which you've removed. To assume that the Boxster brake hardware alone as-sized for a Boxter is sufficient to ensure balance and prevent rear wheel lock is a false premise.

It's nice that your happy, but, I can assure you that there are braking scenarios that can put you and others at risk of having a rear wheel lock event. OEM's don't give away parts that aren't needed.

Please don't take this as a personal attack. It's not. It's just that the advice to replace the proportioning valve with a T is not based in solid autmotive engineering and brake system design principles.

Just want others to have the full story of what has been removed from the proposed system.

Note: Early Boxter was a bit more convoluted with both a proportioning valve and ABS module.

Who said Clay was happy? I delibertly avoided a post after that.

The stock 914 brakes and that valve were purposefully engineered for a reason.

This has been semantically argued before here. (IMG:style_emoticons/default/rolleyes.gif)

[ClayPerrine]

I will say this....

I have Boxster brakes on my car. I took the proportioning valve off and replaced it with a T fitting. The Boxster brakes are already balanced for a mid engine car, and that valve is just unnecessary with the Boxster brakes.


Clay

There is no longer a separate proportioning valve in modern cars. The functionality is now handled by an algorithm called Electronic Brakeforce Distribution (EBD) within the ABS control module that montitors wheel speed and and has a proportioning function built into the controls to limit rear brake pressures even in the event of wheel speed sensor failures.

Boxter brake system would not pass FMVSS regulations without this EBD functionality or a proportioning valve . . which you've removed. To assume that the Boxster brake hardware alone as-sized for a Boxter is sufficient to ensure balance and prevent rear wheel lock is a false premise.

It's nice that your happy, but, I can assure you that there are braking scenarios that can put you and others at risk of having a rear wheel lock event. OEM's don't give away parts that aren't needed.

Please don't take this as a personal attack. It's not. It's just that the advice to replace the proportioning valve with a T is not based in solid autmotive engineering and brake system design principles.

Just want others to have the full story of what has been removed from the proposed system.

Note: Early Boxter was a bit more convoluted with both a proportioning valve and ABS module.

Who said Clay was happy? I delibertly avoided a post after that.

The stock 914 brakes and that valve were purposefully engineered for a reason.

This has been semantically argued before here. (IMG:style_emoticons/default/rolleyes.gif)

The Boxster brakes are undersized for my 4.0L engine. I have a set of 991 calipers to go on the front. That will change the brake bias. I also have an aftermarked manual proportioning valve to go inline on the rear. Because now I am mixing brake sizes, and it will need to be adjusted.

Clay

[914e]

Am I reading the graph correctly a six knees at the lower pressure than a 4? IT seems like it should be the other way around. Since my car will have the same weight as a six I think I should use that setting.

@914e

/6 had 38mm rear caliper pistons on a rotor that was roughtly same OD as the /4 (actually 4mm bigger OD). /4 piston size is 33mm.

/6 therefore makes more brake torque per unit of line pressure input. Therefore the knee point is set lower for the /6.

Yes the engine is a bit heavier on a /6 and that would potentially allow a little more rear brake bias than a /4 but I'm sure it is offset by the larger piston diameter which develops 33% more brake torque (assuming equivalent pad friction).

The main advantage of the larger rear brake is earlier in the stop (toward initial pedal apply). Before weight transfer has fully occurred, the heavier /6 engine, and the 33% more effective rear brakes net you better stopping power via the bigger rear brakes. By the time weight transfer has begun, now you need to limit the more effective rear brakes to even more than a /4 to prevent rear caliper lockup.

If you are at /6 weight but running /4 brakes, I'd stay with the /4 set point.

Having a little more weight on the rear with the /4 brakes and the /4 set point is the more "fail-safe" combination that is less likely to lock up but still sends enough pressure to the rear to not lose rear brake effectiveness like you would with /4 brakes and the /6 set point.

Having said all that. I have no idea where your batteries are, Cg location, etc. What you don't want is a high Cg and/or batteries in the front. If this is the case, you probably would want to think about using the /6 set point. More than anything, those sort of major weight bias and Cg changes would ideally want some real design work done to figure out what the right brake set up is.

@superhawk996
Thanks, I was not aware of difference in piston size now the pressure level make more sense.

The rear batteries will be on both sides of the motor aout where the cylinders would normally be, sitting about 1/4" above stock 4 engine crossmember. If switch to a straight shift linkage and fabricate a new cross bar, I might be able to mount them an inch lower. Each pack enclosed will be around 116 pounds, a third pack will mount across the top of the other two.

Where it gets challenging, is introducing braking regen into the mix. I think setting to the 6 pressure and adding back enough regen to bring it back to 4 level might just work out. I should buy a pressure gauge.

[Superhawk996]

Where it gets challenging, is introducing braking regen into the mix. I think setting to the 6 pressure and adding back enough regen to bring it back to 4 level might just work out. I should buy a pressure gauge.

@914e

IHMO, you can skip the line pressure gauge.

1) Adding line pressure sensors and especially a mechanical gauge screws with the brake pedal feel.

2) It won't help you much unless you were to use it to do a mapping of brake line pressure vs. tractive effort. To get the tractive effort would require wheel force transducers. Wheel force transcucers cost upwards of $100K and then you still need a data acquistion system to feed the wheel sensor inputs into. With these transducers, you could also do a mapping of tractive effort vs. regen demand to understand where it makes the most sense to put the regen to hydraulic brake transition. Cool but not really an option (IMG:style_emoticons/default/sad.gif)

Here's link to wheel force transducers if anyone is curious of what they are, what they look like, or what they do.
https://www.michsci.com/products/transducer...ce-transducers/

Tuning EV's and/or hybrids to get a smooth transitions between regen and mechanical braking has been a historical sore spot for the industry. It's gotten a lot better in the last decade but many of the early hybrids and EV's were really bad with a noticeable transition. And that was with full control over the regen AND the ability to tune the ABS module for that handoff.

Unless you have access to better equipment and instrumentation than I think you probably have, it's going to come down to seat of the pants tuning. The good news is that the transition problem was most noticable on at low speed (like 5-10 mph).

Unless you're trying to milk every last mile out of regen (you might be given your range), you could use regen harder initially (early in stop) and then just back it out completely at low speeds. Alternately, tie your regen to deceleration via accelerometer input and don't regen if you are doing a panic stop above some threshold (say 0.6G).

Either way, those are only brainstorming ideas to address a really tough problem that still isn't perfect in production vehicles.

Since you're in AZ, snow creating really low mu conditions isn't so much of an issue. What ever you settle on in the dry, I'd try in the wet just to make sure that regen isn't creating instability when road coefficient of friction is reduced.

You've got your work cut out for you but it is a cool engineering challenge! (IMG:style_emoticons/default/smilie_pokal.gif)

[ClayPerrine]

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[Superhawk996]

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(IMG:style_emoticons/default/av-943.gif) -- I'll have to own that.

[ClayPerrine]

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(IMG:style_emoticons/default/av-943.gif) -- I'll have to own that.

I won't argue with you. (IMG:style_emoticons/default/poke.gif)


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