[help needed] How does ABS work?

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AndreyB
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How does ABS work?

Post by AndreyB »

More specifically, what is the sequence of ABS pressure modulation valve actions?

Which way and when if anything is moving?
Image

Is the motor constantly spinning?

I would like a bit more details, deeper than this video



I suspect that if ABS wants to intervene in braking it lowers pressure in a line for a short period of time, is it then increasing the pressure a moment later? Are those kicks in the pedal when ABS builds pressure moments after lowering it?
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Re: How does ABS work?

Post by kb1gtt »

Couple of things. Do you know the friction curve? AKA you can get more newtons if you hang out in static friction just before switching to kinetic friction. See below graphic. If you lock up, then release the pressure until you start to roll again, you hang out in this area with maximum braking. As well by keeping the tire spinning, you gain steering control. So basic ABS is that if the wheels are doing more than say 5 MPH, and if a tire locks up, then release pressure to allow the wheel to spin again.

When the pressure is released, your foot will go down, as your foot is the source of the pressure. However if you do this for a prolonged period of time, your foot will hit the floor, and you will loose braking capabilities. To account for this issue, the ABS motor will kick in and replace the fluid that was released by the ABS module. If you find some ice, or sand, you can test this. You'll see your foot goes down, then comes back up after a brief fraction of a second.

Also ABS can be done with an analog system, instead of digital. These are successfully used on air planes every day. However this kind of system causes truck drivers, etc to rely on the brakes. The digital systems are used to scare you, such that you do not rely on them.

As for why you appear to have 3 valves per brake pad. I'm not sure. I know there is a cross over valve, such that if the front left brake line breaks, this valve will engage and block flow to the front left, as well as the rear right. Basically it's usually a metal ball in the brake lines. If you have to much flow out one line, this ball moves to block that loss of fluid, and allows fluid to the other 2 brakes in diagonal. They do the diagonal as it helps prevent your car from spinning out of control.
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AndreyB
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Re: How does ABS work?

Post by AndreyB »

Which way and when if anything is moving?

Is the motor constantly spinning?
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Re: How does ABS work?

Post by kb1gtt »

I believe the motor starts when ABS activates. However I could be wrong. It might spin all the time to ensure safety.
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Re: How does ABS work?

Post by nighthunter »

kb1gtt wrote:
Sun Dec 16, 2018 4:29 am
As for why you appear to have 3 valves per brake pad. I'm not sure. I know there is a cross over valve, such that if the front left brake line breaks, this valve will engage and block flow to the front left, as well as the rear right. Basically it's usually a metal ball in the brake lines. If you have to much flow out one line, this ball moves to block that loss of fluid, and allows fluid to the other 2 brakes in diagonal. They do the diagonal as it helps prevent your car from spinning out of control.
Hi, the crossover valve, is used on cars since 70s and up. It is used as a safety feature, when you press pedal and there will be a massive leak ie. the old brake line cracks/explodes it lets you brake down using the built up pressure. (problem is that is works only once the pressure was built up, relasing brake all other fluid from the two "paired" wheels leaks out). For this most of the fluid reservoirs are inside halved, and there are two main tubes to act as two separate twowheel brakes)
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Re: How does ABS work?

Post by jabran »

I could possibly lend a hand with this. I have launch x431 diagnostic scanner and vagcom between them I should be able to answer some of your questions. I try and get some data towards then end of the week.
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Re: How does ABS work?

Post by AndreyB »

Cool stuff Indirect tire pressure monitoring systems and methods https://patents.google.com/patent/US20140060170A1/en
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Re: How does ABS work?

Post by mk e »

nighthunter wrote:
Sun Dec 16, 2018 3:57 pm
kb1gtt wrote:
Sun Dec 16, 2018 4:29 am
As for why you appear to have 3 valves per brake pad. I'm not sure. I know there is a cross over valve, such that if the front left brake line breaks, this valve will engage and block flow to the front left, as well as the rear right. Basically it's usually a metal ball in the brake lines. If you have to much flow out one line, this ball moves to block that loss of fluid, and allows fluid to the other 2 brakes in diagonal. They do the diagonal as it helps prevent your car from spinning out of control.
Hi, the crossover valve, is used on cars since 70s and up. It is used as a safety feature, when you press pedal and there will be a massive leak ie. the old brake line cracks/explodes it lets you brake down using the built up pressure. (problem is that is works only once the pressure was built up, relasing brake all other fluid from the two "paired" wheels leaks out). For this most of the fluid reservoirs are inside halved, and there are two main tubes to act as two separate twowheel brakes)
I think what you're calling the cross-over valve is the proportioning valve. Its purpose is to reduce the pressure to the rear brakes as line pressure increases This dramatically improves braking in low traction conditions as allows the baseline system bias to be set to match (or nearly match) the vehicle's static weight bias, or lots of weight on the rear tires. As braking force goes up weight shifts forward, unloading the rear tires and you don't ever want the rear to lock before the front as that is an unstable condition so the proportioning valve jumps in and reduces line pressure to the rear so the brake bias remains balanced under all traction conditions.

Dual master cylinders is about redundancy so any 1 failure doesn't leave you without brakes, you'll only have 1/2 the system but that's way better than no brakes at all.

Multiple pistons per wheel in the ABS is about pulse rate and pedal feel. the old systems had 1 and the pedal would bounce around and there was a pretty noticeable noise and vibration through the the car when the system engaged, which caused most drivers to reduce pressure on the pedal leading to cars with ABS being involved in more accidents than those without. Adding pistons smooths out the action to the point that most people can no longer tell when the system engages so they don't release the pedal. Also the higher pulse rate does a better job keeping the wheel turning and at its traction limit, so again you get shorter stopping distance.

I THINK most all ABS systems have eliminated the mechanical proportioning valve so an ABS failure results in a very much compromised braking system.

On the frankenferrari I din't do an ABS install...although I might at some point and added wheel speed sensors all around since I needed them for engine output modulated traction control. The system does have a 2 masters cylinders, set up front and rear not diagonal. This allows the use of a bias adjuster (changes the % pedal pressure applied to the front/rear masters) to set the baseline front/rear braking force since I've messed with weight distribution and changed the calipers and rotors all around so everything has changed....the bias gets set so the rear doesn't lock on a cold wet day. Once that is done, the adjustable proportioning valve is set s the rear doesn't lock on a hot dry day. That's the best that can be done with a mechanical system vs ABS that gets it right at each wheel in all conditions like keeping the front inside tire turning letting you brake a lot deeper into the corner....hence my thoughts of upgrading.
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