RE: What are anti-lock brakes? PH Explains

RE: What are anti-lock brakes? PH Explains

Thursday 19th April

What are anti-lock brakes? PH Explains

Standing on the brakes and still able to steer? You've ABS to thank for that



An anti-lock braking system is designed to prevent a car's wheels from locking up and skidding, even when maximum braking power is being applied. This has several benefits, the most prominent of which are improved stability and handling during emergency braking. This is primarily because, as the wheels are not skidding, the driver can still steer the car in an effort to avoid obstacles.

Modern systems also reduce stopping distances in emergency situations, as they are capable of quickly and effortlessly delivering the maximum available stopping power at each individual wheel without locking them up.

These systems are often dubbed 'ABS', an acronym reputedly derived from 'Anti-Blockier System' - the German name for the first mass-produced automotive anti-lock braking system. The acronym is also widely accepted to stand for 'Anti-lock Braking System'.


How do anti-lock brakes work?

Modern ABS set-ups typically monitor the speed of each wheel and rely on an electronic control unit, a series of valves and a hydraulic pump. These, working together, allow the system to correctly modulate the brake pressure at each wheel.

When the brakes are applied, the system looks at the wheel speeds. If one wheel starts slowing more quickly than the others, suggesting that it is about to lock and slide, the control unit will send signals to an electronically controlled valve regulating brake pressure at that wheel's caliper.

This solenoid valve allows the pressure at that wheel to be reduced until it starts turning again, at which point brake pressure is increased by the pump in order to maintain maximum stopping power. This cycle repeats until the wheels cease locking up or the pressure on the brake pedal is released. Bosch, for example, states that its systems can carry out this cycle up to 40 times a second.

Consequently, an ABS-equipped car can keep every wheel from locking up when the driver stands on the brakes, maintaining steering control and delivering quick, strong and consistent stopping power. The system also grants reliable, controlled braking when slowing on rougher, slippier surfaces.


A brief history of anti-lock brakes

The concept of an anti-lock braking system was first detailed in 1908, with the aim being to stop heavy locomotives skidding and damaging the track or crashing. A French Lieutenant called Pierra Cayla then proposed an anti-lock system for cars in 1923, which was designed to reduce wheel lock and improve steering control in emergency situations.

ABS was first employed in earnest, however, in aircraft. The invention of the jet engine had resulted in heavier, faster aircraft that were harder to land and more demanding on the pilots and tyres. Advances in technology permitted Boeing to equip its B-47 Stratojet with ABS in 1947; a Maxaret system, from Dunlop, was also employed in the 1952 Avro Vulcan.

The technology began filtering back into cars in the 1950s but it wasn't until 1966 that the first production car with ABS arrived, in the form of the Jensen FF. Chrysler then followed in 1970, with the Bendix 'Sure-Brake' set-up, before Mercedes-Benz introduced ABS to a wider audience in 1978. Just 10 years later, around one million Mercedes vehicles were equipped with the life-saving system.

PH Explains homepage

Author
Discussion

RacerMike

Original Poster:

2,042 posts

146 months

Thursday 19th April
quotequote all
Did you not run an almost identical feature a few months ago?!

Fastdruid

5,587 posts

87 months

Thursday 19th April
quotequote all
RacerMike said:
Did you not run an almost identical feature a few months ago?!
I think that was a "history" piece.

Anyway, it missed out one way in which ABS works on modern cars, the ECU knows lots of details about the car and so as it monitors wheel speed if it detects that the wheels are slowing down more is possible (ie not just comparing them) it will release the brakes... and them reapply them etc. They know about the size of the brake pistons and exactly how much fluid is needed along with the latency in the system. This is one way where upgrading to much bigger/better brakes may cause issues. It can confuse the ABS at the limit and cause it to underbrake.

Modern ones however also learn from how the car actually performs and will tailor their response to how the system actually behaves rather than how it should.

Modern systems now as well do clever EBD/EBL (Electronic Brake Distribution/Electronic Brakeforce Limitation) which allows the braking force to be redistributed, typically combined with the DSC /ESC.

Edited by Fastdruid on Thursday 19th April 10:27

Lewis Kingston

177 posts

12 months

Thursday 19th April
quotequote all
RacerMike said:
Did you not run an almost identical feature a few months ago?!
Worry not, you're not misremembering - but that was an 'Origins' feature, which details the history behind the tech in depth – you can read that here: https://www.pistonheads.com/news/general/ph-origin...

These 'Explains' features are just straightforward, short introductory pieces on the tech or hardware in question, for those unfamiliar with them – so you will see similar topics from time to time (for example, one Explains on supercharging, followed much later by an Origins piece). Hope that all makes sense!

DAVEVO9

2,971 posts

202 months

Thursday 19th April
quotequote all

R8Steve

3,694 posts

110 months

Thursday 19th April
quotequote all
Looking forward to next weeks article, 'what is a rear view mirror'.
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EazyDuz

1,067 posts

43 months

Thursday 19th April
quotequote all
Fastdruid said:
I think that was a "history" piece.

Anyway, it missed out one way in which ABS works on modern cars, the ECU knows lots of details about the car and so as it monitors wheel speed if it detects that the wheels are slowing down more is possible (ie not just comparing them) it will release the brakes... and them reapply them etc. They know about the size of the brake pistons and exactly how much fluid is needed along with the latency in the system. This is one way where upgrading to much bigger/better brakes may cause issues. It can confuse the ABS at the limit and cause it to underbrake.

Modern ones however also learn from how the car actually performs and will tailor their response to how the system actually behaves rather than how it should.

Modern systems now as well do clever EBD/EBL (Electronic Brake Distribution/Electronic Brakeforce Limitation) which allows the braking force to be redistributed, typically combined with the DSC /ESC.

Edited by Fastdruid on Thursday 19th April 10:27
How modern?

Fastdruid

5,587 posts

87 months

Thursday 19th April
quotequote all
EazyDuz said:
Fastdruid said:
I think that was a "history" piece.

Anyway, it missed out one way in which ABS works on modern cars, the ECU knows lots of details about the car and so as it monitors wheel speed if it detects that the wheels are slowing down more is possible (ie not just comparing them) it will release the brakes... and them reapply them etc. They know about the size of the brake pistons and exactly how much fluid is needed along with the latency in the system. This is one way where upgrading to much bigger/better brakes may cause issues. It can confuse the ABS at the limit and cause it to underbrake.

Modern ones however also learn from how the car actually performs and will tailor their response to how the system actually behaves rather than how it should.

Modern systems now as well do clever EBD/EBL (Electronic Brake Distribution/Electronic Brakeforce Limitation) which allows the braking force to be redistributed, typically combined with the DSC /ESC.
How modern?
Last 20 years! I should probably update my definition of modern.

romac

56 posts

81 months

Thursday 19th April
quotequote all
R8Steve said:
Looking forward to next weeks article, 'what is a rear view mirror'.
Oh No! So we have to wait longer for the "What is a sun visor?" article. That's gonna be a problem with all this sunshine we are getting. cool

RacerMike

Original Poster:

2,042 posts

146 months

Thursday 19th April
quotequote all
Fastdruid said:
This is one way where upgrading to much bigger/better brakes may cause issues. It can confuse the ABS at the limit and cause it to underbrake.
True to an extent, although most modern systems are less sensitive, as they work on inertia which means that changing the friction level of the pad has less of an effect than it used to. As long as your pad friction level changes by less than 10% you'll probably be largely ok. Very few manufacturers will recalibrate for a different pad (and even different brake sizes will usually share a calibration, but just have the different hydraulic stiffness measurements updated). If you bolt on a set of Carbon Ceramics, it'll really screw things up, which is why if you buy the 'kit' from BMW for CCMs on an M3, you get a new ECU cal with it....

Fastdruid said:
Modern systems now as well do clever EBD/EBL (Electronic Brake Distribution/Electronic Brakeforce Limitation) which allows the braking force to be redistributed, typically combined with the DSC /ESC.
Generally the case for at least 10-15 years. As Max_Torque and I discussed on another thread, the addition of Stability Control means that ABS doesn't have to target vehicle stability as much as it once did. It's possible to have a more aggressive control to achieve shorter braking distances as the Stability Control can 'mop up' any yaw instability you get. It's incredibly difficult for any human to beat ABS in a fixed test, and basically impossible in any real word emergency situation.

Fastdruid

5,587 posts

87 months

Thursday 19th April
quotequote all
RacerMike said:
Fastdruid said:
This is one way where upgrading to much bigger/better brakes may cause issues. It can confuse the ABS at the limit and cause it to underbrake.
True to an extent, although most modern systems are less sensitive, as they work on inertia which means that changing the friction level of the pad has less of an effect than it used to. As long as your pad friction level changes by less than 10% you'll probably be largely ok. Very few manufacturers will recalibrate for a different pad (and even different brake sizes will usually share a calibration, but just have the different hydraulic stiffness measurements updated). If you bolt on a set of Carbon Ceramics, it'll really screw things up, which is why if you buy the 'kit' from BMW for CCMs on an M3, you get a new ECU cal with it....
I was thinking more that the size of brake piston is *known* as is the inertia of the wheel and the entire wheel setup. If you change things massively then you could actually make things worse, the brakes won't release properly as the HU isn't depressurising enough (or too much), the wheel speeds up and slows down faster than expected etc. I know in the case of Audi for example you code to wheel size and brake setup. It has them all calibrated and you just pick which setup you have.

RacerMike said:
Fastdruid said:
Modern systems now as well do clever EBD/EBL (Electronic Brake Distribution/Electronic Brakeforce Limitation) which allows the braking force to be redistributed, typically combined with the DSC /ESC.
Generally the case for at least 10-15 years. As Max_Torque and I discussed on another thread, the addition of Stability Control means that ABS doesn't have to target vehicle stability as much as it once did. It's possible to have a more aggressive control to achieve shorter braking distances as the Stability Control can 'mop up' any yaw instability you get. It's incredibly difficult for any human to beat ABS in a fixed test, and basically impossible in any real word emergency situation.
As I said above I should probably reevaluate my definition of modern... smile I was looking into this in some depth when I was wondering if ABS/DSC could be repurposed for a kit car and specifically looking at the Bosch 5.7 DSC III.... and decided it wasn't a wise idea as the weight, dimensions and brake setup varied so much from the base/donor that how it would behave would be somewhat unpredictable.

99dndd

1,039 posts

24 months

Thursday 19th April
quotequote all
romac said:
R8Steve said:
Looking forward to next weeks article, 'what is a rear view mirror'.
Oh No! So we have to wait longer for the "What is a sun visor?" article. That's gonna be a problem with all this sunshine we are getting. cool
I guess the "What are lug nuts?" article will have to wait.

MagicalTrevor

6,229 posts

164 months

Thursday 19th April
quotequote all
99dndd said:
romac said:
R8Steve said:
Looking forward to next weeks article, 'what is a rear view mirror'.
Oh No! So we have to wait longer for the "What is a sun visor?" article. That's gonna be a problem with all this sunshine we are getting. cool
I guess the "What are lug nuts?" article will have to wait.
This is all a bit 'cart before the horse', isn't it? We've yet to have the 'How to open the car door' instructional video series in 3 parts!

Max_Torque

12,829 posts

152 months

Thursday 19th April
quotequote all
Fastdruid said:
I was thinking more that the size of brake piston is *known* as is the inertia of the wheel and the entire wheel setup. If you change things massively then you could actually make things worse, the brakes won't release properly as the HU isn't depressurising enough (or too much), the wheel speeds up and slows down faster than expected etc. I know in the case of Audi for example you code to wheel size and brake setup. It has them all calibrated and you just pick which setup you have.
it'll still work, because the system is closed loop, attempting to control wheel accel/decel by modulating brake pressure, and it can always get (ultimately) to either full pressure or zero pressure. What changes is how long it takes to get to the optimum point. The open loop calibration is targeted to be as close to optimum as possible, resulting in the least close loop trimming possible. Effectively the system jumps straight in with the calibrated open loop feed forwards control response and then trims with the closed loop control actually get to the optimum setpoint. The closer the calibration, the greater the time spent at the optimum, the shorter the stop. This especially matters as speed climbs, because velocity is distance divided by time! ie you have more distance to slow down ' per second' the slower you are travelling.