4 ms·

I went through to the actual report (http://www.transportenvironment.org/sites/te/files/publications/TE_Mind_the_Gap_2015_FINAL.pdf http://www.transportenvironm

I went through to the actual report (http://www.transportenvironment.org/sites/te/files/publications/TE_Mind_the_Gap_2015_FINAL.pdf http://www.transportenvironment.org/sites/te/files/publicati...) and found an infographic (figure 8) that shows some of the methods manufacturers can use to defeat tests. "Pushing the brake pads fully into the callipers reduces rolling resistance" is apparently one way. That makes me ask the question, why don't we do this already? If it reduces rolling resistance it probably also increases brake-pad life. That'd be a win-win for drivers.

other_herbert 11y ago
As long as the system can quickly return them gently to the rotor surface... If you change your own brake pads you know that after you finish it can take two whole presses of the brake pedal to get the pads to contact the rotors
emp_zealoth 11y ago
The brake pads and jaws get contaminated with dust and the like,making them stick a little. My guess is they get disassembled before test and cleaned perfectly.
- pbhjpbhj 11y ago
  How much benefit is it, wonder if having a small compressed air squirter - like a headlight washer - would pay off (in terms of fuel savings) or not. Would be interesting to know what designs of pads/jaws dissipate the contaminants better?
rplst8 11y ago
The primary reason is that keeping the pads just in contact with the rotor prevents water from collecting on the rotor surface. Wet rotors can affect braking performance and safety.
_ambigu0us_ 11y ago
In car braking systems the pedal movement from depressed to pressed corresponds to a very small (<1mm) pad movement. The pad is just touching when the brake when the pedal is not pressed, and pressing the pedal forces the pad and caliper together, increasing friction. Depressing the pedal does not withdraw the pad. With the brake cylinder fully compressed (as they did for the test), it requires several full pumps of the pedal to bring the pad into contact with the disk. This increases braking distance, which is generally considered a bad thing.
- tonylemesmer 11y ago
  The travel from a single pedal press is small, but its probably a solvable problem. It just needs a different design.
  - simias 11y ago
    If you make the pedal more sensitive (increase the resulting travel per pedal press) you'll make it harder to apply a precise amount of braking power which doesn't sound like a nice feature. I suppose you could have a more clever system that would move the brake faster until it touches the disc and then behave normally but I suppose that for such a critical system constructors and regulators are very conservative. There could also be beneficial side effects to having the pad in contact with the disc at all times, such as preventing water/fluids/dirt to coat it. I'm just guessing here.
    - _ambigu0us_ 11y ago
      > If you make the pedal more sensitive (increase the resulting travel per pedal press) you'll make it harder to apply a precise amount of braking power which doesn't sound like a nice feature. If you've tried hopping in a car from the past few years after driving a much older car you (and you passengers) will have probably noticed this! It's very common in modern cars to have much shorter brake pedal travel, so what would bring you to a gentle stop in an older car will be somewhat more abrupt in a modern car. This is achieved by having a shorter travel pedal, which is then heavily assisted by the brake servos so it is not horribly heavy to apply. This is nominally for safety's sake, as it reduces the time between seeing a obstacle and the brakes being fully applied. And since all recent cars have anti-lock braking and stability systems it doesn't matter so much if the driver can't control the braking pressure quite as precisely. > I suppose you could have a more clever system that would move the brake faster until it touches the disc and then behave normally but I suppose that for such a critical system constructors and regulators are very conservative. Exactly, as I mentioned to the other commenter, it's preferable to have a simple, non powered mechanical system in place to stop the car in the event of bad things happening. Adding extra bits to this system, with the vast R+D cost involved, for marginal increases in fuel economy would be hard to justify for most car manufacturers (especially since the fuel economy saving would benefit the users pockets, not theirs).

other_herbert 11y ago

As long as the system can quickly return them gently to the rotor surface... If you change your own brake pads you know that after you finish it can take two whole presses of the brake pedal to get the pads to contact the rotors

emp_zealoth 11y ago

The brake pads and jaws get contaminated with dust and the like,making them stick a little. My guess is they get disassembled before test and cleaned perfectly.

pbhjpbhj 11y ago
How much benefit is it, wonder if having a small compressed air squirter - like a headlight washer - would pay off (in terms of fuel savings) or not. Would be interesting to know what designs of pads/jaws dissipate the contaminants better?

rplst8 11y ago

The primary reason is that keeping the pads just in contact with the rotor prevents water from collecting on the rotor surface. Wet rotors can affect braking performance and safety.

_ambigu0us_ 11y ago

In car braking systems the pedal movement from depressed to pressed corresponds to a very small (<1mm) pad movement. The pad is just touching when the brake when the pedal is not pressed, and pressing the pedal forces the pad and caliper together, increasing friction. Depressing the pedal does not withdraw the pad. With the brake cylinder fully compressed (as they did for the test), it requires several full pumps of the pedal to bring the pad into contact with the disk. This increases braking distance, which is generally considered a bad thing.

tonylemesmer 11y ago
The travel from a single pedal press is small, but its probably a solvable problem. It just needs a different design.
- simias 11y ago
  If you make the pedal more sensitive (increase the resulting travel per pedal press) you'll make it harder to apply a precise amount of braking power which doesn't sound like a nice feature. I suppose you could have a more clever system that would move the brake faster until it touches the disc and then behave normally but I suppose that for such a critical system constructors and regulators are very conservative. There could also be beneficial side effects to having the pad in contact with the disc at all times, such as preventing water/fluids/dirt to coat it. I'm just guessing here.
  - _ambigu0us_ 11y ago
    > If you make the pedal more sensitive (increase the resulting travel per pedal press) you'll make it harder to apply a precise amount of braking power which doesn't sound like a nice feature. If you've tried hopping in a car from the past few years after driving a much older car you (and you passengers) will have probably noticed this! It's very common in modern cars to have much shorter brake pedal travel, so what would bring you to a gentle stop in an older car will be somewhat more abrupt in a modern car. This is achieved by having a shorter travel pedal, which is then heavily assisted by the brake servos so it is not horribly heavy to apply. This is nominally for safety's sake, as it reduces the time between seeing a obstacle and the brakes being fully applied. And since all recent cars have anti-lock braking and stability systems it doesn't matter so much if the driver can't control the braking pressure quite as precisely. > I suppose you could have a more clever system that would move the brake faster until it touches the disc and then behave normally but I suppose that for such a critical system constructors and regulators are very conservative. Exactly, as I mentioned to the other commenter, it's preferable to have a simple, non powered mechanical system in place to stop the car in the event of bad things happening. Adding extra bits to this system, with the vast R+D cost involved, for marginal increases in fuel economy would be hard to justify for most car manufacturers (especially since the fuel economy saving would benefit the users pockets, not theirs).