4 ms·

Brake-by-wire sounds scary. In my current, recent-ish car, you can still control it - with more effort - if engine and/or electrics are out. I've had the expe

Brake-by-wire sounds scary. In my current, recent-ish car, you can still control it - with more effort - if engine and/or electrics are out. I've had the experience of driving home with a suddenly failed alternator, watching one system after another report offline, until as I turned into my street, the power steering went too. But I was able to safely drive it to my driveway (with 7V remaining on the battery). I'd rather that cars stay that way, and not just because I'm a grumpy old man in training. I do wonder about ABS and always have. If ABS can make the brakes not brake even as you fully stomp the pedal - is there a reasonably conceivable failure mode that would simply prevent you from braking despite perfectly good hydraulics and a stomped pedal?

topspin 1y ago
> If ABS can make the brakes not brake even as you fully stomp the pedal - is there a reasonably conceivable failure mode that would simply prevent you from braking despite perfectly good hydraulics and a stomped pedal? The answer is not simple, as it has changed over time. Early anti-lock systems were so limited that the system would, indeed, fail to utilize the maximum possible braking force. This was known, and yet these were deployed, because the research said that maintaining directional control, the primary benefit of anti-lock, had greater safety value than the compromise of maximum braking performance. Today, however, anti-lock is greatly improved, and anti-lock systems are capable of applying extremely high braking force, even to the point of exceeding thermal design limits (overheat) of the brake system components. The sensors are sampling at higher frequency, the braking models are far more accurate and the computers are faster in current vehicles. So current anti-lock can perform at near the absolute limit. Further, current systems can actually detect panic. Drivers often fail to even use the full braking force available. Current vehicles can detect when sudden, high braking force is applied, switch into "emergency mode" and boost braking force beyond what the driver is demanding. These features started appearing in the early 2000's. Nissan, for instance, introduced "Brake Assist" in 2001, with the 2002 the Altima redesign (L31 platform) and the Maxima. It has the "panic mode" behavior I've described. I personally experienced this once about 15 years ago. Somehow I distracted myself, and when my attention returned I was closing with stopped vehicles at too high a speed and too little remaining distance. Collision was certain and my foot crushed the brake: pure panic, and I never let up. I came to a stop in a blessedly unoccupied left turn lane almost aligned with the stopped vehicles. I recall looking over my hood at the driver of the car I'd nearly hit: between us was a wisp of brake smoke drifting up from passenger fender. I could smell the brakes. The anti-lock did that. If I had had no anti-lock, all directional control would have been lost and my maneuver into the unoccupied lane would have been impossible. If the anti-lock had not applied as much force as it did, I would have been in the intersection, possibly getting t-boned. So they're pretty good today, and I appreciate modern anti-lock designs.
- ejiblabahaba 1y ago
  I think the question was less about the efficacy of ABS, and more about the failure mode. Is it possible for the ABS system to "fail open" unintentionally, such that depressing the brake pedal has no effect whatsoever?
  - topspin 1y ago
    There is no credible way to answer that question. Analysis cannot prove the absence of such a flaw. All we have are records, and those are incomplete. What we can say is that there is no record of ABS “fail-open” flaws plaguing passenger vehicle designs, and the record is now rather long.
Animats 1y ago
> Brake-by-wire sounds scary. "We will have steer by wire and brake by wire over my dead body." - former head of engineering at Chrysler.

topspin 1y ago

> If ABS can make the brakes not brake even as you fully stomp the pedal - is there a reasonably conceivable failure mode that would simply prevent you from braking despite perfectly good hydraulics and a stomped pedal? The answer is not simple, as it has changed over time. Early anti-lock systems were so limited that the system would, indeed, fail to utilize the maximum possible braking force. This was known, and yet these were deployed, because the research said that maintaining directional control, the primary benefit of anti-lock, had greater safety value than the compromise of maximum braking performance. Today, however, anti-lock is greatly improved, and anti-lock systems are capable of applying extremely high braking force, even to the point of exceeding thermal design limits (overheat) of the brake system components. The sensors are sampling at higher frequency, the braking models are far more accurate and the computers are faster in current vehicles. So current anti-lock can perform at near the absolute limit. Further, current systems can actually detect panic. Drivers often fail to even use the full braking force available. Current vehicles can detect when sudden, high braking force is applied, switch into "emergency mode" and boost braking force beyond what the driver is demanding. These features started appearing in the early 2000's. Nissan, for instance, introduced "Brake Assist" in 2001, with the 2002 the Altima redesign (L31 platform) and the Maxima. It has the "panic mode" behavior I've described. I personally experienced this once about 15 years ago. Somehow I distracted myself, and when my attention returned I was closing with stopped vehicles at too high a speed and too little remaining distance. Collision was certain and my foot crushed the brake: pure panic, and I never let up. I came to a stop in a blessedly unoccupied left turn lane almost aligned with the stopped vehicles. I recall looking over my hood at the driver of the car I'd nearly hit: between us was a wisp of brake smoke drifting up from passenger fender. I could smell the brakes. The anti-lock did that. If I had had no anti-lock, all directional control would have been lost and my maneuver into the unoccupied lane would have been impossible. If the anti-lock had not applied as much force as it did, I would have been in the intersection, possibly getting t-boned. So they're pretty good today, and I appreciate modern anti-lock designs.

ejiblabahaba 1y ago
I think the question was less about the efficacy of ABS, and more about the failure mode. Is it possible for the ABS system to "fail open" unintentionally, such that depressing the brake pedal has no effect whatsoever?
- topspin 1y ago
  There is no credible way to answer that question. Analysis cannot prove the absence of such a flaw. All we have are records, and those are incomplete. What we can say is that there is no record of ABS “fail-open” flaws plaguing passenger vehicle designs, and the record is now rather long.

Animats 1y ago

> Brake-by-wire sounds scary. "We will have steer by wire and brake by wire over my dead body." - former head of engineering at Chrysler.