4 ms·

Worse for stopping distances though, especially to the extent that SUVs (very popular in the US) have a higher CoG and therefore the ABS has to be calibrated di

Worse for stopping distances though, especially to the extent that SUVs (very popular in the US) have a higher CoG and therefore the ABS has to be calibrated differently. The car is "safer" in that you won't die, but not safer in terms of avoiding accidents.

simongr3dal 4y ago
Why would the ABS need to be calibrated differently? I thought the car was just monitoring if any wheel is about to stop rolling and then it would ease-up the brake on that specific wheel.
- pxeboot 4y ago
  I believe you are correct. The parent post is probably referring to traction control, or something else more recent than the original ABS.
- gnicholas 4y ago
  I recall learning this in a class on the automotive industry about 20 years ago. We learned about unintended consequences that caused the US automotive market to favor SUVs (see the Chicken tax [1]), and some of the downsides of SUVs. My recollection is that SUVs have longer stopping distances because the ABS has to pause braking more frequently to avoid instability or rollover. It's possible, as another commenter suggested, that I'm recalling ESC, but that wasn't very common in the early 2000s. I think it might have something to do with the fact that the nose of a vehicle dives when braking, and a high CoG exacerbates this. But it's been 20 years, and I've scarcely thought about the course since then! 1: https://en.wikipedia.org/wiki/Chicken_tax https://en.wikipedia.org/wiki/Chicken_tax
  - QuarterReptile 4y ago
    No discussion of the federal government pushing Americans to buy SUVs would be complete without mentioning the favorable depreciation schedule that can (could?) be applied to larger SUVs [1] and the lower standards that they face for fuel efficiency.[2] [1] https://www.section179.org/section_179_vehicle_deductions/ https://www.section179.org/section_179_vehicle_deductions/ [2] https://www.greencarreports.com/news/1102305_will-tighter-cafe-rules-bring-more-trucks-fewer-small-cars https://www.greencarreports.com/news/1102305_will-tighter-ca... (Both links are admittedly lazy, but think of them as entering arguments for further research if you care.)
  - sidewndr46 4y ago
    How exactly would a braking SUV rollover? It'd effectively have to do a front flip. The center of gravity might be high, but that would require circus car level of high to get a front flip from a vehicle like that.
    - Zach_the_Lizard 4y ago
      Corners, particularly those with roads that are banked, make for some interesting side to side rollover fodder. There's that famous Ford Explorer tire issue in 2001 or so that makes for some (dated) examples of SUV rollover accidents.
      - sidewndr46 4y ago
        I don't think ABS is going to help much if someone puts inadequate tires on your vehicle. Also you have it backwards. Banked corners actually reduce the chance of rollover. It's why NASCAR tracks have a large slope. It allows the drivers to brake farther out and longer into the turn before beginning to transition back onto the throttle.
- tsomctl 4y ago
  That was true 20 years ago. The question is how much do you ease up the brake on that specific wheel, and then how much do you reapply it? You need to know the coefficient of friction of the road to know this. Modern controllers are doing a physics simulation of the entire vehicle, and part of this is the weight transfer as you go around a corner (which is partially influenced by the CoG). They then do a physics simulation of the hydraulic brake circuit so it knows how long to run the pump to build up the desired brake pressure. Now of course this is combined with stability control (since that is required by law) and both of them are controlled by the same code. (On a related note, I've spent way too much time reverse engineering the calibration for the ABS for a Silverado truck. Ridiculously huge program, ridiculously complicated, and I have a sneaking suspicion that the source code is a mess.)
  - smilekzs 4y ago
    Software for these controllers are usually codegen'd from a Simulink model (or equivalent dataflow language), which contributes to the difficulty in reverse engineering.
- ukd1 4y ago
  Ya most of the modern systems for "ABS" do way more than just ABS; usually they're also trying to keep steering authority as well, for example. With a higher COG car, you have to be more careful of rollovers, so maybe can't help someone get a super stupid steering angle. Checkout: https://en.wikipedia.org/wiki/Moose_test https://en.wikipedia.org/wiki/Moose_test
letsgo39 4y ago
also maneuverability. If another SUV from the opposite side of the road gets in your lane, good luck avoiding it with a fast reaction.
lilgreenland 4y ago
Braking distance isn't a function of mass. Turns out the momentum from higher mass and the increased friction from higher mass balance exactly. I should say this is true for the physics of sliding friction forces, which aren't super precise.
- kurupt213 4y ago
  I think you are looking at this wrong. It should be surface area of rotor/brakepad interface per lb of vehicle
  - deleted 4y ago
    [deleted]
  - bluGill 4y ago
    Modern brakes can get a lot hotter before brake fade. Until the brakes get too hot it is just a matter of brake pressure. Modern cars universally have disk brakes which shed heat better. Larger vehicles also have bigger tires which leaves more room for brakes. In short while you are not wrong, in practice even the worst numbers are still more than good enough that brake performance isn't an issue.
  - CompelTechnic 4y ago
    Just about any car can brake hard enough to lock up all 4 tires and enter a skid (or activate ABS to avoid this)- the limiting factor to braking distance is friction between the tires and the ground. Scaling for static friction between rubber and asphalt does not follow the friction equation you are taught in high school physics class. Generally, for a given weight of vehicle, a larger tire surface area is able to produce more maximum braking friction. In other terms, minimizing the surface pressure and maximizing the surface area at the contact patch increases maximum braking friction.
- TheCoelacanth 4y ago
  That would be true if your wheels instantly stopped turning when you applied the brakes. The limiting factor for braking isn't the friction between your wheels and the ground, which is weight-dependent, but the friction between your brake pads and rotors, which is not weight-dependent.

simongr3dal 4y ago

Why would the ABS need to be calibrated differently? I thought the car was just monitoring if any wheel is about to stop rolling and then it would ease-up the brake on that specific wheel.

pxeboot 4y ago
I believe you are correct. The parent post is probably referring to traction control, or something else more recent than the original ABS.
gnicholas 4y ago
I recall learning this in a class on the automotive industry about 20 years ago. We learned about unintended consequences that caused the US automotive market to favor SUVs (see the Chicken tax [1]), and some of the downsides of SUVs. My recollection is that SUVs have longer stopping distances because the ABS has to pause braking more frequently to avoid instability or rollover. It's possible, as another commenter suggested, that I'm recalling ESC, but that wasn't very common in the early 2000s. I think it might have something to do with the fact that the nose of a vehicle dives when braking, and a high CoG exacerbates this. But it's been 20 years, and I've scarcely thought about the course since then! 1: https://en.wikipedia.org/wiki/Chicken_tax https://en.wikipedia.org/wiki/Chicken_tax
- QuarterReptile 4y ago
  No discussion of the federal government pushing Americans to buy SUVs would be complete without mentioning the favorable depreciation schedule that can (could?) be applied to larger SUVs [1] and the lower standards that they face for fuel efficiency.[2] [1] https://www.section179.org/section_179_vehicle_deductions/ https://www.section179.org/section_179_vehicle_deductions/ [2] https://www.greencarreports.com/news/1102305_will-tighter-cafe-rules-bring-more-trucks-fewer-small-cars https://www.greencarreports.com/news/1102305_will-tighter-ca... (Both links are admittedly lazy, but think of them as entering arguments for further research if you care.)
- sidewndr46 4y ago
  How exactly would a braking SUV rollover? It'd effectively have to do a front flip. The center of gravity might be high, but that would require circus car level of high to get a front flip from a vehicle like that.
  - Zach_the_Lizard 4y ago
    Corners, particularly those with roads that are banked, make for some interesting side to side rollover fodder. There's that famous Ford Explorer tire issue in 2001 or so that makes for some (dated) examples of SUV rollover accidents.
    - sidewndr46 4y ago
      I don't think ABS is going to help much if someone puts inadequate tires on your vehicle. Also you have it backwards. Banked corners actually reduce the chance of rollover. It's why NASCAR tracks have a large slope. It allows the drivers to brake farther out and longer into the turn before beginning to transition back onto the throttle.
tsomctl 4y ago
That was true 20 years ago. The question is how much do you ease up the brake on that specific wheel, and then how much do you reapply it? You need to know the coefficient of friction of the road to know this. Modern controllers are doing a physics simulation of the entire vehicle, and part of this is the weight transfer as you go around a corner (which is partially influenced by the CoG). They then do a physics simulation of the hydraulic brake circuit so it knows how long to run the pump to build up the desired brake pressure. Now of course this is combined with stability control (since that is required by law) and both of them are controlled by the same code. (On a related note, I've spent way too much time reverse engineering the calibration for the ABS for a Silverado truck. Ridiculously huge program, ridiculously complicated, and I have a sneaking suspicion that the source code is a mess.)
- smilekzs 4y ago
  Software for these controllers are usually codegen'd from a Simulink model (or equivalent dataflow language), which contributes to the difficulty in reverse engineering.
ukd1 4y ago
Ya most of the modern systems for "ABS" do way more than just ABS; usually they're also trying to keep steering authority as well, for example. With a higher COG car, you have to be more careful of rollovers, so maybe can't help someone get a super stupid steering angle. Checkout: https://en.wikipedia.org/wiki/Moose_test https://en.wikipedia.org/wiki/Moose_test

letsgo39 4y ago

also maneuverability. If another SUV from the opposite side of the road gets in your lane, good luck avoiding it with a fast reaction.

lilgreenland 4y ago

Braking distance isn't a function of mass. Turns out the momentum from higher mass and the increased friction from higher mass balance exactly. I should say this is true for the physics of sliding friction forces, which aren't super precise.

kurupt213 4y ago
I think you are looking at this wrong. It should be surface area of rotor/brakepad interface per lb of vehicle
- deleted 4y ago
  [deleted]
- bluGill 4y ago
  Modern brakes can get a lot hotter before brake fade. Until the brakes get too hot it is just a matter of brake pressure. Modern cars universally have disk brakes which shed heat better. Larger vehicles also have bigger tires which leaves more room for brakes. In short while you are not wrong, in practice even the worst numbers are still more than good enough that brake performance isn't an issue.
- CompelTechnic 4y ago
  Just about any car can brake hard enough to lock up all 4 tires and enter a skid (or activate ABS to avoid this)- the limiting factor to braking distance is friction between the tires and the ground. Scaling for static friction between rubber and asphalt does not follow the friction equation you are taught in high school physics class. Generally, for a given weight of vehicle, a larger tire surface area is able to produce more maximum braking friction. In other terms, minimizing the surface pressure and maximizing the surface area at the contact patch increases maximum braking friction.
TheCoelacanth 4y ago
That would be true if your wheels instantly stopped turning when you applied the brakes. The limiting factor for braking isn't the friction between your wheels and the ground, which is weight-dependent, but the friction between your brake pads and rotors, which is not weight-dependent.