Hacker News Reader

Comment by simongr3dal | Hacker News Reader

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.

View on HN

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

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/ [2] 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.)

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.

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.

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.

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

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.)

Software for these controllers are usually codegen'd from a Simulink model (or equivalent dataflow language), which contributes to the difficulty in reverse engineering.

I believe you are correct. The parent post is probably referring to traction control, or something else more recent than the original ABS.

Reply on news.ycombinator.com