> Turns out the momentum from higher mass and the increased friction from higher mass balance exactly.
yes, in the spherical cow sense, but this isn't at all true for real life vehicles and tires. the weight of the vehicle does have a significant impact on traction, here's a quick video explaining tire load sensitivity: https://www.youtube.com/watch?v=kNa2gZNqmT8
related to the above issue (and as GP mentioned), the high CoG of an SUV also poses challenges for braking performance. you want all four wheels of the vehicle to have roughly similar braking force. obviously the front wheels are going to contribute more to braking than the rear, but you don't want a huge disparity. this doesn't matter so much for the abstract question of "how quickly can a vehicle stop?", but it matters a lot for the practical question of "which direction is the vehicle facing?" if the rear wheels are contributing much less of the total braking force, the car is likely to end up sideways. this is bad if you want to do something like "remain in control of the vehicle" while braking.
GP didn't claim that braking distance was larger because of higher mass. Rather, because of the higher centre of gravity the ABS has to be calibrated differently, leading to weaker braking, thus larger braking distance.
I have no expertise in this, but seems prima facie plausible.
I think simongr3dal’s remark isn’t about them taking longer to stop, but about the use of the term calibration.
ABS is a feedback loop: brake faster until just before the wheels start slipping. That doesn’t need calibration.
There may be additional logic in ABS systems for corner cases that requires calibration, though (for example, if your front left wheel has lots more grip than the other ones, can you brake full on it and keep the car going in a straight(ish) line?), but I don’t see how higher CoG would be a factor there.
Ah, fair enough, that makes sense. You're right, if we posit that ABS is purely to prevent slipping, that would imply that it always goes for maximum braking.
> I don’t see how higher CoG would be a factor there.
Could be that the "additional logic" (whether we call it ABS or something else) aims to keep certain values (such as longitudinal or lateral acceleration) within bounds (thus releasing brake pressure when approaching those bounds), and those bounds are tighter with higher CoG.
I remember the Mercedes A-Class (with pretty high CoG) rolled over in the Swedish Moose test initially, until that was fixed with Electronic Stability Control (how?).
“When ESC detects loss of steering control, it automatically applies the brakes to help steer the vehicle where the driver intends to go. Braking is automatically applied to wheels individually, such as the outer front wheel to counter oversteer, or the inner rear wheel to counter understeer”
So, ESC activates the brakes harder than the driver indicates through the controls, while ABS activates the brakes less hard than the driver indicates through the controls.
“Just as ESC is founded on the anti-lock braking system (ABS), ESC is the foundation for new advances such as Roll Stability Control or active rollover protection that works in the vertical plane much like ESC works in the horizontal plane. When RSC detects impending rollover (usually on transport trucks or SUVs), RSC applies brakes, reduces throttle, induces understeer, and/or slows down the vehicle.”
In the end, there can be only one set of commands that get sent to the brakes, throttle, etc, so these systems must be interconnected, making the terms more marketing than indicators of specific systems in the car.
“The concept for ABS predates the modern systems that were introduced in the 1950s. In 1908, for example, J.E. Francis introduced his 'Slip Prevention Regulator for Rail Vehicles'.
In 1920 the French automobile and aircraft pioneer Gabriel Voisin experimented with systems that modulated the hydraulic braking pressure on his aircraft brakes to reduce the risk of tire slippage”
> thus it's always the brake pads stopping the car, not the tires?
The tires are the only thing in contact with the ground. All deceleration force can only come from the grip between tires and the ground[1]. The brake pads in most installations can easily lock up the tires (which means now you only have sliding coefficient of friction instead of rolling friction). In other words, what matters is tire grip.
Buy the grippiest tires you can afford to run.
[1] Well, cars with aero downforce also get a lot of deceleration from that. A Formula 1 car will decelerate at more than 1G merely by lifting off the throttle, without touching the brakes. But none of this is relevant to street cars.
Even race cars running on slicks and weighing 700 kg get better braking by avoiding lockups and using the pads instead. So unless your pads are trash, you better get a proper ABS instead of imagining some magical tire with grip comparable to brake pads.
The above comment seems to be making a distinction that braking force comes from the tires or the brakes. That is not how it works.
The brakes provide friction to resist the spinning of the wheel, but of course only to the point that the tire can provide grip (friction) against the road surface.
Picture trying to brake with slick tires on wet ice. Nearly zero tire grip equals nearly zero braking force. Doesn't matter what kind of brake components are on the car. No grip = no grip.
If you want to be able to stop quickly to avoid accidents you need to maximize grip a the tire-road contact patch and you need to have a braking system powerful enough to fully take advantage of that grip.
Every car sold for decades now already has the latter (unless something is broken, obviously), so the variable you need to control is tire grip.
Thus: Buy the grippiest tires you can afford to run (if you want optimal braking to avoid accidents, that is).
> The brake pads in most installations can easily lock up the tires
> In the United States, the NHTSA has mandated ABS in conjunction with Electronic Stability Control under the provisions of FMVSS 126 as of September 1, 2012.
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.
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.
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.
Yeah, that's why it's as easy to come to a stop on well maintained tarmac as it is on ice, snow or gravel. As long as those brake pads work well that is.
I suggest taking your bike out and practice breaking on various surfaces. You will quickly get what the limiting factor is. It's the same with a car just the tires are much wider and brakes more powerful.
this isn't quite right either. most modern cars have brakes that far exceed the limits of grip for the tires. the brakes themselves should only be the limiting factor if you are riding them down a very long hill or performing multiple hard stops in quick succession (eg, racing).
I should say this is true for the physics of sliding friction forces, which aren't super precise.