I'm not a physicist but I am a bit of a car geek and I've tuned and worked-on cars a fair bit. I am very skeptical of the claim about cars' masses not affecting their stopping distances:
Bigger, heavier cars need bigger brakes. Twenty years ago, this is exactly what they got: Range Rovers et. al. were fitted with much bigger wheels than normal saloon cars that could accommodate large-diameter brake rotors (the bigger the diameter of the rotor the better the mechanical advantage of the brake; there are side-benefits like better heat dissipation, but these don't matter so much for a single emergency-stop). These larger-diameter brake rotors would have much better, multi-cylinder brake callipers and for any given force at the brake pedal the retarding force at the brake would be much greater.
But - this isn't how cars are built today. The new-car market has changed almost beyond recognition in twenty years! Vastly fewer people today buy cars based on performance characteristics (including brake performance) but rather on interior design and software features that have nothing to do with how the car actually drives*. Big three-ton cars are no longer necessarily prestige models (though of course these still exist) - the non-prestige market now demands huge cars too (not necessarily EVs, though EVs are of course part of this demand).
As a result, car manufacturers are producing lower-end huge cars that, to keep manufacturing costs down, share components with regular saloon cars, hatchbacks, etc. from the rest of the manufacturer's range. So there are now cars that're twice the weight of a hatchback but have the exact same single-pot sliding-calliper brakes with smallish rotor diameters that the hatchback does.
2½ ton BMW X5 SUVs have the brakes off a 1½ ton 3-Series. 2 ton VW Tiguan SUVs have the brakes off a 1¼ ton VW Golf. (I'm not trying to pick on German cars specifically, they just happen to be ones I have direct experience of; I'm sure other marques do this at least as much!)
Modern brakes, even the lower-end ones, are sufficiently good that the Golf-spec brakes are able to retard the SUV more than adequately in normal relaxed driving conditions - but if you have to pull-up very sharply, the budget SUV's stopping distance is noticeably further** than that of the Golf. You can easily feel this yourself if you drive a hatchback and a lower-end SUV back-to-back in a fairly sporty manner.
*(I drive a secondhand 420i; whoever specced it from new specced the absolute top-of-the-range infotainment system, driver aids, comfort features, automated this and electric that - and *base-spec single-pot sliding-calliper 290mm-diameter brakes*! To me, that's utter madness; brakes are much more important than a fancy speaker system or whatever and I honestly can't fathom the minds of people who think like that. The point is that car manufacturers can and absolutely do respond to the market forces these people create.
My car felt noticeably under-braked during spirited driving (it would have no doubt felt even more noticeable to any pedestrian I'd hit...) so I replaced the brakes with mid-spec 4-pot fixed callipers with 340mm rotors - not a difficult job but a time-consuming one to do on all four wheels! - these aren't even the highest-spec OEM brakes available for my car, but my stopping distances are nevertheless considerably reduced.)
**(The "longer stopping distance" is another way of saying that the SUV's speed after X yards of braking is higher than the hatchback's, this being what counts for car-pedestrian collisions.)
Stopping distance doesn't have vehicle mass as part of its equation, though. For vehicle mass to matter, the brakes would have to be unable to stop the wheels from turning. Are we seeing brake systems that can't stop wheels from turning at normal speeds?
Thanks for the reply! To answer your question: yes, we *are* seeing brake systems that can't lock-up wheels at legal motorway speeds (in my experience many drivers aren't actually driving at legal motorway speeds, but let's ignore that for now...)
The most clear example I could think of is a motorcycle with a single front disc setup and tyres with an absurdly high coefficient of friction (somehow in excess of 1, for modern tyres!): I know from experience that adding a pillion passenger and luggage significantly increases the stopping distance but doesn't lock-up the wheels (or rather, doesn't cause the ABS to activate) when emergency braking under good environmental conditions (dry tarmac, warm tyres, etc.)
I asked an AI to plug-in some numbers to find out whether an SUV using budget-shared-platform brakes was like this motorcycle case (ie. the brakes aren't strong enough to challenge the tyres' grip and so the -ideal- braking equation doesn't apply) or not. With the caveat that I'm not a physicist and if the AI's calculations were wrong I probably wouldn't be able to tell*, it found that a heavy SUV on budget shared-platform brakes also couldn't brake hard enough to reach the grip-limited point where vehicle mass no longer matters, and so does in fact have a longer stopping distance than a lighter car using the same brakes.
(The margins were closer for the SUV than the motorcycle, so possibly some different assumptions could find brakes capable of locking-up the wheels - but all of this doesn't take into account thermal fade: brakes convert kinetic energy into thermal energy and as they become hotter through repeated stopping they become less efficient, producing less torque. This makes things worse in two ways: firstly the SUV is heavier and thus more kinetic energy is converted into thermal energy, making thermal fade happen sooner and reduce performance more, and secondly the SUV has less (if any!) "brake torque vs. grip" margin and thermal fade could fully eat-up this margin whereas even with thermal fade a lighter car could have margin to spare.)
So is the safest car a very massive car with a small front end? Like those new mail trucks?
I'm not a physicist but I am a bit of a car geek and I've tuned and worked-on cars a fair bit. I am very skeptical of the claim about cars' masses not affecting their stopping distances:
Bigger, heavier cars need bigger brakes. Twenty years ago, this is exactly what they got: Range Rovers et. al. were fitted with much bigger wheels than normal saloon cars that could accommodate large-diameter brake rotors (the bigger the diameter of the rotor the better the mechanical advantage of the brake; there are side-benefits like better heat dissipation, but these don't matter so much for a single emergency-stop). These larger-diameter brake rotors would have much better, multi-cylinder brake callipers and for any given force at the brake pedal the retarding force at the brake would be much greater.
But - this isn't how cars are built today. The new-car market has changed almost beyond recognition in twenty years! Vastly fewer people today buy cars based on performance characteristics (including brake performance) but rather on interior design and software features that have nothing to do with how the car actually drives*. Big three-ton cars are no longer necessarily prestige models (though of course these still exist) - the non-prestige market now demands huge cars too (not necessarily EVs, though EVs are of course part of this demand).
As a result, car manufacturers are producing lower-end huge cars that, to keep manufacturing costs down, share components with regular saloon cars, hatchbacks, etc. from the rest of the manufacturer's range. So there are now cars that're twice the weight of a hatchback but have the exact same single-pot sliding-calliper brakes with smallish rotor diameters that the hatchback does.
2½ ton BMW X5 SUVs have the brakes off a 1½ ton 3-Series. 2 ton VW Tiguan SUVs have the brakes off a 1¼ ton VW Golf. (I'm not trying to pick on German cars specifically, they just happen to be ones I have direct experience of; I'm sure other marques do this at least as much!)
Modern brakes, even the lower-end ones, are sufficiently good that the Golf-spec brakes are able to retard the SUV more than adequately in normal relaxed driving conditions - but if you have to pull-up very sharply, the budget SUV's stopping distance is noticeably further** than that of the Golf. You can easily feel this yourself if you drive a hatchback and a lower-end SUV back-to-back in a fairly sporty manner.
*(I drive a secondhand 420i; whoever specced it from new specced the absolute top-of-the-range infotainment system, driver aids, comfort features, automated this and electric that - and *base-spec single-pot sliding-calliper 290mm-diameter brakes*! To me, that's utter madness; brakes are much more important than a fancy speaker system or whatever and I honestly can't fathom the minds of people who think like that. The point is that car manufacturers can and absolutely do respond to the market forces these people create.
My car felt noticeably under-braked during spirited driving (it would have no doubt felt even more noticeable to any pedestrian I'd hit...) so I replaced the brakes with mid-spec 4-pot fixed callipers with 340mm rotors - not a difficult job but a time-consuming one to do on all four wheels! - these aren't even the highest-spec OEM brakes available for my car, but my stopping distances are nevertheless considerably reduced.)
**(The "longer stopping distance" is another way of saying that the SUV's speed after X yards of braking is higher than the hatchback's, this being what counts for car-pedestrian collisions.)
Stopping distance doesn't have vehicle mass as part of its equation, though. For vehicle mass to matter, the brakes would have to be unable to stop the wheels from turning. Are we seeing brake systems that can't stop wheels from turning at normal speeds?
Thanks for the reply! To answer your question: yes, we *are* seeing brake systems that can't lock-up wheels at legal motorway speeds (in my experience many drivers aren't actually driving at legal motorway speeds, but let's ignore that for now...)
The most clear example I could think of is a motorcycle with a single front disc setup and tyres with an absurdly high coefficient of friction (somehow in excess of 1, for modern tyres!): I know from experience that adding a pillion passenger and luggage significantly increases the stopping distance but doesn't lock-up the wheels (or rather, doesn't cause the ABS to activate) when emergency braking under good environmental conditions (dry tarmac, warm tyres, etc.)
I asked an AI to plug-in some numbers to find out whether an SUV using budget-shared-platform brakes was like this motorcycle case (ie. the brakes aren't strong enough to challenge the tyres' grip and so the -ideal- braking equation doesn't apply) or not. With the caveat that I'm not a physicist and if the AI's calculations were wrong I probably wouldn't be able to tell*, it found that a heavy SUV on budget shared-platform brakes also couldn't brake hard enough to reach the grip-limited point where vehicle mass no longer matters, and so does in fact have a longer stopping distance than a lighter car using the same brakes.
(The margins were closer for the SUV than the motorcycle, so possibly some different assumptions could find brakes capable of locking-up the wheels - but all of this doesn't take into account thermal fade: brakes convert kinetic energy into thermal energy and as they become hotter through repeated stopping they become less efficient, producing less torque. This makes things worse in two ways: firstly the SUV is heavier and thus more kinetic energy is converted into thermal energy, making thermal fade happen sooner and reduce performance more, and secondly the SUV has less (if any!) "brake torque vs. grip" margin and thermal fade could fully eat-up this margin whereas even with thermal fade a lighter car could have margin to spare.)
* But please do feel free to check its calculations yourself! Glad to be corrected if I'm mistaken! https://claude.ai/share/a2218658-78d1-406c-9ef6-5a8f3c9cef1f