4 ms·

> 120 in a Honda? Is that safe The real problem would occur if you had to stop or slow down as quickly as a car that actually had real brakes, like say an AMG.

> 120 in a Honda? Is that safe The real problem would occur if you had to stop or slow down as quickly as a car that actually had real brakes, like say an AMG. But hey... it would be HUGELY hypocritical of me to say that you should Not have done that.

badpun 7y ago
Well, everything is relative - there are a lot of situations where the AMG brakes are not strong enough as well and you crash into someone anyway.
philjohn 7y ago
Plus tyres are typically rated up to a defined speed ...
- lagadu 7y ago
  Indeed but good tyres are rated to speeds in excess of 300km/h which is supercar territory. Hell, my fiesta came from the factory with speed rating Y: 300km/h
willyt 7y ago
I never understood how ‘good brakes’ make a difference as every car has brakes powerful enough to lock the wheels. Why isn’t it the ratio of the mass of the car to the size of the contact patch for the tires that is the overriding factor?
- wjnc 7y ago
  Anti lock braking and wheel surface contact will make big differences between cars in braking behavior. Those AMG not only have better brakes, with possibly better software controlling ABS, but also wider tires. With formulas [1] [1] https://en.m.wikipedia.org/wiki/Braking_distance https://en.m.wikipedia.org/wiki/Braking_distance
  - lagadu 7y ago
    Some also have active aerodynamic braking, sports tyres have softer compounds with more grip which results in better braking and usually larger engines also provide more engine breaking.
    - KozmoNau7 7y ago
      Engine braking is completely irrelevant on a RWD car during hard braking. The weight transfer means barely any braking is needed on the rear wheels, aside from a token amount to keep the car going straight.
- govg 7y ago
  I don't think hard locks on the wheels are a good thing.
- swish_bob 7y ago
  Mass to contact area is a major factor, but it's not the only one. Locking your brakes is not the fastest way to stop. What you want is to apply the maximum friction to the road, this means your brakes have to be letting the disks travel, and at very high speeds your disks and callipers get very hot, very quickly, at which point they stop supplying nearly as much stopping power, and you might in fact no longer be able to lock your brakes. Anti-lock brakes (ABS) are an improvement, because they mean that if you over-brake (locking your brakes) they release them until the wheel starts rotating again, before re-applying the pressure. They're better than locking your brakes, but they're actually less effective than holding your brakes at the optimal pressure. They're just a lot easier to use than it is to apply that pressure manually (particularly as the optimum pressure changes as your speed does and the heat of your disks and callipers).
- KozmoNau7 7y ago
  A lot more goes into braking than simply having enough clamping force to lock the wheels. In a situation when you completely lock the wheels, you're trading the friction between the brake pads and brake discs, for the friction between the tires and the road surface. In other words, instead of the hydraulic clamping force on the discs, you're relying solely on the weight of the car. The interface between tire and road is not nearly as good as the interface between brake pads and brake discs, which wear to match each other exactly, as they're used. So as a good rule of thumb, you never ever want to lock up your brakes. That's why anti-lock brakes are standard. (The main exception is when driving on relatively deep and loose surfaces, such as sand or deep snow, where a locked tire builds up material in front of it.) Undersized discs and pads have less thermal capacity, so they will tend to overheat and lose braking ability. A lot of lower-spec and economy cars will be perfectly capable emergency braking from full speed once, but you absolutely have to let the brakes cool off afterwards, especially if it has unventilated discs and rear drum brakes. That's fine, as long as you're aware of the limitations. In comparison, a larger midrange car (such as a VW Passat) can reasonably handle repeated emergency braking situations without losing any significant amount of braking power, thanks to larger discs and pads. There is also the matter of pedal feel, how confident the brakes feel or if they feel mushy and imprecise, and how the ABS system works. So yes, of course the contact patch matters, but only if the brakes are adequately capable.

badpun 7y ago

Well, everything is relative - there are a lot of situations where the AMG brakes are not strong enough as well and you crash into someone anyway.

philjohn 7y ago

Plus tyres are typically rated up to a defined speed ...

lagadu 7y ago
Indeed but good tyres are rated to speeds in excess of 300km/h which is supercar territory. Hell, my fiesta came from the factory with speed rating Y: 300km/h

willyt 7y ago

I never understood how ‘good brakes’ make a difference as every car has brakes powerful enough to lock the wheels. Why isn’t it the ratio of the mass of the car to the size of the contact patch for the tires that is the overriding factor?

wjnc 7y ago
Anti lock braking and wheel surface contact will make big differences between cars in braking behavior. Those AMG not only have better brakes, with possibly better software controlling ABS, but also wider tires. With formulas [1] [1] https://en.m.wikipedia.org/wiki/Braking_distance https://en.m.wikipedia.org/wiki/Braking_distance
- lagadu 7y ago
  Some also have active aerodynamic braking, sports tyres have softer compounds with more grip which results in better braking and usually larger engines also provide more engine breaking.
  - KozmoNau7 7y ago
    Engine braking is completely irrelevant on a RWD car during hard braking. The weight transfer means barely any braking is needed on the rear wheels, aside from a token amount to keep the car going straight.
govg 7y ago
I don't think hard locks on the wheels are a good thing.
swish_bob 7y ago
Mass to contact area is a major factor, but it's not the only one. Locking your brakes is not the fastest way to stop. What you want is to apply the maximum friction to the road, this means your brakes have to be letting the disks travel, and at very high speeds your disks and callipers get very hot, very quickly, at which point they stop supplying nearly as much stopping power, and you might in fact no longer be able to lock your brakes. Anti-lock brakes (ABS) are an improvement, because they mean that if you over-brake (locking your brakes) they release them until the wheel starts rotating again, before re-applying the pressure. They're better than locking your brakes, but they're actually less effective than holding your brakes at the optimal pressure. They're just a lot easier to use than it is to apply that pressure manually (particularly as the optimum pressure changes as your speed does and the heat of your disks and callipers).
KozmoNau7 7y ago
A lot more goes into braking than simply having enough clamping force to lock the wheels. In a situation when you completely lock the wheels, you're trading the friction between the brake pads and brake discs, for the friction between the tires and the road surface. In other words, instead of the hydraulic clamping force on the discs, you're relying solely on the weight of the car. The interface between tire and road is not nearly as good as the interface between brake pads and brake discs, which wear to match each other exactly, as they're used. So as a good rule of thumb, you never ever want to lock up your brakes. That's why anti-lock brakes are standard. (The main exception is when driving on relatively deep and loose surfaces, such as sand or deep snow, where a locked tire builds up material in front of it.) Undersized discs and pads have less thermal capacity, so they will tend to overheat and lose braking ability. A lot of lower-spec and economy cars will be perfectly capable emergency braking from full speed once, but you absolutely have to let the brakes cool off afterwards, especially if it has unventilated discs and rear drum brakes. That's fine, as long as you're aware of the limitations. In comparison, a larger midrange car (such as a VW Passat) can reasonably handle repeated emergency braking situations without losing any significant amount of braking power, thanks to larger discs and pads. There is also the matter of pedal feel, how confident the brakes feel or if they feel mushy and imprecise, and how the ABS system works. So yes, of course the contact patch matters, but only if the brakes are adequately capable.