4 ms·

Are brakes more powerful than an engine at full speed along with the car (large mass) moving at high speed? Brakes are designed to slow a car not under power.

adamswann 17y ago
Car and Driver magazine had a great article a few months ago in which they tested breaking several different cars (including one of the recalled Toyotas) from 70 mph down to zero -- both with the throttle behaving normally and with it forced to full on. In most cases, the brakes are plenty capable of stopping the vehicle at full throttle (the exception was a 540-hp Roush Stage 3 Mustang travelling at 100 mph with the throttle locked -- in that case, the breaking distance was tripled to a total of ~900 feet). http://www.caranddriver.com/features/09q4/how_to_deal_with_unintended_acceleration-tech_dept http://www.caranddriver.com/features/09q4/how_to_deal_with_u... One of the interesting points in the article was that the Toyota Camry -- with throttle forced wide open -- stops in a shorter distance than a Ford Taurus without any issues. On the other hand, the article also points out that at 100 mph, it wouldn't have been immediately obvious that the breaks were working, even though they were (though at 70, you would have felt an immediate effect).
- sokoloff 17y ago
  The point curiously missing from the C&D article is that a car driving a full throttle will also be generating only a tiny amount of manifold vacuum. In most cars, more vacuum is necessary to provide the power-assist to the brakes, once any stored vacuum is used up. (The brakes will work, but losing power brakes for the average driver will be an upsetting event, especially with a stuck throttle.) They mention "don't turn off the ignition because that will cause you to lose brake assist", but fail to connect the no vacuum at wide open throttle to the same end result. I'm sure the C&D test drivers were willing to mash the pedal and generate the best braking force the car was capable of. The average person, confronted with a stuck throttle, lost power brakes and the resulting "hard" brake pedal may very well not be able to summon the force required to generate full brake pressure. (It's an amazing amount of force compared to what you're used to using. If you're also the type to worry about flipping the car from putting the car in neutral, you're probably not going to bend the seatback applying enough brake pressure to get the job done.) I think I'd have expected better than the average driver from a trained highway patrol officer, though.
  - blueben 17y ago
    You can't have it both ways. Either drivers are pushing the pedal as hard as they can (which every single driver reports) or they aren't because they're scared. Either way, your argument is based on a bad assumption. At full throttle in a non-super or turbocharged vehicle, there is still more than enough vacuum to assist in braking.
    - blueben 17y ago
      Another point I didn't mention. The Prius can detect a no-assist situation and will reroute all braking pressure to the front wheels for maximum effectiveness. So even in the extremely unlikely situation you have no regenerative braking and no vacuum assist, you can still stop the car.
sdh 17y ago
have you ever seen a burnout?
- sokoloff 17y ago
  Yes, but the brakestand type burnout procedure does not use full braking. In a safe location, in a rear drive car with an automatic, try the following: In drive, heavy brake pressure with the left foot, roll into the throttle with your right foot. Get to full throttle (over a second or two; don't take forever), and gradually release some of the brake pressure. You'll find a sweet spot where the torque of the drivetrain is able to overcome the rear brake(s) and spin your right rear tire (or both of them if you have a tight limited slip diff). Your partial pressure on the front brakes, which are not driven, will hold the car stopped. You can further come off the brakes to let the car rollout of the burnout, or get out of the throttle and stay put. Obviously, this is far harder on the car than driving to the corner store for some milk, but it's an effective way to clean off the tires at the dragstrip, a good way to get attention at the local teen hangout on Friday nights, and a great way to get a ticket for "ostentatious display".

adamswann 17y ago

Car and Driver magazine had a great article a few months ago in which they tested breaking several different cars (including one of the recalled Toyotas) from 70 mph down to zero -- both with the throttle behaving normally and with it forced to full on. In most cases, the brakes are plenty capable of stopping the vehicle at full throttle (the exception was a 540-hp Roush Stage 3 Mustang travelling at 100 mph with the throttle locked -- in that case, the breaking distance was tripled to a total of ~900 feet). http://www.caranddriver.com/features/09q4/how_to_deal_with_unintended_acceleration-tech_dept http://www.caranddriver.com/features/09q4/how_to_deal_with_u... One of the interesting points in the article was that the Toyota Camry -- with throttle forced wide open -- stops in a shorter distance than a Ford Taurus without any issues. On the other hand, the article also points out that at 100 mph, it wouldn't have been immediately obvious that the breaks were working, even though they were (though at 70, you would have felt an immediate effect).

sokoloff 17y ago
The point curiously missing from the C&D article is that a car driving a full throttle will also be generating only a tiny amount of manifold vacuum. In most cars, more vacuum is necessary to provide the power-assist to the brakes, once any stored vacuum is used up. (The brakes will work, but losing power brakes for the average driver will be an upsetting event, especially with a stuck throttle.) They mention "don't turn off the ignition because that will cause you to lose brake assist", but fail to connect the no vacuum at wide open throttle to the same end result. I'm sure the C&D test drivers were willing to mash the pedal and generate the best braking force the car was capable of. The average person, confronted with a stuck throttle, lost power brakes and the resulting "hard" brake pedal may very well not be able to summon the force required to generate full brake pressure. (It's an amazing amount of force compared to what you're used to using. If you're also the type to worry about flipping the car from putting the car in neutral, you're probably not going to bend the seatback applying enough brake pressure to get the job done.) I think I'd have expected better than the average driver from a trained highway patrol officer, though.
- blueben 17y ago
  You can't have it both ways. Either drivers are pushing the pedal as hard as they can (which every single driver reports) or they aren't because they're scared. Either way, your argument is based on a bad assumption. At full throttle in a non-super or turbocharged vehicle, there is still more than enough vacuum to assist in braking.
  - blueben 17y ago
    Another point I didn't mention. The Prius can detect a no-assist situation and will reroute all braking pressure to the front wheels for maximum effectiveness. So even in the extremely unlikely situation you have no regenerative braking and no vacuum assist, you can still stop the car.

sdh 17y ago

have you ever seen a burnout?

sokoloff 17y ago
Yes, but the brakestand type burnout procedure does not use full braking. In a safe location, in a rear drive car with an automatic, try the following: In drive, heavy brake pressure with the left foot, roll into the throttle with your right foot. Get to full throttle (over a second or two; don't take forever), and gradually release some of the brake pressure. You'll find a sweet spot where the torque of the drivetrain is able to overcome the rear brake(s) and spin your right rear tire (or both of them if you have a tight limited slip diff). Your partial pressure on the front brakes, which are not driven, will hold the car stopped. You can further come off the brakes to let the car rollout of the burnout, or get out of the throttle and stay put. Obviously, this is far harder on the car than driving to the corner store for some milk, but it's an effective way to clean off the tires at the dragstrip, a good way to get attention at the local teen hangout on Friday nights, and a great way to get a ticket for "ostentatious display".