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#1
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Simultaneous Application of Gas and Brake Pedals
Nomen Nescio wrote:
> Now look at a modern car. The brake and accelerator pedal have > little or no difference in height. Is is entirely possible for > the right foot to press on both pedals at the same time. The heavy duty floor mat (Mopar brand, which I use in the winter) in my 300M tends to creep forward and up against the center console - which means it gets up and behind (and to the right) of the accelerator pedal. This reduces the amount of foot-space to the immediate right of the accelerator pedal and moves the right foot a little to the left instead of being centered on the accelerator pedal. I've found that in this position I brush against the *back* of the brake pedal when pulling back on the gas. All in all I'd have to agree that there should be more spacing between the gas and brake pedal. But I think that it's a manditory design criteria that the brake system of any car is supposed to be able to over-power the engine in all situations. Back in the days when you had a spring that pulled back on the throttle plate, if that spring broke you could have WOT (wide-open-throttle). I can't imagine the braks system of any car not being able to stop the wheels from turing - even in that situation. |
#2
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Couldn't in my 1967 GTO. The engine torque far overpowered any brake pressure I could place on the brake pedal. Now the car wouldn't actually move (the front brakes kept it in place)...but it would sure billow plenty of smoke from the spinning rear tires! |
#3
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On Sat, 22 Jan 2005 22:38:54 -0500, "James C. Reeves"
> wrote: > >Couldn't in my 1967 GTO. The engine torque far overpowered any brake >pressure I could place on the brake pedal. Now the car wouldn't actually >move (the front brakes kept it in place)...but it would sure billow plenty >of smoke from the spinning rear tires! > And if it had been front wheel drive? That's the rub with many of todays high powered vehicles. You have antilock brakes that are made as small as they can get away with to keep the weight down (and since they have antilock, it is hard to overwork them anyway) and now we have cars with more horsepower than the old muscle cars. The power brakes are engine vacuum operated, and the vacuum goes for a dump when the engine is under load. So, yes, there are MANY cars on the road today that would have a hard time restraining the engine with the brakes even well below full throttle. |
#4
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> wrote in message
... > And if it had been front wheel drive? Isn't more braking power put to the front wheels of a car due to the weight distribution properties during stoping? I don't know the ratios, though. > That's the rub with many of todays high powered vehicles. You have > antilock brakes that are made as small as they can get away with to > keep the weight down (and since they have antilock, it is hard to > overwork them anyway) How do you figure? Antilock does not help with heat tolerance or dissipation. > and now we have cars with more horsepower than > the old muscle cars. The power brakes are engine vacuum operated, and > the vacuum goes for a dump when the engine is under load. Yes, but you should still have pressure for at least a couple brake presses stored up in the system - same as if the engine stops while driving. -- Scott Ehardt http://www.scehardt.com |
#5
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In article >,
"Scott Ehardt" > wrote: > > wrote in message > ... > > And if it had been front wheel drive? > > Isn't more braking power put to the front wheels of a car due to the weight > distribution properties during stoping? I don't know the ratios, though. Yup, the ratios are appx 85/15 for FWD and appx 60/40 for RWD > > That's the rub with many of todays high powered vehicles. You have > > antilock brakes that are made as small as they can get away with to > > keep the weight down (and since they have antilock, it is hard to > > overwork them anyway) > > How do you figure? Antilock does not help with heat tolerance or > dissipation. > > > and now we have cars with more horsepower than > > the old muscle cars. The power brakes are engine vacuum operated, and > > the vacuum goes for a dump when the engine is under load. > > Yes, but you should still have pressure for at least a couple brake presses > stored up in the system - same as if the engine stops while driving. Correct, there is a vacuum check valve in the booster inlet that should prevent the vacuum from dumping out when the engine is under load. It usually takes between 8 and 10 (and sometimes more) pedal pumps to deplete the stored vacuum in a brake booster with the engine not running. |
#6
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On Sun, 23 Jan 2005 05:18:01 GMT, "Scott Ehardt"
> wrote: > wrote in message .. . >> And if it had been front wheel drive? > >Isn't more braking power put to the front wheels of a car due to the weight >distribution properties during stoping? I don't know the ratios, though. > >> That's the rub with many of todays high powered vehicles. You have >> antilock brakes that are made as small as they can get away with to >> keep the weight down (and since they have antilock, it is hard to >> overwork them anyway) > >How do you figure? Antilock does not help with heat tolerance or >dissipation. > No, but because antilock brakeswork smoother if they don't lock in the first place, manufacturers tend to install smaller less effective brakes on cars with antilock as standard. >> and now we have cars with more horsepower than >> the old muscle cars. The power brakes are engine vacuum operated, and >> the vacuum goes for a dump when the engine is under load. > >Yes, but you should still have pressure for at least a couple brake presses >stored up in the system - same as if the engine stops while driving. |
#7
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"Scott Ehardt" > wrote in message om... > > wrote in message > ... >> And if it had been front wheel drive? > > Isn't more braking power put to the front wheels of a car due to the > weight distribution properties during stoping? I don't know the ratios, > though. > >> That's the rub with many of todays high powered vehicles. You have >> antilock brakes that are made as small as they can get away with to >> keep the weight down (and since they have antilock, it is hard to >> overwork them anyway) > > How do you figure? Antilock does not help with heat tolerance or > dissipation. I agree. If anything it would make it worse (assuming locked brakes vs. ABS stop). If the wheels aren't locked while stopping, all the energy is going into frictional heat between pads and the rotors. If the wheels are locked, there is no energy being released as heat in the brakes, they aren't moving, so it all goes to the tires and the pavement. |
#8
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#9
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On Sun, 23 Jan 2005 08:15:23 -0500, Matt Whiting
> wrote: wrote: > >> On Sat, 22 Jan 2005 22:38:54 -0500, "James C. Reeves" >> > wrote: >> >> >>>Couldn't in my 1967 GTO. The engine torque far overpowered any brake >>>pressure I could place on the brake pedal. Now the car wouldn't actually >>>move (the front brakes kept it in place)...but it would sure billow plenty >>>of smoke from the spinning rear tires! >>> >> >> And if it had been front wheel drive? >> That's the rub with many of todays high powered vehicles. You have >> antilock brakes that are made as small as they can get away with to >> keep the weight down (and since they have antilock, it is hard to >> overwork them anyway) and now we have cars with more horsepower than >> the old muscle cars. The power brakes are engine vacuum operated, and >> the vacuum goes for a dump when the engine is under load. > >Horsepower doesn't matter much in this case, it is torque that matters >and only a few cars today have torque ratings above the muscle cars of >the 60s. > > >> So, yes, there are MANY cars on the road today that would have a hard >> time restraining the engine with the brakes even well below full >> throttle. > >I guess it depends on how you define many. I don't think any four >cylinders and probably precious few V-6s can do this. Sure, the large >V-8s probably can generate enough torque to overcome the brakes on the >drive wheels, but I'd have to try it to be sure. > >The logic that suggests that few cars can do this is simple. Look at >how long it takes (in time, not distance) to accelerate a car to 60 MPH. > That tells you how fast energy is being put into the motion of the >car. Most cars take 6 or more seconds. Now look at how long it takes >to stop the same car from 60 MPH. It will often be half this time or >less. This tells you that you can remove that same amount of energy >with the braks about twice as fast (or more in most cars) as you can put >it in with the engine. This gives you a rough suggestion that the >brakes are substantially more powerful than the engine. > >Now, of course, you have to factor in that the engine is working on >typically only two wheels and thus may be wheel spin limited initially, >but that only applies to cars that are fairly high performance. The >brakes are working on all four wheels, however, mostly on the front due >to weight transfer. Even so, I'll bet that only a few vehicles have >engines with sufficient torque to overcome the brakes on even two >wheels, and certainly won't overcome all four as the Audi proponents >originally claimed. > >Keep in mind that most torque convertors stall at less than 2,000 RPM so > you can't consider the engines peak torque, but must look at the >torque available at whatever the stall RPM is for that car's TC. This >will typically be much less than the peak torque. > > > >Matt And the torque at the wheels is typically in the range of 9 to 15 times crankshaft torque with a standard transmission, and higher with a torque converter equipped car. From a dead stop not many cars can overcome the brakes, but when attempting to stop at speed, even relatively low speed, and hitting the accelerator at the same time, it is a bit different story. |
#10
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