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Old 07-01-2004, 08:50 PM   #46
cinqyg
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Join Date: May 2004
Location: Midlands, England
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Quote:
Originally Posted by ChrisV
Reaction time is in the driver, not which type of transmission used. The question was about which CAR can stop quicker, and the truth is that the interface between the tires and the road determines ultimate stopping ability. Your reaction time scenario is so variable that a bad driver in a Porsche 911 could stop slower than a good driver in a Corolla, but that doesn't mean the Corolla itself stops better than the Porsche.

With properly operating brakes, engine braking will never stop faster than applying pressure to the brake pedal. If applying pressure to the brake pedal can lock up the tires (or engage the ABS system in cars so equipped), then no amount of additional braking force (such a engine braking OR additional pedal pressure) will stop a car any shorter.


your doing it again your bad for my blood presure.

and i still think puting the car in rever will stop the car quicker.
why dont we fit retro rokets and parachute. or just drive it into a wall that will stop a car quicker than any brakes.
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Old 07-03-2004, 06:15 AM   #47
jcutsh
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Quote:
Originally Posted by ChrisV
With properly operating brakes, engine braking will never stop faster than applying pressure to the brake pedal. If applying pressure to the brake pedal can lock up the tires (or engage the ABS system in cars so equipped), then no amount of additional braking force (such a engine braking OR additional pedal pressure) will stop a car any shorter.


Ok I am back for some more punishment.

ChrisV,
You mention ABS equipped cars, which are certainly touted as being able to stop a car faster.
However the great advantage with ABS is they do not lock the wheels, allowing the car to stop under full control. Locking the wheels at 80 mph can be detrimental to oneís health resulting in total loss of control of the car. That coupled with the fact that most car braking systems are biased 65-70% to the front and only 35-30% to the rear, means that rear brakes arenít applied with the same force as the front. The front brakes always lock first, and continue that way until the car slows enough for the rear brakes to lock. Any external force applied under these conditions, (such as engine braking) has to help in stopping the vehicle.

Now I hadnít thought this out as far as FWD vs RWD, but it seems to me that weight distribution of the car is a factor also. RWD is just heavier to the back wheels, which would increase friction of the tires on the road. Many times I have brought FWD cars to a hard stop and feel the rear end bouncing, which certainly doesnít help stopping power. I can see no advantage in a FWD car. Brakes on the front of a fwd are generally larger to overcome residual Hp produced by the engine. We all try to stop our car in an efficient and safe manner. That usually does not include locking up the wheels. I have watched test drivers while testing braking distances on new cars and never once did they lock up the wheels. They all came to full controlled stops.

I make this post not to be facetious but to try to understand why this idea is so prevalent in some of our drivers. Myself being one. I grew up in east Tennessee, at a time when there were very few or no automatics. When the shine haulers first tried automatics their biggest complaint was. ďYou canít stop the damn things.Ē Maybe that just stuck.

I try to approach a problem like this in milliseconds. There has to be a starting point in order to compare the two so letís use the first half second starting at the word STOP.

Automatic rwd
.1sec. The brain hears the signal and moves the leg toward the brake. The engine is running 3500 rpm deprived of fuel starts to return to idle, but for most cars this takes from .5 to 1.5 full seconds.
.2 sec. The leg applies pressure to the brake peddle. Engine rpm is now 2500rpm and still propelling the car forward.
.3 sec. The foot forces the peddle to the floor and the brake fluid pressure builds on the pistons at the wheels. The engine rpm is now 2000 rpm and forward propulsion is still being applied.
.4 sec. The front brakes apply full on. The rear brakes canít apply enough pressure to overcome the momentum of the car and the power of the engine which is at 1500rpm and is still driving the wheels forward.
.5sec. The brakes are fully applied wheels are locked. Engine rpm is 1000 and pressure drops in the transmission and torque convertor.

Manual rwd
.1 sec. The brain hears the signal and moves both legs one toward the brake and one toward the clutch. The right hand grabs the gearshift. The engine is running 3500
.2sec. The feet, in one smooth motion apply pressure to the brake peddle and clutch disengaging the engine stopping forward driving power to the rear wheels. The car starts slowing even before the brakes are applied.
.3sec. The gearshift is pulled to a lower gear and the clutch is released, and the rear wheels are now controlled by engine braking. The pressure is being applied to the pistons at the wheels. The car is slowing down bigtime.
.4 sec. The front brakes apply full on the rear brakes apply because the engine is helping to overcome momentum of the car. The car has already lost 50% speed. The brakes are probably locked full on at this point.

Yes I made all of this up, but it explains how I was trained to approach a problem in college in the 60's. If you have a better way, please tell me. There will be those of you who say, but the engine is not producing power. This power is produced by to the dynamic Hp stored in the hydraulic fluid of the torque convertor. mass of the crankshaft, pistons, and rods . It only takes 25-40 Hp to move a car at 60mph. It takes +300Hp to get there in 6 seconds. Any modern engine producing less then 40 Hp isnít running.

As we can see above, the engine can continue to drive the car forward even though the foot has been removed from the gas, at 60mph, this could account for 50 to 80 feet of extra travel. Now if you factor in ABS for this little scenario, a system designed not to lock the wheels, then engine braking becomes a greater advantage. Also factor in the fact that some automatics provide a small amount of engine braking and the problem compounds again.

Where does the practical begin, and the theory stop? I had dozens of experiences in my life where practical won out over theory. But I believe a good theory can always be proven, we just donít understand how to explain them yet. Logic has been a guiding force in my life, but sometimes logic canít be explained easily. Donít bet any money on this, I have been wrong before, and I will probably be wrong again. Itís the learning process that counts. I would love to see a real test performed on the subject. Pushing the envelope of thought on the board, should be a good thing. If I am proven wrong then a good theory will come forth and we will all benefit. We could all beat ourselves to death on this and get no further toward the truth. Maybe we should call the ďMyth BustersĒ
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