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  #1  
Old 02-26-2002, 10:29 AM
lperei01
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Automatic transmission cars brake worse...

Before you recommend me psychiatric treatment I start to explain the point:

Recently a friend of mine attended a driving course (from Audi) where they pointed out that the most efficient way to use the brakes with ABS system was to press down simultaneously the brake (of course) and the clutch pedal.

Now, I did not understand the logic behind pressing the clutch pedal. I thought that the engine (idling after you start to brake) would always contribute to slow down the carůAs far as I know the procedure applies to any car with ABS system.

Does this mean that automatic transmission cars are condemned to brake worse than its manual transmission counterparts? What can be done to overcome this limitation (if any)?
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  #2  
Old 02-26-2002, 12:12 PM
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No because there's slippage designed into the automatics torque converter allowing you to stop / lock the wheels without stalling the engine or shifting to neutral. With a manual, of course, you have to step on the clutch

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  #3  
Old 02-26-2002, 12:26 PM
lperei01
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I believe you Mark, but still, what is the logic behind pressing the clutch?
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  #4  
Old 02-26-2002, 12:28 PM
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So you don't kill the engine and therefore lose power assists.
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  #5  
Old 02-26-2002, 12:34 PM
lperei01
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Could that happen with an ABS system? Doesn't the ABS allow the wheels to turn enough for the engine to keep running?
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  #6  
Old 02-26-2002, 12:54 PM
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yal yal is offline
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The ABS doesn't allow the wheels to do anything except stop

The Automatic transmission has an ingenious thing know as a torgue converter which senses engine speed relative to actual drive shaft speed and adjust everything accordingly. An idividual with a manual tranny accomplishes the same thing with the clutch.
The amazing thing about the automatic tranny in its basic form is that it does not use sensors to accomplish what it does. When you hit the brakes you force the car to stop the converter unlocks the drive shaft from the engine allowing the brake to accomplish its job.

It is likely that an automatic transmission does brake worse than a manual because the converter does not completely disconnect the engine from the drive shaft at full stop. There is still forward motion on an auto. But the difference in the full stop is not perceptable. You should be more concerned with the weight of the auto compared to the manual if you are searching for the discrepancies in braking performance.

Go here, read and see if some of your questions can be answered..

http://www.howstuffworks.com/automatic-transmission.htm

Last edited by yal; 02-26-2002 at 01:05 PM.
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  #7  
Old 02-26-2002, 01:05 PM
lperei01
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YAL,

Although I'm not a techy I do know what ABS is for:p

My point was: If the ABS doen't allow the wheels to stop (for more than some miliseconds; you see I know something about the issue ) it wouldn't also kill the engine...

I still do not undestand why should one step the clutch. Anyway thank you for your reading suggestion, which I'll immediately follow
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  #8  
Old 02-26-2002, 01:20 PM
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There will be no measurable difference, you step on the clutch so that the engine is still running when you come to a stop.
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  #9  
Old 02-26-2002, 01:24 PM
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yal yal is offline
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I am not trying to patronize you or anything just trying to explain what I know...i'm no techy either

Thing is if you don't step on the clutch you are likely locked into a gear. Stepping on the clutch disconnects the engine from the driveshaft which still has momentum when you come to a stop. If your cruising along in gear and you hit the brakes the manual tranny, unlike the auto, can't compensate for the sudden change in speed between the engine and driveshaft on its own.
Off all the manual trannies I have tried not many of the engines could move the car by themselves in first gear without given gas.

I guess if you were fast enough you could hit the brakes hard without the clutch, ABS kicks in, and just before you get to the point of no return (stall speed of the gear your in) you can punch the gas. You can be off again without ever touching the clutch. I think this could be more likely while in first gear. Is this what you mean?
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  #10  
Old 02-26-2002, 01:42 PM
lperei01
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I think I got it now. The recommendation of stepping on the clutch would just avoid your engine to shut down.

Nevertheless I've driven manual transmission cars for more than 12 years (in Portugal less than 1% of the cars are automatic; I got my automatic 300 SL last summer) and never had this type of problem: upon reaching a complete stop I just hit the clutch.

What I understood from the conversation with my friend was that you could actually stop in a shorter distance if you step the clutch simultaneously with the brake pedal. I assumed that the modern ABS sytems could somehow be fooled by the engaged transmission, thus reducing the ABS efectiveness...
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  #11  
Old 02-26-2002, 02:13 PM
lperei01
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Yal:

In what concerns your observation about reducing the auto wheight, I'm taking measures now: my wife will never again be allowed to seat in the car while driving

Actually after your observation I was curious about quantifying the effect of driving arround with your wife on the braking distance. A quick calculation revealed me that a 5% change in the total weight of the car could actually make a difference of more than 10 meters if your are bringing the car from 100 Mph to a complete stop. Wifes out of the cars?:p
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  #12  
Old 02-26-2002, 02:54 PM
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yal yal is offline
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Hahaha Apologise to your wife for me, I shouldn't have enlightened you!
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