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Old 11-21-2004, 05:30   #1 (permalink)
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confused Down shifting to reduce stopping distance

How does down shifting from D to 3 or 2 help stopping distance if you are not touching the accelerator at all and are braking?
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Old 11-21-2004, 10:23   #2 (permalink)
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Re: Down shifting to reduce stopping distance

The engine slows the car down. Using 3 or 2 gives a direct drive from the engine to the wheels.
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Old 11-21-2004, 20:06   #3 (permalink)
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Re: Down shifting to reduce stopping distance

Just wondering, is this bad for the 4-speed electronic auto?
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Old 11-21-2004, 20:32   #4 (permalink)
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Re: Down shifting to reduce stopping distance

Should be fine, as long as you don't do it at 5000 revs every time you hit a set of lights Engine braking is useful but can damage the box if done excessively from high revs. You are placing alot of load on the box.
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Old 11-21-2004, 21:57   #5 (permalink)
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Re: Down shifting to reduce stopping distance

Hi, I dont think much of using the gearbox to slow down a modern car. That is what the brakes are for! This seems to come from the days when cars had terrible brakes and ppl were taught to use the gearbox to slow cars down. Brake pads are much cheaper than gearboxes, especially automatic transmissions and modern cars have fantastic brakes.
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Old 11-21-2004, 22:47   #6 (permalink)
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Re: Down shifting to reduce stopping distance

I still don't understand... when you are not touching your accelerator and instead have your foot on the brakes, can the engine still drive the wheels or is there some sort of clutch (I know auto's don't have a clutch, but maybe something similar?) that disconnects the engine from the wheels?

Say you have your foot on the brakes, the engine RPM should be very low (around idle, or 1000rpm) but since you have your foot on the brakes the engine is "disconnected" and that 1000rpm is just idleing, not to drive the wheels. How then does changing the gears help slow you down?

Auto's give me a headache, can't figure out how they work!
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Old 11-21-2004, 23:01   #7 (permalink)
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Re: Down shifting to reduce stopping distance

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Originally Posted by TheTechGod
I still don't understand... when you are not touching your accelerator and instead have your foot on the brakes, can the engine still drive the wheels or is there some sort of clutch (I know auto's don't have a clutch, but maybe something similar?) that disconnects the engine from the wheels?

Say you have your foot on the brakes, the engine RPM should be very low (around idle, or 1000rpm) but since you have your foot on the brakes the engine is "disconnected" and that 1000rpm is just idleing, not to drive the wheels. How then does changing the gears help slow you down?

Auto's give me a headache, can't figure out how they work!
No no. When your auto is in "D" it is turning the drive shaft through a one way clutch, so on overrun (braking, or any time when the wheels are driving the tailshaft) the shaft freewheels in the gearbox. When you use a gear manually in an auto (like when you select "2") then the gear is engaged and the one way clutch is bypassed, giving a direct drive, therefore on overrun, when the wheels are driving the tailshaft, they are also fighting against the engine. This is exactly the same as engine braking in a manual. The car has a better stopping distance because the wheels are driving the engine with a large amount of resistance (the engine is trying to slow down because the throttle is closed). It's sort of hard to describe, but is a relatively simple concept - just think of it exactly like a manual box.
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Old 11-22-2004, 00:24   #8 (permalink)
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Re: Down shifting to reduce stopping distance

I think I understand most of what you are saying, except the part about "When you use a gear manually in an auto (like when you select "2") then the gear is engaged and the one way clutch is bypassed, giving a direct drive"

Is the one way clutch always engaged when driving/braking UNLESS you change the gear manually?
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Old 11-22-2004, 01:12   #9 (permalink)
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Re: Down shifting to reduce stopping distance

Brakes slow cars, thats what there for stopping or slowing.
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Old 11-22-2004, 01:17   #10 (permalink)
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Re: Down shifting to reduce stopping distance

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Originally Posted by sbutler
Brakes slow cars, thats what there for stopping or slowing.
Oh you mean that pedal next to the accelerator can be used to slow the car down? Cool! (joke)

Seriously, I just want to know WHY down shifting helps slow it down, how does it work?

I never suggested down shifting instead of braking...
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