On Wed, 11 Feb 2009 09:14:57 +0100, scott wrote:

>> By downshifting, what happens is the wheels aren't stopped, but the
>> torque applied through the drivetrain slows them down as the engine
>> revs come down.
> 
> The brakes do the same.  And in both cases, as soon as the negative
> torque (either from engine or brakes) is higher than the friction
> between the tyre and road, the wheels lock.  As I said in my other post,
> maybe it's easier to control small amounts of negative torque with the
> gears, but still you're only applying it to 1 or 2 wheels, not 4, so
> you're never going to be able to slow down as quickly.

Um, no, the brakes don't do the same - if you have disc brakes, the 
brakes stop the wheels when fully engaged (I don't have drum brakes so I 
can't comment on those).  Conversely, by downshifting, you increase the 
engine revs and the wheels are not locked - they still turn.

Now, when you're on dry pavement, the wheels normally don't lock because 
the car's momentum overcomes the friction on the brakes, bringing you to 
a gentle (or sometimes less-than-gentle) stop.  On ice, though, it takes 
less friction of the brake pads against the wheel to stop the wheels 
because it takes less to overcome the friction of the wheel against the 
ground.

So it's easier to lock the wheels up and skid.

However, you're right that you won't slow down as quickly by using the 
engine to slow the vehicle down.  The point isn't to slow down quickly, 
it's to slow down safely.

You have to combine this with driving more defensively as well - not 
following the car ahead of you too closely, for example.  If you don't 
give yourself more space to stop, you'll run into someone else.

But that gets into another whole area of "stupid driver tricks".

>> In that case, pumping the breaks was an effective way to slow the car
>> down.  Similar effect to a car with ABS (which I really miss in my
>> current car, my last one had ABS but the new one doesn't), but the
>> pulsing isn't rapid.
> 
> They make cars without ABS?

Yes.  My 1999 Saturn SC1 has no ABS.  :-)

Jim

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