On Fri, 13 Feb 2009 13:53:30 +0200, Eero Ahonen wrote:

> Jim Henderson wrote:
>> 
>> 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).
> 
> Yes, also on drum brakes. But why on earth would you engage brakes
> fully, if you're not trying to stop the car as quickly as possible?

Because slamming on the brakes on ice doesn't have that effect.

>> Conversely, by downshifting, you increase the engine revs and the
>> wheels are not locked - they still turn.
> 
> Yes, if you'll do it carefully enough (see the similarity to brakes?)
> :). It's possibly a bit easier to be carefully enough, since the maximum
> braking power is very limited.

Yes, but that's the reason to do that.  Similarly, for going down steep 
inclines, using the engine to slow the vehicle down rather than the 
brakes keeps you from burning the brakes out.

> And yes, I actually managed to found one +point to engine braking - with
> FWD car it practically ever won't spin the car, since rear wheels won't
> lock.

Yep, that's another big advantage.

>> 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.
> 
> Either you're having really bad brakes (repair them!) or you're not
> anymore engaging them fully, when driving on dry pavement (so why to do
> it on ice?).

I don't do it on ice.  You can (and I can) lock the wheels driving at 50 
MPH with the brakes, but that also is likely to cause an uncontrollable 
skid - though a shorter one - and often that is more desirable than 
running into whatever it is that you're braking hard to avoid.

>> 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.
> 
> Yes, the scale is smaller, but it's still there.

Sure, I wouldn't disagree with that.

Jim

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