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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?
> 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.
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.
> 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?).
> 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.
> So it's easier to lock the wheels up and skid.
Definedly.
> Jim
-Aero
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Jim Henderson wrote:
>
> I wouldn't do 4th to 2nd. Always one gear at a time, otherwise as you
> state, you can cause the wheels to lock up.
I do that (downshifting 2-3 gears at once) every day while slowing down
from the motorway - either 6-5-3-2, 6-4-2 or 6-3-2, depending on place,
traffic etc.
And I've managed to lock driving wheels up even without downshifting, by
just lifting the gas pedal (icy, Volvo 740 and Nokian HKPL4 (studded and
very good winter tires)). Pressing clutch to get traction back and using
brakes (=braking with front wheels) were the savior of that situation.
But yes, usually with FWD car it's pretty hard to mess up with engine
braking, we certainly agree on that.
> Jim
-Aero
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scott wrote:
>
> Maybe in some cars with sensitive brake pedals it's easier to control
> the small amount of negative acceleration through the
> clutch/gears/accelerator than through the brake pedal?
>
Language barrier again :), I'd think sensitive brake pedal as the ones
that give you the most information. But yes, that's true. Top on that,
engine braking is more economical than using the brakes.
-Aero
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Jim Henderson wrote:
>
> It depends on the road conditions. For example, this morning as I turned
> left into the parking lot at the office, the car's momentum overcame the
> friction of the tires on the ice, and I ended up not turning as per usual
> but sliding instead. 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.
>
> If a car has ABS, you can get away with hitting the breaks, because the
> ABS system will prevent the wheels from locking up.
>
And if you'd had ESP, it should've braked for you (with just left
wheels, to make the car turn) ;). For some reason (dunno why) Audi's
(tested with B7 and B8 A4's) don't seem to do this - if you turn the
wheels too quickly on ice and slide forwards without turning, the ESP
won't realize it and does nothing.
> Jim
-Aero
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> And if you'd had ESP, it should've braked for you (with just left
> wheels, to make the car turn) ;).
It's funny when my car does this, if you leave all the ESP stuff turned on
and then try to fishtail the car after approx 0.1 seconds of sliding you can
feel it pull on the brake on just one wheel that gets the car back in shape.
> For some reason (dunno why) Audi's
> (tested with B7 and B8 A4's) don't seem to do this - if you turn the
> wheels too quickly on ice and slide forwards without turning, the ESP
> won't realize it and does nothing.
My changing-down-on-ice experiments were exactly to see if the ESP system
could cope with it, but alas it couldn't and I just slid with a wiggly back
end until it gripped again. I guess the ESP programmers didn't include a
function to blip the throttle if it detected engine braking was locking the
rear wheels :-)
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> Top on that,
> engine braking is more economical than using the brakes.
Depends how expensive it is to replace the clutch :-)
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scott wrote:
>
> It's funny when my car does this, if you leave all the ESP stuff turned
> on and then try to fishtail the car after approx 0.1 seconds of sliding
> you can feel it pull on the brake on just one wheel that gets the car
> back in shape.
Ahh, Audi takes more time, it lets the slide start but then crunches the
car back to line.
BMW?
> My changing-down-on-ice experiments were exactly to see if the ESP
> system could cope with it, but alas it couldn't and I just slid with a
> wiggly back end until it gripped again. I guess the ESP programmers
> didn't include a function to blip the throttle if it detected engine
> braking was locking the rear wheels :-)
I don't even need to brake with the engine - if I have too much speed
for a corner, keep the throttle steady and turn quickly, car goes
straight ahead and ESP does nothing.
-Aero
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scott wrote:
>> Top on that,
>> engine braking is more economical than using the brakes.
>
> Depends how expensive it is to replace the clutch :-)
>
Ummm.. no, bad clutch-usage ain't dependent on engine braking. :-)
-Aero
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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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On Fri, 13 Feb 2009 14:01:18 +0200, Eero Ahonen wrote:
> Jim Henderson wrote:
>>
>> I wouldn't do 4th to 2nd. Always one gear at a time, otherwise as you
>> state, you can cause the wheels to lock up.
>
> I do that (downshifting 2-3 gears at once) every day while slowing down
> from the motorway - either 6-5-3-2, 6-4-2 or 6-3-2, depending on place,
> traffic etc.
Ah, I've never driven a 6-speed vehicle. From higher gears it would
probably be less of an issue, but dropping from 3 to 1 can redline the
engine, at least IME.
> And I've managed to lock driving wheels up even without downshifting, by
> just lifting the gas pedal (icy, Volvo 740 and Nokian HKPL4 (studded and
> very good winter tires)). Pressing clutch to get traction back and using
> brakes (=braking with front wheels) were the savior of that situation.
Yes, effective use of the clutch in that type of situation would also
help - since it would engage/disengage the engine from the drivetrain.
> But yes, usually with FWD car it's pretty hard to mess up with engine
> braking, we certainly agree on that.
:-)
Jim
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