> 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.

> 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?

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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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Jim Henderson wrote:
> a gentle (or sometimes less-than-gentle) stop. 

I once gave myself a nosebleed by borrowing my mother's car. I was used to 
driving a dinky manual without power breaks. I borrowed the behemoth car my 
parents drove, and as I coasted up to the stop sign, I stomped the clutch 
all the way to the floor, of course. Except their car had very strong power 
breaks and was an automatic with no clutch pedal. Ouch.

-- 
   Darren New, San Diego CA, USA (PST)
   "Ouch ouch ouch!"
   "What's wrong? Noodles too hot?"
   "No, I have Chopstick Tunnel Syndrome."

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On Wed, 11 Feb 2009 10:42:40 -0800, Darren New wrote:

> Jim Henderson wrote:
>> a gentle (or sometimes less-than-gentle) stop.
> 
> I once gave myself a nosebleed by borrowing my mother's car. I was used
> to driving a dinky manual without power breaks. I borrowed the behemoth
> car my parents drove, and as I coasted up to the stop sign, I stomped
> the clutch all the way to the floor, of course. Except their car had
> very strong power breaks and was an automatic with no clutch pedal.
> Ouch.

Ouch indeed.  Kinda the reverse of what nearly everyone I know who 
learned to drive a stick went through when not timing the clutch properly 
when shifting.  Of course, you lurch forward (usually) rather than 
backwards, so a nosebleed isn't so much of a problem as whiplash. ;-)

Jim

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Jim Henderson wrote:
> Ouch indeed.  Kinda the reverse of what nearly everyone I know who 
> learned to drive a stick went through when not timing the clutch properly 
> when shifting.

Oh, I did that too. This was years after I learned to drive a stick. I just 
wasn't really paying attention to the controls as such. Just, you know, drivin'.

-- 
   Darren New, San Diego CA, USA (PST)
   "Ouch ouch ouch!"
   "What's wrong? Noodles too hot?"
   "No, I have Chopstick Tunnel Syndrome."

Post a reply to this message

On Wed, 11 Feb 2009 11:56:50 -0800, Darren New wrote:

> Jim Henderson wrote:
>> Ouch indeed.  Kinda the reverse of what nearly everyone I know who
>> learned to drive a stick went through when not timing the clutch
>> properly when shifting.
> 
> Oh, I did that too. This was years after I learned to drive a stick. I
> just wasn't really paying attention to the controls as such. Just, you
> know, drivin'.

Yep, I managed it when I tried driving a stick for the first time in 
years myself.  Takes a few tries to get the hang of it again.

At least when I drove in England, I was driving an automatic.  I don't 
know that I'd get used to shifting with the lever on the wrong side (Yes, 
Stephen, the left side is the WRONG side <g>).

Jim

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On 11 Feb 2009 15:02:24 -0500, Jim Henderson <nos### [at] nospamcom> wrote:


>At least when I drove in England, I was driving an automatic.  I don't 
>know that I'd get used to shifting with the lever on the wrong side (Yes, 
>Stephen, the left side is the WRONG side <g>).
>

Of course it is, it isn't the American Way ;)
-- 

Regards
     Stephen

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On Wed, 11 Feb 2009 20:23:40 +0000, Stephen wrote:

> On 11 Feb 2009 15:02:24 -0500, Jim Henderson <nos### [at] nospamcom> wrote:
> 
> 
>>At least when I drove in England, I was driving an automatic.  I don't
>>know that I'd get used to shifting with the lever on the wrong side
>>(Yes, Stephen, the left side is the WRONG side <g>).
>>
>>
> Of course it is, it isn't the American Way ;)

<Grumble> I shoulda expected THAT. ;-)

(Haven't you noticed my spelling?)

Jim

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> Um, no, the brakes don't do the same - if you have disc brakes, the
> brakes stop the wheels when fully engaged

Of course, because through the brakes you are applying more torque than the 
tyre/road surface can provide, so the wheel locks.

> Conversely, by downshifting, you increase the
> engine revs and the wheels are not locked - they still turn.

Only because *usually* the negative torque applied from the engine is not 
enough to overcome the grip between the tyre/road.  Try downshifting on ice 
or even on wet tarmac violently (eg 4th to 2nd at 40mph) and you will lock 
the wheels (certainly audible on wet/dry tarmac).

BTW the two effects add up, so if you are already braking hard with the 
pedal, downshifting hard at the same time will likely lock the wheels 
easily, this is why racing drivers always "blip" the throttle while braking 
and changing down, to avoid the engine braking that could lock the wheels.

> 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.

Yes, and the same applies to engine braking.  Next time it's wet or snowy 
try changing down (a couple of) gear(s) and letting the clutch out quickly, 
you'll hear/feel the driven wheels lock until the car slows enough for the 
wheels to grip again.  Obviously on snow and ice this happens easier.

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On 11 Feb 2009 17:52:18 -0500, Jim Henderson <nos### [at] nospamcom> wrote:

>On Wed, 11 Feb 2009 20:23:40 +0000, Stephen wrote:
>
>> On 11 Feb 2009 15:02:24 -0500, Jim Henderson <nos### [at] nospamcom> wrote:
>> 
>> 
>>>At least when I drove in England, I was driving an automatic.  I don't
>>>know that I'd get used to shifting with the lever on the wrong side
>>>(Yes, Stephen, the left side is the WRONG side <g>).
>>>
>>>
>> Of course it is, it isn't the American Way ;)
>
><Grumble> I shoulda expected THAT. ;-)
>
>(Haven't you noticed my spelling?)
>

No, you are asking a dyslectic about spelling? :)

Anyway I am used to reading American spelling now and translate into proper
English, automatically. It is BTW one way. :)
-- 

Regards
     Stephen

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