On Thu, 12 Feb 2009 10:55:08 +0100, scott wrote:

>> 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.

Exactly my point.

>> 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).

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.  What you're looking to do is 
*gradually* slow down, so doing things that work contrary to that goal 
like downshifting two gears are obviously going to not achieve that goal.

> 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.

Well, see my above paragraph.  Sure, if you do things that are contrary 
to the goal of gradually slowing the vehicle down, you're going to wind 
up in trouble.

>> 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.

I've used engine braking enough that I don't need to "try it", and like I 
said, of course you can *make* it do it, but that's not the goal.  The 
goal is to use the tools available in a way that is appropriate to get a 
gradual slowdown in speed, not a sudden change that's going to send you 
out of control.

Jim

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