http://www.metoffice.gov.uk/weather/uk/advice/snow.html

"If braking is necessary, pump the brakes don't slam them on."

...unless you have friggin ABS! :-P

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> "If braking is necessary, pump the brakes don't slam them on."
> 
> ...unless you have friggin ABS! :-P

BTW ABS doesn't work below about 5 or 10 mph...

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scott wrote:
>> "If braking is necessary, pump the brakes don't slam them on."
>>
>> ...unless you have friggin ABS! :-P
> 
> BTW ABS doesn't work below about 5 or 10 mph...

Really?

Any specific reason?

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>> BTW ABS doesn't work below about 5 or 10 mph...
>
> Really?
>
> Any specific reason?

I imagine it's because if the ABS system sees all four wheels go from 5mph 
to 0mph it can't tell whether you've actually come to a stop or you are 
skidding on ice.  At 50mph it's obvious if the wheels all stop within a 
fraction of a second you are skidding, at low speeds it's not so easy to 
work out.

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scott wrote:
>>> BTW ABS doesn't work below about 5 or 10 mph...
>>
>> Really?
>>
>> Any specific reason?
> 
> I imagine it's because if the ABS system sees all four wheels go from 
> 5mph to 0mph it can't tell whether you've actually come to a stop or you 
> are skidding on ice.  At 50mph it's obvious if the wheels all stop 
> within a fraction of a second you are skidding, at low speeds it's not 
> so easy to work out.

Mmm, OK.

I must admit, I've always wondered how the hell ABS actually works.

But not as much as power steering. This, as far as I can tell, should be 
logically impossible. And yet it apparently works. How baffling!

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Invisible wrote:
> Uh... WTF? It's not exactly like the Arctic out there! :-P

There are large numbers of people who have difficulty driving in 
anything more slippery than rain.  This is most likely in areas which 
are known for the mildness of their winters.

Add to this the general temperament of the government to view itself as 
some kind of mother to the people, and you get urgent warnings telling 
people to regard as wild uncontrollable dangers things that in fact 
present a small and manageable risk.

I grew up in Michigan, and can tell you that even as experienced with 
the snow as they are there, the first significant snowfall of the winter 
is a refresher course in winter driving for just about everybody.

Regards,
John

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> I must admit, I've always wondered how the hell ABS actually works.

A speed sensor on each wheel, if one drops too quickly compared to the 
others, or they all drop too quickly than what is "usually possible" a 
solenoid is opened that releases the brake pressure until the wheels speeds 
are back to normal, then the solenoid is closed.  The process might then 
repeat if the driver is still pressing the brake pedal hard - this is the 
"pulsing" sensation you feel.

Note that modern cars are much more complex, with ABS being part of a much 
larger control system, capable of regulating the braking force to each 
individual wheel (to allow each wheel to be giving the maximum stopping 
power, and to prevent skids, wheel-spin etc).

> But not as much as power steering. This, as far as I can tell, should be 
> logically impossible. And yet it apparently works. How baffling!

There's a part in the steering system that twists or bends by a specific 
amount depending on how hard you tug at the wheel.  A set of valves then 
controls some pressurised hydraulics based on how much force you are putting 
into the wheel, which helps you steer.

Newer cars have a torque transducer and an electric motor with some clever 
software.  This saves on fuel as there is no hydraulic compressor having to 
be run the whole time (usually you only need power assist at low speeds).

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>> I must admit, I've always wondered how the hell ABS actually works.
> 
> A speed sensor on each wheel, if one drops too quickly compared to the 
> others, or they all drop too quickly than what is "usually possible" a 
> solenoid is opened that releases the brake pressure until the wheels 
> speeds are back to normal, then the solenoid is closed.  The process 
> might then repeat if the driver is still pressing the brake pedal hard - 
> this is the "pulsing" sensation you feel.
> 
> Note that modern cars are much more complex, with ABS being part of a 
> much larger control system, capable of regulating the braking force to 
> each individual wheel (to allow each wheel to be giving the maximum 
> stopping power, and to prevent skids, wheel-spin etc).

...not to mention traction control... ;-)

>> But not as much as power steering. This, as far as I can tell, should 
>> be logically impossible. And yet it apparently works. How baffling!
> 
> There's a part in the steering system that twists or bends by a specific 
> amount depending on how hard you tug at the wheel.  A set of valves then 
> controls some pressurised hydraulics based on how much force you are 
> putting into the wheel, which helps you steer.
> 
> Newer cars have a torque transducer and an electric motor with some 
> clever software.  This saves on fuel as there is no hydraulic compressor 
> having to be run the whole time (usually you only need power assist at 
> low speeds).

The part I can't figure out is that, as far as I can tell, hooking up a 
motor that turns the steering wheel in the same direction as you're 
trying to turn it should generate a positive feedback loop. I can't 
figure out how turning the wheel doesn't cause the powerful motors to 
rip the wheel out of your hands and put the car into full lock before 
snapping the steering column in half...

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John VanSickle wrote:
> There are large numbers of people who have difficulty driving in 
> anything more slippery than rain.  This is most likely in areas which 
> are known for the mildness of their winters.

I did notice that in the videos of people pushing cars, all the cars 
seemed to be attempting to drive uphill at no more than walking pace - 
shorly not a sensible thing to do in snow?

> Add to this the general temperament of the government to view itself as 
> some kind of mother to the people, and you get urgent warnings telling 
> people to regard as wild uncontrollable dangers things that in fact 
> present a small and manageable risk.

Yeah, maybe. I guess the government doesn't want to hear the whole "why 
didn't you warn people?" thing, or "why didn't you take action?". (OTOH, 
it didn't seem to be a problem for New Orleans...)

> I grew up in Michigan, and can tell you that even as experienced with 
> the snow as they are there, the first significant snowfall of the winter 
> is a refresher course in winter driving for just about everybody.

I can remember 1995 or so, there was a lot of snow one day. About 20 cm 
or so. Not huge, but a lot. Nobody seemed to really pay much attention. 
A few people got slightly stuck or something, but nobody considered it a 
big deal. Yet today, it's a catastrophy... hmm.

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> ...not to mention traction control... ;-)

Er yeh, that's what I meant by wheel-spin.  OOC I looked up the instructions 
for how a dealer fits the "USB Audio" upgrade to my car (I was just 
wondering if the cables were already there and I simply needed to fit a 
suitable plug - sadly not).  Anyway, it appears my car has a network of 
fibre optic cables o_O.  To fit the upgrade, apparently involves ripping out 
half the car, making some connection to a box of opto-electronics in the 
boot, then fitting a "USB hub" under the drivers seat and fitting the USB 
socket in the centre console, and of course fitting cables between all of 
them.  The 250 pound price tag doesn't sound so bad now!

> The part I can't figure out is that, as far as I can tell, hooking up a 
> motor that turns the steering wheel in the same direction as you're trying 
> to turn it should generate a positive feedback loop. I can't figure out 
> how turning the wheel doesn't cause the powerful motors to rip the wheel 
> out of your hands and put the car into full lock before snapping the 
> steering column in half...

If you simply program the motor to apply the same torque you are applying 
with your hands to the wheel, then there is no feedback, it will just feel 
like it takes half the force to turn it.  Of course in reality it is more 
complex, the multiplier is probably dependant on speed, and maybe other 
factors like how sharp you are turning.  And maybe there is some limit the 
torque you apply needs to go above before the motor kicks in?

Something like:

If AppliedTorque > 10 Then
 MotorTorque = (AppliedTorque - 10) * CalculateAssistFactor( VehicleSpeed , 
SteerAngle )
Else
 MotorTorque = 0

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