On 2014-10-28 09:32:26 +0000, Roland Perry said:

The bit about the brakes absorbing the power is in the situation where
the 'slipping' wheel is *both* being braked (in your scheme) and *also*
applying power to the road surface.

I suppose you will lose a small amount of power through friction (and
warming up the brakes) in that situation.

Neil
--
Neil Williams
Put my first name before the @ to reply.

On 2014-10-28 09:39:29 +0000, Roland Perry said:

Is ASC also controlling the throttle?

In some cases yes. However even in traditional traction control where
it isn't, it still works.

If I'm driving a car with both wheels *just* about to start spinning
and I floor the accelerator, what prevents them starting to spin?

If *both* wheels spin, what is needed is to reduce the throttle. The
braking works where *one* wheel is spinning, to divert power from one
side to the other. If you braked *both* wheels, you would be fighting
the engine and burning off the excess in the brakes, but that's where
the option of controlling power comes in. Of course cars are only
fitted with one engine, so reducing power is a far blunter instrument.

FWIW the brakes on modern cars are perfectly strong enough to fight
against the accelerator - try pulling off against the handbrake and see
how far you get. Though I can see that if used for extended periods
they might overheat.

Having the brakes absorbing the excess power for a few seconds if the
objective is to stop a skid on a roundabout (which is the sort of
scenario the video clip posted earlier is simulating), but I'm
wondering about how long they'd survive if the car was being driven
enthusiastically up an Alpine pass with power applied for very long
periods.

Just like one not fitted with traction control, you have to drive a car
with some mechanical sympathy if you want it to last any length of
time. It's a safety feature, not one designed to protect the car
against poor driving. I suppose the risk is that it makes poor driving
*less* visible until the point your brakes overheat.

But even so...you wouldn't be making much progress up the pass even
with ASC if there was so little traction that the brakes kept needing
to be applied on both driven wheels. So soon enough you'd give up.

Neil
--
Neil Williams
Put my first name before the @ to reply.

On Tue, 28 Oct 2014 11:27:11 +0000, Neil Williams
wrote:

On 2014-10-28 09:39:29 +0000, Roland Perry said:

Is ASC also controlling the throttle?

In some cases yes. However even in traditional traction control where
it isn't, it still works.

If I'm driving a car with both wheels *just* about to start spinning
and I floor the accelerator, what prevents them starting to spin?

If *both* wheels spin, what is needed is to reduce the throttle. The
braking works where *one* wheel is spinning, to divert power from one
side to the other. If you braked *both* wheels, you would be fighting
the engine and burning off the excess in the brakes, but that's where
the option of controlling power comes in. Of course cars are only
fitted with one engine, so reducing power is a far blunter instrument.

FWIW the brakes on modern cars are perfectly strong enough to fight
against the accelerator - try pulling off against the handbrake and see
how far you get. Though I can see that if used for extended periods
they might overheat.

In my 2007 model car, the system is called DSC, and I've found this
description of how it works in an online forum:

"The Dynamic Stability Control (DSC) system includes the:

- anti-lock brake system.
- yaw/lateral control.
- full speed traction control.

The DSC system manages the braking system to enhance the driver
control of the vehicle.

The DSC system continually monitors the steering wheel angle, master
cylinder brake pressure, front and rear wheel speeds, vehicle yaw and
lateral rate acceleration.

The yaw/lateral rate sensor supplies a signal to the DSC module, via a
serial link, which monitors the vehicle's rate of acceleration from
its central axis in a sideways direction, and also the vehicle's
angular rotation around its central axis.

The driver input parameters are continually monitored via the brake
master cylinder dynamic stability control sensor, the brake pedal
travel sensor and the steering wheel angle sensor.

DSC is enabled/disabled via the traction control ON/OFF switch.

Self-diagnosis of the DSC system is provided via the instrument
cluster message centre.

Traction control is an additional function added to the ABS/DSC
system. The vehicle's driven wheels are continually monitored for
wheel spin relative to the calculated reference speed and to each
other. If wheel spin is detected, the traction control function
intervenes independently of the driver, applying brake pressure to the
slipping wheel and reducing the engine drive torque supply. Meanwhile,
brake pressure is modulated by the traction control until traction is
re-established. Traction control brake actuation is diminished above
40 km/h (25 mph). Above this speed traction control relies primarily
on engine torque reduction.

Traction control is enabled/disabled via the traction control ON/OFF
switch. When the switch is in the (OFF) position, the amber traction
control warning lamp solidly illuminates within the instrument cluster
message centre. The traction control is automatically activated when
the ignition is switched on. Self-diagnosis of the traction control
system is also provided via the instrument cluster message centre.

The traction control brake intervention is automatically disabled
whenever the brakes exceed a temperature limit. The traction brake
intervention will remain disabled until the brakes have cooled,
irrespective of ignition switch position or ignition switch cycling."

From
http://www.jaguarforums.com/forum/ge...ow-works-3853/

I dare say more modern cars than mine have more sophisticated systems,
including features like torque vectoring.

On Tue, 28 Oct 2014 11:19:41 +0000, Neil Williams
wrote:

On 2014-10-28 09:20:43 +0000, Roland Perry said:

The concept I've having difficulty with is that braking a wheel causes
that wheel to transmit more power to the road. Let's say it's a long
uphill slippery road with 100HP from the engine; does this braking
activity swap the 100HP from one driven wheel to the other and back as
each one encounters a slipperier side of the road?

Ah, I see. Essentially, yes, that's how it works, using a side-effect
of how a diff functions.

With a normal diff, if one wheel spins all power is lost via that
wheel, none goes to the one with traction (like my water example, all
the power goes the easiest way it can, which is a free-spinning wheel).
Braking therefore allows the power to be passed to the *other* wheel,
with the aim that when the brake is released that wheel might have had
chance to gain some grip and try again.

I *think* it's also the case that maximum traction is gained at the
point *just before* a wheelslip, which stopping the spin to try again
from that point will help, though I admit I find that quite hard to get
my head around. (ABS mainly prevents wheels locking up for another
reason - to allow them to keep their ability to steer).


Yes, so ASC would first brake a spinning wheel to a standstill, then
reduce the brake force until the wheel just starts spinning again,
after which it increases the brake force, and so on. In effect, it
keeps the wheel near the point of maximum adhesion through letting it
turn at the optimum speed by modulating the brakes.

One important point is that this is available at all times (unlike a
lockable diff), and it doesn't stop the car turning (unlike a locked
diff).

On Tue, 28 Oct 2014 09:28:23 +0000, Roland Perry
wrote:

In message

, at 04:47:11 on Mon, 27 Oct 2014, Recliner
remarked:
If you want 75% of the power put on the road through the left wheel,
and 25% through the right wheel, how does braking the right wheel
achieve that without absorbing some of the engine power?

It will need to apply a small force to the wheel, but I can't see why
it would absorb any significant power as that would just go to the
other wheel via the diff.

That sounds fine if you aren't attempting to put any power on the road
through the 'spinning' wheel. I'm looking at the case where you want
about half the power that would otherwise be sent through the rubber to remain.

With ASC, you would be putting some power through the wheel that would
wastefully spin with a locking diff.

If the diff is locked both driving wheels rotate at the same speed. If
one is spinning, very little power is "lost" - the only place it can be
dissipated is warming up the tyre/road surface and if slippery/icy
that'll be very little. The rest of the power inevitably goes to the
wheel with grip.

Agreed, but the opportunity is also lost to put some limited power
through the wheel with low adhesion.

But the key thing is you can't have a locked diff on a normal road
car, as it would ruin the tyres and make the car undrivable on curves.
Off-road vehicles can usually lock the centre diff and possibly the
rear diff, but that's a conscious decision taken when driving
off-road, not a default setting. So if a car unexpectedly hits a
slippy patch, even cars with lockable diffs will be unprepared. But
ASC is ready and waiting to spring into action at all times.

On 2014-10-28 15:32:20 +0000, Recliner said:

But the key thing is you can't have a locked diff on a normal road
car, as it would ruin the tyres and make the car undrivable on curves.
Off-road vehicles can usually lock the centre diff and possibly the
rear diff, but that's a conscious decision taken when driving
off-road, not a default setting. So if a car unexpectedly hits a
slippy patch, even cars with lockable diffs will be unprepared. But
ASC is ready and waiting to spring into action at all times.

There is always the option of a limited-slip diff. But that's way more
complicated, which is what makes ASC a better option - every modern car
already has the hardware, it's just software.

Neil
--
Neil Williams
Put my first name before the @ to reply.

Neil Williams wrote:
On 2014-10-28 15:32:20 +0000, Recliner said:

But the key thing is you can't have a locked diff on a normal road
car, as it would ruin the tyres and make the car undrivable on curves.
Off-road vehicles can usually lock the centre diff and possibly the
rear diff, but that's a conscious decision taken when driving
off-road, not a default setting. So if a car unexpectedly hits a
slippy patch, even cars with lockable diffs will be unprepared. But
ASC is ready and waiting to spring into action at all times.

There is always the option of a limited-slip diff. But that's way more
complicated, which is what makes ASC a better option - every modern car
already has the hardware, it's just software.

Yes, LSDs are now confined to just a few high performance cars, as ASC is
so much cheaper and easier. The electronics can also deliver different
characteristics for use in different conditions, without changing the
hardware.

In message , at 11:27:11 on Tue, 28
Oct 2014, Neil Williams remarked:

FWIW the brakes on modern cars are perfectly strong enough to fight
against the accelerator - try pulling off against the handbrake and see
how far you get.

That's a fight between the clutch and the handbrake. Repeat the exercise
at 60mph with the throttle floored, and see how effective the handbrake
is then!

Having the brakes absorbing the excess power for a few seconds if the
objective is to stop a skid on a roundabout (which is the sort of
scenario the video clip posted earlier is simulating), but I'm
wondering about how long they'd survive if the car was being driven
enthusiastically up an Alpine pass with power applied for very long periods.

Just like one not fitted with traction control, you have to drive a car
with some mechanical sympathy if you want it to last any length of
time. It's a safety feature, not one designed to protect the car
against poor driving.

It seems you are coming round to my point of view, which is that ASC is
a form of ABS+ for emergencies, and not traction control for everyday
use.

Do you regard 4WD as a merely a "safety feature" to get you out of a
skid, and not something to use to increase your traction whenever
required, even if for extended periods?

I suppose the risk is that it makes poor driving *less* visible until
the point your brakes overheat.

But even so...you wouldn't be making much progress up the pass even
with ASC if there was so little traction that the brakes kept needing
to be applied on both driven wheels. So soon enough you'd give up.

I'd be expecting at least half of full power from the engine to be
usefully reaching the wheels.

--
Roland Perry

In message , at 13:23:19 on
Tue, 28 Oct 2014, Recliner remarked:

Traction control brake actuation is diminished above 40 km/h (25 mph).
Above this speed traction control relies primarily on engine torque
reduction.

So a sort of "rev limiter" really, and they've stopped using the brakes
to burn of the excess. That accords with my own 'sniff test' wrt the
engineering aspects involved.

Back in the day I had a somewhat powerful car that could spin the rear
wheels at 60mph on a dry road in 3rd. You could tell it was doing that
from the way the engine surged. Of course, do that going round a corner
and it was a choice of applying opposite lock or ending up in the ditch.

In fact the first time I took the car to be serviced the chap
congratulated me for having made it that far without putting it in a
ditch! Once you got the hang of it, it was a case of being able to steer
both ends, the front with the wheel and the back with your right foot.

--
Roland Perry

In message , at 15:32:20 on
Tue, 28 Oct 2014, Recliner remarked:
If the diff is locked both driving wheels rotate at the same speed. If
one is spinning, very little power is "lost" - the only place it can be
dissipated is warming up the tyre/road surface and if slippery/icy
that'll be very little. The rest of the power inevitably goes to the
wheel with grip.

Agreed, but the opportunity is also lost to put some limited power
through the wheel with low adhesion.

In practice the power gets applied through as many of the wheels as do
have adhesion.

But the key thing is you can't have a locked diff on a normal road
car, as it would ruin the tyres and make the car undrivable on curves.
Off-road vehicles can usually lock the centre diff and possibly the
rear diff, but that's a conscious decision taken when driving
off-road, not a default setting. So if a car unexpectedly hits a
slippy patch, even cars with lockable diffs will be unprepared. But
ASC is ready and waiting to spring into action at all times.

The times I found locked diffs the most useful was driving on snow/ice
in the heart of winter where you could pretty much expect never to come
across a patch of dry road. Those are the sort of conditions where
modern hatchbacks with front wheel drive just sit there literally
spinning their wheels.
--
Roland Perry