On 2014-10-27 09:14:39 +0000, Roland Perry said:

In message , at 08:35:17 on Mon, 27
Oct 2014, Neil Williams remarked:
Braking a wheel isn't the same as locking a diff. Apart from anything
else, the locked diff still powers both wheels.

Though achieves nothing by doing so, as it is spinning.

It means some power can be transmitted as soon as adhesion returns, but
most importantly because the diff is locked the other wheel is still
getting power.

Braking the wheel that is spinning causes the other wheel to get all
the power with a regular diff.

Though we may be heading off at a tangent here - a diff lock clearly
has significant advantages (if it didn't, 4x4s would not be so fitted),
but also major disadvantages in terms of cost, complexity and weight,
given that most car drivers will need what it provides once every blue
moon. OTOH traction control and similar can be provided using exactly
the same hardware as ABS which every car already has - just requires
additional software in the ECU.

Neil
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Neil Williams
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On 2014-10-27 09:20:02 +0000, Roland Perry said:

It can only do that if the controlling element is in the differential.
Otherwise all the torque is still going to the wheel, but the brakes
are stopping the wheel from rotating too fast, which means those brakes
are absorbing the power, which is thus not (purposely it seems)
available at the road surface.

You clearly *don't* understand how a (non-locking) differential works.

Neil
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Neil Williams
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On 2014-10-27 09:22:46 +0000, Roland Perry said:

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.

Then you apply partial braking, the effect of which is to send it to
the other wheel (with a small loss via friction).

Neil
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Neil Williams
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Roland Perry wrote:
In message , at 08:37:12 on Mon, 27 Oct
2014, Neil Williams 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. The brakes don't lock the wheel if
there's some traction to be had, but just slow it enough to grab whatever
limited traction is available on the slippy surface.

Roland Perry wrote:
In message
, at 03:39:08 on Mon, 27 Oct 2014, Recliner remarked:

ASC supersedes simple traction control, though it's sometimes called
traction control (that was the name used when I first got it, many years
ago, before the more sophisticated ASC superseded it). It reduces the
torque going to a wheel without traction,

It can only do that if the controlling element is in the differential.
Otherwise all the torque is still going to the wheel, but the brakes are
stopping the wheel from rotating too fast, which means those brakes are
absorbing the power, which is thus not (purposely it seems) available at the road surface.

All the torque that the available adhesion on each tyre can support is
independently monitored and delivered to each wheel. Trying to deliver any
more torque would cause the wheel to spin, thus reducing the power
transmitted. So, by absorbing some of the power in the brake, more can be
successfully transmitted through the wheel.

And the really good thing is that all this achieved with no extra hardware,
just clever electronics using the existing ABS system that all modern cars
have. So it doesn't increase the weight or mechanical complexity of the car
at all, and has an insignificant impact on costs of already highly
computerised modern cars.

So it's not only more effective than a locking or limited slip diff, but
also cheaper, simpler and lighter.

Roland Perry wrote:
In message , at 08:35:17 on Mon, 27 Oct
2014, Neil Williams remarked:
Braking a wheel isn't the same as locking a diff. Apart from anything
else, the locked diff still powers both wheels.

Though achieves nothing by doing so, as it is spinning.

It means some power can be transmitted as soon as adhesion returns, but
most importantly because the diff is locked the other wheel is still getting power.

Even better with ASC, where both wheels are providing whatever traction
they can.

Roland Perry wrote:
In message
, at 03:39:08 on Mon, 27 Oct 2014, Recliner remarked:
Not permanently locked ones, but a brake in the diff (rather that at the wheel).

What would be gained by duplicating the function?

Braking a wheel isn't the same as locking a diff. Apart from anything
else, the locked diff still powers both wheels.

No it's not the same, but it's what traction control is. And ASC can still
power both wheels, independently reducing torque on each wheel to the level
that the tyre's grip can sustain. That provides more traction than a simple
locked diff.

A locked diff is providing traction via the non-slipping wheel, except
the "slipping" one isn't - because it's rotating at the same speed as the
"non-slipping" one - and is therefore well placed to start providing
traction as soon as road adhesion returns to that side.

With ASC, each wheel is provided with just enough traction to stop it
spinning, so *both* wheels are providing whatever traction they can, and
neither is either spinning or locked. That's much better than a locked diff
which is, at best, optimising traction for just one wheel.

On 2014-10-27 09:44:36 +0000, Neil Williams said:

On 2014-10-27 09:22:46 +0000, Roland Perry said:

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.

Then you apply partial braking, the effect of which is to send it to
the other wheel (with a small loss via friction).

Imagine, say, you have a mains-pressure water tap with a Y piece added
to it. The Y piece is large enough to take the maximum flow from the
tap on either side (the cross-section of each side of the Y piece being
the same as or greater than that of the pipe feeding the tap). You
place your hand over one side of the Y piece completely - all the water
goes out the other way (this is the effect of braking one wheel fully
with a non-locked diff). You release the hand a bit and some water can
flow out of the "blocked" side - this is the effect of partially
braking the wheel. You will note that in neither case is significant
force applied to your hand.

You partially block both sides - such that the "engine" is having to do
some work. In that case, there is force applied to both sides (on one
side the road, on the other side the balancing effect of the brakes),
though. This is what it would be like when the system applied partial
braking. I guess what you have is that the brake on the "spinning"
side is having to apply the same force as the wheel is applying to the
road on the other side, which as it can stop the vehicle it's probably
more than capable of.

So I suppose a bit of both. But the fact is (a) it works and (b) it's
simpler and cheaper than locking or limited slip diffs.

Neil
--
Neil Williams
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Neil Williams wrote:
On 2014-10-27 09:44:36 +0000, Neil Williams said:

On 2014-10-27 09:22:46 +0000, Roland Perry said:
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.
Then you apply partial braking, the effect of which is to send it to
the other wheel (with a small loss via friction).

Imagine, say, you have a mains-pressure water tap with a Y piece added to
it. The Y piece is large enough to take the maximum flow from the tap on
either side (the cross-section of each side of the Y piece being the same
as or greater than that of the pipe feeding the tap). You place your
hand over one side of the Y piece completely - all the water goes out the
other way (this is the effect of braking one wheel fully with a
non-locked diff). You release the hand a bit and some water can flow out
of the "blocked" side - this is the effect of partially braking the
wheel. You will note that in neither case is significant force applied to your hand.

You partially block both sides - such that the "engine" is having to do
some work. In that case, there is force applied to both sides (on one
side the road, on the other side the balancing effect of the brakes),
though. This is what it would be like when the system applied partial
braking. I guess what you have is that the brake on the "spinning" side
is having to apply the same force as the wheel is applying to the road on
the other side, which as it can stop the vehicle it's probably more than capable of.

So I suppose a bit of both. But the fact is (a) it works and (b) it's
simpler and cheaper than locking or limited slip diffs.

I think it's more than than that: it's certainly simpler, cheaper and
lighter than a locking or limited slip diff, but I think it's better too,
as some limited amount of torque can be delivered through the low friction
wheel by carefully modulating or pulsing the braking force on it.

It also works when both wheels have very little friction: with a locking
diff, they'd probably both spin, but with ASC some small amount of traction
may be deliverable through one or both wheels.

On Sun, Oct 26, 2014 at 03:06:25PM +0000, Roland Perry wrote:

ABS-style braking is the opposite of traction control. The former
automates the stopping of vehicles, that latter the acceleration.

They're both acceleration, just with a different sign :-)

Consider the case where you've got one wheel slipping - on ice, for
example. Without a diff lock or some other form of traction control lots
of power will go to that wheel and be wasted, instead of going to the
wheel that isn't slipping and where that power can actually be used.

To stop that happening you'd want to either automagically apply the diff
lock (but most vehicles don't have one, and adding one adds expense,
weight, and maintenance costs) or automagically apply a brake (which
modern vehicles already have automatic control over). So it's obvious
that it would be implemented by selectively braking individual wheels.
The only thing that makes it the opposite of ABS is that ABS
automatically *disengages* the brake, whereas traction control would
automatically *engage* it.

In other words, traction control is ABS with a different sign.

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