Hill descent

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J

Jeremy Mortimer

Guest
I recently read (I think it was in an article on the Difflock site on
driving in winter, but I can't find it now) that if one wheel loses grip
while descending a hill using engine braking with the diffs unlocked, the
vehicle can run away.

Is this true? I'm getting a headache trying to work out the effect of
differentials in this situation. Can someone more mechanically minded than
me explain?

Jeremy
 
On or around Wed, 26 Jan 2005 08:33:33 GMT, Jeremy Mortimer
<[email protected]_this.org> enlightened us thusly:

>I recently read (I think it was in an article on the Difflock site on
>driving in winter, but I can't find it now) that if one wheel loses grip
>while descending a hill using engine braking with the diffs unlocked, the
>vehicle can run away.
>
>Is this true? I'm getting a headache trying to work out the effect of
>differentials in this situation. Can someone more mechanically minded than
>me explain?

with all the diffs unlocked, 1 wheel losing grip can lose you all the drive,
and also all the engine braking.

with the centre diff locked, one wheel on each axle has to turn. It's still
possible to get stuck and still possible to slide on downhills, but at least
2 wheels will grip, or looking at it another way, 2 wheels have to lose grip
for it to stop going forwards. This is why the "cross-axle" situation stops
you going anywhere - one front wheel and the opposite rear wheel are
unloaded, and don't grip.

adding a rear locking diff means, when it's locked, that both rear wheels
and one front wheel have to turn. Thus 3 wheels have to lose grip to stop
you.


diff lock shouldn't be used in grippy situations. In slippery conditions,
engage diff lock, especially for climbing or descending. If the
slipperyness is intermittent, make sure you disengage it again afterwards.

finally, downhill on ice, you've 2 tons of steel and nothing's really gonna
make much odds - you just have to keep it lined up and hope that the ice
finishes before you run out of road... diff lock might give you a slight
advantage.
--
Austin Shackles. www.ddol-las.fsnet.co.uk my opinions are just that
"All animals are equal, but some animals are more equal than others"
George Orwell (1903 - 1950) Animal Farm
 
Austin Shackles <[email protected]> wrote in
news:[email protected]:

> On or around Wed, 26 Jan 2005 08:33:33 GMT, Jeremy Mortimer
> <[email protected]_this.org> enlightened us thusly:
>
>>I recently read (I think it was in an article on the Difflock site on
>>driving in winter, but I can't find it now) that if one wheel loses
>>grip while descending a hill using engine braking with the diffs
>>unlocked, the vehicle can run away.
>>
>>Is this true? I'm getting a headache trying to work out the effect of
>>differentials in this situation. Can someone more mechanically minded
>>than me explain?
>
> with all the diffs unlocked, 1 wheel losing grip can lose you all the
> drive, and also all the engine braking.

Thanks. I understand the principle when the wheels are driven, but I wasn't
sure about its applicability to engine braking.

I've thought about it some more, and it seems to me that it *doesn't* apply
to engine braking.

To simplify the case, consider a 2-wheel drive car, stationary and in gear.
If loss of grip loses you engine braking, then you should be able to lift
one of the driven wheels of that car and have it free-wheel on the other.
I'm almost certain the diff will not allow that. Am I wrong?

This has real-world significance. It's not uncommon to have ice under one
side of the car and not the other; under those conditions I would like to
know whether I can rely on engine braking or whether I need descent
control. I like descent control, but I like engine braking better (yes, I
know I can use both).

Jeremy
 


Austin Shackles wrote:

>
> with the centre diff locked, one wheel on each axle has to turn. It's still
> possible to get stuck and still possible to slide on downhills, but at least
> 2 wheels will grip, or looking at it another way, 2 wheels have to lose grip
> for it to stop going forwards. This is why the "cross-axle" situation stops
> you going anywhere - one front wheel and the opposite rear wheel are
> unloaded, and don't grip.
>
> adding a rear locking diff means, when it's locked, that both rear wheels
> and one front wheel have to turn. Thus 3 wheels have to lose grip to stop
> you.
>

When descending slippery roads be aware that locked diffs can get you
into trouble.
Especially in corners where at least one but maybe all the wheels will
not turn at the appropriate speed. So you will loose control and end up
off-road.
Which is maybe where you want to be but not initiated by the car.
4WD with all diffs open would be the best solution here. You do not need
the locks to get traction. Gravity will help you here.
Realise that part-time 4WD's with a T-case do not have this option.
Kind regards,
Erik-Jan.

--

http://www.fotograaf.com/trooper
 
In message <[email protected]>, Austin Shackles
<[email protected]> writes
>On or around Wed, 26 Jan 2005 08:33:33 GMT, Jeremy Mortimer
><[email protected]_this.org> enlightened us thusly:
>
>>I recently read (I think it was in an article on the Difflock site on
>>driving in winter, but I can't find it now) that if one wheel loses grip
>>while descending a hill using engine braking with the diffs unlocked, the
>>vehicle can run away.
>>
>>Is this true? I'm getting a headache trying to work out the effect of
>>differentials in this situation. Can someone more mechanically minded than
>>me explain?
>
>with all the diffs unlocked, 1 wheel losing grip can lose you all the drive,
>and also all the engine braking.
>
>with the centre diff locked, one wheel on each axle has to turn. It's still
>possible to get stuck and still possible to slide on downhills, but at least
>2 wheels will grip, or looking at it another way, 2 wheels have to lose grip
>for it to stop going forwards. This is why the "cross-axle" situation stops
>you going anywhere - one front wheel and the opposite rear wheel are
>unloaded, and don't grip.
>
>adding a rear locking diff means, when it's locked, that both rear wheels
>and one front wheel have to turn. Thus 3 wheels have to lose grip to stop
>you.
>
>
>diff lock shouldn't be used in grippy situations. In slippery conditions,
>engage diff lock, especially for climbing or descending. If the
>slipperyness is intermittent, make sure you disengage it again afterwards.
>
>finally, downhill on ice, you've 2 tons of steel and nothing's really gonna
>make much odds - you just have to keep it lined up and hope that the ice
>finishes before you run out of road... diff lock might give you a slight
>advantage.
Diff lock shouldn't be used on tarmac/concrete or similar surfaces but
should be engaged when off metalled roads - be prepared for the
unexpected!!

Also remember when descending if you do start to slide then use your
right foot and accelerate down the hill until traction is regained.

Whatever you do don't lock up the brakes.
--
hugh
Reply to address is valid at the time of posting
 
In message <[email protected]>, Erik-Jan Geniets
<[email protected]> writes
>
>
>Austin Shackles wrote:
>
>>
>> with the centre diff locked, one wheel on each axle has to turn. It's still
>> possible to get stuck and still possible to slide on downhills, but at least
>> 2 wheels will grip, or looking at it another way, 2 wheels have to lose grip
>> for it to stop going forwards. This is why the "cross-axle" situation stops
>> you going anywhere - one front wheel and the opposite rear wheel are
>> unloaded, and don't grip.
>>
>> adding a rear locking diff means, when it's locked, that both rear wheels
>> and one front wheel have to turn. Thus 3 wheels have to lose grip to stop
>> you.
>>
>
>When descending slippery roads be aware that locked diffs can get you
>into trouble.
>Especially in corners where at least one but maybe all the wheels will
>not turn at the appropriate speed. So you will loose control and end up
>off-road.
>Which is maybe where you want to be but not initiated by the car.
>4WD with all diffs open would be the best solution here. You do not need
>the locks to get traction. Gravity will help you here.
>Realise that part-time 4WD's with a T-case do not have this option.
>Kind regards,
>Erik-Jan.
>
Locking a front diff on tarmac is asking for trouble. You will simply
lose all steering - even with power assistance you will not turn the
wheels - believe me I've been there.!! Locking a rear diff will put
sever stress on your rear drive train in corners if the wheels grip.
Similarly locking centre diff will put sever strain on the drive train
if the wheels have grip and can easily lead to breakage. In my case the
centre diff was still engaged even though the light was out (Defender).
Having KAMM diff locks with strengthened half shafts the hub bolts on
the rear left half shaft sheared off. Luckily I realised what had
happened and stopped to extract the half shaft which by then was
protruding about 9 - 12 inches out from the side of the vehicle
otherwise I would have made quite a mess of a few parked vehicles in the
town I was about to enter. (I still had drive cos the centre diff was
still locked)
--
hugh
Reply to address is valid at the time of posting
 
On or around Wed, 26 Jan 2005 10:17:10 GMT, Jeremy Mortimer
<[email protected]_this.org> enlightened us thusly:

>To simplify the case, consider a 2-wheel drive car, stationary and in gear.
>If loss of grip loses you engine braking, then you should be able to lift
>one of the driven wheels of that car and have it free-wheel on the other.
>I'm almost certain the diff will not allow that. Am I wrong?

nope. engine braking is not normally the same sort of power transmission as
driving, so it's less common, but one wheel on the slippery = no engine
braking. Or 2 wheels, on yer landy with the difflock engaged.

--
Austin Shackles. www.ddol-las.fsnet.co.uk my opinions are just that
"Nessun maggior dolore che ricordarsi del tempo felice nella miseria"
- Dante Alighieri (1265 - 1321) from Divina Commedia 'Inferno'
 
On or around Wed, 26 Jan 2005 12:59:16 +0100, Erik-Jan Geniets
<[email protected]> enlightened us thusly:

>
>
>Austin Shackles wrote:
>
>>
>> with the centre diff locked, one wheel on each axle has to turn. It's still
>> possible to get stuck and still possible to slide on downhills, but at least
>> 2 wheels will grip, or looking at it another way, 2 wheels have to lose grip
>> for it to stop going forwards. This is why the "cross-axle" situation stops
>> you going anywhere - one front wheel and the opposite rear wheel are
>> unloaded, and don't grip.
>>
>> adding a rear locking diff means, when it's locked, that both rear wheels
>> and one front wheel have to turn. Thus 3 wheels have to lose grip to stop
>> you.
>>
>
>When descending slippery roads be aware that locked diffs can get you
>into trouble.
>Especially in corners where at least one but maybe all the wheels will
>not turn at the appropriate speed. So you will loose control and end up
>off-road.

true, but one wheel losing grip will lose all your engine braking, so if you
were relying on that to slow you down, you'll rapidly go faster. Basically,
going down slippery slopes is always risky, and if you're talking about ice,
downright dangerous. I think in general difflock in would be better than
difflock out, but it rather depends on how much grip there is. in a
situation where there's no grip at all, then it'll make no difference.
--
Austin Shackles. www.ddol-las.fsnet.co.uk my opinions are just that
"Nessun maggior dolore che ricordarsi del tempo felice nella miseria"
- Dante Alighieri (1265 - 1321) from Divina Commedia 'Inferno'
 
Austin Shackles <[email protected]> wrote in
news:[email protected]:

> On or around Wed, 26 Jan 2005 10:17:10 GMT, Jeremy Mortimer
> <[email protected]_this.org> enlightened us thusly:
>
>>To simplify the case, consider a 2-wheel drive car, stationary and in
>>gear. If loss of grip loses you engine braking, then you should be
>>able to lift one of the driven wheels of that car and have it
>>free-wheel on the other. I'm almost certain the diff will not allow
>>that. Am I wrong?
>
> nope. engine braking is not normally the same sort of power
> transmission as driving, so it's less common, but one wheel on the
> slippery = no engine braking. Or 2 wheels, on yer landy with the
> difflock engaged.

OK, now I've definitely got a headache.

I have a Lego set with a differential in it at home - I'm going to try it
this evening. (I also have a Disco, and I'm in a region which is currently
rather full of icy hills, but I think I'll try the Lego first :)

With a drive shaft connected through a diff to an axle with a wheel at each
end, and with the drive shaft and one wheel held stationary, the other
wheel should turn freely, n'est-ce pas? We'll see....

Jeremy
 
On Wed, 26 Jan 2005 13:55:26 GMT, Jeremy Mortimer
<[email protected]_this.org> wrote:

>
>With a drive shaft connected through a diff to an axle with a wheel at each
>end, and with the drive shaft and one wheel held stationary, the other
>wheel should turn freely, n'est-ce pas? We'll see....
>
>Jeremy

Not sure this is the question you are asking, but the question is "If
the propshaft and one wheel are locked, will the diff allow the other
wheel to turn?" The answer is "No".

The diff allows distribution of torque and rotation in one of two
different directions. There is always a mathmatical relationship
between the speed of the various shafts (goverened by the ratio of the
diff) but if the speed of two of the shafts is zero, the third MUST
also be zero.

Assuming simple mechanical diffs, of course. Limited slip diffs are a
different kettle of halibut.

David
 
We had 30 in of snow.On flat ground there was an improvement on snow covered
streets with the center diff locked, also improved stopping effect of brakes
which is good.



"hugh" <hugh@[127.0.0.1]> wrote in message
news:[email protected]...
> In message <[email protected]>, Austin Shackles
> <[email protected]> writes
>>On or around Wed, 26 Jan 2005 08:33:33 GMT, Jeremy Mortimer
>><[email protected]_this.org> enlightened us thusly:
>>
>>>I recently read (I think it was in an article on the Difflock site on
>>>driving in winter, but I can't find it now) that if one wheel loses grip
>>>while descending a hill using engine braking with the diffs unlocked, the
>>>vehicle can run away.
>>>
>>>Is this true? I'm getting a headache trying to work out the effect of
>>>differentials in this situation. Can someone more mechanically minded
>>>than
>>>me explain?
>>
>>with all the diffs unlocked, 1 wheel losing grip can lose you all the
>>drive,
>>and also all the engine braking.
>>
>>with the centre diff locked, one wheel on each axle has to turn. It's
>>still
>>possible to get stuck and still possible to slide on downhills, but at
>>least
>>2 wheels will grip, or looking at it another way, 2 wheels have to lose
>>grip
>>for it to stop going forwards. This is why the "cross-axle" situation
>>stops
>>you going anywhere - one front wheel and the opposite rear wheel are
>>unloaded, and don't grip.
>>
>>adding a rear locking diff means, when it's locked, that both rear wheels
>>and one front wheel have to turn. Thus 3 wheels have to lose grip to stop
>>you.
>>
>>
>>diff lock shouldn't be used in grippy situations. In slippery conditions,
>>engage diff lock, especially for climbing or descending. If the
>>slipperyness is intermittent, make sure you disengage it again afterwards.
>>
>>finally, downhill on ice, you've 2 tons of steel and nothing's really
>>gonna
>>make much odds - you just have to keep it lined up and hope that the ice
>>finishes before you run out of road... diff lock might give you a slight
>>advantage.
> Diff lock shouldn't be used on tarmac/concrete or similar surfaces but
> should be engaged when off metalled roads - be prepared for the
> unexpected!!
>
> Also remember when descending if you do start to slide then use your right
> foot and accelerate down the hill until traction is regained.
>
> Whatever you do don't lock up the brakes.
> --
> hugh
> Reply to address is valid at the time of posting


 
On or around Wed, 26 Jan 2005 15:28:39 GMT, "news" <[email protected]>
enlightened us thusly:

>We had 30 in of snow.On flat ground there was an improvement on snow covered
>streets with the center diff locked, also improved stopping effect of brakes
>which is good.

you get a slight sort-of anti-lock effect - locking both front wheels will
transmit braking to the back through the centre diff, and then one of them
will lock.

--
Austin Shackles. www.ddol-las.fsnet.co.uk my opinions are just that
"Plus ça change, plus c'est la même chose"
Alphonse Karr (1808 - 1890) Les Guêpes, Jan 1849
 
In message <[email protected]>, Jeremy
Mortimer <[email protected]_this.org> writes
>Austin Shackles <[email protected]> wrote in
>news:[email protected]:
>
>> On or around Wed, 26 Jan 2005 10:17:10 GMT, Jeremy Mortimer
>> <[email protected]_this.org> enlightened us thusly:
>>
>>>To simplify the case, consider a 2-wheel drive car, stationary and in
>>>gear. If loss of grip loses you engine braking, then you should be
>>>able to lift one of the driven wheels of that car and have it
>>>free-wheel on the other. I'm almost certain the diff will not allow
>>>that. Am I wrong?
>>
>> nope. engine braking is not normally the same sort of power
>> transmission as driving, so it's less common, but one wheel on the
>> slippery = no engine braking. Or 2 wheels, on yer landy with the
>> difflock engaged.
>
>OK, now I've definitely got a headache.
>
>I have a Lego set with a differential in it at home - I'm going to try it
>this evening. (I also have a Disco, and I'm in a region which is currently
>rather full of icy hills, but I think I'll try the Lego first :)
>
>With a drive shaft connected through a diff to an axle with a wheel at each
>end, and with the drive shaft and one wheel held stationary, the other
>wheel should turn freely, n'est-ce pas? We'll see....
>

>Jeremy
Jack up the rear of the Disco with it in gear. That simulates 100%
engine braking. Now, turn one wheel forwards to simulate the gripping
wheel and watch what happens to the other wheel which has no grip cos
its in mid air - it goes backwards. So the engine braking is having zero
effect on the forward motion.

BICBW

--
hugh
Reply to address is valid at the time of posting
 
On Wed, 26 Jan 2005 19:37:37 +0000, hugh wrote:

> Jack up the rear of the Disco with it in gear. That simulates 100%
> engine braking. Now, turn one wheel forwards to simulate the
> gripping wheel and watch what happens to the other wheel which has
> no grip cos its in mid air - it goes backwards.

But that isn't exactly what is happening on the road, 'cause the
engine is running and the prop shaft rotating not locked...

What happens to the engine revs when one wheel looses grip under
engine braking, no throttle, do they drop to idle? After all the
engine is no longer being forced round by the speed of travel. But
that doesn't seem to fit with the counter rotating wheel effect
described above, as the counter rotating wheel revolves at the same
RPM as the rotating one.

My brain hurts...

--
Cheers [email protected]
Dave. pam is missing e-mail



 
I would like to fit air lockers front and back at some point - mate has em
on his 5.0 V8 110 bloody awesome off road, but its got over 3" lift and twin
shocks per wheel.


"Austin Shackles" <[email protected]> wrote in message
news:[email protected]...
> On or around Wed, 26 Jan 2005 15:28:39 GMT, "news" <[email protected]>
> enlightened us thusly:
>
>>We had 30 in of snow.On flat ground there was an improvement on snow
>>covered
>>streets with the center diff locked, also improved stopping effect of
>>brakes
>>which is good.
>
> you get a slight sort-of anti-lock effect - locking both front wheels will
> transmit braking to the back through the centre diff, and then one of them
> will lock.
>
> --
> Austin Shackles. www.ddol-las.fsnet.co.uk my opinions are just that
> "Plus ça change, plus c'est la même chose"
> Alphonse Karr (1808 - 1890) Les Guêpes, Jan 1849


 
On Wed, 26 Jan 2005 06:13:18 -0600, hugh wrote
(in article <[email protected]>):

> In message <[email protected]>, Austin Shackles
> <[email protected]> writes
>> On or around Wed, 26 Jan 2005 08:33:33 GMT, Jeremy Mortimer
>> <[email protected]_this.org> enlightened us thusly:
>>
>>> I recently read (I think it was in an article on the Difflock site on
>>> driving in winter, but I can't find it now) that if one wheel loses grip
>>> while descending a hill using engine braking with the diffs unlocked, the
>>> vehicle can run away.
>>>
>>> Is this true? I'm getting a headache trying to work out the effect of
>>> differentials in this situation. Can someone more mechanically minded than
>>> me explain?
>>
>> with all the diffs unlocked, 1 wheel losing grip can lose you all the drive,
>> and also all the engine braking.
>>
>> with the centre diff locked, one wheel on each axle has to turn. It's still
>> possible to get stuck and still possible to slide on downhills, but at least
>> 2 wheels will grip, or looking at it another way, 2 wheels have to lose grip
>> for it to stop going forwards. This is why the "cross-axle" situation stops
>> you going anywhere - one front wheel and the opposite rear wheel are
>> unloaded, and don't grip.
>>
>> adding a rear locking diff means, when it's locked, that both rear wheels
>> and one front wheel have to turn. Thus 3 wheels have to lose grip to stop
>> you.
>>
>>
>> diff lock shouldn't be used in grippy situations. In slippery conditions,
>> engage diff lock, especially for climbing or descending. If the
>> slipperyness is intermittent, make sure you disengage it again afterwards.
>>
>> finally, downhill on ice, you've 2 tons of steel and nothing's really gonna
>> make much odds - you just have to keep it lined up and hope that the ice
>> finishes before you run out of road... diff lock might give you a slight
>> advantage.
> Diff lock shouldn't be used on tarmac/concrete or similar surfaces but
> should be engaged when off metalled roads - be prepared for the
> unexpected!!
>
> Also remember when descending if you do start to slide then use your
> right foot and accelerate down the hill until traction is regained.
>
> Whatever you do don't lock up the brakes.
>

Ok, here are a couple of things I got from two land rover guys who were in
the Camel GT.
1.In low traction low speed situations, Keep your foot lightly on the brake.
This keeps everything tight and acts almost like a traction control device.
I have also been told to just lightly tap the brake.
2. Also in reduced traction circumstances like a heavy rain the center diff
can be locked when you are going mostly straight. The long curves on the
interstates are straight enough. And keep the tire pressures all the same.
This is easier on the differential.

Any hints about driving on ice are appreciated. Dallas rarely gets snow. It
does get some very bad ice storms. Last time we had ice on the road I was
driving a honda insight. Two seater hybrid. I tried to stop at five MPH and
went into a slide. I am just a little bit worried about driving the Discovery
because of the weight.

I have been reading that some times are better in the ice than others. The
real hard compound high milage tires getting very glassy in cold weather.
There is also a rating other than the Mud +Snow on the side of the tire which
is a snowflake on a mountain. It might just be U.S. though.


http://www.tirerack.com/winter/tech/severe.jsp

Most of the people I have talked to with full locking rear differentials say
the vehicle is hard to hand on snow and ice.

Hud

96 Discovery

 
On or around Wed, 26 Jan 2005 20:40:42 +0000 (GMT), "Dave Liquorice"
<[email protected]> enlightened us thusly:

>But that isn't exactly what is happening on the road, 'cause the
>engine is running and the prop shaft rotating not locked...

does on steep downhills off-road though, when there's next-to-no weight on
the arse end. seen one do it on Sunday last.

--
Austin Shackles. www.ddol-las.fsnet.co.uk my opinions are just that
"Festina Lente" (Hasten slowly) Suetonius (c.70-c.140) Augustus, 25
 
In message <[email protected]>
Jeremy Mortimer <[email protected]_this.org> wrote:

> I recently read (I think it was in an article on the Difflock site on
> driving in winter, but I can't find it now) that if one wheel loses grip
> while descending a hill using engine braking with the diffs unlocked, the
> vehicle can run away.
>

If you don't have your centre diff locked, yes. The effect is
most alarming - wheels sliding all over the place.

> Is this true? I'm getting a headache trying to work out the effect of
> differentials in this situation. Can someone more mechanically minded than
> me explain?
>
> Jeremy

Keep the center diff locked and refrain from using the brakes
and you won't have too much trouble - you'll certainly be better
off the almost everyone else!

Richard

--
www.beamends-lrspares.co.uk [email protected]
Running a business in a Microsoft free environment - it can be done
Powered by Risc-OS - you won't get a virus from us!!
Helping keep Land Rovers on and off the road to annoy the Lib Dems
 
hugh <hugh@[127.0.0.1]> wrote in
news:[email protected]:

> In message <[email protected]>, Jeremy
> Mortimer <[email protected]_this.org> writes
>>Austin Shackles <[email protected]> wrote in
>>news:[email protected]:
>>
>>> On or around Wed, 26 Jan 2005 10:17:10 GMT, Jeremy Mortimer
>>> <[email protected]_this.org> enlightened us thusly:
>>>
>>>>To simplify the case, consider a 2-wheel drive car, stationary and
>>>>in gear. If loss of grip loses you engine braking, then you should
>>>>be able to lift one of the driven wheels of that car and have it
>>>>free-wheel on the other. I'm almost certain the diff will not allow
>>>>that. Am I wrong?
>>>
>>> nope. engine braking is not normally the same sort of power
>>> transmission as driving, so it's less common, but one wheel on the
>>> slippery = no engine braking. Or 2 wheels, on yer landy with the
>>> difflock engaged.
>>
>>OK, now I've definitely got a headache.
>>
>>I have a Lego set with a differential in it at home - I'm going to try
>>it this evening. (I also have a Disco, and I'm in a region which is
>>currently rather full of icy hills, but I think I'll try the Lego
>>first :)
>>
>>With a drive shaft connected through a diff to an axle with a wheel at
>>each end, and with the drive shaft and one wheel held stationary, the
>>other wheel should turn freely, n'est-ce pas? We'll see....
>>
>
>>Jeremy
> Jack up the rear of the Disco with it in gear. That simulates 100%
> engine braking. Now, turn one wheel forwards to simulate the gripping
> wheel and watch what happens to the other wheel which has no grip cos
> its in mid air - it goes backwards. So the engine braking is having
> zero effect on the forward motion.
>
> BICBW

You're not wrong. This is exactly the answer I was looking for, and in
fact I confirmed it with my Lego set last night. I hadn't expected to see
the wheel go backwards - most counter-intuitive.

I presume that the real-world implications of this are that it is a good
idea to lock the centre diff (but taking note of someone's comment
elsewhere in this thread that this can cause a wheel to break grip in a
bend), and also a very good idea to use descent control.

As I said, this is not academic - we have a steep hill leading down from
our house, which may have ice on one side and tarmac on the other. It
also has a house wall across the bottom, so it's no place to have the
vehicle run away!

Has anyone actually seen a wheel go backwards in these circumstances? I
guess you'd need good traction on the other side, so it's probably more
likely on ice patches than in mud....

Jeremy

 
On Wednesday, in article
<[email protected]>
[email protected]_this.org "Jeremy Mortimer" wrote:

> Austin Shackles <[email protected]> wrote in
> news:[email protected]:
>
> > On or around Wed, 26 Jan 2005 08:33:33 GMT, Jeremy Mortimer
> > <[email protected]_this.org> enlightened us thusly:
> >
> >>I recently read (I think it was in an article on the Difflock site on
> >>driving in winter, but I can't find it now) that if one wheel loses
> >>grip while descending a hill using engine braking with the diffs
> >>unlocked, the vehicle can run away.
> >>
> >>Is this true? I'm getting a headache trying to work out the effect of
> >>differentials in this situation. Can someone more mechanically minded
> >>than me explain?
> >
> > with all the diffs unlocked, 1 wheel losing grip can lose you all the
> > drive, and also all the engine braking.
>
> Thanks. I understand the principle when the wheels are driven, but I wasn't
> sure about its applicability to engine braking.
>
> I've thought about it some more, and it seems to me that it *doesn't* apply
> to engine braking.
>
> To simplify the case, consider a 2-wheel drive car, stationary and in gear.
> If loss of grip loses you engine braking, then you should be able to lift
> one of the driven wheels of that car and have it free-wheel on the other.
> I'm almost certain the diff will not allow that. Am I wrong?

Yep, you're wrong.

The average rotation speed of the two wheels will match the speed of the
prop shaft, modified by the drive ratio. Zero propshaft speed will mean
the wheels have to rotate in opposite directions, that's all. And the
forces applied take the path of least resistance.

--
David G. Bell -- SF Fan, Filker, and Punslinger.

"I am Number Two," said Penfold. "You are Number Six."
 
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