Hill descent

On Thu, 27 Jan 2005 08:10:00 GMT, Jeremy Mortimer
<[email protected]_this.org> wrote:

>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.

The real world implication is not to park a series lr on a hill with
one wheel on wet leaves and expect the handbrake to keep it there.

AJH

hugh wrote:


>
> >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.

I'll have a definite headache as well by now.

If I remember well I would not be able to turn a wheel forward because
of the engine compression. It is in gear after all?
Out of gear it will act as in your description.
I'll probably be wrong here. Maybe a bad memory ?????
Kind regards,
Erik-Jan.

In message <[email protected]>, Dave
Liquorice <[email protected]> writes
>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...
>

No it's the extreme case purely for the purpose of illustrating the
action of the diff. Engine braking relies on the torque generated on the
prop shaft by the rotating rear wheels due to their forward motion being
counteracted by the compression of the engine as it is forced to spin
faster than it would normally do for the throttle position, which is
fully closed.

If one wheel loses grip there is less torque being applied tot he prop
shaft as it can be transmitted through the diff to the slipping wheel.
Hence no decelerating torque applied to the prop shaft by the engine.
>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...
>
You're not alone.
--
hugh
Reply to address is valid at the time of posting

In message <[email protected]>, StaffBull
<[email protected]> writes
>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.
>
Had them on my ex mil 200 tdi 90. Brilliant in mud, you really can get a
lot further in before you get stuck. Front one's not so effective
climbing as weight shifts to rear. In fact in some cases it was better
to leave the front one out on a steep climb to get better steering
control.
--
hugh
Reply to address is valid at the time of posting

In message <[email protected]>, Disco Duck
<[email protected]> writes
>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
>
Best vehicle I ever drove on snow was a Daf with its rubber belt
constantly variable transmissions ratios. The rear wheels could be
simultaneously in totally different ratios according to whether they
were gripping or not.
--
hugh
Reply to address is valid at the time of posting

Might concentrate getting one on the back then if its more use there and
it'll half the cost!!
"hugh" <hugh@[127.0.0.1]> wrote in message
news:[email protected]...
> In message <[email protected]>, StaffBull
> <[email protected]> writes
>>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.
>>
> Had them on my ex mil 200 tdi 90. Brilliant in mud, you really can get a
> lot further in before you get stuck. Front one's not so effective climbing
> as weight shifts to rear. In fact in some cases it was better to leave the
> front one out on a steep climb to get better steering control.
> --
> hugh
> Reply to address is valid at the time of posting

On Thu, 27 Jan 2005 09:46:45 +0000, [email protected] wrote:

> The real world implication is not to park a series lr on a hill with
> one wheel on wet leaves and expect the handbrake to keep it there.

Surely that applies to any vehicle, not just a series LR, where the
handbrake acts on the prop shaft not the (rear) wheels?

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

On Thu, 27 Jan 2005 21:01:03 +0000 (GMT), "Dave Liquorice"
<[email protected]> wrote:

>On Thu, 27 Jan 2005 09:46:45 +0000, [email protected] wrote:
>
>> The real world implication is not to park a series lr on a hill with
>> one wheel on wet leaves and expect the handbrake to keep it there.
>
>Surely that applies to any vehicle, not just a series LR, where the
>handbrake acts on the prop shaft not the (rear) wheels?

Yes, although I cannot think of others atm. The thing that catches
people out with the lr system is when you jack them up, especially
with a hilift, on sloping ground..

AJH

On Thu, 27 Jan 2005 07:42:40 +0000, Austin Shackles wrote:

>> 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.

You mean counter rotating wheels on the same axle?

This is a very interesting, complex and dynamic situation. One where
your instinctive reaction to the run away, ie brake, is not correct.
It requires definate thought and action when gently sliding towards a
tree to lift off the brake let alone accelerate...

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

On Thu, 27 Jan 2005 18:57:39 +0000, hugh wrote:

> If one wheel loses grip there is less torque being applied tot he
> prop shaft as it can be transmitted through the diff to the slipping
> wheel.

So the clues to one wheel losing grip, downhill, under engine braking,
is simultaneous drop in revs and acceleration. Eeeekkk...

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

On Thu, 27 Jan 2005 21:27:53 +0000, [email protected] wrote:

> Yes, although I cannot think of others atm.

Discovery, mine has the characteristic lurch and "settlement" when on
a hill. Presumably RRs as well? When people refer to "series Land
Rovers" I think of those vehicles that predate the Defender...

> The thing that catches people out with the lr system is when you
> jack them up, especially with a hilift, on sloping ground..

The big clue is the large chock supplied with the wheel brace and jack
handles... B-)

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

On or around Thu, 27 Jan 2005 21:20:05 +0000 (GMT), "Dave Liquorice"
<[email protected]> enlightened us thusly:

>On Thu, 27 Jan 2005 18:57:39 +0000, hugh wrote:
>
>> If one wheel loses grip there is less torque being applied tot he
>> prop shaft as it can be transmitted through the diff to the slipping
>> wheel.
>
>So the clues to one wheel losing grip, downhill, under engine braking,
>is simultaneous drop in revs and acceleration. Eeeekkk...

yep, if you've free diffs everywhere. If it happens on a series or a
locked-centre-diff RR/Disco/90/110 then you've got 2 wheels on the ice. If
there's ice on 1 side of the road only, and the other 2 wheels are on dry
ground, then at this point you brake, being ready to compensate with the
steering wheel for the off-centre braking effect. If there's ice all over
the road, you pray that there's a clear patch for it to slow down on again
before the next corner...

--
Austin Shackles. www.ddol-las.fsnet.co.uk my opinions are just that
Confidence: Before important work meetings, boost your confidence by
reading a few pages from "The Tibetan Book of the Dead"
from the Little Book of Complete B***ocks by Alistair Beaton.

On Fri, 28 Jan 2005 07:41:15 +0000, Austin Shackles wrote:

>> So the clues to one wheel losing grip, downhill, under engine
>> braking, is simultaneous drop in revs and acceleration. Eeeekkk...
>
> yep, if you've free diffs everywhere.

Disco II, so free diffs everywhere... not (yet) looked underneath to
see if the center diff lock spigot is present or not.

OK I have HDC but encountering this single sided ice is just for short
sections on public roads that are otherwise treated and clear. So
getting into the low box to engage the HDC is not particulary
convient. I know where most of these frozen run offs occur but just
occasionally there is a new one depending on the recent rain/snow melt
levels.

> If there's ice all over the road, you pray that there's a clear
> patch for it to slow down on again before the next corner...

If it's that bad you don't need a corner, it'll be sliding down the
chamber, as well as the hill, and into the ditch/hedge/wall/tree (in
order of preference)... B-(

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

On Thu, 27 Jan 2005 22:21:02 +0000 (GMT), "Dave Liquorice"
<[email protected]> wrote:

>On Thu, 27 Jan 2005 21:27:53 +0000, [email protected] wrote:
>
>> Yes, although I cannot think of others atm.
>
>Discovery, mine has the characteristic lurch and "settlement" when on
>a hill. Presumably RRs as well? When people refer to "series Land
>Rovers" I think of those vehicles that predate the Defender...

Yes the problem will happen with all the landrover products, including
the permanent 4wd ones, but maybe less so with viscous centre diff. I
meant I couldn't think of another firm that used a prop shaft parking
brake.

AJH

In news:[email protected],
Erik-Jan Geniets <[email protected]> blithered:
> hugh wrote:
>
>
>>
>>> 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.
>
> I'll have a definite headache as well by now.
>
> If I remember well I would not be able to turn a wheel forward because
> of the engine compression. It is in gear after all?
> Out of gear it will act as in your description.
> I'll probably be wrong here. Maybe a bad memory ?????
> Kind regards,
> Erik-Jan.

Both rear wheels raised!

--
The enemy of my enemy is my fiend?

If at first you don't succeed,
maybe skydiving's not for you!

In message <[email protected]>, Dave
Liquorice <[email protected]> writes
>On Thu, 27 Jan 2005 18:57:39 +0000, hugh wrote:
>
>> If one wheel loses grip there is less torque being applied tot he
>> prop shaft as it can be transmitted through the diff to the slipping
>> wheel.
>
>So the clues to one wheel losing grip, downhill, under engine braking,
>is simultaneous drop in revs and acceleration. Eeeekkk...
>
Once experienced, never forgotten. It's something to practice on play
sites. Find a very steep short slope with good run off area. You'll soon
get the feel of it.
--
hugh
Reply to address is valid at the time of posting

In message <[email protected]>,
[email protected] writes
>On Thu, 27 Jan 2005 22:21:02 +0000 (GMT), "Dave Liquorice"
><[email protected]> wrote:
>
>>On Thu, 27 Jan 2005 21:27:53 +0000, [email protected] wrote:
>>
>>> Yes, although I cannot think of others atm.
>>
>>Discovery, mine has the characteristic lurch and "settlement" when on
>>a hill. Presumably RRs as well? When people refer to "series Land
>>Rovers" I think of those vehicles that predate the Defender...
>
>Yes the problem will happen with all the landrover products, including
>the permanent 4wd ones, but maybe less so with viscous centre diff. I
>meant I couldn't think of another firm that used a prop shaft parking
>brake.
>
>AJH
Perhaps the original idea was to have a brake clear of the mucky wet
stuff. Don't forget this was long before the days of disc brakes. Also
they were NOT designed to go into DEEP claggy stuff, just normal farm
tracks and fields.
--
hugh
Reply to address is valid at the time of posting

On Fri, 28 Jan 2005 15:12:06 +0000, hugh <hugh@[127.0.0.1]> wrote:

>>Yes the problem will happen with all the landrover products, including
>>the permanent 4wd ones, but maybe less so with viscous centre diff. I
>>meant I couldn't think of another firm that used a prop shaft parking
>>brake.
>>
>>AJH
>Perhaps the original idea was to have a brake clear of the mucky wet
>stuff. Don't forget this was long before the days of disc brakes. Also
>they were NOT designed to go into DEEP claggy stuff, just normal farm
>tracks and fields.

I wasn't meaning to particularly criticise the arrangement, in the LR
prop shaft brake can have other advantages (useful in the event of
brake fade?) and with 50s parking brakes being generally poor the
4.7:1 advantage must have been worth something.

Once you understand the possibility you can get around it by leaving
it in gear, engaging 4wd or locking the diff (mind not so easy with
vacuum operated ones)

AJH

GbH wrote:

>
> Both rear wheels raised!
>
Yeah, that was also where I was refering to.
Kind regards,
Erik-Jan.

hugh wrote:

> >
> Best vehicle I ever drove on snow was a Daf with its rubber belt
> constantly variable transmissions ratios. The rear wheels could be
> simultaneously in totally different ratios according to whether they
> were gripping or not.

And it even has the Hill Descent (snow/ice) button on the dash.
Kind regards,
Erik-Jan.