Hill descent

On or around Wed, 26 Jan 2005 08:33:33 GMT, Jeremy Mortimer enlightened us thusly:

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 wrote in news: snipped-for-privacy@4ax.com:

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

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.

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.

In message , Austin Shackles writes

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.

In message , Erik-Jan Geniets writes

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)

On or around Wed, 26 Jan 2005 10:17:10 GMT, Jeremy Mortimer enlightened us thusly:

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.

On or around Wed, 26 Jan 2005 12:59:16 +0100, Erik-Jan Geniets enlightened us thusly:

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 wrote in news: snipped-for-privacy@4ax.com:

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

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.

On or around Wed, 26 Jan 2005 15:28:39 GMT, "news" enlightened us thusly:

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.

In message , Jeremy Mortimer writes

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

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...

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.

On Wed, 26 Jan 2005 06:13:18 -0600, hugh wrote (in article ):

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.

1. 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.

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" enlightened us thusly:

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.

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

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

hugh wrote in news:bXlQe7KBG$ snipped-for-privacy@raefell.demon.co.uk:

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