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#26 05-12-07 23:17:13

dog
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Registered: 29-10-06
Posts: 813

Re: Driving through floods

How exactly are your vehicles laid out then?

I've heard a lot of offroad-capable vehicles, mostly part time 4x4's which are built solely 'for the people of Surrey who live up a lane which sometimes has leaves on it', get in to trouble when a set of wheels encounters little or no traction. Even though all 4 wheels are supposed to be driven, if both rear wheels or both left wheels (for example) get stuck in a bog or lift off the ground, the differentials send all the power to them and not the two wheels which remain on the ground. Seems to defeat the object of 4WD, no?

I have also heard that the Americans fixed this with the original Hummer by using "torque sensitive" differentials on the front and back, which although are fully mechanical, and more intelligent than open diffs and limited slip diffs. The wheel with least traction will always receive power, no matter what, but the diff can detect slip and send much more power to the wheel which has the traction. The center diff is manually lockable, like your Landy by the sounds of it, so it can be driven on the road normally as 2WD and then driven offroad as 4WD.

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05-12-07 23:17:13

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Re: Driving through floods




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#27 06-12-07 09:21:36

timmanwaringadi
Verified Member
From: Devon
Registered: 29-08-06
Posts: 2,406
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Re: Driving through floods

The Land Rover is a full time 4x4, and when the diff lock is out it is still driving in 4WD. I think the Hummer is too, but many vehicles -especially the Japanese off roaders are selectable 4x4 which means that you flick a switch/lever and the other axle is connected to the drive.

In full time 4x4 vehicles, a diff is needed between axles because the front axle (steering axle) will need more power as it travels further. Locking both axles at the same speed would introduce so much stress into the situation that things go ping! Off road however wheels can slip - allowing the pressure to escape. To ensure that one axle doesn't stop getting power they all have a centre diff - either a locking diff like the Land Rover Defender, or a viscous diff (which locks up when the axles speed difference becomes too high) like the Range Rover/BMW.

With regards to wheels slipping then if both wheels to the left of the vehicle are on ice, and the right wheels on good grip like tarmac, then yes, all the power will go via the diffs to the wheels which are slipping. The centre diff only makes sure that the power is split 50/50 between axles and once it gets to the axles you still have the weakness of a simple diff which could send the power to a wheel with no grip.

The way that most Land Rovers get around this problem is by having Traction Control, which I think you know about - we've discussed it before (uses abs brakes to sense difference in wheelspeed, slows the spinning wheel with the breaks thus evening up the pressure on the diff and ensuring that some of the power goes through to the other wheel). The only problem with TC is that it slows you down whenever you get wheelslip - this means that on surfaces where wheelslip is inevitable (peat bog, soft sand, wet soft grass) you can often lose all of the momentum that you need to get through. As you can't usually swith the TC off, most off road drivers prefer not to have it, and rely on their own skills to avoid or get through the situation.

The Torsen diffs are a mechanical device, so not intelligent at all, but astonishingly tough (and equally expensive). It simply (through a very complex combination of cogs and clutches etc - check out how stuff works for a good explanation) doesn't allow full slip (ie 100% of power to 1 wheel and none to the other) - as soon as the differential gets too high it starts to lock up. You can also get air-locked axle diffs for competing with.

I think that just about covers it!

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#28 06-12-07 13:28:12

dog
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Registered: 29-10-06
Posts: 813

Re: Driving through floods

I thought you were free to disable traction control whenever you wanted. I'm not sure about offroaders but I've been in a few BMW's and they all have buttons which lets the driver disable things like TC, ESP etc.

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#29 07-12-07 08:54:20

timmanwaringadi
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From: Devon
Registered: 29-08-06
Posts: 2,406
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Re: Driving through floods

You've got some rich friends dog - like with most of these things, it's only the top of the range models and very expensive makes that you can do this with.

The BMW's aren't off roaders - their 4x4's are ok as posh tow vehicles, but they're awful off road. The mercs are much better, and a few of those allow you to turn off traction control.

The majority of cars that get taken off road however are much less luxurious and don't have the same benefits!

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#30 07-12-07 13:16:54

dog
Verified Member
Registered: 29-10-06
Posts: 813

Re: Driving through floods

Not sure I'd want to drive an X5 off road either! Or that new Porsche 4x4 piece of ***** either!

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#31 07-12-07 14:54:54

timmanwaringadi
Verified Member
From: Devon
Registered: 29-08-06
Posts: 2,406
Website

Re: Driving through floods

I hate to admit it, because I think it looks like a chav tonka toy, but the Porsche is actually pretty good.

Shame you have to get an orange tan and a peroxide before they'll sell you one.

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