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Thys

Discovery I and the Central Diff Lock

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I was watching a DVD on Safe 4X4 Driving Tips. In one of the chapters the presenter used a Disco I with central diff lock, and a Disco II with traction control. He then pullen both vehicles on a ramps with free running rollers on both rear wheels and one front wheel, the remaining front wheel had an obsticale against the wheel. The Disco II could drive over the obsticale, but the Disco I did not move; question then is what is the purpose of the old central diff lock?

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Hi,

The car has three diffs - one in the centre and one on each axle. Their purposes are

The axle diffs allow the outer wheel to go faster when the car is turning a corner since the outer wheel has further to go.

The centre diff allows the front axle to go faster when the car is turning a corner since the front axle has further to go.

With this setup and nothing locked, if any ONE wheel loses traction completely then we can expect that any drive goes to that wheel and you are stuck. (not quite true but would be in a perfect theoretical world). This is much the same as a normal car with only one driven axle - if you completely lose traction on ONE wheel you are stuck.

With the centre diff locked, it requires one wheel ON EACH AXLE to lose traction before you get stuck. Therefore (ignoring other factors) you are twice as well-off as a single axle drive car. It is not difficult however, to contrive of ways to get these cars stuck even with the centre diff locked - one of which is the rollers you describe (although you would only need a set of rollers under one wheel from each axle), and another more normal one, is getting the car 'cross-axled' - the situation here is that due to terrain, diagonally opposing wheels are on hollows and hence spinning uselessly. This is why so many people spend s much effort on long travel suspension - to keep all four wheels on the ground!

Now... In the case of traction control, by comparing the reported speeds of all four wheels, the computer will ascertain that three wheels are spinning and apply the brakes to them. By the action of the three (free) differentials, this will transfer the drive to the remaining wheel and hence you will pull free.

Hope this helps...

Roger (copy & pasted)

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What you saw there with the roller stand is one of the advantages of the traction control Disco II has.

Now here is a movie with the advantage a lockable center diff has: When to engage 4x4 (47MB avi - better use right click > Save As ...).

That is the case with the old Disco I with center diff lock versus traction control without a lockable center diff the Disco 2 has.

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Hi,

The car has three diffs - one in the centre and one on each axle. Their purposes are

The axle diffs allow the outer wheel to go faster when the car is turning a corner since the outer wheel has further to go.

The centre diff allows the front axle to go faster when the car is turning a corner since the front axle has further to go.

With this setup and nothing locked, if any ONE wheel loses traction completely then we can expect that any drive goes to that wheel and you are stuck. (not quite true but would be in a perfect theoretical world). This is much the same as a normal car with only one driven axle - if you completely lose traction on ONE wheel you are stuck.

With the centre diff locked, it requires one wheel ON EACH AXLE to lose traction before you get stuck. Therefore (ignoring other factors) you are twice as well-off as a single axle drive car. It is not difficult however, to contrive of ways to get these cars stuck even with the centre diff locked - one of which is the rollers you describe (although you would only need a set of rollers under one wheel from each axle), and another more normal one, is getting the car 'cross-axled' - the situation here is that due to terrain, diagonally opposing wheels are on hollows and hence spinning uselessly. This is why so many people spend s much effort on long travel suspension - to keep all four wheels on the ground!

Now... In the case of traction control, by comparing the reported speeds of all four wheels, the computer will ascertain that three wheels are spinning and apply the brakes to them. By the action of the three (free) differentials, this will transfer the drive to the remaining wheel and hence you will pull free.

Hope this helps...

Roger (copy & pasted)

Thanks Roger, this helps a lot, I will be more cautious when negotiating axle twisting terrain, i had a false scence of security up to now.

Regards

Thys

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What you saw there with the roller stand is one of the advantages of the traction control Disco II has.

Now here is a movie with the advantage a lockable center diff has: When to engage 4x4 (47MB avi - better use right click > Save As ...).

That is the case with the old Disco I with center diff lock versus traction control without a lockable center diff the Disco 2 has.

Thanks, I will look this movie up!

Thys

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