Thread: Centre Diff Lock

Originally Posted by isuzurover

Sorry but this shows a lack of understanding.

A cross axle diff lock is different to a front-rear diff lock.

With the CDL unlocked you can spin one front OR rear wheel and lose all drive. With the CDL locked you need to spin one front AND one rear wheel.

With the CDL locked the cross axle differentials still work fine...

If you are driving in a (mainly) straight line there will be no CDL windup.

The new Ashcroft centre (lockable) LSD is of course the best option. The fact that those who have fitted the LSD have reported greatly improved dirt road driving with the LSD (over an open diff) is evidence that those with an open diff should lock it when driving on high speed dirt/gravel roads.

Oh I understand exactly how diffs and difflocks work. I also understand that if turning corners is part of your driving, the inside and outside AS WELL as the front and rear need to travel a different distance. Instead of this difference being accounted for in the differentials (that's what they're designed for after all) it must be taken care of by tyre slip in a locked situation, that's the point I was trying to make. I don't think wheels slip enough on a gravel road to wear out centre diff thrust washers, more likely they'll become hammered by windup pressure.

Originally Posted by Naks

As soon as you leave tar, engage CDL and reduce tyre pressures.

That's a bit extreme. My driveway isn't tar.

I much prefer the wording in the plate in my rangie. "Engage Diff Lock when traction is likely to be lost"
Or something like that.

Drifting gravel roads counts as losing traction.

The point being that all Land Rover operated and endorsed driving instruction teaches that "high speed" (generally less than 90km/h) driving on gravel roads are safer with the CDL engaged.

The CDL makes a significant difference to the longitudinal stability of the car and assists in negating over steer. There is little or no chance to encounter wind up on any short piece of hard surface, especially if you're not turning the vehicle. It's also good habit to have your own mini check list for entering and exiting any loose surface, which in turn will prompt you not to forget your CDL engaged.

As Dougal said, any surface likely to cause loss of traction very much includes "high speed" dirt road driving. Realising that you've lost traction at 90km/h (touch of the brakes, tightening turn, patch of pea gravel etc) doesn't give you much opportunity to then assess the situation and decide to engage your diff lock...

Cheers,

Lou

Being new to 4x4 I'm just ploughing through Robert Pepper's 4WD Handbook in which he says having the centre locked is fine on pretty much anything other than bitumen (or similar high traction surfaces). Certainly my (limited) experience is the 90 handles much better in the dirt with it on.

Maybe the debate is really about semantics, when is a dirt road not dirt? I tried looking it up but my head spun with all the different types: dirt, gravel, macadam, murram & laterite, aggregate, combo's of all ...

There are situations though where there is very little traction and you are best without the diff lock in as the extra scrubbing will cause you to break traction.

Ice is one of the few and even then, whether to lock or not depends more on what's at the edge of the ice than the ice itself.

Tom Sheppards "The Land Rover Experience" book which accompanied the training courses of the same name in the early 90's says:

When to use diff lock. A simple rule-of-thumb is to use diff lock when you use the low range transfer gears on any loose or slippery surface. Use of low range generally means that you are on difficult or uneven ground and wanting a high tractive effort. Such ground usually has uneven traction characteristics as well as an uneven surface and you will need all the traction (grip) you can get. Locking the centre differential will precluded loss of traction resulting from potential front/rear differences in grip.

High range diff lock? It is possible to engage diff lock in high range too. When, if ever, is this desirable? It will be useful to engage diff lock in high range when driving on potentially difficult surfaces such as wet grass, mud, loose or packed snow, or lose sand on tracks or in open desert.

When to de-select diff lock? It is important to de-select diff lock when on any hard grippy surfaces such as tarmac or concrete (wet or dry) whether you are in high or low range.

Which is roughly what Robert Pepper is advising. You can safely lock on anything that isn't a hard grippy surface.

I seem to recall being taught to engage Low Range and Lock CDL when doing the LROCV 4wd basics course. I usually will lock the CDL in high if the track/road has a loose surface or slippery surface.

cheers
Paul

Originally Posted by Dougal

Ice is one of the few and even then, whether to lock or not depends more on what's at the edge of the ice than the ice itself.

At the risk of going off topic - ice is the edge case to just about all driving techniques and driving aids.

I have had the unfortunate pleasure (admittedly a many years ago now, with likely less advanced ABS tech) of finding out how ABS and ice play together - hint: it does not end well for the car.

In my instance the ABS gave a few kicks, decided there was no traction and let the car free wheel in to the nearest solid object in front of me. Once I realised what was going on the handbrake only slowed me a bit, but certainly did a better job than freewheeling. Then again by that stage steering was only useful as something to keep me (the driver) busy on the way to the inevitable.

Has anyone tested a Puma ABS on ice?

Back in 2004 I did a double pirouette in a brand new Discovery 2 on a country lane in Worcestershire. Luckily it was before the farmer's annual hedge trim and there was a lot of foliage and a silted up shallow ditch to break the momentum...

Safe to say slightly primitive TC and ABS is just about useless on black ice.

Disco 3 and 4 is a completely different kettle of fish though...

Cheers,

Lou