Differential

[Spinal]

Gents (and the rare lady that hides here),

I need some information regarding diff locks.

A car I'm looking at has two options, central diff or rear diff (or both). What does each do, which do I want, what are the pros/cons -etc?

Basically, I'm looking for a diff-101...

My understanding is that the central diff turns the rear wheels (or not) depending if it is engaged or not. The rear diff locks the two rear wheels together, making them turn together - right?

What if there is no rear diff, what happens to the rear wheels?

M

 

[DITTRICH]

At the risk of being completely stupid...

A four-wheel drive (4WD) vehicle will have at least two differentials (one in each axle for each pair of driven roadwheels), and possibly a centre differential to apportion torque between the front and rear axles. In some cases (e.g. Lancia Delta Integrale, Porsche 964 Carrera 4 of 1989[10]) the centre differential is an epicyclic differential (see below) to divide the torque asymmetrically, but at a fixed rate between the front and rear axle. Other methods utilise an 'Automatic Torque Biasing' (ATB) centre differential, such as a Torsen—which is what Audi use in their quattro cars (with longitudinal engines).

I read it on the internet so it must be true

 

[tim.100]

Centre diff locks the f&r prop shafts at the same speed

Rear diff locks the rear half shafts at same speed

Either will ensure at least two wheels are driven. A 4wd car with open riffs can be reduced to 1wd in rough terrain!

 

[Ted]

Exactly as Tim.mcd said.

An example is a Disco.
This runs in permanent 4wd, so needs a diff on front and rear axles.
Without a central diff lock you could get 1wd if enough of the wheels are slipping - a central diff lock will allow one front and one rear to drive.

Discos without front/rear diff locks are a maximum of 2wd in the really rough stuff, but a G-Wagen can have the three diff locks to give true 4wd

Earlier G-Wagens (IIRC) had selectable two or four wheel drive so only had the two (front and rear) diff locks. Pretty sure that the later ones were permanent 4wd and had the three. in 2wd no centre diff is needed as the front and rear are disconnected. In 4wd the front and rear are directly connected so no diff lock required.

Often people get confused with freewheeling hubs on the frons. Their purpose is purely to reduce the drag caused by the front diff/halfshafts for road use and give improved fuel consumption. They are engaged on the rough stuff.

 

[Stratman]

How differentials work

 

[Spinal]

So why would a 4x4 manufacturer NOT put 3 diffs? Is it purely down to cost, or are there other disadvantages?

 

[Ted]

I believe that it is down to cost AND the fact that a '4wd' still performs pretty well with the possibility of two wheels losing traction - especially as they are well separated (front/rear).
Also, if only one wheel has good traction there is a hell of a lot of strain on the driving halfshaft and associated hardware - I am guessing that all halfshafts would need to be pretty robust and therefore more costly.

Edit: PS, don't ever lock a front diff on tarmac. I did and it makes the steering (or lack of) very interesting.

 

[tim.100]

So why would a 4x4 manufacturer NOT put 3 diffs? Is it purely down to cost, or are there other disadvantages?

The 3rd diff would go on the front axle, where its operation makes steering very non-linear and unpredictable in some situations. So the answer is actually product liability.

Plus differentials are complex and thus expensive!