SUV 4wd technical question

[Swordman]

Hey folks, I'm hoping someone can answer these questions.

In an SUV with switchable 4wd (like a Nissan X trail), am I right in thinking they have a locking centre diff? So, if one wheel loses traction, only that axle spins away the power and the other axle provides the traction?

The Suzuki Jimny has vacuum operated 4wd; does that operate the same way? And has anyone got experience with the traction control of the new Jimnys; are they as good as lockers?

[010101]

Vacuum 4wd engagement is temporary mechanical connection of the propshaft between the front and rear diffs.
Centre diff lock needs a centre diff, which is needed because the front and rear axle diffs are permanently connected, unlike vacuum engagement.

[KevinCamaroSS]

Swordman said:

Hey folks, I'm hoping someone can answer these questions.

In an SUV with switchable 4wd (like a Nissan X trail), am I right in thinking they have a locking centre diff? So, if one wheel loses traction, only that axle spins away the power and the other axle provides the traction?

The Suzuki Jimny has vacuum operated 4wd; does that operate the same way? And has anyone got experience with the traction control of the new Jimnys; are they as good as lockers?

A large number of the modern part time 4wd SUVs use electronics to send the power where it is needed, no locking diffs to be seen anywhere.

[spookly]

There are lots of different systems for 4wd, even with just one manufacturer. Look at Jeep for example, in the WJ alone there were 2/3 different types of system.

Best looking into the specific system for the vehicle you're interested in.... try... dunno... google?

[300bhp/ton]

Swordman said:

Hey folks, I'm hoping someone can answer these questions.

In an SUV with switchable 4wd (like a Nissan X trail), am I right in thinking they have a locking centre diff? So, if one wheel loses traction, only that axle spins away the power and the other axle provides the traction?

The Suzuki Jimny has vacuum operated 4wd; does that operate the same way? And has anyone got experience with the traction control of the new Jimnys; are they as good as lockers?

Did you find an answer for this?

Essentially the answer to your question is No.

But it's a LARGE topic and would require a lengthy in-depth reply to cover more comprehensively.

[Hainey]

If you want a proper old school locking centre diff the your in series one discovery territory. It was electronics from then on.

[300bhp/ton]

Hainey said:

If you want a proper old school locking centre diff the your in series one discovery territory. It was electronics from then on.

Not 100%.

There are many types of 4wd system.

The simplest as found on Series Land Rover's, many Jeeps and most Jap 4x4's is simply a 2wd/4wd selector on the transfer case. All this does is engage the front output shaft as well as the rear.

When Land Rover introduced the Range Rover, they knew their standard axles weren't up to the task of a heavier vehicle and V8 power. Instead of improving the axles, they decided to drive both of them and split the power. They used a different transfer box, one with a differential in. This means it can still send power to both the front and output shafts of the box, but unlike a regular 4wd setup, the output shafts can spin at different speeds. This is needed for use on road/tarmac, as the front and rear wheels need to be able to spin at different speeds to each other.

Classic Range Rover's do not have a diff lock. The centre diff is a viscous unit, a bit like an LSD. It cannot attain full lock up.

When Land Rover introduced the Ninety/One Ten. They introduced a new transfer box. This one wasn't a viscous unit, but had a diff lock, so in the locked state does exactly the same as older 4wd systems do, it locks the front and rear output shafts together, so that they rotate at the same speed.

The advantage however is, it can be unlocked and operate as a normal open differential with 50:50 split. It is not akin to an LSD however. Which is why a classic Range Rover and a Defender will drive slightly differently on and off road.

This transfer box is known as the LT230 (Leyland Transmissions) for the Land Rover.

Land Rover used this same transfer box on the Discovery 1 and Discovery 2 (although the D2 they disabled or removed the locking function on most models).

Range Rover's continued with a non locking chain centre diff in the p38a and early L322's. Although traction control was introduced over this time period. The TCS however is designed to work on an axle level, as the axle's also have open diffs.

The D3, RRS and facelift L322 (when they switched to Jag engines) all use a locking centre differential. However it is electronically controlled and can vary how much it is locked and uses clutch packs to achieve this.



Understanding how power is transmitted front to rear is critical in understanding an AWD system.



Vehicles like a Freelander, Evoque or X-Trail use a completely different kind of system. They are normally predominantly FWD and when the electronics detect wheel slip, they will via viscous couplings and electronic controlled clutch packs, engage drive to the rear wheels. A very different system to a traditional awd transfer box. However this system suits FWD vehicles with transverse mounted engines. The downside is they are often less robust and usually have a FWD bias.



The next critical thing to understand is axle differentials. Splitting the power front and rear is the same concept, but you then have two other places to split the power, the diff of each axle. Most axles are "open", to allow each wheel to rotate at different speeds when cornering. But off road this can cause issues.

If you lift the front right wheel and rear left wheel off the ground on most 4wd vehicles (Defender/Jeep/Jimny/Toyota), what will happen is those wheels off the ground will spin uselessly and the vehicle won't move. This is because it is splitting the power 50:50 front to rear. But that power will follow the path of lease resistance. The wheels off the ground have no real resistance, so the open diffs mean the power will go to those wheels.


Limited slip diffs and locking diffs can all get around this problem. Although may cause other things to contend with.

This will affect all types of 4wd system, so an X-Trail is just as likely a victim in such a scenario.


A modern alternative to this is an off road biased traction control. You'll find it on most modern Land Rover's, Jeeps and others.

It will mimic a locking or LSD axle diff. As you spin the wheels in the air, it will brake them, which generates load, this in turn will force the opposite wheel on the same axle to rotate. TCS is very affective, although can cause high brake wear. But will require wheel spin to work.


Vehicles like the Freelander 1 use this to very good affect. And on snow, wet grass or sand, when equipped with traction control can easily outperform a Defender with open axle diffs.



On the Discovery 2, Land Rover decided the let the traction control do all the work. So it not only loads individual wheels, but will do so to the entire axle, meaning the open diff can drive both output shafts better off road. In practice it doesn't work quite as well as having a locking centre diff as well. But does have certain advantages.


The Jimny essentially has the same 4wd setup as a Series Land Rover. 2wd and 4wd. And as far as I know open diffs. Although aftermarket locking diffs are an option IIRC. If it now has TCS (which must be fairly new), then it will be even more capable.

However 4wd layout is only part of what makes a good off roader. Suspension type, design and travel play critical roles as does the design of the rest of the vehicle.

An X-Trail is probably ok on slippery surfaces such as sand, snow, wet grass. And has the potential to perform fairly well (even better if latter ones have TCS as well as their adaptive AWD system).

But for proper off road use the rest of the X-Trail is many notches below a Jimny or a traditional Land Rover or Jeep.

[Hainey]

Great explanation and I learned a lot. Thank you.

[Derek Chevalier]

300bhp/ton said:

Classic Range Rover's do not have a diff lock. The centre diff is a viscous unit, a bit like an LSD. It cannot attain full lock up.

Viscous coupling was only introduced in late 80s I believe

https://www.ifixit.com/Wiki/The_History_of_Land_Ro...

[Swordman]

Thanks for the 300bhp/ton! I've now bought a Jimny and I've also bought some maxtrax boards for desert dune bashing - should be fun!