Thread: All wheel drive vs 4 wheel drive vs ?

Originally Posted by one_iota

I'm shifting to the positive!

I started this thread to find out more about the vehicle and in particular, get some clarity on the traction control system.

As always on AULRO there have been some interesting contributions here some of them on the mark and others speculative. After all, there are very few of these vehicles "on the road" here so information is thin and based on very little hard engineering. I wish JLR would be more open about this particular aspect: "Sell it to me!"

For me, if I make the move it will be at least 12 months away so plenty of time to get some "real world" perspectives. As I've said previously we Defender traditionalists are conservative and resistant to change. I think I'm up to the challenge.

For years I have driven long hours across paddocks and open scrub. The centre diff makes a huge difference as wheels just climb over tussocks rather than bashing into them and fighting them via lack of power distribution in a centre locked tranmission (as is the case with every other 4wd on the market). Wheels can rotate and climb over tussocks (grass stumps) at different rates to avoid building up stress. It would be a pity to see Landrover abandon this advantage.

Originally Posted by DiscoJeffster

You have it backwards.

Well... one of us now must, and it'll be interesting to try and figure out who!

Understeer comes from 4WD systems using a standard centre differential where torque is evenly split front to rear 50:50. Two problems occur. When the vehicle has excess torque applied - e.g. fast cornering with a foot-load of throttle, the front wheels which are trying to steer push wide as the torque they're trying to put the ground causes a outward slippage. This is exacerbated with the rear wheels also pushing the same torque forwards, driving the car off line.

Understeer comes from the front wheels losing traction, making them unable to apply a lateral force component to the front of the vehicle. Doesn't matter whether the front, rear or both axles are being driven, that's what understeer is. The difference between RWD and AWD (stick with open diffs and even splits for now) is that where a RWD's lateral steering force is entirely a component of the amount of grip (friction) the tyre can support *across* its contact patch, an AWD can add to that lateral force by applying rotational torque *along* the contact patch. What happens next is a product of many variables but boils down to AWD with open diffs giving you the potential for better steering performance provided you don't apply so much torque that you break front traction.

Spinning out the rears on a RWD is a whole different game and can be used to glorious effect. But I want to point out that in every scenario where we are turning successfully without any wheels losing traction, without understeer or oversteer, *the front wheels are spinning faster than the rear wheels*. They must, right? Because they're making a larger circle than the rears. This is why differentials exist at all.

In the system presented here (and note the diagram specifically calls out better driving dynamics), the rear wheels will drive the vehicle causing an effective yawing motion to improve the cornering. The front wheels are not trying to apply torque hence can focus on steering angle. This is why rear wheel drives are better around a track than a front wheel drive in any road car comparison. In the case where too much torque is applied to the rear wheels or the angle of the vehicle is off line, the front wheels can be engaged by the centre transfer case and apply small amounts of torque to bring the car back into line and improve overall traction.

First up, never believe marketing over engineering.

In the system presented here, the vehicle is shown is a variant of the old, old school 4WD. It's got a permanently driven rear shaft with 1:1 speed ratio from the TC. And it has a mechanism whereby the front drive can be nearly instantly connected and disconnected from the rear drive, with the same 1:1 ratio. And here's the key point: with this mechanism alone it's completely impossible to apply forward-driving torque to the front wheels during a turn, because without a centre diff there's no way for the front drive shaft to spin faster than the rear. In fact the opposite is true – if you engage the front drive during a turn you will force the front wheels to slow their spin below what is needed to keep pace with the turn, you will introduce a lateral force component *opposite* to the direction of turn, and you will contribute to understeer.

These are exactly the same reasons why traditional 4WD must be disengaged when driving on bitumen. The lack of a differential makes it unsuitable for use on high-grip road surfaces and reduces rather than enhances turning performance.

This system is nearly identical to the Subaru STi which has a torque bias dial in the vehicle where you can specify how much bias you'd like in the system front to rear.

It's NOTHING like the Subaru AWD system, which is and always has been based on having a centre differential.

It's a clever system and I would not hesitate having it on my vehicle, but as I previously said, it is obviously another thing to go wrong, and expensive to repair I'm sure.

I wouldn't be surprised if it's cheaper than the auto-locking centre diff it's replacing, though it does have two clutches (centre and front) instead of two. And for the kind of driving I came to love in my four years with the Disco 4... hell no I don't want to take the step backward to one of these things.

Ask yourself why the more on-road performance-oriented P400 and V8 Defenders are still using the older centre diff arrangement rather than this.

EDIT: I asked myself that too! Turns out Jaguar created this system in order to maintain RWD sports car experiences with an extra degree of "I can still move when it's icy and drift like a champ" thanks to judicious applications of torque to the front wheels. Now I'm still asking the same question but finding it puzzling from the opposite direction.

I just watched a bunch of videos on the Jaguar IDD system. Learned three things:

1. When initially driving away it engages the clutches to pre-emptively avoid wheel slip, just like LR locking centre diff has done for twenty years

2. In all other cases, the front is only given torque when wheel slip is detected somewhere. Note this, they only give torque to the fronts during fast turns when rear wheel slip occurs, ie oversteer. That makes sense, because at that point it's fine for the front and rear drive shaft to be turning at the same speed. The fronts can be pulling around the corner while the rear is spinning, and this works a treat for opposite lock.

3. They also have traction control like terrain response and I think can do left/right torque vectoring by braking – again with the emphasis on the rear wheels.

I'm not saying it's a total piece of crap. I'm just saying that it's not what I want in a Defender, especially a diesel one. Would have made more sense to me to put this RWD sports car augmentation into the big petrol models and left the diesels to crawl sure-footed over every terrain they encounter especially when it's not bitumen.

TB,

I understand that your Defender's centre diff locks in response to the loss of traction and delivers 50/50 front and rear that circumstance. Is this via a clutch pack like the rear E-diff?

If the Jaguars system responds to lack of traction in the rear when cornering hard would a similar system modified for off-road driving also respond to the loss of traction and transfer some of the drive to the front? It seems this apportionment according to the JLR info is between 0 and 100% of 50%.

I like the torque that my 4.4 TDV8 L322's centre diff provides to the front wheels when cornering, pulling the front around the bend and especially noticeable uphill where lesser vehicles seem to always have to cut the corner on a particular corner. The centre diff clutch pack wouldn't be engaged at all but the drive afforded by the diff works well for the heavy vehicle.

Originally Posted by one_iota

I understand that your Defender's centre diff locks in response to the loss of traction and delivers 50/50 front and rear that circumstance. Is this via a clutch pack like the rear E-diff?

If I'm not mistaken, the centre differential is a planetary type rather than the pinion type we see front and rear. But yes, it uses an electronically-controlled clutch pack, as does the optional rear locker even though it's on a different type of differential. This is the same system that's been available since at least the intro of the L319 Discovery 3 and L320 RRS.

50/50 is what I have when all wheels have grip and the diffs are *un*locked. That's one big difference from the new system, which is 100% rear drive until the clutch engages.

If a front wheel starts to spin in my Defender, without any lockers it would suck up 100% of the available torque and I wouldn't move. Locking the centre makes it like that diff doesn't exist, so suddenly 100% of torque is available at both front and rear diffs. Because that front wheel still has no traction, the front axle can't offer any forward progress but all that torque is also available at the rear and provided neither of those tyres breaks grip we just move.

The Jaguar system moves off from a standstill with the clutch pre-emptively locking the front and rear drive shafts together. The benefit there over RWD is that if a rear wheel breaks grip the front can still pull the vehicle forward. In that instance, that's how you get 100% of torque to the fronts – because the rear has lost traction but there's no differential function in the centre.

Both the drivelines we're talking about here will suffer the same fate when getting cross-axled, and both have the same remedy: use the rear locker if fitted, and/or apply brakes to the spinning wheels.

So they're functionally very similar when the need is to overcome one wheel's loss of traction. They differ greatly when all wheels have traction and you want to make a turn.

If the Jaguars system responds to lack of traction in the rear when cornering hard would a similar system modified for off-road driving also respond to the loss of traction and transfer some of the drive to the front? It seems this apportionment according to the JLR info is between 0 and 100% of 50%.

Like I said before, the only way you can get 100% to the front is when at least one of the rears have no grip. And when turning with the Jaguar system, putting ANY torque to the front requires rear wheel slip (not necessarily spin, just enough slip to account for the lack of an actual differential). Or if the rear has more traction, the front wheel must slip (understeer). That's because when turning the front wheels have further to travel than the rears. If you lock them together in a turn then either the fronts must skid forwards or the rears must spin out.

And repeating yet again... the centre clutch without a differential makes the Defender *exactly* like an old school 4WD when that clutch is engaged. It works. You'll still use traction control and rear diff locker to maintain progress when one or more wheels doesn't have grip. But it won't be quite as smooth and assured as what we've been used to for the past couple decades of Discovery and RR.

The centre clutch isn’t just on off - it’s variable so as long as the electronics don’t call for a complete lock, you will still have slippage and variability front to rear that you get with a standard centre diff. I would be quite sure the drive dynamics takes steering angle into account and wouldn’t lock it completely during cornering even under a traction loss event causing poor dynamics when cornering.

Originally Posted by DiscoJeffster

The centre clutch isn’t just on off - it’s variable so as long as the electronics don’t call for a complete lock, you will still have slippage and variability front to rear that you get with a standard centre diff. I would be quite sure the drive dynamics takes steering angle into account and wouldn’t lock it completely during cornering even under a traction loss event causing poor dynamics when cornering.

If you apply that clutch during cornering it will act as a brake on the front driveshaft, not an aid to propulsion. You're seeming to miss the point still that when in a turn with full traction the front wheels have to travel further and therefore faster than the rears. You simply cannot "apply torque" using only a clutch in this situation. Only partially applying it means you're braking it more gently and extracting energy from the front via friction in the clutch.

Only when the rears are spinning faster than the front can the clutch be applied to direct torque to the front during a turn. So, oversteer situations only. Great fun. If you're in a sports car. I bought a Defender.