Thread: All wheel drive vs 4 wheel drive vs ?

Originally Posted by TB

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.

Oh hey, new thought in the spirit of "yes, and..."

If we think about understeer, we can't just close the clutch and get more drive to the front because of the reasons I've already explained. But maybe, given that we're already sliding forwards on the front wheels, they can select which wheels are sliding and which are driving! If you lock the centre clutch, then apply just the right amount of brake to each of the inside wheels, you can make those inner wheels have all the skid action while the outer wheels get the exact right rotation speed for front/rear courtesy of the open diffs – and then you can drive both front and rear on the outside to help power around the corner.

You could do the same thing with a locking centre diff, by the way. But it's at least one way the clutch-only arrangement might actually make a positive contribution to a turn at speed.

Originally Posted by TB

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.

Let me be abundantly clear TB. No more than 50% will be delivered to the front, on that we agree. However this system delivers between 0 and 50% to the front depending on demand. It is in a sense is a limited-slip differential. I could imagine that when travelling in a straight line the front driveline is disengaged...the is no need for a differential. When turning there is a requirement to allow for differential rotation at least between front and rear. Could the variable torque distribution provided by the transfer case clutch plates provide this?

I'm (mostly) with TB on this... A differential is very different from a clutch when it comes to distributing power. I think the point TB is making but doesn't seem to be being adopted is the fact that a differential will enable the output speed to be higher than the input speed. So if the input to the diff is turning at say 100 rpm, the rear could turn at 98 rpm but the front would turn at 102 rpm! This is fundamentally different from 50:50 torque split and is the whole point of a differential.

I'd say the new system will bias on-road performance by giving the car more of a rear wheel drive feel through the corners (as the fronts generally won't do anything other than actually brake the front axle when clutch is engaged). This "might" have a similar effect to left foot braking.

A couple of points though:

1. Clutch systems are reactive not proactive - LR admit this saying that it takes 0.15s to transfer from 100:0 to 50:50
2. I bet the clutch system on its own is cheaper than a center diff and clutch system

All that being said, bench racing doesn't prove anything, the real answers will come from the driving experience.
Either way, I wouldn't let the new system put me off the Dxxx Defenders - at least until proven otherwise.

I think you guys are going in a bit deep here.

The centre diff may aid understeer/oversteer but not control it

understeer/oversteer is controlled by suspension settings and tyre pressures

if you want to decrease your understeer increase your front tyre pressures or decrease your rear tyre pressures or conversely soften or harden your front/rear suspension or both

for some strange reason many here operate their vehicles with major front/rear tyre pressure differences which cooks up understeer.

no differential will compensate for that.

btw my Ranger is almost identical to a 130 (yes you guys hate hearing that) except for rear leafs and my front/rear tyre pressures are always the same.

Originally Posted by CrustyNoodle

Either way, I wouldn't let the new system put me off the Dxxx Defenders - at least until proven otherwise.

One shouldn't forget that LR removed the ability to lock the D2 centre diff, only to reinstate the facility a couple of years later.

Originally Posted by one_iota

Let me be abundantly clear TB.

Wouldn't dream of stopping you.

No more than 50% will be delivered to the front, on that we agree.

Er... no, we don't
Both a locking differential and this new clutch can deliver up to 100% of torque to the front, when the rears have no traction.

However this system delivers between 0 and 50% to the front depending on demand.

True, when at least one wheel on both front and back still has traction.

It is in a sense is a limited-slip differential.

That's a real stretch, though I possibly understand what you're trying to get at. It's sorta, in a vague way, like half of an LSD, in that it has a way to react to the rear wheels spinning out and increase torque to the fronts when that happens.

I could imagine that when travelling in a straight line the front driveline is disengaged...the is no need for a differential.

Quite. That's the stated purpose of the design – to allow the front driveline to stop spinning, reducing friction and saving fuel.

When turning there is a requirement to allow for differential rotation at least between front and rear. Could the variable torque distribution provided by the transfer case clutch plates provide this?

Well, no, not in the slightest. The clutch does exactly the opposite: it acts to force the front shaft to go at the same speed as the rear. That right there is the specific bee that got in my bonnet and made me start asking WTF are they thinking.

Originally Posted by zilch

Attachment 171548

Originally Posted by TB

...
Both a locking differential and this new clutch can deliver up to 100% of torque to the front, when the rears have no traction.

The documentation posted by Zilch says that "0-50/50-100 (% front rear)".

I read that to mean maximum of 50% can be applied to the front and a minimum of 50% to the rear. The rear will always be driven regardless of its state of traction with between 50-100% of torque. Likewise, 0-50% for the front.

If you're saying that, because the rears have no traction and the fronts do, 100% torque is "applied" to the front then, that's different. With 50/50 torque split the rear's are still being driven which they wouldn't be if no torque was being sent to them.

Did I read you right?

SBD4

Yes, the 100% front torque scenario is when the rears have no traction. And yes, there is always a tiny fraction of torque used to overcome the inertia of the drivetrain and the wheel itself so that it's not strictly possible to have 100% go to the front but I chose to ignore that detail unless somebody brought it up because it doesn't help understand what the clutch mechanism is actually doing.

But, I would say no that's not a 50/50 split when you're in that situation. With the centre clutch locked, and assuming that the clutch itself doesn't slip, the final distribution of torque depends firstly on the traction at the wheels and secondly whatever the traction control system is doing with the brakes. The clutch plays no active role in controlling that split though, which I think might be the point you are making. The clutch can't make the vehicle a front-wheel-drive with the rears disconnected from the engine.

The "Engineering Explained" channel on YouTube has a number of videos on differentials and AWD systems which I found helpful in decoding the discussion around all this. I want to highlight, yet again, that apart from JLR a clutch system like this seems to only ever be used on a front-drive vehicle to recruit the rears for additional drive. That way around makes sense because the rear drive shaft would always be moving equal to or slower than the front, and so applying the clutch would always be an accelerating and never a braking force, and allows that application to be partial and progressive up to the 50/50 point. Flipping it around the other way it is largely reduced to being an on/off switch for old school 4WD. See if you can find a scenario where the Defender's rear drive shaft would be moving faster than the front, such that applying the centre clutch would allow a partial transfer of torque rather than an immediate 50/50 or 100/0 engagement.

Finally, that LR document that was shared here, while it contains some very useful information, was not released without marketing dept's stamp of approval. They made sure to convey the story they wanted people to hear in a way that encourages the emotions they want you to feel about the product. But if you just look at the bare facts of it: it's a clutch that can clamp the front driveshaft to the rear one so that instead of RWD you have 4WD. Work forwards from there rather than backwards from the marketing.