Freewheel hubs on Range Rover axles

There's a Pinz Repair Manual (about 125 MB) here if anyone wants to delve a bit further into how it goes together:

It's quite large but worth every byte, an excellent scan that's better quality than my printed manual! Cheers for the pointer. They didn't OCR it either, which is good, I've seen a few technical manuals that have been wrecked by OCRing.

Yes, I acknowledged those reasons! :) And possibly the more important reason: it's an interesting project.

But I think you overestimate both those problems - the tyre scrub really

*isn't* a big issue, and my 150 Defender's turning circle was around 20% worse than a standard 110's. (The legendary size of Land Rover turning circles is mostly hangover from the Series days; the coil sprung vehicles have turning circles much the same as other utility vehicles with similar wheelbases. E.g 110 Defender has a turning radius of 6.4m, S70 Land Cruiser 6.5m.)

Anyway, my point was just that such a conversion *would* undoubtedly solve those problems, but the complexity and relative flimsiness of a further steering axle doesn't seem to be a good trade-off .

The tipping I was referring to was simply the massive oversteer that rear steering introduces! It can't be corrected - it's just a consequence of steering from the back. Commercial 4-wheel steering systems are not very successful (because of the poor trade-off between the problems they solve versus the problems they introduce). But in any case, they generally only operate at very low speeds to avoid the dangerous oversteer. On some implementations, they switch direction for high speed steering, and turn the rear wheels very slightly in the same direction as the front.

I certainly take your point about the sideways forces, though. All the wheel-bearings on a 6x6 take a hammering when you do tight turns - I would imagine the sideways forces must be huge. (Certainly the tyres deform far more noticeably than they do when you traverse a side-slope, for example).

Yes, sideways scrub isn't a really big issue, except when turning tightly. When making small turns, the sidewalls simply flex, and then unflex again when straightening out. As you say, the long wheelbase of lorries makes for a poor turning circle, and therefore less scrub.

Duh! I just realised (looking at the pic of the 8x8 with 3-axle steering) that you must have intended steering the middle axle, in the same direction as the front, rather than the rear-most axle in the opposite direction (?)

That would certainly nullify my argument about oversteer. On the other hand, it restricts your turning-radius gain. Your 100/140 vehicle would turn like a 4-wheeled 140 (and pretty much the same as a non-rear-steer

6-wheeled 100/140, other than the very slight increase due to tyre scrub).

Incidentally, one of the pleasures of driving a 6x6 is how stable it is on the road. There is very little wandering compared to a 4-wheeled vehicle. It'd be a pity to lose that :)

(Of course the other side to that coin is that when you get into some slippery mud, a 6x6 does have a tendency to go straight, no matter where you steer!)

He probably didn't, the problem with that kind of steering is that you have to reduce how much your front axle steers because the next steering axle can only steer as much as the mechanism allows, but it always has to steer sharper than the one closer to the front. So if you can't increase the steering lock on the middle ones then you have to reduce it at the front.

They're great for towing too, apparently. Never tried it in mine.

Try doing it with three locked-up axle diffs!

I've tended to find in the pinz that if the rear axles are locked and the front isn't, it doesn't affect manoeverability that much, the front locker tends to make the biggest difference.

I did have trouble in some ruts once though, I needed to steer out while going backwards but it wasn't interested, it just ignored the front wheels. I just cut the side of the ruts in front of the front wheels and drove up the cuts, locking the front diff allowed it to climb the sides OK even on the bald tyres I had on it at the time.

Isn't it the other way around? i.e. the front wheels turn the sharpest, and each successive axle turns less.

You'd also have to have different Ackerman angles on each steering pair of wheels, to ensure that all 6 (or 8) wheels describe different arcs about the same centre-point, regardless of how much steering lock is applied.

Doh! That's what I said, it's just not what I typed!

Same issue though, the standard lock on one set of axles means you can't use all the lock on the other ones.

On an 8x8, particularly one in which all the wheels are equidistant from each other such as the Escaro, the easiest thing to do would be to have steering axles at front and rear with fixed axles in the middle.

The whole "oversteer" issue would be a matter for driver training, as you'd still be describing a path along the same arc as you would be if you had steering at the front three axles. The back of the vehicle would swing out, but it does on a conventional vehicle if you have a large rear overhang too.

I'll bet there's some law somewhere banning it all though!

I had 3 ARB's in the Defender, so I know just what you mean. You mentioned the logical order of engaging axle-drives and lockers: it's an interesting issue with a lockable centre diff. Depending on the traction requirement, I could choose between (as a first step):

1) engaging the centre (logitudinal) diff lock ("proper" 4x4) 2) engaging the third axle (6x6) 3) locking the axle (transverse) diff locks on the front and/or middle axle.

I generally found the most sensible order, as conditions got difficult, was:

1) Engage the centre (logitudinal) diff lock 2) Engage 6x6 3) Engage one or both of the rear transverse locks 4) Engage the front transverse lock

Interestingly, I started off assuming that it would be better to engage

6x6 before locking anything. But of course, the open logitudinal diff always equalises the torque between the front axle and the rear *pair* of axles, so it was a pretty certain bet that 6x6 with all diffs unlocked would result in a front wheel losing grip first.

That was a bit of a pity - for long distances in sand or on slippery surfaces, the best configuration is to have the maximum number of driven wheel, but all open differentials. The only way (I can see) to achieve that with 6x6 is to have the open longitudinal diff split the rear/front torque in a 2:1 ratio, instead of 1:1.

Bonus question: Can you bias a differential simply by varying the relative sizes of the side-gears?

I was going to ask what the configuration of drive distribution was on yours, you beat me to it. The diff between front and rear is always an issue, hard to do a 6x6 drivetrain with full differential capability! It's almost easier to do it with 8x8 as you can have cascading diffs, one diff feeding two other diffs which feed the axles. Torque biassing diffs could probably do it nowadays with a 6x6 drivetrain.

I doubt it, you'd end up having two output shafts wanting to run at different speeds, so you'd either have to build the diff so that it could compensate for that (putting bearings on the frame-mounted gears) or put gearing to reduce the speed again after the diff, but that would then multiply the torque back up...

Mmm... you would have to gear it back down, but that would be the whole point.

The standard 1:1 diff sends equal torque to front and rear, at any speed. If we use unequal spiders, we can make the rear propshaft travel at twice the speed of the front, but still with the same torque?

Then we gear down the rear diffs by changing the cw/pinions, and get the same speed, but twice the torque.

So, let's say a front wheel loses traction at 1000Nm of torque. With a standard centre diff, the total front shaft torque will be 2000Nm, which will therefore limit the total rear shaft torque to 2000Nm, or only

500Nm per rear wheel (on the 6x6, all diffs unlocked, and assume for simplicity, diff ratios of 1:1). Total useful torque is 4000Nm.

But with the modified diff, the rear shaft will be delivering its 2000Nm twice as fast, and then the diffs will convert this to 4000Nm at the same speed at the front, giving us our per-wheel torque of 1000Nm, same as the front. Total useful torque is 6000Nm.

Or is there a flaw in the reasoning?

In the day job, the longitudinal diff. on a 6x6 is an epicyclic 40/60 split differential (lockable). We don't have any differential between the two rear axles. There's a cross-axle diff. lock on each axle.

Trying to achieve unequal torque splitting with bevel gearing would be a challenge (at least, I've never seen an example).

Not sure, I'm mostly in the dark on torque calculations, although I'm assuming that if gearing down increases torque, then gearing up will decrease it, so doubling the speed would halve the torque. I'm not certain about that in this case though.

Probably, there usually is in the simple answers to complex problems ;-)

What vehicle is this out of interest?

And can a torque biassing diff operate in that mode full time?

There's probably a good reason for that ;-)

I really must look into how torque biassing diffs work.

What make and model of truck?

LOL I know the feeling.

True, but it does mean you don't *lose* any steering lock, you just don't gain as much as you might otherwise.

Uh uh, I don't think so. With the middle axle being fixed, you'd

*counter* steer the rear axle, and your turning radius would be much smaller. The centre of the arc would be in line with the middle axle, and much closer to the vehicle (for any given angle of the wheels).

With the rear axle being fixed, and both other axles turning the same way, the centre of the arc would be in line with the rear axle, and further away.

Since the overall turning radius is the radius of the biggest arc (the front outside wheel), it follows that you'll be turning much more tightly.

The oversteer isn't due to the tighter arc, though. It's for the same reason you lose control if you try to reverse the car very fast. (And that's not just because you have to twist around in the seat to see behind you!)

No doubt!

It depends on what you're adding and where. If for example you added an axle to the back of a 110 and made the original 110 rear axle steer, you'd increase the turning circle. If you added a new axle between the 110's original front and rear axles and made the new axle steer then you wouldn't lose anything.

Yes, but that's just a case of steering less, so you gear the steering down and train the driver. What I meant is not the exact same steering arc, but an arc, and you control the tightness of it with the steering wheel. It would be odd with the back swinging out but it should be easy to get used to. It took a while to get used to steering a forward control vehicle where you swing around out the front but fairly quickly you end up not noticing the difference any more.

.. so you steer less ;-)

Unfortunately I can't find anything at moment that will describe the function etc. and which is in the public domain. The link doesn't go very far!

Yes.

On or around Tue, 20 Mar 2007 14:43:11 GMT, snipped-for-privacy@none.com (Madhatchetsbrother) enlightened us thusly:

remember that it's in addition to the normal rear axle, and only supporting the last few feet of the loadspace, unless you've loaded a very hevay item right at the back.

and the more recent front axles are pretty tough, the only problems you normally get are those caused by huge mileage or neglect.

On or around Tue, 20 Mar 2007 15:42:45 GMT, snipped-for-privacy@none.com (Madhatchetsbrother) enlightened us thusly:

no, countersteering the back one. the 8x8 has fixed axles in the middle and steering ones at each end, AFAIK, and they're rarer than hen's teeth. it's basically (very roughly) 2 90s put back to back, but each with it's rear axle in between the 2 of the other one. It's driven by 2 T-boxes, linked together by a shaft - the front t-box drives axles 1 and 3 and the rear one drives 2 and 4. The only thing that's possibly suspect is the fact that half the axles are being driven in reverse.