Freewheel hubs on Range Rover axles

Oh, nothing big then ;-)

I wouldn't imagine that truck would on a single gallon either!

Reply to
Ian Rawlings
Loading thread data ...

I think you two are on about different 8x8s, the pic I posted was of a custom-made Haflinger 8x8 which steered the first three axles, whereas you are on about the Escaro, which was then taken over by Steward and Stevenson, and I think someone else after them, gradually losing all landy components on the way. Apparently they had a lot of trouble with the front axle as it was overloaded.

Reply to
Ian Rawlings

On or around Tue, 20 Mar 2007 19:18:53 +0000, Ian Rawlings enlightened us thusly:

yeah, I was. I'd forgotten your haffy, or more liekly assumed it was a front-and-back steerer.

and yes, it was take over by a transpondian lot.

but the pictures I've seen of the transmission are more or less as I described.

Reply to
Austin Shackles

The characteristics of steering from the rear of the vehicle can't be compensated for (at least, no where near *fully* compensated for) by changing minor steering geometry, or changing driver technique.

When you steer the rear of the car, the rear of the car goes in the "wrong" direction, and the car has an overwhelming urge to pivot around its outside front wheel, and continue going in the wrong direction even when you straighten the steering wheel.

Try reversing your car fast through some cones - it's not just technique, it really is impossible to maintain control once you reach some critical speed!

Reply to
Madhatchetsbrother

On or around Wed, 21 Mar 2007 11:31:09 GMT, snipped-for-privacy@none.com (Madhatchetsbrother) enlightened us thusly:

yeah, but that's for 2 reasons:

1) you're effectively steering the rear wheels ONLY. 2) the geometry is going backwards and so is the caster angle, so the steering, instead of self-centring, self-uncentres.

in the case of my proposed 6x6, neither applies:

1) although the rear-most wheels are steering, the vehicle "pivots" around the "middle" axle, which is in fact near the back. The front end steers normally, the "middle axle follows in the same line as it always did on the original 4-wheel vehicle and the rear one follows a curve that makes it follow the middle one. Envisage a 6x6 with no rear steer and a lift-axle at the back. The rear steering axle will follow the line the lift-axle described would follow when lifted.

2) the geometry on the rear axle can be (and would be, I reckon) set to self-centre when going forwards, thus adding to the self-centring effect.

Reply to
Austin Shackles

Bear in mind though that if the rearmost axle is closer to the pivot point than the front one, in order to follow the line it'd have to steer a lot more.

Reply to
Ian Rawlings

On or around Wed, 21 Mar 2007 20:53:14 +0000, Ian Rawlings enlightened us thusly:

no it won't. turn it round, imagine the steering axle just in front of the rearmost axle. it only has to steer a little bit.

If you happen to find an 8-legger wagon or a "chinese" 6 to look at, you'll find the axle furthest from the fixed one steers the most.

if it's behind the fixed axle, as I plan, then it still only steers a bit, just in the oposite direction.

Look at it another way: the longer the wheelbase, the more steering angle you need on the steering wheels to allow it to turn the same size circle.

It may not be linear: if you take a hypothetical case where the front axle can adopt a 90 degree steering angle, then all other steering axles must be at 90 degrees as well. If you did that, you'd have more problems than steering angles though :-)

Reply to
Austin Shackles

See below

A relatively minor issue, but yes, it wouldn't make anything easier.

This is correct - the closer the rear axles are to each other, the

*more* controllable the vehicle will be. But in that case, the rear axle steers very little - it merely supports the rear of the vehicle (the part which would be called overhang if the axle was lifted or absent). And so the turning circle reduction would be very small. (On a standard 6x6, the turning circle usually falls somewhere between the longer and shorter possibilities, i.e. a 100/140 will turn like a 120).

No, the rear follows a path *outside* the middle axle's (as you state below).

This is the crux of the matter. It's about the mass that is on the outside of the pivot (i.e. the total mass that is behind the non-steering axle). As that mass swings outside the circle, its momentum wants it to continue in that direction, hence the staggering amount of oversteer. (I'm tempted to say the words "angular momentum", but I fear that can is *really* full of worms).

Regarding your point that steering a car backwards is worse because you are steering the rear wheels only: Yes, that is true - steering the front wheels too would *improve* things by moving the pivot point further back. Keeping the overall turning radius constant, controllability would improve the more we shift the pivot point backwards. Best case: pivot point in line with the rear axle i.e. front-wheel-steering alone.

But the salient point for our discussion (the effect of counter steering a 6-wheeler) is that - for any given amount of front-wheel steer - to the extent that you steer the *rear* wheels, you will induce oversteer. Three effects all contribute to this:

1)The pivot point is shifted forwards 2)The proportion of the mass behind the pivot point rises 3)The radius of the turn tightens

(It seems like point 1 and 2 are the same, but I was trying to say that not only is more of the mass behind the pivot, but also that this mass is further away).

The only one you can mitigate by design is point 3, but this is the very reason for doing the exercise!

Anyway, I am pretty sure that any vehicle with counter-steering axles would have handling that would charitably be described as "hairy"! Otherwise wouldn't they steer trucks like that? :)

Reply to
Madhatchetsbrother

Given that I've got it wrong twice now, I think I'll give up thinking about it ;-)

Quit while you're behind, that's what I say!

Reply to
Ian Rawlings

On or around Thu, 22 Mar 2007 15:25:26 GMT, snipped-for-privacy@none.com (Madhatchetsbrother) enlightened us thusly:

some of 'em do. some 3-axle semi-trailers have self-steering rear axles, also there's an interesting artic with a pivot both ends, which is apparently capable of getting to places that neither a standard artic nor a standard rigid will go.

I know one chap who used to run a short artic instead of a rigid 'cos it was more manoeuvrable.

But since lorries are generally longer, the scrub problems on the back are less severe - and also, they generally don't drive the front axle and as such can get much more extreme steering angles, making for better turning circle, so I doubt the extra cost and complexity makes it worthwhile. On a landy-type private project, cost and complexity are not much problem and also the benefits are greater.

4x4s (except the sierra, which still has the 2-WD style geometry at the front) tend to suffer (as to front-wheel-drive things) with limited steering angles caused by the need for the flexible drive, which makes for poor turning circle.
Reply to
Austin Shackles

Let's see any truck that has rear counter-steering designed to work at speeds over 30mph...

No, they do make some trucks with more than one axle steering. What they don't do is make them counter-steer.

Ok Austin, I don't know why you don't seem able to concede that there is an oversteer problem *inherent* in counter-steering, due to the pivot-point being further forwards relative to the centre-of mass. It doesn't render the concept useless, but it does make it wildly unsafe at road speeds. That's why all 4-wheel-steering systems are designed to steer both axles in the same direction at speed.

My advice (free and worth every penny!) if you're building a project vehicle is to understand *in advance* what you can reasonably expect from the result.

I was wrong about Pinzgauer diffs, you're wrong about counter-steering being benign :)

Reply to
Madhatchetsbrother

On or around Sat, 24 Mar 2007 08:09:06 GMT, snipped-for-privacy@none.com (Madhatchetsbrother) enlightened us thusly:

It should handle the same as a vehicle with a long rear overhang. It won't handle the same as it did originally, this is true, and I wouldn't expect it to. But some commercials especially have a LOT more rear overhang than most

4x4s and cars - IIRC you're allowed 60% of the wheelbase maximum (I've not actually checked that, though) - and they don't all crash as a result. I'm not denying the handling will be different, but I doubt it'll fall over at the slightest provocation which is what you seem to be suggesting.

Now, if I were proposing a 4-wheel thing with countersteering rear axle, that would be different, and you'd be right, it would have "interesting" handling, same as things like loaders and fork-lifts that steer from the rear only.

and I agree with you, see above. but I'm not proposing a 4-wheel one.

The only way to settle this is to build one. I've not got the money right now, though - might manage an RC model though, which is a thought, and would be a cheaper way of finding out if it's workable...

maybe. but unless you have actual experience of the configuration I'm suggesting, I venture to question how you know?

coming back to the stability thing: suppose you take a 100" rover and extend the body so that the rear overhang is 60", the maximum allowed by C&U regulation 11. The resulting vehicle will have different handling characteristics from the original, but I doubt it would become uncontrollable, unless you loaded it in such a way as to take all or most of the weight off the front axle. My proposed 3rd axle will be underneath that overhang and will track so that the whole vehicle behaves like a 4-wheeler with a long overhang, except that by putting that axle there, you won't now (easily) be able to load it so as to unstick the front axle from the road.

Looking at it another way: if the 3rd axle were contrived like the ones on the back of the 3-axle semi-trailers so that it self-steered, how would it contribute negatively to the stability of the thing, compared with the

4-wheel case with long overhang? it can't, it's purely passive. This would in fact work fine for going forwards[1] - the main reason for steering that axle positively rather than having it self-steer is that it will then work equally well in reverse. I don't know how those self-steer ones on semi-trailers work in reverse - I tend to assume they have something that locks them straight, unless they have some extreme cunningness that lets the self-steer backwards as well. [1] though I don't think you could drive it - which is the other point about having it positively steered.
Reply to
Austin Shackles

How about the GPV 8x8x8 truck, max speed 70MPH, steering is mechanical and works full time, with the rear two axles counter-steering.

formatting link
I also spotted some papers that apparently detailed research into counter-steering axles on commercial vehicles to reduce tyre scrub but couldn't read them without spending cash to download them.

There were a few more that I came across in a brief search but it was hard to tell if they used counter-steer at highway speeds, but it's very possible, at least one vehicle is known to do it and it's something that's been looked into for commercial vehicles. There's also no reason why not as long as all the axles steer in a manner that keeps them on the same arc, as Austin says it's no different to having a large rear overhang.

Have a look around using terms like "8x8x8" and "6x4x4" or other combinations, "6x4x4" for example means six wheels, four driving, four steering. Most 6x4x4s would steer the front two of course.

Reply to
Ian Rawlings

Quite correct: if we disregard the problem of front axle lightness, then a 6x6 with rear counter steering will handle (and turn) much the same as an identical vehicle with the rear-most axle absent altogether, if such a thing were feasible - i.e. huge oversteer.

And in reality, the third axle is not something just added to a vehicle under the existing overhang. On your proposed 100/140, for example, I would surprised if the laden centre of mass weren't roughly over the middle axle, i.e. almost half the mass will be behind the pivot point.

It's not the absolute mass of the overhang that matters, as much as where the *centre* of mass is, relative to the pivot point (the real or imaginary centre axle). So in a very long truck, a few feet of overhang doesn't have nearly the same adverse effect as the same overhang on a shorter vehicle (although you certainly would want to go slowly through the roundabouts either way).

Yes, you can always make the handling of a 4ws counter-steer *less* hairy: firstly, steer the rear axle less (that will determine/be determined by where you put your middle axle if you have one);secondly, increase the "front pair" wheelbase, which will shift mass forward relative to pivot point. But both these measures work against the very improvement in turning circle you are trying to gain. To the extent that you tighten the radius of the turn by introducing rear counter steer, you will induce oversteer.

Hmmm.. so you're saying that the presence of the non-steering middle axle makes all the difference? The problem of the oversteer is simply caused by the amount of mass that swings further outside the circle, not by the number of wheels in contact with the road. Once the vehicle is in its oversteer-induced sideways skid though, I suppose the extra pair of wheels would help retard its progress through the hedge! :)

As long as you haven't yet actually lost control of the vehicle (i.e. as long as the wheels are all following their designated paths, rather than skidding or lifting), the presence of the middle axle is not hugely relevant.

The *principles* are really pretty straighforward matters of mass and inertia. Like counter-steering from the rear induces oversteer! :)

FWIW, I did research the steering (and wheelbase etc.) options with Foley and others when I designed our 6x6 vehicle, though for expedition purposes I would have rejected 4ws even if it hadn't presented a handling problem.

All agreed, except for the bit about the handling of the 60" overhang! A big proportion of the car's mass behind pivot point means that once again, you have a huge mass further outside the circle when you turn, and everything will end up in the hedge.

Notwithstanding the interesting GPV military trucks that Ian identified, there are good reasons that commercial trucks either have 2ws, or they have same-direction multi-axle steering, despite the advantages offered by rear counter-steering. Counter-steering the rear makes a truck far more manoeuvrable than same-direction steering, does the same job eliminating scrub, and is no more complex or difficult to implement. Yet

4 and 6ws (same direction) trucks are common, versus an (almost) total nonexistence of 4 and 6w counter steer trucks.
Reply to
Madhatchetsbrother

All the Mercedes urban rubbish trucks here have a counter steering rear axle - and they happily do 60mph+. And the setup isn't some add-on, it's a factory Merc configuration.

Reply to
EMB

And cross-axle diff-locks!

formatting link
Huge link, but from the blurb;

"The 26t versions are available with electronically-hydraulically controlled mid or rear steer axles, the 6x4 configuration features a hub reduction bogie with inter axle and cross axle differential locks."

Mind you there's tons of trucks with rear-steering axles, but what whassisface is insisting on is that it can't be done at over 30MPH. I noticed he didn't comment on the 70MPH one with mechanical counter-steering that is active all the time.... Seems he wants to argue with the metal ;-)

Reply to
Ian Rawlings

MotorsForum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.