4x4 2WD the same as any other not 4x4 car?

pstuart

ILW wrote: »

I used to have an old 1.3 Suzuki SJ. It would go anywhere.

I used to have a wife who would do the same.

ILW

pstuart wrote: »

I used to have a wife who would do the same.

Was she Japanese?

goonarmy

alastairq wrote: »

The big secret is about keeping moving...and biggest aspect to aid that, is ground clearance.

{Hence why, pre-WW2 [and some post WW2] vehicles fared better on rough roads, tracks, etc that were more commonplace, than 'modern' vehicles...they had skinnier, but much 'taller' wheels}

The most recent new vehicle that offers the advantages of ground clearance and stronger suspension, but with the option of buying [more cheaply] with either 2wd only [fwd]...or with 2/4wd....is the Dacia Duster.

Tata used to offer such an option with their ancient pickups and SUVs [or whatever you modernistic youngsters call them these days?]

For a lot of customers, only the ground clearance was needed....without the extra expense of a 4wd option which might only be used once or twice in a lifetime.

As for the De[a]fender?

Well, I spend a lot of my working time coping with them..admittedly one's with a lot more interior space inside than the commercial versions. All they have, as far as traction aids are concerned, is an inter-axle diff lock, which gives positive 4wd.
Ie, the front axle [not wheels] is positively tied to the back axle....not free to turn faster or slower, which it is without the diff lock engaged [for normal road use...effectively, one-wheel drive]

The humble Toyota Hilux is streets ahead of the Defender in that particular aspect...in that it offers 2wd for normal [good grip] use, 4wd [high & low ratio] for 'off' road use.....plus [the killer for L/rover] a rear axle [cross] differential lock.

95% of any non-tarmac [ie, waterproof road surface] driving needs only positive 4wd at the most.

Only when the going gets silly, and wheelspin halts proceedings, does a cross diff lock [across an axle] prove beneficial [to overcome the problems a differential creates]....beneficial, that is, until both wheels on that axle start to slip together..then it's time for reverse gear.

So---''ground clearance'' really is 'king'....

[belly-out, and it won't matter how many wheels can turn together, one is stuck!]



If one simply needs to cope with some non-tarmac/concrete surfaces [the odd building site, for example, or rough farm track....or even, speed humps?] on occasion, then go for a 2wd-only option, & save money and possibly some fuel as well.

However, if regularly trying to persuade horse boxes out of silly grassy, muddy fields, then a 4wd option is perhaps best?

However, 4wd isn't the panacea for slippery surfaces.

It is simply [like diff locks] a tool to do a job.

And like any tool, it is only as good as the person using it.


I have recently joined the ranks of scruffy so-called 4wd-ers....having committed myself wholeheartedly to an ancient Daihatsu Fourtrak diseasel!

[Bought, for its ability to tow two whole Barratt estates down a road....3.5 tonnes....might have to consider tachograph rules if it ever gets to tow that sort of weight??]

It has the option of 2, or 4, wheel drive [hi & low range]....4wd being 'positive'....[ie, no diff in the transfer box, like the prehistoric L/rover]...but ..honestly, I cannot visualise any particular circumstance during its normal usage, where I'd actually need to use 4wd.

It's currently returning, with care, around 33 mpg's....which on my income is just about tolerable.

I wish the tyres weren't so darned wide, however.....

What is this positive your post talks of?

alastairq

What is this positive your post talks of?

Positive drive occurs when neither end can turn faster or slower than the other.

For example, Land Rover Defenders have both front & rear axles driven full time.

However, to avoid wind-up when driving normally, going round corners, a [third] differential unit is placed within the transfer gearbox, between front and back axle propshafts.

Because, when going round corners, the front axle needs to turn faster than the back one...[the back takes a shortcut, check out tyre marks when wet?}....the central differential, in the transfer 'box, allows this to happen.

[a bit like the diff in one's ordinary FWD transmission allows one front wheel to run faster than the other, when the need arises, is going round corners? Except that, the issue is longitudinal in the land rover [& others], instead of lateral, as with cars, etc.]

When both front & back axle are driven positively [4wd], the front axle cannot turn faster than the back. Eventually this might lead to wind-up on the transmission, if negotiating good grippy ground.

This wind-up can manifest itself in enhanced wear & tear on the transmission, and tyres.....it can be noted when trying to disengage the inter-axle diff lock on the Defender...and it won't 'come out'. to release it, the vehicle needs to by driving slowly either forwards or backwards....in a straight line...until the warning light goes out [the diff lock is released].....the most vehicle-sympathetic method.

Vehicles with ''2 or 4wd'' systems, have positive 4wd ..ie no differential unit in the transfer box.



''Cross diff locks'' work within an axle line..ie between the wheels of an axle.

It is the differential that allows wheel spin on slippery ground.

Thus, if one wheel spins, the other ceases to drive..we all have experienced that at some time.

But, if we could have 'locked' the differential, the other wheel, so far doing nothing, would then get equal drive [both wheels locked together]...and would be the wheel to shove one forwards again.



Lorry drivers driving LGVs with a rear, 4-wheel 'bogie'.....[3 or 4 axle lorries]...where the rear bogie has both axles driven , all the time.....have a similar problem. To allow one axle t turn slightly faster or slower than the other , in the bogie...ie when cornering....there is an additional differential unit within hte drive line, ie between the two driven axles.

However, drive over a nasty little hump, or a sharp dip in the road surface, where one entire axle& wheels can leave the ground...this dangling axle starts to spin freely..the other[driven] axle simply stops driving altogether. Very embarrassing!

So the driver would have to 'lock' that inline differential [inter-axle diff lock] to get both axles driving equally..and continue moving.

Sorry it's a bit long-winded, but all this stuff about 4wd, etc, really needs simplifying, to be understood properly.

And who wants a box full of jelly in one's driveline? [dig at Freelanders?}

goonarmy

alastairq wrote: »

Positive drive occurs when neither end can turn faster or slower than the other.

For example, Land Rover Defenders have both front & rear axles driven full time.

However, to avoid wind-up when driving normally, going round corners, a [third] differential unit is placed within the transfer gearbox, between front and back axle propshafts.

Because, when going round corners, the front axle needs to turn faster than the back one...[the back takes a shortcut, check out tyre marks when wet?}....the central differential, in the transfer 'box, allows this to happen.

[a bit like the diff in one's ordinary FWD transmission allows one front wheel to run faster than the other, when the need arises, is going round corners? Except that, the issue is longitudinal in the land rover [& others], instead of lateral, as with cars, etc.]

When both front & back axle are driven positively [4wd], the front axle cannot turn faster than the back. Eventually this might lead to wind-up on the transmission, if negotiating good grippy ground.

This wind-up can manifest itself in enhanced wear & tear on the transmission, and tyres.....it can be noted when trying to disengage the inter-axle diff lock on the Defender...and it won't 'come out'. to release it, the vehicle needs to by driving slowly either forwards or backwards....in a straight line...until the warning light goes out [the diff lock is released].....the most vehicle-sympathetic method.

Vehicles with ''2 or 4wd'' systems, have positive 4wd ..ie no differential unit in the transfer box.



''Cross diff locks'' work within an axle line..ie between the wheels of an axle.

It is the differential that allows wheel spin on slippery ground.

Thus, if one wheel spins, the other ceases to drive..we all have experienced that at some time.

But, if we could have 'locked' the differential, the other wheel, so far doing nothing, would then get equal drive [both wheels locked together]...and would be the wheel to shove one forwards again.



Lorry drivers driving LGVs with a rear, 4-wheel 'bogie'.....[3 or 4 axle lorries]...where the rear bogie has both axles driven , all the time.....have a similar problem. To allow one axle t turn slightly faster or slower than the other , in the bogie...ie when cornering....there is an additional differential unit within hte drive line, ie between the two driven axles.

However, drive over a nasty little hump, or a sharp dip in the road surface, where one entire axle& wheels can leave the ground...this dangling axle starts to spin freely..the other[driven] axle simply stops driving altogether. Very embarrassing!

So the driver would have to 'lock' that inline differential [inter-axle diff lock] to get both axles driving equally..and continue moving.

Sorry it's a bit long-winded, but all this stuff about 4wd, etc, really needs simplifying, to be understood properly.

And who wants a box full of jelly in one's driveline? [dig at Freelanders?}

You sound like you nearly know what you are talking about.



Positive !!!!!!!

alastairq

Teaching off-road driving forms part of my job.....[usually when the student spends too much time driving on the grass verge....................]

How would YOU describe it?

[Best you get on to MAN...that's how they describe it too]

goonarmy

alastairq wrote: »

Teaching off-road driving forms part of my job.....[usually when the student spends too much time driving on the grass verge....................]

How would YOU describe it?

[Best you get on to MAN...that's how they describe it too]

I know how to describe it. Thats how i know your nearly right, ever so close.

kaya

AWD is all wheel driveline a subaru or BMW x5 and permanently drives all 4 wheels supplying more power to whichever wheel needs it most . A 4x4 or selectable 4wd vehicle such as a shogun will allow you to switch between 4wd or 2wd and usually have the ability to lock the diff's which will supply equal power to all of the wheels and feature a hi-lo ratio selectable gearbox , there is a world of difference between the capabilities of the two different classes of vehicles and also a huge price difference

alastairq

I know how to describe it. Thats how i know your nearly right, ever so close.

Problems arise when folk...in 'describing' what happens in any given driveline, end up generalising.

A 4[or more] WD driveline, which does not have a differential unit in the transfer gearbox [or in the driveline], drives both [all] axles positively..ie all at the same speed.

If a differential is present in the driveline [not counting the differentials in each axle..between the wheels on that axle]....then the axles cannot receive a positive drive......one axle can rotate at a different speed to the other...

Worst case scenario, if one entire axle loses grip, and spins, the others cease to propel the vehicle......due to the action of the inter-axle differential.

Lock that inter-axle differential, and the result is positive drive to all axles, without one being able to turn at a different speed to another. [as per L/rover defender]

This in itself does not prevent wheelspin....and loss of drive to its opposite wheel.

The only way to restore drive to a wheel in that axle, is to lock the differential located in that axle....cross diff lock.

With road cars, such as Subaru's, etc....technology is brought into play, but the drivelines are nowhere near as 'robust' as those found in a dedicated off-road vehicle. [to do with shock loadings on difficult terrain]

Whilst differential locks overcome the problems presented by a differential, when on slippery surfaces [wheelspin, loss of drive]....they have their downsides as well....for example, affecting steering [increasing turning circles].....and creating wind-up in the entire chassis, if the ground proves to be grippier than first thought.

(95% of folks' off road driving won't probably need anything more than positive 4wd.....ie selecting 4wd, or, in a Defender, locking the inter axle differential]


Only when deliberately seeking difficult ground to drive over,, will axle diff locks [cross diff locks] be of benefit.

Providing, of course, one has sufficient ground clearance?

Which is one reason why an Austin 7 can go where a LAnd Rover cannot?

sharnad

Kia sportages are great in the snow when you turn the 4x4 on