What happens if you engage 4WD without locking the front wheel hubs? Is there any effect whatsoever? The handbook appears to say that it is alright to drive at speeds of up to 87 mph (4th gear) in 4WD. I had a Volkswagon Transporter Caravelle that was stuck in 4WD (switch was broken)(front wheel hubs not locked) and it handled brilliantly.

- (#11171) Mike, 27 May 04 14:45

I think with the townace when you engage 4wd the power is put though to the front axle, but unless you lock the hubs it will not drive the wheels, so no 4wd...

- (#11171) neil (torbay), 27 May 04 15:35

Neil. Since posting the above message I have found the following in 'Ace Answers' from a Dave Bright.

I'm sure someone will enlighten me if I'm wrong, but I had thought that when the 4wd is engaged, power is transmitted to the front wheels. As there is a differential on the front axle, both front wheels can turn at different speeds. This is good on roads, because going round corners results in the inside wheel turning slower than the outside wheel. This is not good though when in 4 wheel drive in slippery conditions. If one front wheel hits a
slippery bit, it spins and reduces the drive to the other wheel. The lockable hubs are used to prevent the front wheels from turning independently, so if one of them hits a slippery bit, the other one still has drive.

His thoughts then are my thoughts now. Does anybody else agree with Dave Bright?
- (#11171) Mike, 28 May 04 02:55

No they don't. I’m afraid I was wrong and managed to find out when I got stuck on a muddy-gravel drive last winter.

The front hubs have to be locked to have any drive at the front. The 4wd is then switched on or off from the 4wd button on the dash board. (This is a fuel economy thing)

You can't run these things in 4wd on tarmac roads, it puts to much strain on the engine, gearbox, tyres etc. So to allow for changeable surfaces, i.e. in winter when going from snow covered roads to treated roads, the 4wd can be switched on and off from the drivers seat. Because 4wd adds to the load on the engine, it also causes a drop in fuel economy. The front hubs allow this to be disconnected, so that there is no extra load when 4wd is not needed like in the summer.
Hope that helps to explain it.

- (#11171) dave Bright (Bournemouth), 28 May 04 03:07

Dave. Just read your response. I am disappointed, but thanks for clearing up the matter, once and for all.
Regards, Mike Siddall

- (#11171) Mike, 28 May 04 13:36

It works like this:
The 4WD button on the dash works on the transfer box behind the gearbox and when engaged sends drive to the front axle. This will then drive the front diff and the front axle shafts. However if the front hubs are not locked then the axle shafts are not connected to the wheels and are not driven. When you lock the hubs they lock the wheels to the shafts and allow 4Wd. Thus to get 4WD you need the hubs locked and 4wd engaged. This is a fuel economy measure. If you are not likely to need the 4WD then you unlock the hubs.Otherwise when you are driving in 2WD the front wheels are having to turn the front axle shafts, front diff and front propshaft for no reason, increasing drag on the driveline and wasting fuel.
Also, as there is no centre diff like you find in permanent 4Wd vehicles (like Range Rovers for instance) then you cannot use it on dry tarmac. This is because when cornering the front wheels go at different speeds from the rears and you will get what is called "wind up" in the driveline. This is a bad thing and will knock out all your U/J's, scrub your tyres away, and could even damage your gearbox. The diffs in the axles will not stop this as they only correct for speed differentials on their own axles, not front to rear.
Hope this clears it up.

- (#11171) John Shaw, 29 May 04 01:29

OK. Thanks everone. I've 'got it' now. Cheers

- (#11171) Mike, 29 May 04 03:28

hi Mike
I did post a message saying sorry to have disapointed you, but NTL crashed and didnt come back. I'm in London right now so I have no idea if it's back up yet.

For those interested in why this happens its because usually constant 4wd uses some sort of viscous coupling to allow for the difference between front and rear wheel speeds. As these things are usually just rear wheel driven, the front and rear tyres ware at different rates, so the back will turn faster than the front. The advise given in the hand book says rotate the tyres and spare. This results in a more even ware on all the tyres, but slight differences will always be there. This is what causes extra load on a system that is already stressed at the best of times.

Regards
dave Bright
- (#11171) dave Bright, 29 May 04 06:41

The viscous coupling is to stop the transmission loading up, the front wheels and back wheels rotate at the same relative speed or the tyres would get warm and begin to smoke, what does differ is the wheels rotational speed when going round a corner, the outside wheels need to rotate much faster than the inside ones.
By engaging front wheel drive there is more mass for the engine to turn and as we know it is accelerating this mass that uses more fuel, this is also one of the reasons for the freewheeling hubs, the other is wear and tear on the front tranny, was it landrover who developed these or a dissatisified landrover owner?
Rgds

- (#11171) Master_Simon, 30 May 04 03:12

The viscous coupling or centre differential on 4x4 vehicles is there for exactly the purpose that Dave Bright explains. The front & rear axles DO actually turn at different speeds to each other, apart from if you are going in a dead straight line on a perfectly smooth road, that doesn't happen very often. On the Townace, with 4WD & hubs locked, you must be travelling in a dead straight line on a smooth road, otherwise the axles will try to run at different speeds & this will cause meshing of the gears in the differentials, eventually leading to diff wind up. If you care to look at how any 2 wheel steer vehicle goes around a corner, not only will the outside wheels travel further than the inside wheels on each axle, but the front wheels will also travel further than the rear wheels due to rear wheel 'cut in'. It is for this reason you need to account for differing axle speeds. The freewheeling hubs are only put there to cut fuel consumption, aid acceleration & cut down on noise, otherwise you could just leave the front axle spinning from the turning of the front wheels & rely on the fact that the front prop shaft is not connected to the transmission due to being de-selected at the transfer box.

Permanent 4x4 cars need this centre diff to get around all of this.

As regards accelerating the mass, the mass of the vehicle remains the same, unless of course you remove the front axle & transfer box when you don't need 4WD. What you mean is that you have increased friction when in 4WD, i.e. more friction withing all the transmission, plus the engine is also having to overcome the extra drag from things such as the front axle oil etc.

So Master_Simon please have a think about the distances that each wheel turns.

Regards Rob.

- (#11171) Rob Drinkwater, 30 May 04 10:00

I shall have to take your word for it that the wheels travel at different speeds in a straight line, how this can be with the accompaniment of screaming tyres and smoke don't know, and as I said, in a corner there are different wheel speeds involved , that doesn't actually take that much thinking about.

Now where you are wrong is when you say about the mass, yes the overall weight of the vehicle will stay the same, please give me some credit, but what will change is the rotational mass, now if we remove say 25kilos ( just a figure plucked from the air) from the drive line, the vehicle will accelerate faster, this is why an old tuning tip was to lighten the flywheel, it aids pick up and this stands good in this example too, and my lawyer says I should mention that by lightening the flywheel does indeed enable higher revs, but this is a useful by product.

If you want me to get my uni notes out, I shall, I may not be an expert in the relative rotational velocities of the wheels of a vehicle moving in a straight line but I do understand physics, thank you

- (#11171) Master Simon, 2 Jun 04 01:51

Oh dear, this seems to have started argument, and that wasn’t my intention. I was just trying to explain how tyre wear will result in the wheels turning at different speeds. When all the tyres are new, the diameter and circumference are the same. Once there are a few thousand miles on them, the front tend to wear more than the back because they are more loaded and do the steering. If the front tyres have 5mm of tread and the back have 10mm of tread, the diameter and circumference is different. This means that the front wheels will have to rotate more than the rear wheels to cover the same distance. Without a coupling to allow for this difference, something will have to give.

- (#11171) dave Bright (Bournemouth), 2 Jun 04 02:15

Excellent a 4WD thread. I don't have much call for using the 4WD on my townace and its been redundant since I acquired the vehicle over 2 years ago.

I remember at the outset waggling the 4WD stick up and down just to check it moved, which it did, but have noticed last week that it now seems stuck.

The only work done in the area was over 12 months ago when I needed a new exhaust manifold gasket and I took the centre cover off as well as the seats and may have disengaged a cable or two as well.

I have checked the transmission oil level (which was a bit low) and topped up, but the 4WD stick appears stuck.
Any suggestions on the cause? (and sorry for highjacking your thread!)

Phil

- (#11171) Phil, 2 Jun 04 02:37

You are totaly correct Dave the front and rear wheels do turn at different speed, some manufacturers of 4x4 vehicles recomend a difference in tread wear front to rear to be no more than 2mm in depth Vauxhall being one manufacturer, i have replaced no less than 15 transfer boxes on small vauxhall 4x4's (mainly cavalier)due to odd tread wear, however some people get so wrapped up in what is correct on paper they can't see the practical.

- (#11171) COLIN (mechanic), 2 Jun 04 06:04

Not had much use myself for the 4WD, though the two occasions I have it's proved excellent. I live at the top of a very steep hill. One winter I was the only bugger to get up it. I smirk relentlessly watching 1 Discovery and 1 Suzuki fail it miserable the the same day.

The second occasion I was at Talacre beach in north Wales. The carpark is simply a sand bed. Some poor bugger and his wife had buried their VW camper axle deep the night before and finished spending the night there. Backed up the towny, attached his tow rope, engaged 4WD and pulled it out without a slip of a tyre, very satisfying.

- (#11171) Dave B, 3 Jun 04 17:09