Thread: When is Four Wheel Drive engaged?

Originally Posted by Blknight.aus

ITs sort of a one way sprag clutch that drives the front axle and it was setup so that when the back wheels slipped the front wheels would recieve drive from the sprag. IT was lockable with a sliding collar which is what the ring was for.

And its a freewheeling unit not a flywheel..

Thanks - I guess I need to google what a sprag clutch is.

Nahh, its a one way clutch. When torque is applied in one direction it is transmitted when applied in the other direction nothing...

Theres 3 or 4 different types that do the same thing in about the same ways with different minor advantages...

But delving into them here would be a hijack.

OK looked it up and I think I understand - so the car was not constant 4wd as douted but rear wheel drive and when the rear wheels slipped, so turning faster than the front wheels the sprag clutch activates engaging the front wheels.

I was confused because the early landies were marketed as constant 4wd when they were rear wheel drive with an automatic on demand awd system - while the mechanicals are different the result is the same as the Viscous Coupling fitted range rovers (rear wheel drive and only AWD when the rear wheels slip) and the Freelander being front wheel drive and only awd with full torque to all wheels when the front wheels slip.

Cheers

Garry

you need to remove two words from your post....


the I think....

Mate for a lay description youve nailed it.

Originally Posted by garrycol

OK looked it up and I think I understand - so the car was not constant 4wd as douted but rear wheel drive and when the rear wheels slipped, so turning faster than the front wheels the sprag clutch activates engaging the front wheels.

I was confused because the early landies were marketed as constant 4wd when they were rear wheel drive with an automatic on demand awd system - while the mechanicals are different the result is the same as the Viscous Coupling fitted range rovers (rear wheel drive and only AWD when the rear wheels slip) and the Freelander being front wheel drive and only awd with full torque to all wheels when the front wheels slip.

Cheers

Garry

NO. They are full time four wheel drive, with both front and rear wheels driven at the same speed. When driving straight all wheels are driving. When turning, because the front wheels take a longer track, the rear wheels only drive unless the rear wheels slip, in which case the front wheels drive as well. Effectively (forwards only not reverse) it is the same as a locked centre diff or selectable four wheel drive engaged, not like a viscous coupling, as it does not allow power to be given to front and rear axles turning at different speeds.

The advantages over a centre diff were:-

1. No need for a separate lock except for reverse.

2. The free wheel unit was already in production in the company, reducing tooling costs.

The reasons for dropping it have never been explained that I know of, but were probably because the free wheel unit in this application proved less than reliable, possibly because the setup allowed full low range torque to be applied to the rear drive during turns intermittent gripping of the rear wheels could then put high shock loads on both the free wheel unit and the rear diff and drive shaft.

John

Originally Posted by DiscoDave

This sounds like a different system entirely to the Series III or have I got it wrong? When does my Series I have 4WD engaged?

Dave

Sorry for coming to this thread very late, but I have been lazy or busy or something. (Or feel treasonous )

Yes the early Series 1's had a different 4 wheel drive system. In fact they were actually a full time four wheel drive with an integral freewheel unit on the front output shaft which allowed the front propellor shaft to rotate faster than the rear one. This only happened when cornering, where the front outside wheel takes the longest path so rotates faster to cover the distance until the end of the corner where all wheels are traveling in the same direction at the same speed and drive is applied equally to all wheels.

The issue came when you wanted to have 4wd in reverse. In this case the freewheel unit needed to be disengaged (or locked) into 4wd.

From the begining of the 1948 production model to gearbox number 06104014 (VIN 06103884) in the early part of the 1950 model the freewheel lock was actually a small metal ring recessed into the floor on the RHS of the gearstick behind the transfer box lever. At gearbox number 06104015 the "ring pull" was replaced by a yellow knob that looked the same as ther later yellow knobs on the part-time 4wd transmissions. The function remained the same as the ring pull chain to engage 4wd in reverse.

The selectable part-time 4wd transmission came early in 1951 at gearbox number 16102314 (note that in 1951 there were multiple destination codes so the gearbox number is the best guide.)

Onto the body styles the first of the yellow button/freewheel unit models still had the headlamps behind the grill and the wide transom with the "sprung shovel" shaped seat backs although the "fishplate" that held the bumper bar on had been replaced by the later style bar where the mounting brackets were now part of the bar. By the introduction of the part-time selectable 4wd transmission, the Land Rover had the 7" Lucas headlamps, the spade shaped folding seat backs and the 2 1/4" wide front springs with the shackles at the rear of the spring. So essentially the elements that remained to the end of Series 3 production.

Hope this helps.

Regards
Diana

To go back a bit - a freewheel is what the drive hub of most road bicycles have, allowing drive to be applied in one direction only (forward) while the wheel can travel faster than the drive chain (or shaft in the case of a Series I).

One implication not mentioned yet is that engine braking in an early Series I would only transmit to the rear wheels, making driving down a steep slope a rather interesting idea.

actually I was going to suggest the simple is it all wheel drive or not test....

jack up all wheels one at a time with the vehicle in gear, park brake off and see if you can spin the wheel in each direction...

IF you only move all 4 wheels far enough to take up driveline slack you have a full time 4 wheel drive.

If a wheel can be rotated freely its not a full time four wheel drive.

The early S1 fails this test as you can freespin the front wheel forwards when the freewheel lock is not engaged. In realitly the only time this might happen is if you are doing a steep descent under engine braking (which means you should have the freewheeler locked up anyway) and the backs break away. (as previously mentioned.