hows it work

[jutwit]

hi guys , ive got a 4 wheel drive freeda, and was wondering how it works, is it 25% drive to each wheel constantly, front rear split of 40/60 or similar , or is it one of those trippy systems that sense slippage and move power around for the best grip?

[dave_aber]

It's basically rear wheel drive.

As you drive along, the rear wheels drive the Bongo along, and the Bongo pushes the front wheels along. The front axle and propshaft transmit this motion back to the centre viscous coupling. As long as the front wheels are moving a the same speed as the rears, the VC sees little or no 'shear' - difference in speed.

If the VC senses shear, i.e. if the rear wheels are spinning on ice, and the fronts are not moving, then the fluid in the VC heats up, and this causes it to lock the rear drive to the front drive - so power passes to the front wheels as well, in a 50/50 split. At lower shear levels, lower fluid temps, the torque split will be less, like 25% front, 75% rear. It can never be more than 50% to the front.

Read all about it

In practice, it works really well. Driving on snow last winter on a long gradual hill, I tried a bit too much power, deliberately. Initially I felt the rear lose traction slightly, then a tiny but noticeable shake on the steering wheel as the fronts started to pull, and all was well again.

The 2 downsides are - you have to lose grip on the rears before the fronts take some power, and once you have lost grip you may not get it back again. Also, unless you have a LSD in the rear axle (some have, some haven't) then it's possible to get stuck with 2 wheels spinning - one front and one rear. I also experienced this last winter trying to get into my drive over a 'hump' of snow. With a LSD in the back it's still possible to get stuck, but you need to lose traction on both rears and one front. (Audi Quattro's in comparison can drive with 1 wheel in traction and 3 slipping).

[waycar8]

or is it one of those trippy systems that sense slippage and move power around for the best grip?

Some have limited slip diff (optional extra), if yours has a limited slip diff there will be a yellow sticker type thing on the rear diff somewhere

[dave_aber]

Talking of LSD's - is the drive ratio in the LSD axles the same as the plain diff ones?

Thinking of trying to get hold of a LSD axle, and want to make sure that it's the same ratio as my existing front diff before I tear the VC apart.

[jaylee]

It's basically rear wheel drive.

As you drive along, the rear wheels drive the Bongo along, and the Bongo pushes the front wheels along. The front axle and propshaft transmit this motion back to the centre viscous coupling. As long as the front wheels are moving a the same speed as the rears, the VC sees little or no 'shear' - difference in speed.

If the VC senses shear, i.e. if the rear wheels are spinning on ice, and the fronts are not moving, then the fluid in the VC heats up, and this causes it to lock the rear drive to the front drive - so power passes to the front wheels as well, in a 50/50 split. At lower shear levels, lower fluid temps, the torque split will be less, like 25% front, 75% rear. It can never be more than 50% to the front.

Read all about it

In practice, it works really well. Driving on snow last winter on a long gradual hill, I tried a bit too much power, deliberately. Initially I felt the rear lose traction slightly, then a tiny but noticeable shake on the steering wheel as the fronts started to pull, and all was well again.

The 2 downsides are - you have to lose grip on the rears before the fronts take some power, and once you have lost grip you may not get it back again. Also, unless you have a LSD in the rear axle (some have, some haven't) then it's possible to get stuck with 2 wheels spinning - one front and one rear. I also experienced this last winter trying to get into my drive over a 'hump' of snow. With a LSD in the back it's still possible to get stuck, but you need to lose traction on both rears and one front. (Audi Quattro's in comparison can drive with 1 wheel in traction and 3 slipping).

Nice post Dave.. Certainly explained this 4X4 stuff to me!

I have no limited slip diff & though i roughly understand the principles the red highlighted has dispelled any notion of witchcraft beneath my chariot!

[dave_aber]

The other thing to add is that the "rear wheels spinning on ice, fronts stationary" is the extreme case. In practice, as soon as there is any difference between axle speeds, the VC will shear a little bit, heat up a little bit and lock a little bit. It's a gradual progressive process, not as if you have to get the rears spinning, wait a bit, then 'BANG' the fronts join the party.

Even on dry roads, when cornering the fronts travel further than the rears, so the front axle speed is slightly higher than the rear - and the VC will begin to bite up. I believe this is what can cause "castling" on the front tyre treads as the front inside wheel is being encouraged to slow down to make the axle speeds the same.

[jutwit]

cheers guys, best not be entering the camel trophy then!!!

[T9sus4]

Excellent explanation! I think the Bongo Fury forum in itself is a viscous coupling device: whenever there is a difference between what I read and what I know, then some of the knowledge from the forum gets transferred to my brain.

[technophobe]

Does this mean that on a normal road only the rear wheels are driving, and the front drive will only engage if there is skid or slip?

[dave_aber]

The fronts only have power if there is difference in speed between the axles. If there is a skid/slip/wheelspin then this happens. Also during cornering, the front wheels naturally have to track through a longer arc, so consequently must turn faster - so the VC will begin to lock up in corners as well, with no skid/slip/spin.

Read all about it