Stuck in the Mud

[Mr Halibut]

We spent the latter part of the Christmas Hols on a campsite in Cornwall. When trying to reverse the caravan off the hardcore pitch I had to drive onto the grass with the rear drivers side wheel part way on a tarmac path. I started to reverse with my head out of the window and nothing was happening although my foot was on the gas, the rear drivers side wheel did not move at all. My wife then told me to stop what I was doing because the rear passenger side wheel was digging its way down and spraying mud.

Eventually I gave up because no matter if I went forwards or backwards only the passenger rear wheel was spinning. (A friend with an X-Trail moved it out for us in the end).

It is a Rear wheel drive but I don't understand why only the passenger side wheel is spinning when the other side had all of the traction. Do I have a unique one wheel drive system?

[trevd01]

You have just had a perfect illustration of what a differential does!

Loss of traction

One undesirable side effect of a differential is that it can reduce overall torque - the rotational force which propels the vehicle. The amount of torque required to propel the vehicle at any given moment depends on the load at that instant - how heavy the vehicle is, how much drag and friction there is, the gradient of the road, the vehicle's momentum and so on. For the purpose of this article, we will refer to this amount of torque as the "threshold torque".

The torque on each wheel is a result of the engine and transmission applying torsion, a twisting force, against the resistance of the traction at that wheel. Unless the load is exceptionally high, the engine and transmission can usually supply as much torque as necessary, so the limiting factor is usually the traction under each wheel. It is therefore convenient to define traction as the amount of torque that can be generated between the tire and the ground before the wheel starts to slip. If the total traction under all the driven wheels exceeds the threshold torque, the vehicle will be driven forward; if not, then one or more wheels will simply spin.

To illustrate how a differential can limit overall torque, imagine a simple rear-wheel-drive vehicle, with one rear wheel on asphalt with good grip, and the other on a patch of slippery ice. With the load, gradient, etc., the vehicle requires, say, 2000 Nm of torque to move forward (i.e. the threshold torque). Let us further assume that the non-spinning traction on the ice equates to 400 Nm, and the asphalt to 3000 Nm.

If the two wheels were driven without a differential, each wheel would push against the ground as hard as possible. The wheel on ice would quickly reach the limit of traction (400 Nm), but would be unable to spin because the other wheel has good traction. The traction of the asphalt plus the small extra traction from the ice exceeds the threshold requirement, so the vehicle will be propelled forward.

With a differential, however, as soon as the "ice wheel" reaches 400 Nm, it will start to spin, and then develop less traction ~300Nm. The planetary gears inside the differential carrier will start to rotate because the "asphalt wheel" encounters greater resistance. Instead of driving the asphalt wheel with more force, the differential will allow the ice wheel to spin faster, and the asphalt wheel to remain stationary, compensating for the stopped wheel by extra speed of the spinning ice wheel. The torque on both wheels will be the same - limited to the lesser traction of 300 Nm each. Since 600 Nm is less than the required threshold torque of 2000 Nm, the vehicle will not be able to move.

An observer will simply see one stationary wheel and one spinning wheel. It will not be obvious that both wheels are generating the same torque (i.e. both wheels are in fact pushing equally, despite the difference in rotational speed). This has led to a widely held misconception that a vehicle with a differential is really only "one-wheel-drive". In fact, a normal differential always provides equal torque to both driven wheels (unless it is a locking, torque-biasing, or limited slip type).

http://en.wikipedia.org/wiki/Differenti ... _device%29

[Muzorewa]

That's how a differential works, you need it to enable the wheels to go at different speeds around corners. That's where a 4wd version would be better, although even then you only need to lose traction on one wheel of each axle to be stuck.

[Mr Halibut]

I guess this is where an limited slip diff becomes a bonus. In future I need to keep a couple of grippy mats in the boot!

Thanks for the replies

[scanner]

It is a Rear wheel drive but I don't understand why only the passenger side wheel is spinning when the other side had all of the traction. Do I have a unique one wheel drive system?

To get what you thought you had, you need a limited slip diff.


http://en.wikipedia.org/wiki/Limited_slip_differential

Some Bongos have them, others don't.

[the-bongo-mongo]

never get that problem with the deli !! diff locks etc......(he awaits to be shot ! ) only joking...the bongo i have here is 4wd and it spins in the muddy stuff too but obviously not as much....i usually look for flat ground to camp on as any incline at all with rain like weve had here and you will have bother with a heavy rwd vehicle like the bongo....choose your park up point carefully !

[vanvliet]

Some folk on the forum think that because they have a 4WD Bongo they can somehow tackle rough / slippy terrain in a top heavy , low ground clearance vehicle using tyres designed for normal road motoring. Others are more realistic - - -

[mikeonb4c]

I guess this is where an limited slip diff becomes a bonus. In future I need to keep a couple of grippy mats in the boot!

Thanks for the replies

I have a pair of thes inexpensive Fiamma jobs:

http://tinyurl.com/2h9g98

They live (unused to date) very unobtrusively, slipped under the boot mat by the tailgate, ready for deployment if/when needed.

[vanvliet]

Good idea Mike . Might get a pair

[The Great Pretender]

LSD conversions available if enough want them. Or make your own

[helen&tony]

Ahh, Vanvliet
I couldn't agree more....the Bongo is a great vehicle, make no mistake, but an "off-roader"....no way. It's just got (the 4x4) a traction control system, which might help in a muddy field...the ground clearance is definitely not suitable for off- road, and, as you say, the tyres , for most purposes, are as good as ice-skates in the gooey stuff. I've done a lot of off-roading in Land Rovers, etc. and been a member of Land Rover clubs, and in my mind , if you want a 4x4, then buy a Landie. I've also had , in 1981, an imported Lada Niva, which was a gem off road, and I'll be saving my money for a long wheelbase one for winter use. The Deli, i've had no opportunity to drive, but if it has the same running gear as a Pajero, I should imagine it is pretty good. There is an alternative for a good 4x4 just for fun, and that is a Liaz. There are loads of them over here, and they aren't dear, and our neighbour has one with a Merc diesel fitted. THey look like a Type 2 on steroids, have simple solid axle transmission, and ground clearance to make a Deli weep. No power steering, so I can't drive one , as I have old elbow injuries. I bet they would turn heads if customised.
Cheers
Helen

[the-bongo-mongo]

this is what you want to go off road camping !!

[helen&tony]

MMM
Very nice too
Helen

[the-bongo-mongo]

i want it !!

[The Great Pretender]

this is what you want to go off road camping !!

Naa you wouldn't want to get it dirty.

BTW my wife thinks a 3 star hotel is a tent.