Some notes about off-road driving

Getting stuck!

Differentials and wheel spin

Losing traction on one or more wheels is the most common reason for getting stuck when driving off-road (in other words, your wheels spin). This applies as much to four wheel drives as it does to two wheel. It is quite possible to get stuck in a conventional four wheel drive when just one wheel loses traction - the culprits are the differentials.

The differential (diff) is designed to allow your car to drive round corners. When you go round a corner the outer wheel has to travel further than the inner (this means it also has to turn faster). If both wheels are connected by a solid axle so that they turn at the same speed, then something has to give - either one or more tyres will skid/ scuff or the axle will break.

This problem only concerns the driven wheels, as these are the only ones that are connected together. The diff solves the problem with a neat bit of engineering by allowing the two wheels to rotate at different speeds, while still driving both. This is a great trick, but off-road it has an unfortunate side effect in that when a wheel loses traction, it spins with less resistance, and the diff transfers all the torque to this wheel. This means that the wheel that has good grip gets no torque, leaving the car motionless with one spinning wheel.

With four wheel drives, the picture gets much more complicated since each axle will have its own diff. If there is no connection between the two axles, for example with a twin engined vehicle, then so long as the wheels on one axle have traction then progress can be made. However, it is not uncommon to spin a wheel on each axle, and this will render the vehicle stuck - this is known as being "Cross-Axled", and is common if trying to cross a big ditch diagonally - you may get diagonally opposite wheels in mid air, or with very little grip.

Most four wheel drive vehicles are single engined, and to get the power to both axles there is a central propshaft. As with the axle between wheels, there is a diff on the propshaft (centre diff) which makes it possible for front and rear axles to turn at different speeds - necessary where there's good grip, to compensate for small differences in tyre diameter, and when turning. Again, here if one wheel spins, it will not only take all the torque from the other wheel on the axle but also from the other axle. Not all 4wds use a centre diff - but if you don't have one - don't use 4wd on road.

To get around the problem caused by a spinning wheel, the most common solution is to lock one or more diffs. If you lock the diff, it cannot transfer the torque, and so drive is maintained even with loss of traction to one wheel. With most four-wheel drives, you only get a diff-lock on the centre diff. This is ok until you spin a wheel on each axle - and then you are stuck. Some cars come with the ability to lock one or both axles as well (and there are some aftermarket items which do this as well). If you can lock all the diffs, you would need to spin all four wheels before getting stuck.

The diff locks that are most useful are those on the centre diff and on the rear axle. If you have a completely locked front diff, even in mud, while you may be able to get forward motion, you will not be able to steer - and this can make the difference between getting out and staying stuck.

The biggest downside of locking a diff is that you can only do it off-road, or when driving in a straight line. Many off-roaders keep the diff locked and this results in a torque wind up in the drivetrain which will sooner or later snap one or more driveshafts.The most sensible way to use a diff lock is to only use it when very necessary.

Limited slip diffs could be a better solution as they allow the wheels some ability to move at different speeds but still allow power to reach both wheels. However, they are rare in 4wd applications, as a completely locked diff is both simpler and more helpful off-road. On high-performance road cars, however, they are often used with success.

There are other systems out there such as computerised differential control and viscous couplings which all help avoid these problems, but they are outside the scope of this piece.

OK, so diff locks are great for four wheel drives - what about the two wheel drives? A diff lock will help you to get a 2wd where normally a 4wd would be needed, but it won't do as good a job as proper 4wd.

So - what other alternative is there? The trials fraternity use what they call fiddle brakes. These are levers in the car which allow you to apply a brake to individual drive wheels. So if a wheel spins - brake it - that goes some way to equalising the torque split, and can help.

OK, but what about the rest of us who don't have fiddle brakes? There is no easy answer but I have had some success by applying the handbrake gently. It only works if your handbrake works on the driven wheels (Sorry, most fwd drivers!) and it's a bit fiddly, but it's better than getting out and muddy!

(Thanks to Adrian Chapman for correcting my laymans description above and rewriting it in a technically correct format)

Ground Clearance

Your off-road ability is limited by the amount of space under your car. You have to be aware of what is the lowest point under the car (often the diff on a four wheel drive), and what are the other vulnerable items (normally the exhaust).
You can only go where the gaps under your car are higher than the obstacles on the ground. Sounds obvious but when driving on tracks, often the vehicle that created the ruts will have more ground clearance than you! Rut creators are most often lorries or tractors. If lorries then not only will the ruts be deep but they will be wide apart, which means you have to drive with one wheel in a rut and the other out. This is dangerous as it gives your car a sideways tilt - ok on the flat, but if the road slopes to one side or you are driving too fast then it can turn your vehicle over. Other problems to consider include fuel starvation. You need to choose a route that minimises this problem and also protects the vulnerable parts.

That aside, the biggest problem is grounding the vehicle. When viewed from the side, most four wheel drives appear to have huge ground clearance. Look from the front and its quite a different story - the diff will often take out over half of the apparent clearance. Getting the diff stuck is known as high centering - for obvious reasons I think. There is no easy cure except pushing/ towing.

If you are in a 2CV, the flat bottom can act as a sledge which will skip over this sort of obstacle. If you still get stuck, then get out of the car - this will raise the car by a couple of inches - push off and away you go (sit there spinning the wheels will form ruts which you will have to dig your way out). An alternative approach I have not tried is to adjust the idle speed to slightly faster than normal and leave the car running in first gear. Push and rock the car until it clears the obstacle and then run alongside and get back in (I don't think this would work too well on the narrow and twisty lanes in the UK).

Attack Angle

An important factor in driving off-road (and I mean off all roads/ tracks etc) is the ability of the car to climb slopes and come down off slopes without getting stuck. The more overhang there at the front or back, the more likely you are to get stuck.
If you want to drive up a steep bank and the front of the car hits the bank - you get stuck. If you had less overhang then the wheels would start to drive up the bank before the front hits.
The worst culprit is the rear overhang. You get the front wheels up or down and the the back of the vehicle hits the ground. If you look at some of the four wheel drives on the market you will see that the only place you could take them off-road would be on a flat field.

Independant Suspension/ Axle Articulation

Two related areas here - its the ability of the suspension to keep the wheels on the ground (and therefore still delivering power). For example, the modern Jeep has massive power but also has a rigid suspension system with which it is easy to end up with one or more wheels off the ground. (implications)
This is a common problem with leaf spring vehicles (the main problem with leaf spring vehicles is that they are very uncomfortable off road). Its a system which is cheap, reliable and easy to repair, but it is best on load carrying vehicles such as lorries. The rigidity of the axles is a major problem off road.

Independant suspension means that the wheels are not connected to the other wheels so they are free to move up and down as needed. With this system the wheels will nearly always be on the ground. 2CV owners will be aware of this ability - when jacking up on the body sills, it will tilt to over 30 degrees before the wheels lift off the ground.

Crossing Water

The 2CV was never designed for crossing deep water and has certain problems as a result. See below for some notes by Adrian Chapman, a 4x4 2CV driver regarding this issue. You will see that the Africar intended to resolve some of the problems by using a different engine and gearbox and by increasing the ground clearance.

"To get water in the air filter, you're either in a Dyane (take the Turbo tube off, if the water's going to be fan deep), or giving it some. If you hit the water fast and sudden, then water will go everywhere, and coil/HT leads are a likely, especially if the HT leads are old. If you've gone in carefully, and the water's gradually got deeper, then the most likely cause is points box, especially if you've backed off and lost the bow wave.

Even assuming the seal round the lid is there and good, there are plenty of water ingress points. It's not designed for this treatment! Lumenition is less prone than points to water failure, and the french Multic magnetic kit is meant to be pretty much waterproof. That just moves your weak point higher. The Visa engine is better, as there isn't a points box. It's still possible to fill the "brain" with water, though, even if it is mounted on the dashboard, inside the car. You could seal it inside a tupperware sandwich box high on the bulkhead.....

Assuming you aren't immediately assailed by bits of fan as soon as it hits the water, then your problems are likely to come when the gearbox gets immersed. There's plenty of little breather holes in the casting, which will make your gearbox oil a lovely emulsion. Add to that the wheel bearings not being sealed, and not much liking life after all their grease has washed out.

If you do go through a nice deep flood, and make it out (or don't), then PLEASE change engine oil and filter, change gearbox oil, and grease kingpins, driveshafts, wheel bearings, knife edges and suspension cans asap.

If not, you may find life not quite so wonderful shortly after. This is, of course, assuming that your chassis is utterly rustproof.

If your air filter is full of water, then your engine oil is also very likely to be, with the breathing system in place. I'd also be checking the fan VERY carefully....

Do NOT even contemplate reversing through the water. You WILL flood it all.

If you've really cocked up, and the motor has hydraulic locked, through the water level getting to the air intake (very very unlikely on a 2cv, but quite likely on other stuff), DO NOT even attempt to turn the motor over or start it. Take all the plugs out, and spin it over on the starter. Then, before you start it, change the oils."

Adrian
2cv 4x4.

Last update: 09/Nov/00	Contact: acadiane@free.fr