Want to know if your SUV is capable of taking on the rough stuff? Jake discusses the off-road essentials that every proper 4×4 needs.
In earlier times there was a clear distinction between off-road vehicles and everyday family cars. These days this distinction is very blurred. Some cars have all the attributes of a four-wheel-drive product, but they transmit power only through two wheels. There are even so-called crossovers that look vaguely like off-road vehicles but have none of their qualities.
What attributes are essential in an off-road vehicle? Here’s my list:
1. Good ground clearance is absolutely essential, and generous approach, break-over and departure angles should be part of the design package as well. Protection of vital areas by skid plates is a bonus, but these can be added after purchase.
2. Generous up-and-down wheel travel, controlled by suitable spring rates and matching damping, is not absolutely essential, but many expensive off-road designs offer these features. Less expensive 4×4 vehicles, based on one-ton commercial vehicles (bakkies), usually exhibit a bumpy ride, but they are still very capable. The actual springing medium, whether by coils, torsion bars, leaf springs or compressed air, is not as important as the geometry (the actual path followed by the wheels while they travel up and down). There will always be a controversy over the choice between independent and non-independent suspension. The former offers more comfort and often less wheel spin. The latter design, involving the use of beam axles, is more rugged and allows a wheel to travel upwards without reducing the ground clearance between the wheels.
3. The engine must develop a generous amount of torque at low revs. This can be achieved in the design stage by choosing suitable valve timing, but it also depends on other factors such as the number of cylinders. When comparing engines of the same capacity, a four-cylinder unit will develop more torque at low speed than a six. Four cylinders will also deliver the firing strokes further apart than a six, making it slightly less prone to wheel spin. Motorcyclists know that a single suffers far less from wheel spin in soft sand than a twin. A four is almost unmanageable in sand. To see this, go for a ride on a solid but wet beach on a single-cylinder motorcycle, and then go back and examine your tracks. You’ll find a deep indentation corresponding to every power stroke. There won’t be any other indentations because the flywheel keeps the engine going for the rest of the time. This shows that firing strokes may cause wheel spin. The others don’t. The conclusion is that the lower the number of firing strokes (cylinders), the less chance there is of wheel spin occurring.
4. A good off-road vehicle should not be fitted with the same gearbox as a family saloon. It should have a range of specially chosen ratios, arranged in high and low ranges. Gearboxes and final drives multiply the engine’s torque in proportion to the ratio being employed, but leave the power unchanged, apart from a small amount of loss due to friction. This means that if an engine delivers 120 Nm at 1 800 r/min and it is transmitted through a 3:1 first gear the gearbox, the output shaft will rotate at 1800/3 = 600 r/min and experience 120 x 3 = 360 Nm of torque. If this torque goes through a 4:1 final drive, each drive-shaft (and wheel) will rotate at 600/4 = 150 r/min and get one-half of (360 x 4) = 1440/2 = 720 Nm of torque. This explains why it’s easier to get wheel spin in first gear than in any other gear and why first gear should only be engaged when no other gear will do.
5. An open differential allows each wheel to rotate at its own speed during cornering, but sends 50% of the torque to each driven wheel. Unfortunately, this value depends on the amount of grip at the wheel with the least amount of traction. If one wheel loses traction, both wheels will get one-half of almost nothing and the vehicle will come to a stop. A limited slip or locked differential is therefore desirable. The former supplies extra friction inside the differential unit to slow down the amount of spin and increase the usable torque. The latter forces the two wheels to rotate at the same speed, but the amount of torque each wheel gets is now separately determined by the grip at each wheel.
6. Tyres play a very important role. Universal tyres are not very good for serious off-road motoring, while specialised mud or snow tyres are noisy and wear out quickly on a freeway. This is a problem of choice that every motorist will have to face and deal with according to his or her needs. Many drivers find chunky all-terrain tyres to be the best compromise.
7. Off-road vehicles based on bakkies usually have a separate chassis, and many enthusiasts think this is the best construction. You can feel the chassis twist on a rough road, with the result that the bumps don’t jar as much as they would in a unit-constructed (monocoque) vehicle. The latter construction, usually found in soft-roaders based on front wheel drive cars, is more rigid and often tends to be more uncomfortable.