How to: Choose a Suitable Off-roader

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Choosing the best off-road vehicle for your needs is not easy. However, if you are realistic about the regularity and severity of the off-road activities you are likely to pursue, it's a much simpler task to choose the right recreational vehicle — one that will suit your budget.  

There are many different kinds of off-road terrain — and only the exorbitantly expensive single-purpose 4x4s can cope with every kind of obstacle. The rest of the vehicles on the market are compromises and often based on family sedans or hatchbacks or light commercial vehicles to make them more affordable. Many of them are still very capable, provided you stick to dirt roads (even badly maintained ones) and frequently-used tracks. It follows that your choice of vehicle will depend on how you intend to use it.

What kind of off-roading do you intend to do? 

The following classification starts off with vehicles that are only really suitable for good dirt roads and progresses up the scale to more expensive and more capable off-roaders. The technical terms used are explained towards the end of the article.

2WD crossovers/compact family cars

These 2WD versions of off-road vehicles (usually based on front wheel drive family car platforms) are surprisingly popular. They offer the good ground clearance and visual appeal of an off-road vehicle without the extra expense and complication of 4WD.  

The 2WD Suzuki Vitara is a good example of a compact family car that's dressed up to look trail-ready.

4WD crossovers/family cars

These are front-wheel-driven vehicles that engage the rear wheels automatically via a multi-plate clutch or a viscous coupling when the front wheels start to spin. This is a very convenient 4WD system, but has the undesirable feature that in sand or mud the vehicle has to just about come to stop before 4WD comes into action. The MPV- and car-like qualities of these types of vehicles make them very popular with families.

Vehicles such as the Toyota RAV4 operate in 2WD until conditions require all-wheel traction.

4WD Double-cab bakkies and lifestyle SUVs

These very capable vehicles usually have a solid driveshaft between the front and rear axles, but are equipped with manually selectable high-range 2WD and 4WD as well as low-range 4WD. The lack of a differential between the front and rear axles implies that low-range 4WD should only be engaged on soft ground to avoid drivetrain damage.

Double-cab bakkies might be cumbersome by passenger car standards, but they offer surfeit off-road ability.

Permanent four-wheel drives

These vehicles are characterised by having a 3rd differential between the front and rear axles to avoid driveline stress. This differential is often fitted with a locking mechanism and, in this mode, it requires the same caveat as when driving a part-time 4WD in low range.

The Range Rover's impressive on- and off-road prowess can be attributed to its dynamic permanent AWD system.  

Ultimate off-road vehicles

These combine permanent 4WD and dual-range transmissions with manually selectable diff-locks inside each of three differentials (front, rear and central). There are not many such vehicles around, because electronic wheel spin control or automatically engaged limited slip differentials are taking over. The Toyota Land Cruiser, Nissan Patrol and Mercedes-Benz G-Class are good examples.

A general explanation of a typical 4WD system:

Understanding respective driveline components

The differential unit, fitted between the left and right wheels in the same axle, or between the front and rear axles, contains gears that allow one side to rotate faster than the other during cornering. This is necessary to compensate for the fact that the outer wheels travel a greater distance than the inner wheels.

This prevents driveline stress but has one serious disadvantage: the torque each wheel gets depends not only on what the engine can deliver, but also on the grip between the tyre and the road at the wheel with the LEAST traction. The result is that a spinning wheel will cause the vehicle to come to a complete stop because neither wheel will get any torque.

This video explains the controls on a modern bakkie-based 4WD vehicle:

The foregoing description refers to what is known as an open differential, and there are three ways that its behaviour can be modified to suit off-road conditions.

  • A limited slip differential (LSD) introduces extra friction into the unit, usually in the form of small multi-plate clutches inside the differential unit. These plates are forced together in proportion to the torque being applied and reduces the tendency for one wheel to spin.
  • A differential lock forces the wheels to rotate at the same speed, but this does not mean that they get the same torque. The torque each wheel gets will be in proportion to the traction that exists at that wheel, because the delivered torque is proportional to the grip. To see this, do the pencil test: Grip a pencil between your fingers on one hand, and try to apply torque with your other hand. You’ll find that the amount of torque you are able to apply with one hand depends on how hard you can grip the pencil with your other hand. 
  • This means that a diff-lock should only be used on soft ground, otherwise the situation may arise that one wheel has lost all traction but the other wheel may have enough traction to take all the torque the engine can deliver, resulting in a broken axle shaft.
  • Electronic wheel-spin control brakes any spinning wheel in an attempt to regain traction. For ordinary off-road driving, not much wrong with such a system, but it does mean that instead of avoiding wheel spin one should sometimes deliberately provoke wheelspin to get the system to respond, and that’s not the ideal in sand or mud.

The Mercedes-Benz G-Class is equipped with multiple differentials to facilitate its legendary "go-anywhere" ability.

Petrol or turbodiesel?

Diesel engines are generally more suitable for off-road use. The absence of an electric ignition system means that they’re less affected by water and in turbocharged form they develop more torque than an equivalent naturally aspirated (non turbo) petrol engine. They consume about 15 to 20% less fuel than petrol engines but will usually require more frequent servicing and most motors require 50 ppm (passenger car-grade) fuel.   

Petrol engines are less affected by infrequent servicing and low quality fuel. This makes them a better choice for trips to parts of Africa where fuel quality is suspect. They’re also significantly cheaper to buy and repair.

Manual or automatic?

Many purists prefer the positive control of a manual gearbox, but for general use an automatic is very convenient. However, it’s worth remembering that any vehicle with an automatic transmission cannot be push-started and should only towed for a very short distance without removing the driveshaft.

On the other hand, an automatic transmission is an engine-saver, because it will not allow large throttle openings in a high gear, or excessive revs in a low gear. In addition, most modern units have lock-up clutches that make for a solid drive above 20 kph, so they do not waste as much fuel as the older units.

This explains how you can get stuck while in 4WD mode:

And lastly, new or used?

Off-roaders, when they're used to their full potential, operate under much tougher conditions than their road-biased passenger car counterparts, so it goes without saying that new vehicles — backed by complete manufacturer warranties and service/maintenance plans — offer buyers the peace-of-mind that there are no potentially questionable ownership histories to the vehicles that they're buying.

However, buying a well-maintained used vehicle often represents the best value for money option, if you're wary... In the case of crossovers/family cars, which are operated in ways similar to conventional road-biased passenger cars (most of the time), check that the vehicles' service histories are complete, that the body and paintwork have been well maintained and be on the lookout for signs of excessive wear and tear, which could point to neglect or hard use. If you suspect that the vehicle you're considering has been subjected to off-road routes that are tougher than the vehicle were designed to deal with (rough, unkempt gravel roads for example), excess stone chips in the paintwork, graunched rims and a pockmarked underbody will be dead giveaways. During a test drive, suspension or wheel issues should announce themselves through squeaks or knocks emanating from the vehicle's extremities.

Water splashes look spectacular but extended submersion can be damaging to a 4X4's differentials.

When pondering a more heavy-duty 4WD vehicle be mindful of the fact that knowledge of the off-roader's ownership history and, preferably, the balance of warranty and service/maintenance plans are paramount... out of all these vehicles' components, differentials take the most strain and some all-wheel drive systems' breathers, which vent air pressure that builds up in drivetrain assemblies such as the differentials, transmission and transfer case due to internal friction, can be overwhelmed during water crossings. When driving through water, a vacuum is caused by the drivetrain assembly being cooled rapidly. This vacuum shuts the breather valve, causing air to be drawn into the assembly through the seals. If the seals are still submerged when that happens, water could be drawn into the drivetrain, which can contaminate the oil. Also be wary of the fact that when vehicles' air filters absorb moisture due to being subjected to water, it could lead to "engine breathing problems"; a petrol motor will tend to run too rich and a turbodiesel is likely to overheat due to being starved of air.

When driving through water, a vacuum is caused by the drivetrain assembly being cooled rapidly. This vacuum shuts the breather valve, causing air to be drawn into the assembly through the seals. If the seals are still submerged when that happens, water could be drawn into the drivetrain, which can contaminate the oil. Also be wary of the fact that when vehicles' air filters absorb moisture due to being subjected to water, it could lead to "engine breathing problems"; a petrol motor will tend to run too rich and a turbodiesel is likely to overheat due to being starved of air.

There is stiff competition in the family car market; all-wheel-drive configuration tends to be reserved for more expensive derivatives.

And finally, on the topics of petrol versus turbodiesel and manual versus automatic, turbodiesels used to offer significant torque advantages over naturally-aspirated petrol engines, but now that turbocharging is more common in the case of the latter, the advantage is limited to (purportedly) better fuel consumption. Bear in mind, however, that service intervals with many diesel-engined vehicles are shorter and more expensive, which is not of concern when a vehicle is still under its service plan, but a consideration when it isn't/about to run out of it.

The question of manual and automatic comes down to personal preference. Advancements in the efficiency of automatic transmissions have realised smaller penalties in terms of fuel consumption (compared with manual 'boxes). Manual transmissions are more cost-effective to buy, but bear in mind that the lifetime of their clutches and driveline components will largely depend on how previous owners used/abused their leftmost pedals...       

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