GROUND CLEARANCE AND SUSPENSION
It is a bad idea to take a vehicle manufacturer’s minimum ground clearance claim and base off-road performance on it. For example, a Land Rover Freelander has similar clearance to a Mercedes Geländewagen. The Freelander’s independent suspension enables it to have a reasonable clearance which is measured from the chassis. In the case of the Mercedes, the clearance is measured from the differential housing, hung below its live axles that move up as the wheels ride over obstacles. So when these vehicles go off-road, the Mercedes’s clearance increases and with the Freelander it decreases. It is the suspension type and design that enables a vehicle to keep its clearance off-road, or lose it. In the case of the Freelander and most vehicles with independent suspension, it loses it. In the case of the live axles on the Mercedes, it keeps it. The result: The Freelander has inadequate clearance for driving off road and the Mercedes, which while it is not over endowed with clearance, must rate as one of the best off-roaders of all time.
In addition, clearance should be measured not only under the lowest point of the chassis but in front of, behind and between the axles as well. The front and rear overhangs (approach and departure angles), wheelbase in relation to wheel size (break-over angle) and centre of gravity (roll-over angle) are important factors which affect a vehicle’s off-road ability.
The maximum angle a vehicle can approach an obstacle without any part of the vehicle striking that obstacle.
The maximum angle a vehicle can leave an obstacle without any part of the vehicle striking that obstacle.
The maximum angle a vehicle can ride over without striking the obstacle between its axles. The longer the wheelbase the larger this angle is. On some vehicles, parts of the transmission protrude below the chassis and this has a detrimental effect on the break-over angle. So, if you are fitting protective equipment or towing apparatus to your vehicle, it is important to consider the negative effect it may have on these angles and therefore the vehicle’s off-road ability.
The choice of wheelbase should be determined by the kind of work the vehicle is likely to undertake and the loads to be carried. Long wheelbase vehicles can carry heavier payloads and have a higher seating capacity. They handle better on the road, on corrugations and on fast unsurfaced roads.
Short wheelbase vehicles have a few advantages when off-road. An improved break-over angle is the most significant. They are generally lighter, more manoeuvrable and more economical. Short wheelbase is a disadvantage on gravel roads and corrugations as they tend to have less straight-line stability and are more prone to slide.
This is the angle at which a vehicle will roll when traversing a slope at right angles. This value is a result of the distance of the vehicle’s centre of gravity above the ground. Anything above 40° is good and below 35° is poor.
CHOICE OF SUSPENSION
No compromise made to improve off-road ability or on-road comfort is more noticeable than those made to the suspension. The type and rating of the springs, the configuration of the axle location and the axle design all have a significant effect on a vehicle’s ability off-road and comfort both off and on the road.
Two types of axles are fitted to off-road vehicles – independent and live/solid beam axles.
Solid/live axles versus independent axles
If the vehicle is going to spend most of its time in the bush or will be worked hard in very rough country, rigid, solid beam axles, also known as ‘live axles’, are stronger and perform better than independent suspension.
When a wheel on a solid axle rides over an obstacle and lifts, it lifts the part of the vehicle closest to the ground (the differential) with it, thereby increasing ground clearance and clearing the differential over the obstacle. Because solid axles are very heavy, independent suspension reduces the unsprung weight contributing to ride comfort on-road.
With independent suspension, as a wheel rides over an obstacle the differential is left in a vulnerable position closer to the ground. Although independent suspension is able to offer superior axle articulation because the axle is independent of the differential, this is rarely the case with the current range of vehicles. In general, vehicles with the best axle articulation are those with solid axles and coil springs front and back.
No single compromise to the suspension system is more noticeable than axle articulation. Axle articulation is the suspension’s ability to allow the wheels to move vertically, to drop into deep ruts and follow the contours of the ground without leaving it and losing traction. Articulation is therefore very important to an off-road vehicle but to a road cruiser it is a curse because it allows the body to roll uncomfortably as the vehicle is cornered. In general, independent suspension gives less articulation and body roll than does solid axles.
Springs AND SHOCKS
Three types of springs are fitted to off-road vehicles – coil springs, leaf springs and torsion bars. Solid beam axles are either fitted with leaf or coil springs while independent axles are fitted with coil springs or torsion bars, or both. Another system, based on pneumatic cylinders in place of springs, permits variable ride-height adjustment from the cab. This highly sophisticated system is controlled by a computer and is fitted to top-spec 4x4s like the Range Rover and Land Cruiser V8.
Coil versus leaf springs
Coil springs make for a better ride both on and off the road. This is because they absorb vibration better than leaf springs and suspension designers can take advantage of unrestricted axle articulation offered by coil springs.
Coil spring designs require axle location arms to locate the axle to the chassis – a job which leaf springs do themselves. These arms come in the form of radius arms at the front, trailing arms at the rear and panhard rods or similar to locate the axle laterally. These suspension systems can absorb irregularities in the road surface so efficiently that vehicles get damaged often long before the driver realises the damage he is doing. One of the philosophies behind maintaining the production of 4X4s with leaf spring suspension for so long was the fact that an uncomfortable ride limits the driver’s endurance before limiting the vehicle’s.
Some vehicles, often those equipped with leaf springs, have heavy duty nylon straps attached to the chassis and looped around the axle at each hub. These prevent spring and shock breakages where suspension travel over uneven ground allows the axle to drop too far.
Shock absorbers control the oscillation of the road springs. When operating on rough surfaces they work hard because axle travel is greatly increased which in turn increases shock absorber temperatures. Shock absorbers are a vital part of the suspension system and in most cases, those supplied by the vehicle manufacturers are the minimum required for safety and vehicle controllability.
If you use your 4×4 off-road and on gravel roads and are considering improving the ride, handling and off-road performance, upgrading the shock absorbers is the first thing to consider. Gas shock absorbers and other suspension modifications are discussed in detail in Chapter-3.
These diagrams illustrate the variations in suspension systems fitted to off-road vehicles.
Front coil springs with a solid axle are almost always combined with a similar setup on the rear. This setup offers the best combination for off-road ability. Examples: Land Rover Defender, Mercedes Geländewagen and Unimog, Toyota Land Cruiser, 80 and 105 GX, newer Nissan Patrol, first and second generation Range Rovers and the first Toyota Hilux. The unusual combination of solid axles all-round with coils on the front and leaf springs on the back is found on the Toyota Land Cruiser 70 and 79 series.
Front independent coil springs or torsion bars with solid rear axles are found on vehicles such as the first and second Mitsubishi Pajero, Isuzu Trooper, Ssangyong Musso and bakkie-based vehicles like the Nissan Hardbody and Sani, Ford Ranger, Mazda Drifter, Isuzu KB and Frontier, all but the first generation Toyota Hilux.
Leaf springs on front and back axles are found on older designs such as the Land Rover series I,II and III, first Toyota Hilux, all early Land Cruisers, Suzuki SJ40, Jeep CJ, old Chevrolet Blazer, first Nissan Patrol, SVM and even the current Ford F250. This design is very old fashioned and not seen much any more.
Many years ago I predicted that all-four wheel independent suspension would become the most popular solution for 4×4 designers and this is now evident in most modern 4x4s, both those designed to go off-road and those that just look as if they can. Until the mid nineties just about the only vehicle fitted with this was the VW Syncro Bus. Then the Mitsubishi Pajero paved the way and today it is found on almost all ‘soft-roaders’. Now the third generation Range Rover, Discovery-3 and the Jeep Grand Chreokee among many others have all taken this route. This is because this system is best for on-road comfort and safety. While it works well off road, the penatlies are load carrying ability and axle articulation.
As engineers have developed systems they have managed to improve the design of independent suspension so that it can almost match the off-road performance of solid axles. While the die-hards (I am one of them) who love solid axles off road hold onto their older designed 4x4s for as long as possible, it will not be long before we are forced into accepting that this is the suspension of the future. Already, if we want a Land Rover with solids axles our choice is limited to Defender, and Toyota lovers are limited to Cruiser pick-up. The choice of station-wagons with pure solid axle suspension is now limited to just the Nissan Patrol.