It’s easy to get confused by the range of different four-wheel-drive and off-roading options on offer right now. Our resident expert, Doctor Diesel, takes a look at the different technology employed underneath today’s all-wheel-drive vehicles.
Whether you live at the end of a private track, still need to get to work in wintry conditions, or just crave the added security of four-wheel drive, it’s possible you are considering buying a car with four driven wheels. Engineers and designers have been incorporating all-wheel-drive – with varying degrees of success – for almost as long as vehicles have been in existence. We say ‘vehicles’ because the first contraptions wouldn’t be too familiar to us, or be very useful for popping to the shops.
One of the earliest designs came from a British engineer who patented a four-wheel drive system for a traction engine in 1893, with no less than three differentials and four-wheel steering. Over in Germany in 1899, a bright-eyed 25-year old fellow called Ferdinand Porsche designed a vehicle with an electric hub at each wheel, a layout still widely, and incorrectly, regarded as ‘futuristic’ by journalists today. His device was shown at the 1900 World Exhibition in Paris. But, the car market was highly fragmented and specialised in those days, so while various prototypes, racers and military vehicles were equipped with four-wheel drive, the wider population would have known very little about their existence.
It was World War II which created both the necessity and provided the means of mass production for the first truly iconic four-wheel drive, the Willys Jeep, with the Land Rover following shortly behind in 1948. These lightweight and basic off-roaders could go almost anywhere, but a design brief which included being parachuted out of cargo planes meant they didn’t exactly boast creature comforts. But, a post-war Willys knew a more comfortable, upmarket model should be popular and introduced the Jeep Wagoneer in 1963. This was the first SUV (a term not even coined at the time) and its innovations included independent front suspension and the first automatic gearbox in a production four-wheel drive. The luxurious Range Rover arrived in 1970, several years after the Jensen Formula Ferguson took the title of ‘first four-wheel drive production sports car’. It cleverly used different gearing for its front and rear axles to send 40 per cent of the power to the front wheels and 60 per cent to the back, a level of power distribution still common in supercars today. Here we take a look at today’s modern four-wheel drive systems in more detail. We’ll also delve into the world of electronic assistance systems, and even whether you need four-wheel drive at all.
This is commonly found in rugged off-roaders like the Land Rover Defender where serious off-roading is the primary goal. It’s a robust, mechanical way of delivering power to all four wheels, but requires heavy components, adding significant weight and affecting fuel economy through both this added mass and extra friction found in the driveline. All vehicles have differentials to allow the inside and outside wheels to spin at different speeds in corners, but off-roaders normally feature locking differentials to prevent all the engine’s power being sent to a spinning wheel with no grip. Permanent four-wheel drive vehicles also have a central differential to accommodate the small differences in speed of the front and rear wheels during cornering. In off-road driving, the central differential can often be locked for greater traction. Few modern performance-biased cars employ permanent 4×4 because of its impact on fuel economy, with the notable exception being rally-bred Subaru models including the WRX which prioritises road holding and durability over low running costs.
A part-time four-wheel drive vehicle has a transfer case, which splits power between the front and rear axles and is found in vehicles like the Jeep Wrangler. It is a simple and cost-effective system, but is no longer popular with designers because it requires the driver to choose the right settings, often bringing the vehicle to a halt to do so. The driver is normally required to manually select between two-wheel drive (where drive to one axle is disengaged), four-wheel drive high ratio, or four-wheel drive low ratio, whereby power is distributed via a set of low range gears in the transfer case, providing high torque at low ground speeds. In older vehicles, and pick-up trucks, a second selector is found next to the gear lever, but newer models often replace this with a button on the dashboard. Part-time 4x4s cannot operate in four-wheel drive at high speeds on tarmac roads because they do not have a central differential. Driving in four-wheel drive on the road can result in a binding sensation during turns, increased component wear and tyre scrubbing. The mechanical differentials can also clunk and graunch during operation, which is undesirable in a premium model. Low gear ratios are desirable for activities like rock crawling, where high engine outputs are required, but at very low speeds. Low ratios are also useful for descending steep slopes, where speed can be more safely controlled with engine braking rather than wheel braking, which can cause the vehicle to skid and lose control.
The vast majority of vehicles marketed as ‘four-wheel drive’ spend most of their time driving two wheels, with the ability to send power to all four wheels reactively. This includes Audi’s quattro, Volkswagen’s 4Motion and Mercedes’ 4Matic systems, amongst others. This technology is perfect for the majority of four-wheel drive customers, who need the extra grip occasionally, but spend most of their time driving on the road and prioritise running costs. If we take the Volkswagen Tiguan as one representative example, it’s fitted with a transversely mounted diesel engine which sends power to the front wheels in normal driving, with a resulting improvement in economy and a significant reduction in CO2 emissions. If wheel speed sensors detect slippage from either wheel at the rear axle, a central Haldex clutch pack locks up, energising a prop shaft and sending power to the rear axle. While ‘reacting’ to slippage might seem less desirable in off-road applications than permanent 4×4, modern systems are able to distribute torque rearwards in milliseconds. Systems like this are also favoured in high-performance applications, where shuffling power between the axles improves traction during acceleration and can even alter a car’s mid-corner balance by sending more power to the outside rear. This is a technique referred to as thrust vectoring, employing physics akin to paddling a boat faster on one side to turn without slowing down. Part-time 4×4 is usually of a more compact and lightweight construction than full-time 4×4 components.
Hybrids with 4×4
Hybrid vehicles use one or more electric motors to supplement the power of an internal combustion engine, so why not use this extra power source to propel a freewheeling axle? That’s exactly what manufacturers including Lexus and Peugeot have done with the RX 450h and 3008 and 508 HYbrid4s. These aren’t models you’ll bump into on the Rubicon Trail anytime soon, but torque from an electric motor can be delivered instantly to the rear wheels at lower speeds, aiding traction to get you out of potentially sticky situations. Because electric motors are fairly compact and require little cooling, they can be located in the rear of larger vehicles with minimal modifications to the passenger compartment. The electric motor fitted in the Peugeot 508 RXH HYbrid4 may only have 36bhp, but it can also muster 148lb ft of torque almost instantaneously, providing useful additional grip when driving on snow or dirt roads. This is a great solution for business customers who need a car with low emissions for it to be affordable, but may also need mild off-roading abilities from time to time.
Do you need 4×4 at all?
If you don’t expect to go too far off-road, but want extra security in wintry or muddy conditions, you may not need four-wheel drive at all. A front or rear-wheel drive vehicle can benefit greatly in challenging situations from having winter, all season or mud tyres fitted. In fact, research has shown a front-wheel drive car fitted with winter tyres will often perform better in snow than a four-wheel drive fitted with summer tyres. A four-wheel drive equipped with summer tyres can actually be quite dangerous in snow, because it has greater traction to accelerate up to speed, but doesn’t have any extra braking or cornering grip than a conventional car.
The compound of winter tyres is rich in natural rubber, which stays supple at temperatures below seven degrees Centigrade, allowing them to grip the road better. The tread is also covered in grooves to give them more bite. Several front-wheel drive crossovers also forgo four-wheel drive and instead reply on electronic differentials to improve grip. The Peugeot 2008 with Grip Control and the Fiat Panda Trekking are fitted with electronic differentials which brake a spinning front wheel to send torque to the tyre with greater traction.
If you are really serious about maximising grip, the biggest improvement comes from fitting specialised off-road tyres. These are designed to work with specific surfaces, such as mud, snow or sand and while they are usually road legal, they can reduce fuel consumption, impact road handling and have a lower speed rating than conventional road tyres. Mud tyres have a deep and aggressive tread pattern designed to claw into soft surfaces, as well as durable sidewalls to absorb impacts. Sand tyres have fewer tread blocks but they are even larger and can resemble paddles to help drive the vehicle forwards. For extra pliancy and grip, off-road tyres are often used at far lower pressures than road tyres, and experts will adjust the tyre pressures depending on the terrain and weather conditions for maximum traction.
With so many vehicles on the market, deciding what kind of driver you are and what conditions you will face are essential steps to choosing your ideal four-wheel drive vehicle. Once you have an idea of these, consulting an off-road expert is a great idea. With years of experience and knowledge, they will be able to guide you towards the right four-wheel drive set-up for your needs. They may even have a range of vehicles to show you how each system works and what their capabilities and limits really are. For most people, an automatic four-wheel drive system will be far more capable than they’ll ever need. In fact, a tutor will tell you the limiting factor is normally the person behind the wheel, and a few lessons will quickly improve your off-roading awareness and ability immeasurably.
This takes two main forms, adjustable suspension which softens and stiffens the dampers, and a second system which uses airbags and a compressor to raise and lower ride height. It’s the second system which is most useful for off-roaders, because it can drastically change the characteristics of a vehicle. The Range Rover Sport is a prime example, because its air suspension allows it to go from a high-performance road car with a relatively low centre of gravity to a serious off-roader thanks to its half-a-metre of adjustment.
Around view cameras
‘Bird’s eye view’ cameras are increasingly popular, particularly for large SUV’s, thanks to their usefulness when attempting to manoeuvre such a big vehicle. Clever software takes real time footage from fish-eye cameras mounted in the door mirrors, front bumper and tailgate, and stitches them together over a top-down view of the car so you can see exactly how you are positioned. This tech can be as useful for safely crossing a log bridge or avoiding jagged rocks in tight manoeuvres, as it is parking in Knightsbridge.
Hill descent control
Land Rover developed Hill Descent Control (HDC) for use in the Freelander to compensate for the fact it was designed without a low ratio gearbox. By using its anti-lock braking system the HDC system, it brakes individual wheels to control a vehicle’s speed as it descends a steep slope. It’s technology which has been widely adopted by rivals and the latest set-ups even allow you to adjust the descent speed via the cruise control settings on the steering wheel. Hill Descent Control requires you to take your feet off the brake and accelerator and simply steer until you reach the bottom of the slope, with the car taking care of the rest.
An open differential allows power to be lost to a spinning wheel with no grip. Locking differentials ensure engine power is split across the axle, so even if one wheel has zero grip, the other wheel will continue to turn. Traditional locking differentials have to be manually activated by the driver for low speed off-roading and disabled for road use. Modern systems lock the differentials automatically when the anti-lock braking (ABS) system senses wheel spin, with the best even able to decipher what type of surface you are driving on. To save weight and complexity, a mechanical locking differential can be mimicked by the ABS system braking a spinning wheel to transfer torque across the axle. This setup is commonly referred to as an electronic differential and examples of its application include the Fiat Panda 4×4.
More commonly found in full-size SUVs, the low-range (or low-ratio) gearbox is an additional set of gears located in the transfer case which make full engine power and torque available at very low speeds. These gears are desirable for rock crawling, where the vehicle needs lots of power to climb up and over large obstacles at low speeds. Low-ratio gears can also aid towing heavy objects like another stuck vehicle and help descend slippery slopes using only engine braking.
In the latest off-road vehicles, old-fashioned levers and buttons have been replaced by a dial, allowing you to choose between settings for likely scenarios like sand, mud and ruts, grass, gravel and snow and rock crawling. Land Rover first introduced Terrain Response with the Discovery 3 in 2004, in a bid to make off-road driving simpler. Selecting the terrain type on the dial would quickly alter the ride height, adjust the differentials, gearbox settings and throttle response. Terrain Response 2 added an automatic setting which uses data from wheel sensors to detect terrain automatically.
Serious off-roading vehicles are designed to cross streams and rivers, but have a maximum wading depth, usually dictated by the air intake of the engine. A wading sensor tells the driver the depth of water the vehicle is in, but should be used with caution, as they cannot tell the depth of water ahead of the vehicle. Land Rover’s system uses sensors in the door mirrors to determine how deep the water is and display a visual representation on the dashboard, along with an alarm if the water approaches the vehicle’s maximum wading depth.
If you imagine an SUV driving along a flat road towards a ramp, the maximum approach angle is the greatest possible steepness of the ramp the SUV can drive onto without fouling its bodywork (usually the front bumper). In the case of the Land Rover Defender it’s around 47 degrees, while for a Skoda Yeti it is 19 degrees.
Now imagine an SUV is driving off a downwards ramp onto the flat, as if disembarking a ferry. Its maximum departure angle is the permissible steepness of the ramp you can drive off with no bodywork contact.
When traversing a hump or a table top-style obstacle there’s a chance the underneath of the car between the axles can bottom out. The breakover angle is measured between the tyre tread and the middle of the vehicle’s chassis, so a short wheelbase Land Rover 90 has a larger breakover angle than a long wheelbase Defender 130.