Do you know which wheels are powering your car?

Citroen Avant

What you need to know:

  • This sends power to the back wheels only. It used to be the universal system for all passenger cars, whatever their size, until Citroen changed the trend with their “Traction Avant” model in the 1930s. RWD remains dominant among bigger luxury cars and heavier commercial vehicles.

These are all “drive” configurations; determining which wheels the engine power is sent to. There are four main options; two of them are 2WD systems; two are 4WD systems. When it comes to which is “best”, the answer will always be another question; best at what? 

Front-wheel drive (FWD)

This system sends the engine power to the front wheels. It is now predominant in passenger cars, especially those with small and medium-sized engines.  They are cheaper to make, lighter, have more power and are fuel efficient. They also offer more precise steering on good roads.

Downsides

They come with more complex engineering therefore less tolerant of maladjustment or defects. There is also more workload on front wheels (to roll, to deliver power, to brake, go up and down and steer), are not suitable for very high power levels and their handling control is more demanding (less fun) at extreme speeds or in slippery road conditions.  The system is also not good at mud-plugging and steep climbing and is less balanced weight distribution.

Rear-wheel drive (RWD)

This sends power to the back wheels only. It used to be the universal system for all passenger cars, whatever their size, until Citroen changed the trend with their “Traction Avant” model in the 1930s. RWD remains dominant among bigger luxury cars and heavier commercial vehicles.

Plus points

There is more relaxed steering, easier handling and control, work and weight better shared between front and back, simpler engineering and less fussy about adjustment. It is, therefore, more durable and reliable and has better physics for mud-plugging and steep climbing.

Downsides

There is more weight and cost, loss of inside space (prop shaft tunnel), more power loss between engine and drive wheels (the “drive train”), so fractionally higher fuel consumption and lower performance.

Four-wheel-drive (4WD)

This is usually the same as RWD in general use, but the driver can “select” to send power to the front wheels as well. This was tried as soon as Citroen introduced a way to transmit power to wheels that also steered, and 4WD became mainstream through Jeep and Land-Rover for utility vehicles during and after World War II.

Plus points

There is more grip. Beyond that, nothing solely attributable to the 4WD transmission itself, more to do with other design features ( such assize, clearance, big wheels and big engines, among others), which meet the usage demands and preferences, all part of the 4WD market demand package.

Distinction must be drawn between ordinary cars with 4WD transmissions and 4WD estates which (whether large and luxurious or small and Spartan or the other way around) are built for utility in difficult on-road and off-road conditions. To further distinguish between estates and SUVs, those with monocoque (single shell) construction are built to be used like “cars”, and those with chassis are built for rougher conditions, harder work and often bigger loads.

Car-type 4WD

Pioneered by Subaru, most publicised by the Audi Quattro, and now an option offered by most brands (all the way from the Toyota Probox to Rolls Royce). The only plus point is extra grip, which is insignificant at ordinary speeds on good roads in fair weather, but offers much improved control at high speeds and equally superior traction on twisty roads and loose or slippery or sticky surfaces.

Downsides

Everything else is much the same as the equivalent 2WD car but the 4WD transmission and drive train adds to purchase price, weight and running cost. None of these have the clearance, wheel size, suspension travel or heavy-duty construction to qualify as purpose-built off-roaders and workhorses. Few have an extreme low-ratio option.

Big estate/utility-type 4WD

These include the Land-Rover, Toyota Land-Cruiser, Nissan Patrol, Isuzu Trooper, Jeep, Hummer and Mercedes Gelandewagen, among others. Though many of these now have luxury variants that no owner is likely to use for severe expeditions and fieldwork, their design and construction can do the rough and tough stuff, competently, reliably and durably. Almost all have selectable low ratio. At the large and luxurious end, as much or more than their extra grip, these options are popular for their status image, their size and power, their ground clearance and driver eye level, their handling management of rough roads, their can-do assurance on slippery roads, and the sense of extra safety their size and weight is thought to deliver. True for minor shunts. Sometimes true, sometimes not, for big prangs. Check the NCAP ratings in year of manufacture,

SUV-type 4WD: These fit somewhere between the car and estate categories, usually but not always nearer to the car end of the spectrum.

All-wheel drive (AWD)

This option sends power to all four wheels all of the time, without a manual option to disengage the front. It is sometimes called “Permanent” or “Full” 4WD, and in the most sophisticated designs it is used with computerised control management systems. It is most used at the car end of the spectrum, and is so superior in competitive sport performance that all elite rally cars are now AWD with all the bells and whistles that the latest technology can offer.

All 4WD/AWD options come with penalties of price, weight and running cost compared with their nearest 2WD equivalents. 

Much variety

In terms of driving experience, or the car’s capability, if you are driving moderately on a smooth and grippy road surface, and especially at low speeds in heavy traffic (which is how most of us motor most of the time) the various drive configurations make no difference. They only become noticeable (never mind highly significant or essential) for driving on twisty roads at very high speeds and/or in extreme weather conditions, or on slippery and sticky roads, or completely off road (boulders, ditches, heavy vegetation and precipitous climbs and descents).

Subaru Forester 4WD

Also, any of the drive systems can be fitted to any vehicle, big or small, lightweight or heavy duty, ground-hugging or on stilts, luxury limo or rough-and-raw utility, passenger or cargo. And for most buyers, one or more of those other characteristics will be as crucial to choice as the drive system.

So, be very clear in your mind that styles and dimensions and weights and engine sizes and shapes and ride-heights and heavy-duty specifications are pros and/or cons in their own right, separate from and irrespective of the drive configuration.

You might rarely or never expect to need 4WD, but if you need a very big station wagon, or want high ground clearance or an elevated view, or weight and torque power for towing, or extra-robust construction for rough roads, 4WD is likely to be there. For some motorists it is an essential. For others (the majority?) it is an incidental extra bonus, not a primary reason for choice.

One big difference

There is only one generic difference between 2WD and 4WD; grip or the potential traction they can deliver between the tyres and the road surface. But for people who do remote safaris or expeditions of fieldwork, it can be all-important to make the difficult easy and to make the unlikely possible.

Even if it is used for only one percent of their total mileage. And, just as importantly, the low range gearing 4WD often comes with can be vital.

Front or back?

There are a number or reasons FWD is the first-choice “norm” for most cars. The driving wheels “pull” the car along (like a horse and cart) instead of “pushing” it. The drive and steering are working together in the same direction and the rest follows, making steering more precise. The drive train is also shorter – fewer power-sapping components and joints, less metal, less cost, less weight, and no need for a gearbox/prop shaft tunnel making a big bump in the floor pan.

In counterpoint, using just two wheels to do just about everything does put more stress on all the systems involved, and is therefore less tolerant of one of the processes being out of kilter. The rear wheels, meanwhile, have almost nothing to do except keep the back bumper off the ground.

There are important differences between FWD and RWD in the optimum technique for cornering at high speed. In simplistic terms with FWD, you slow down to apex speed before the bend and power through it with “point and squirt”. Misjudgment tends to cause understeer. Bad mistake and you hit the bank on the outside of the bend.

With RWD you enter a bend faster and scrub speed off on the bend by letting the tail hang out as a braking effect, and go full power again when the road and the car body are lined up again after a little power off and “opposite lock” (steering into the direction of the tail skid). Misjudgment tends to cause oversteer. Bad mistake and you skid/spin.

The FWD method is more efficient, but misjudgments have more sudden consequences and are harder to correct. RWD is likely to skid sooner but more gently and is easier to correct, and the physical sensation is widely regarded as more “fun”, such as the sport of drifting.

FWD holds its line (the back follows the front) better on flat surfaces with some grip (pulling the car rather than pushing it is better in this context), but gives less grip when climbing steeply - the dynamic weight is thrown backwards, reducing weight, hence compromising grip at the all-important front.

For the same reason RWD gives better grip on steep climbs and when powering through a boggy patch. RWDs that also have rear engines (VW Beetle, Porsche) are particularly good in mud.

 Four-Wheel Drive: a griping tale

Thanks to greater traction, 4WD and AWD cars can drive you round the bend more quickly and with greater levels and options for control on slippery or sticky surfaces, all other things being equal. Which, as noted, they are usually not. A high centre of gravity and long suspension travel is not a recipe for sharp, fast bends.

The differences…and the differentials

Any “drive” system must have differentials to allow wheels on the left and right to rotate at different speeds when they are turning a corner (the wheels on the outside of the bend have to travel further and thus rotate more often). In the process, differentials automatically send variable proportions of power to each wheel on the same axle (from as balanced as 50 percent to left and 50 percent to right, to as extreme as 0 percent to one and 100 percent to the other).

 There can also be differentials at the gearbox, which make the same decisions between front and back. Differentials types include full-slip or limited-slip, and some have the option to engage a diff lock.

The mechanical full-slip versions send power to the wheel that is offering least resistance, so if one wheel is on slippery ice (or in a muddy ditch) and the other is on grippy sandpaper, diffs will send power to the one on ice. It will spin easily (but not move you anywhere) while no power will be sent to the one on a grippy surface.

With a full-slip diff you can rev as hard as you like and you will still be stuck. With a limited slip diff, some power will still go to the wheel on the sandpaper, and off you go. With a locking diff, equal power will be sent to both wheels, no matter what. Much improved traction but almost impossible to change direction. If all the wheels are rotating at exactly the same speed, the car will go in a straight line.

Computerised AWD traction management systems and torque converters can send differing proportions of power to front and back (and each wheel individually) and are able to change their mind several times per second as to the balance of power distribution for best grip and control. 

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