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Driving Wheels - Front, Rear or All-Wheel Drive?

words - Joe Kenwright
Those drivers who draw on a fraction of their car's capabilities would never know if their car was front, rear or all wheel drive.
Each has clear advantages and disadvantages, some of which can be a matter of life and death depending on the situation. Joe Kenwright looks at all the issues surrounding the different combinations of wheels which do the driving and steering.

FRONT WHEEL DRIVE

What is front-wheel drive (FWD)?
Front-wheel drive cars are so-called because the engine's power and torque are supplied to the front wheels via a short driveshaft. The front wheels have the double task of putting power to the ground as well as controlling the direction of the vehicle (steering). The rear wheels are basically there to keep the car's bum from dragging down the road. Okay, oversimplification, but you get the picture.

Some examples of front drive cars
Small cars: Ford Laser, post-1985 Toyota Corolla, Holden Astra, Honda Civic, post-1981 Mazda 323, VW Golf, Peugeot 306, Hyundai Excel
Medium cars: Holden Vectra, Honda Accord, pre-1994 Subaru Liberty, Peugeot 405, Toyota Camry, Audi A4, Saab 9-3
Family cars: Mitsubishi Magna, Audi A6, VW Passat, Nissan Maxima, Volvo 850, Saab 9-5
Sporty cars: Honda CR-X, post-1985 Toyota Celica, Nissan NX, Mini, Renault Clio Sport

Did you know: The original VW Beetle was rear drive but the 2001 New Beetle has front drive Golf mechanicals hidden where the boot used to be. Because many Beetle fans dismissed it as an imposter, sales never reached projections.

From the driver's seat
If a driver applies too much power while a front drive car is cornering, then it will tend to run wide or steer less than what the driver expected hence it understeers. If the driver releases the accelerator at this point, the back will come around and steer more than the driver expected hence the term oversteer. This is generally accepted as being more forgiving for the average driver.

Typical crash situations
Overshooting a corner then ploughing head-on into a tree or pole or oncoming traffic; overshooting a right turn and hitting the kerb or traffic lights; overshooting the centre line into oncoming traffic while turning left in a T-intersection; overshooting the closest lane while entering a highway; leaving the road sideways after backing off too severely mid-corner.

Survival tactics
Better to avoid trouble in the first place. Slow down before the corner, match accelerator and steering to the corner, increase acceleration only after the car has settled in its new direction and you can see what is ahead. If you must back off to sharpen up the cornering line, do so smoothly.

Upsides and downsides
Upside: Because all the mechanical components are located ahead of the windscreen, the car can be made lighter, cheaper and more economical.

Downside: Small front drive cars only need a lightweight rear section to support the wheels and suspension. In the absence of minimum rear crash standards, many provide token rear crash protection.

Upside: Because the complete engine, transmission and front suspension can be installed in one process, front drive cars are cheaper to build than rear drive cars.

Downside: Tightly packed engine bay and extra complexity can generate repair costs that exceed the value of the car at an earlier age and contain greater risks for the used buyer.

Upside: In wet, slushy conditions, the extra weight over the driven front wheels can generate extra traction and surer steering.

Downside: Steep hills, heavy loads and towing will reduce traction in a front drive car.

Upside: Front drive leaves more space for passengers and luggage with a deeper load area.
Downside: Mechanical parts packed tightly in the front often generate larger turning circles, longer front overhangs and higher repair costs in a low speed crash. Lighter rear suspensions can sag under heavy use.

Upside: Fewer mechanical parts spread throughout the car make it easier to isolate noise and vibration for higher refinement levels at a lower price.

< i>Downside: Concentration of engine, transmission, air-conditioning, steering, differential and drive shafts over front wheels plus dual steering and traction functions chop out front tyres two to three times faster than rear tyres. Front brakes also work harder and rear brakes can rot through lack of work.

Upside: Standard power steering and clever engineering has eliminated torque-steer (the tendency for the engine's torque to steer the car independent of the steering) once common in front drive cars.

Downside: The steering usually ends up feeling like it is winding up a rubber band while robbing the driver of valuable feel and feedback - the most common complaint against modern front drive models.

Upside: Latest fuel injection, multi-valve and turbocharger technology has boosted power outputs of compact front drive engines to levels of much bigger engines found in rear drive cars.

Downside: There is a limit to how much power can be fed through the front wheels before steering is lost. As front drive manufacturers reach that limit, they are forced to add all-wheel drive which negates the original cost, space and weight advantages. Because compact front-drive engines have to be revved harder to deliver equivalent power, automatic transmission and air-conditioning can affect them more at city speeds.

Trouble spots
  • Driveshafts: Four constant velocity (CV) joints are required to allow the front wheels to provide traction while steering. Their longevity depends on protective rubber boots which can split and generate catastrophic failure.
  • Engine Mounts: Front-drive engine mounts have to stop the engine from rotating around the front wheels. If they break, the flailing engine can snap expensive exhaust systems and damage other parts.
  • Underbody Damage: Extra front overhang and lower ground clearance can generate crushed or cracked front structural members, cracked transmission cases and missing or damaged splash trays. Cracking the transmission case can cause some brand new European hatches to be written-off.
  • Steering: Drivers who insist on steering a front drive car while the car is stationary will generate premature wear in all steering parts when most of the vehicle's weight is on the front wheels.
  • Brakes and Tyres: Heavy front brake wear generates regular rotor replacement as well as brake pads. Rapid front tyre wear dictates front to rear rotation at every major service.
  • Transmission: On some front drive cars, the engine has to be removed for a simple clutch or transmission repair.
  • Heat Damage: Tightly packed mechanicals are vulnerable to heat soak under Australian temperatures causing gaskets, hoses and seals to harden and leak, expensive electronics failures and cracked plastic parts.

REAR WHEEL DRIVE

What is rear-wheel drive (RWD)?
In this instance the engine's power and torque is sent to the rear wheels, leaving the front wheels just to steer the car. This approach is more commonly found in sports cars, but is also used quite heavily in Australian family cars.

Some examples of rear-drive cars
Small cars: BMW 3-series hatchback is the smallest currently available
Medium cars: BMW 3-series, Mercedes-Benz C-class, Lexus IS200
Family cars: Holden Commodore, Ford Falcon, BMW 5-series, Volvo 9-series, Mercedes-Benz E-class
Sporty cars: Mazda MX-5, Toyota MR2, Nissan 200SX, Honda S2000

Did you know: BMW resurrected the Mini name for its first front-drive small car to protect its rear-drive heritage. It will soon be joined by the new BMW 1 Series; the first small rear-drive five-door hatch since the 1977-80 Mazda 323, but not even BMW could create room for a spare tyre.

From the driver's seat
If a driver applies too much power while a rear drive car is cornering, the back will force the car into a tighter cornering line or break loose hence the term oversteer. If the driver releases the accelerator before the car completes the turn, the car will try to run wide or understeer. This is the opposite to what a front drive car will do and is generally accepted as being less forgiving for the average driver.

Typical crash situations
After the tail breaks loose in a roundabout or corner, the car hits an edge and rolls or hits a solid object from the side; driver overcorrects a sideways slide and slams into oncoming traffic; driver applies too much power or braking when the car is slightly sideways and spins into surrounding objects; the driver enters a corner too fast and slides straight ahead into a solid object under brakes.

Survival tactics
Better to avoid trouble in the first place. Enter a corner slower than you expect and create enough margin to maintain some power through the back wheels to hold a neutral cornering line. Apply extra power only as the corner starts to straighten and you can see what is ahead.

Upsides and Downsides
Upside: Mechanical components are usually laid out sequentially front to rear for better weight distribution and easier repair.

Downside: Main propeller shaft requires an intrusive centre tunnel through the middle of the cabin while the rear luggage floor has to be raised to fit over rear drive.

Upside: Longitudinal engine location allows installation of bigger unstressed engines with easier access, reduced overhang and extra space for front wheels to turn for tighter turning circle.

Downside: Because less space is dedicated to passengers and luggage, a rear drive car must be bigger and heavier to match a front drive car.

Upside: Steering and traction functions are split between front and rear wheels for more even tyre and brake wear and front suspension optimised for steering.

Downside: Rear axle designs that provide good grip and consistent rear wheel location while traction is fed to the rear wheels are complex and costly. All base Falcons prior to 2002 BA series; all base Commodores prior to 2001 VXII series and all early Japanese and European rear-drive models were compromised in this area until new multi-link or double wishbone rear suspension was fitted.

Upside: Mechanical components spread from front to rear dictate a stronger structure past the windscreen and more even weight distribution with or without a load.

Downside: Heavier body structure requires more fuel and power for equivalent performance. Fast-spinning drive shafts under the cabin and behind the back seat can add to the noise and vibration occurring under the bonnet and is expensive to isolate.

Upside: Because steering and traction are separated, more power can be fed through the rear wheels without reducing the front wheels' ability to steer.

Downside: If the rear wheels lose traction or the power changes the rear suspension geometry, the car can steer from the rear outside the driver's control. In slushy conditions, the front wheels can dig in and cause the rear wheels to skid.

< i>Upside: Rear drive allows the driver to adjust the way a car corners with a combination of accelerator and steering which can be more enjoyable and ultimately safer on sealed road surfaces.

Downside: This requires higher skill levels and leaves less margin for driver error or inexperience.

Upside: The more a rear drive car is loaded or steeper the hill, the better its traction.

Downside: A lightly loaded rear drive car will lose traction in boggy level ground far earlier than a lightly loaded front drive model.

Upside: Spread of rear drive mechanical parts front to rear generates higher ground clearance.

Downside: Less room to hang fuel tanks and mufflers can leave these parts lower.

Trouble spots
  • Rear Tyres: Quality tyres are critical in maintaining predictable rear grip on choppy or wet surfaces. Different brands and types on each wheel can cause the car to swap ends or slew sideways.
  • Rear axle bushes and shock absorbers: Worn shock absorbers allow the tyres to leave the road and flick the car sideways. Broken or unsuitable bushes will cause the car to steer from the rear.
  • Leaking rear brakes: Uneven braking at the rear can cause the car to slide sideways or spin.
  • Worn Driveline: Long propeller shaft under cabin will generate clunks and vibrations if out of balance or universal joints worn. Burnouts and brutal acceleration will generate clunks and bangs in differentials and transmissions that are amplified by the cabin.
  • Body Frame Damage: Tendency for rear wheel drive cars to leave the road sideways and hit something can generate serious but unseen front and rear axle alignment problems or cracked and broken structural parts.
  • Towing Damage: Heavy towing without the correct towbar can stretch the body to the point where the doors and boot may no longer open or shut correctly.

ALL WHEEL DRIVE

What is all-wheel drive (AWD)?
Increasingly prominent in the 21st century, all-wheel drive refers to any system where all four wheels play a role in getting power to the ground. But not all all-wheel drive systems are created equally; the differences are often in how they divide the engine's output between the axles, and what type of system does the dividing.

For example, Honda's CR-V compact SUV is a front-wheel drive vehicle until wheelspin is sensed, and then the rear wheels are brought into play. Although this qualifies the CR-V and others like it, as a "permanent all-wheel drive" because it is permanently engaged, the amount of drive that goes to the rear wheels is not permanent and varies considerably.

What is four-wheel drive? Exactly the same as all-wheel drive, but more often used in conjunction with 'offroad' capable vehicles like the Nissan X-Trail, Toyota LandCruiser and Mitsubishi Pajero. There are only three out of dozens of different all-wheel drive systems where the amount of drive fed to each axle is constant:
  • Part-time all-wheel-drive system -- where you manually select all-wheel drive. This locks front and rear wheels into receiving equal traction but it is called "part-time" because you can only use it in slippery conditions. This is the original all-wheel drive system. For offroad, it is still a good system when you know exactly what is happening.
  • Permanent all-wheel drive (1) -- has a centre diff lock that locks the front and rear wheels into behaving like a "part-time all-wheel drive" in slippery conditions. The Mitsubishi Pajero was a pioneer in offering a choice of rear-wheel drive only, permanent all-wheel drive and part-time all-wheel drive in the one system.
  • Permanent all wheel drive (2) -- that is engineered to feed traction to front and rear wheels in constant proportions. The best all-wheel drive sports cars have this system so the driver knows exactly how the car will behave at high speed.
Things to know
  • Very few all-wheel drive passenger cars on the Australian market provide enough traction to all wheels on a dry, sealed surface to make a difference at highway speeds. Most were developed as a traction aid for Northern Hemisphere winters. You cannot rely on a car's all-wheel-drive system to get you out of trouble in a highway cornering situation. In some cases, it may make it worse.
  • Most all-wheel-drive systems based on a front-drive car direct traction to the rear wheels only after the front wheels have started to skid. This can take so long in some systems that it is useful only for driving out of the ditch you skidded into.
  • Even models from the same manufacturer have totally different all-wheel-drive systems depending on transmission and application and behave very differently. If you must place your life in the hands of an all-wheel-drive system, find out what it can and cannot do beforehand.
  • Because the torque-split front to rear can vary according to the surface and which wheels are spinning, an all-wheel-drive car may not behave the same in every situation.
  • All-wheel-drive systems must add weight and reduce performance when the number of driveline components double. All-wheel-drive models usually suffer the combined downsides of both front and rear-drive cars while offering advantages only under provocation or on slippery or loose surfaces. The notable exceptions are certain sports models engineered with a rear-drive bias while feeding some drive through the front wheels.
  • Many crossover vehicles such as the Honda CR-V are normally two-wheel drive in their home market but feature all-wheel drive and raised suspension to exploit Australia's generous duty concessions originally intended for primary industry. The Subaru Forester currently attracts 10 per cent less duty than the mechanically identical Impreza. Australians were quick to recognise the extra value in these SUVs over similar two-wheel-drive models but their extra height can make some less stable.

THE BOTTOM LINE

No matter which wheels are driving your car, some factors are constant.
Uneven tyre wear, wrong tyre pressures, worn steering or suspension, faulty alignment and unsuitable modifications can drop you into big trouble.
Treat every car as different until you get the feel of how it behaves. If there are grey areas in your driving where you are not sure of what to do or you have just generated a close shave, consider an expert driving course.
Driving is one area where you never stop learning.


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Powered By Motoring.com.au Published : Thursday, 15 April 2010


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