What Drives You…
Or, more appropriately, what drive system does your car have? Here in the northeast, with winter comes the great debate among car guys (and gals) over which drive system – All Wheel, Four Wheel, Front or Rear Wheel – is the best in the snow and ice. But it’s really a debate without a clear winner, as each of the four drive systems found on modern vehicles has its advantages and disadvantages.
For the professional Security Driver, the question as to which system is best comes down to the same three factors that determine success in a behind-the-wheel emergency – the driver, the vehicle, and the environment. For those protection practitioners who have some input as to which vehicles will be used to transport the Principal from Point A to Point B safely and securely, a little knowledge can go a long way towards making an informed decision. For those who are providing secure ground transportation in a vehicle selected by someone else –it is essential for the driver to understand what type of drive system that vehicle (or those vehicles, as the case may be) has and how the vehicle in question can be expected to perform in a wide range of conditions and environments if he or she expects to use the most of that vehicle’s potential. This is particularly important if and when the driver is confronted by a behind-the-wheel emergency, be it a potential crash or a deliberate attempt to stop the vehicle.
Rear Wheel Drive
As the name implies, in vehicles with rear-wheel drive systems just the rear wheels receive power from the engine. From the earliest years of the automobile through the early to mid-80’s this was the most common drive configuration. By then, front-wheel drive systems were becoming more popular with manufacturers for a number of reasons.
The advantage to rear-drive is that it segregates the steering from the drive function, with the front wheels doing the former and the rear wheels the latter. A limited slip differential often referred to as positraction (which was actually GM’s brand name for their limited slip differential), which sends available power (torque) to the wheel with the most traction, is common on rear-wheel drive vehicles.
The disadvantage inherent to rear-wheel drive cars is especially pronounced when climbing a hill, as weight transfers from the front of the car to the rear, making it harder for the rear wheels to push the vehicle up the hill. This often leads to tire slippage or wheel spin as the transferred weight exceeds the tires capacity to grip the road surface.
Front Wheel Drive
Again, as the name implies on vehicles with front-wheel drive systems just the front wheels receive power (torque) from the engine. This is accomplished through a transaxle assembly that takes the place of the separate transmission and differential (axle) assembly on a rear wheel drive vehicle. One of the most significant benefits of front-wheel-drive cars is the additional cabin space they offer. Without a center transmission and driveshaft tunnel, you can comfortably seat people in a front-wheel drive car that has smaller overall dimensions than a rear wheel drive car with the same seating capacity. Limited slip differentials in front wheel drive cars are rare, so generally, the transaxle sends torque to the wheels with the least traction. Today’s front wheel drive cars are typically equipped with electronically controlled traction control systems which overcome this disadvantage by allowing power to be transmitted to the opposite wheel.
The disadvantage of front-wheel drive cars is that the traction and steering duties are both falling to two wheels. When front wheel drive cars lose traction due to surface conditions such as wet (or snow covered) pavement they not only lose the ability to transmit power to the ground via the tires, they also lose the ability to steer.
Four Wheel Drive
In general, 4WD was available on trucks and early SUVs and was the standard by which winter driving as well as off-road capabilities were measured for many years. Like rear wheel drive vehicles, most of the time four-wheel drive vehicles transmit power to the rear wheels. Unless equipped with a limited slip differential, power would be transmitted to the wheel with the least amount of grip. If that were the case, it is possible for the vehicle to lose grip at the rear drive wheel on nearly flat surfaces when the 4WD system is not engaged.
The four-wheel drive system includes a transfer case which is connected directly to the transmission that sends power to the front differential when engaged. Without some kind of locking differential, four-wheel drive vehicles would only send power to one rear wheel, and the opposite front wheel if the surface wouldn’t allow for much traction.
Modern four-wheel drive systems utilize electronic transfer cases, automatic locking hubs, and automatic locking differential, making them much more user-friendly than older versions. While 4WD systems remain the standard for off-roading, they still have some of the same idiosyncrasies that they exhibited in the past – the systems are not for use on dry pavement, and must be shifted into 4WD when traction is limited by the driver.
All-wheel drive (AWD) vehicle utilize a center differential instead of a transfer case to transmit power to each of the vehicles four wheels. There are a number of different types of center differentials, including torque sensing, non-locking, multiple clutches, and multi-clutch coupling systems.
The advantage of all-wheel drive systems have over other types of drive systems is – as the name implies – that all four wheels have the potential to be powered at any given time. But that doesn’t necessarily mean that they are, just that they can be.
In fact, under normal driving conditions, most all-wheel drive systems operate with a bias toward either the front or rear wheels, sometimes as much as 100 percent. In dry conditions, under acceleration, these systems tend to transmit the most torque to the rear wheels. Conversely, at steady speeds, these systems tend to transmit more torque to the front wheels. When traction is limited due to surfaces conditions such as water, snow, or ice snow on the roadway all-wheel drive systems interface the traction control, antilock brake, and certain engine controls sensors to determine which wheels have the most traction and transmit the most torque to those wheels.