Four Wheel Drive Vehicles Verus Alternatives
As we detailed in the Drivetrain Section, effective driving on snow and ice requires traction. You get better traction with front wheel drive than with rear wheel drive. You get better traction when you’re working with four wheels than with two. The reasons why are explained below.
Front wheel drive (FWD) is when the engines power is routed to the front wheels. FWD designs are cheaper than alternatives. These designs also place the engine weight over the front wheels. This creates … test question from Drivetrain Section … better traction. This traction helps when climbing hills. It also provides some help on slicker conditions because the FWD vehicle are steering using the front wheels.
Rear wheel drive (RWD) is when the engines power is routed t the rear wheels. Test question from above … what types of vehicles might benefit from weight on the rear tires? If you guessed trucks when loaded because the heavy load create rear weight that creates better rear wheel traction, you’d be correct. RWD vehicles also split the driving and steering functions allowing designers to optimize suspension for better handling. This particular feature is why performance cars usually have RWD. In snow and ice, RWD is not so good. The front wheels provide the steering, but the weight in the back tends to lift the front wheels a bit. This means … question from above … less weight, lower coefficient of friction, or simply put, bad traction.\
AWD is when the engine provide
All Wheel Drive (AWD)
d power to each wheel. There are a variety of AWD designs, but the goal is generally similar: allocate power to wheels to create the traction needed for a variety of road conditions. An AWD system might deliver power to one set of wheel, versus the other. But the system can sense slippage in one axle and divert power to the other in hopes of finding better traction.
Different manufacturers offer different types of AWD systems. They are designed to accomplish different things. Most AWD systems kick on only when slippage starts. As quickly as it kicks in, its still kicking in after an event.
A GNASA requires a design that is optimized for snow and ice. There are some AWD designs that are optimized for snow and ice.
Subaru has a great AWD design. Its called symmetrical AWD and is different than part-time AWD designs. Suburi’s AWD system is always delivering torque to both the front and rear wheels. Sensors can determine where the most torque is needed and deliver it. The AWD system uses a center differential so that front and rear wheels can turn at different speeds. This helps keep the car stable over changing road conditions.
Four-Wheel Drive (4WD)
Four-wheel drive (4WD) systems are optimized for severe off-road driving conditions. GNASA’s don’t require off-road driving. But my greatest GNASA’s have always involved roads that stop being roads. With enough snow and ice on a road, you’re essentially off-road. You’re on a road, but your car vehicle won’t know it. I’ve found that 4WD systems are best in these conditions.
Components of All Wheel and Four Wheel Drive
As discussed in the Drivetrain Section, better traction for snow and ice is created when you have individual control over as many of the wheels as possible. 4WD systems have two differentials, front and rear, and a transfer case. The part-time systems have locking hubs (ability to turn the system on and off). AWD are usually always on. Both systems are likely to have integrated advanced electronics. These electronics allow the vehicle to better optimize the available traction.
The differentials transmit torque from the driveshaft or transmission to the drive wheels. They also enable the wheels to spin at different speeds. This is critical when making turns because outside wheels follow a different path than inside wheels as do front and rear wheels. Essentially each wheel is spinning at a different speed. The speed difference between inside and outside wheels is enabled by the differential. All-wheel drive systems can vary the speeds between front and rear wheels by the transfer case.
The transfer case splits power between the rear and front axles on a 4WD vehicle.
On part-time systems, the transfer case is what locks the front-axle driveshaft to the rear axle driveshaft forcing the wheels to spin at the same time. This means that the wheels needs to slip, dynamic wheel slip. On any slick surface like ice or snow, this works well. On concrete, not so much. Statis slip with locked front and rear axles on concrete create jerky turns and lots of wear and tear. In part-time 4WD systems, the transfer case may have an additional set of gears giving the vehicle a low range. The extra gear gives the vehicle a extra torque. This helps 4WD systems climb and descend very steep hills, travel on snow and ice covered roads.
All-wheel drive systems have a device that can differentiate speeds of rear and front wheels. This could be a center differential or other gearset. AWD vehicles use these systems to manage traction over a variety of road conditions. The ability to manage spin on all four wheels is often critical when driving for long periods over roads alternating between ice, snow and packed snow.
Choosing between a 4WD and an AWD can be a bit tricky. True off-roaders often prefer 4WD because of low gears and the torque these create. Snow drivers get this benefit when they climb a very steep snowy or icy driveway. On the other hand, AWD usually performs better over varying road conditions. If the road is alternating between ice, snow and packed snow, an AWD vehilce will adjust traction to the wheels better than a 4WD.
Wheels in a car are bolted to a hub. 4WD systems usually have front wheel locking hubs. When the 4WD is not engaged, the hubs are locked, disconnecting the front wheels from the front differential, half shafts and driveshaft. This allows the car to function in two-wheel drive by stopping the differential, half-shafts, and driveshaft from spinning. This saves wear and tear on these parts and improves fuel economy.
I had a consulting job in the1990’s working with a legendary digital communications engineer. He ran a 400 plus engineering company that developed all kinds of communications technologies for NASA and the military. The engineers were experts at wireless communications and telemetry. They were also expert at using, manipulating and programming sensors. The company developed software and hardware that integrated positioning information from satellite and sensors that could support sophisticated planning tasks. These tasks might be types of simulations, virtual reality applications, and communication and tracking applications. Imagine a battlefield or orbiting space debris. How do you track the objects, locate the objects relative to each other, predict the objects path? Their systems and expertise allowed them to do this, and turn this kind of scenario into a simulation.
My first task was writing a market forecast report. I identified two area for massive growth: telecommunications and telematics. Telematics integrates vehicle technologies, wireless technologies, telecommunications technologies, sensor technologies, instrumentation and computer science.
I bring this up because advanced electronics applications involving vehicles and sensors is decades old. Engineers have been placing sensors, or controls, on things that move or change since the 1920’s. The modern concept of “self-driving” vehicles might seem new, but the basic engineering that makes it possible is really old. Many of the component parts that make it possible are also quite old. This all brings us back to ensuring you have a great snow and ice vehicle.
Advanced electronics make this possible. These include sensors and controllers. A vehicle system to manage braking might involve speed sensors to measure wheel speed, and a controller (computer) to interpret sensor data and tell mechanical devices what and when to do something (i.e., valve to reestablish pressure). In other words, sensors track and report data, a controller interprets that data and sends instructions to other parts of the car based on that data.