Mr. Green Car: What’s driving you? — part three, four-wheel-drive
Editor’s note: The following is part three in a series. Part one, “What’s driving you?”, appeared in the Feb. 6-12 issue. Part two, “Front-wheel drive,” appeared in the Feb. 20-26 issue.
By Allen Penticoff
In the previous two Mr. Green Car columns, I discussed the nature of rear-wheel-drive and front-wheel-drive vehicles. This is the third of a four-part series on common drive trains. This time, four-wheel-drive (4×4, as it is commonly abbreviated) will be the topic.
Other than rear-wheel-drive (RWD), four-wheel-drive is the oldest type of drivetrain. In the early days of motorized transportation, horse-drawn vehicles were still common and paved roads were a rarity. Much of the year, these dirt roads were nearly impassable, particularly with the early RWD cars.
Bramah Joseph Diplock, a British engineer, patented a four-wheel-drive system in 1893. Ferdinand Porsche built a four-wheel-driven electric car in 1899. Europeans were experimenting with 4×4 in the early 1900s, while in the United States The Four Wheel Drive Auto Company (FWD) in Clintonville, Wis., would begin production of 4×4 cars in 1908 that led to 15,000 of its Model B trucks that were sold to British and American forces for use in World War I.
Trucks needed 4×4 to make it anywhere, and the military needed them for combat in the First World War. (FWD is an abbreviation for the company that is still in use and should not be confused with front-wheel-drive).
Four-wheel-drive is distinguished from all-wheel-drive (AWD) in that in common usage, the extra two driven wheels, usually the front, are chosen by the driver when they need to be engaged — whereas with AWD, all four wheels are driven all the time. Sometimes this type of four-wheel-drive is referred to as “part-time” four-wheel-drive.
Most 4×4s start out life as a rear-wheel-drive vehicle (Subarus being a notable exception among several), with a front differential and transfer case added to provide for driving all four wheels. Instead of the driveshaft leading to the rear differential, there is a “transfer case” attached to the output shaft of the transmission. From there, are two driveshafts — one goes to the front differential and one goes to the rear differential. The transfer case often has the option of a neutral position, where no power is transferred to the wheels, a two-wheel drive position (2H), a low-range 4×4 gear set (4L) for when tough heavy-duty use is needed (such as snow plowing) and a high 4×4 range (4H) for normal driving. The transfer case may have a floor-mounted manual shift lever, or a dash-mounted switch. The transfer case may allow for the difference between what is happening with the front axle and rear axle, or it may be fixed. Some newer transfer cases are electronically controlled and sense slippage in the drive wheels and automatically engage the other axle — this feature is called “automatic four-wheel-drive.”
Another common feature of 4×4 vehicles is that there may be “hubs” on the front wheels that allow the wheel to be disconnected from the axle when four-wheel-drive is not needed. This reduces wear and tear on the front drive system, and reduces friction caused by the front wheels turning the steering u-joints, or CV joints, differential and driveshaft.
Older 4×4s tend to have manual hubs, where the driver must stop the vehicle and turn a selector on each hub to engage them. Newer 4×4s have auto-locking hubs that are engaged when the front drive system is producing power to the wheels. My old AMC Eagle Wagon was a mix of 4×4 and AWD. It had a curious need to roll forward a bit to engage the hubs once four-wheel-drive was selected. Thus, if you were already stuck in snow and not in 4×4 mode, you could not get the front wheels to help you out. You needed to decide on engaging 4×4 before conditions needed it. Other Eagles were full-time 4×4, and thus among the first AWD cars available. It was awesome in deep snow.
Despite its name, four-wheel-drive is not necessarily driving all four wheels when you need them. Because it is common to use two standard differentials without limited-slip clutches, it is possible to get one front wheel spinning on ice and one rear wheel spinning on ice, and you are not going anywhere. Watch someone plowing snow — you can sometimes see this happening. To be true four-wheel-drive, all the wheels need similar traction. Conversely, in a 4×4 system without a slipping transfer case, if there is not some slippage of the tires, as on dry pavement, the system will bind and jump, as it cannot accommodate the difference in what the front tires are doing and the rear tires are doing. With these systems, it is not recommended to use 4×4 on dry pavement or at high speeds. Another optional feature, seen on off-road vehicles, is to have “locker” differentials. With the front and rear differential locked — there is no accommodation for the wheel speed variations; all slippage must be between the tire and the surface. However, you do get all four wheels providing power to the ground, no matter what. This feature is popular for driving in soft sand.
Four-wheel-drive can add a lot of weight and complexity to a vehicle. The extra parts require maintenance, adding expense. They typically get much lower gas mileage than their RWD counterparts. 4×4s are still preferred for heavy-duty and off-road use. Thus, Jeeps still have 4×4 rather than AWD, and trucks of all sorts use 4×4 rather than AWD. AWD systems tend to be less robust and, therefore, found on passenger cars — although in most situations, AWD can provide better traction than 4×4s.
I’ve lived with both AWD and 4×4 vehicles and can attest to their maintenance expense and fuel consumption. My 1979 Chevy Suburban with 4×4 and a 350 engine got a pretty lousy 10 mpg compared to my 1990 Suburban with RWD that gets 15 mpg. It’s not a straight comparison, but it is close. Thus, if you choose to have four-wheel-drive, you should have a real and regular need for it, otherwise you are wasting a lot of fuel, polluting more and incurring unnecessary expense for infrequent need of that capability.
From the March 6-12, 2013, issue
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