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Simple Guide to Understanding Your Car's "Drive" System

BMW's all wheel drive system is called X-Drive and it works very well.
BMW's all wheel drive system is called X-Drive and it works very well.
(c) D.Miliano 2014 all rights reserved

Knowing how your car gets the power to the ground may seem like a "no brainer" to some of you reading this, but understanding exactly what you have and how it works could help improve your safety - especially in inclement weather conditions. Here is a simple guide to what "drive" systems exist and how each works along with their plusses and minuses.

BW's answer to All Wheel Drive
(c) D.Miliano 2014 all rights reserved

Rear Wheel Drive - Time was, this was the most common type of system in our cars. The engine was up front. The transmission was behind it, a long drive shaft road in the hump that ran down the middle of our car and the rear wheels, through a thing called a differential, provided the propulsion. Note, it's not called two wheel drive because in most cases it isn't. It's actually one wheel drive - something you discovered if you were trying to get out of a muddy ditch or a snow drift. The differential, unfortunately, in most cases, sends power to the wheel with the least grip. An option on some cars - especially performance cars or muscle cars of the 1960'ss and '70's - was something called limited slip differential or posi-traction. This bit of mechanical magic would sense slippage and lock the two wheels together, further improving chance that your wheels would get traction in slippery conditions. (Watch the movie My Cousin Vinny for more details on posi-traction!)

BMW's current line of cars with rear wheel drive cars have a form of posi-traction but it's backed by electronic wizardry instead of pure mechanical means. This configuration - engine up front, drive wheels out back - gives the driver a sporty feel and simple mechanicals. For Reference: All of NASCAR racers are Rear Wheel Drive. In today's market place, many BMWs and Mercedes Benz cars are rear wheel drive as are Camaros, Mustangs and Dodge Daytonas. It's a good system for the majority of drivers in the majority of situations.

Front Wheel Drive - Cars with front wheel drive have the engine in the front but instead of a transmission and drive shaft going out back they have a trans-axle along side the engine. In this system, the front wheels both provide drive and steering. The weight over the front wheels, some feel, gives better traction, but as anyone who has driven a front wheel drive car in the rain or snow knows, when you accelerate quickly, the weight shifts backward and the front tires - tasked with both steering AND providing forward traction - spin like crazy.

Front Wheel Drive was invented and became popular because it offered better "volume packaging" and easier assembly. With all of the mechanical components in the "box" in the front of the car, that left a box in the middle for the passengers and a box out back for the trunk. Simple, efficient and less expensive to design and build. Most modern cars with Front Wheel Drive have electronic technology to limit start up wheel spin as described above and some even have limited slip differential technology too. However, for people interested in pure performance, asking the front wheels to do all of the steering AND the forward "drive" and most of the braking isn't the recipe for success. This explains why very few race cars are front wheel drive. While it works very well on the street, it's not the best if you go serious racing. Most modern economy cars such as Toyota's Camry, Honda's Civic and Accord, Nissan's Maxima and the cars from Kia and Mazda are front wheel drive. It works well and, because of the heavy forward weight bias, creates something called "understeer" in a corner. That is a condition where your car "plows" if you enter a turn too fast. Most experts claim that this is a safer situation and easier for the driver to correct than it's opposite condition, "over steer" - that's where the car rotates or spins, leaving the road back end first.

Four Wheel Drive - Years ago, this technology was used mostly for pick-up trucks. This system is fairly crude (when compared to modern alternatives). The vehicle lives most of its miles in rear wheel drive mode until the driver takes it off road or is confronted with a low grip situation like mud or snow. Then, the driver has to operate a lever that links the transmission and the rear wheels to a device called a transfer case that sends "drive" power to the front differential and the front wheels. This process, back in the day, was done with levers and in many cases had to be done at a full stop. Today, these systems still exist but are more sophisticated with electronic actuation and a lack of the need to stop before shifting into four wheel drive mode. The system, like rear wheel drive, has limitations since, unless posi-traction is part of the mechanicals, the vehicle still only has two wheel drive. However, once four wheel drive is engaged, since the drive wheels are in the front and the back, it works well in most situations.

For serious off-road use, some systems have a locking differential feature and a low range gearing that makes all four wheels turn together, regardless of the road conditions. This gives the vehicle the maximum traction and power possible but limits maneuverability and the speed the vehicle can travel (usually less than 10 MPH). Intended for serious off road use, it has limited value on most street applications. Users of these systems have to be aware of how these systems work, when they are best to use and when they should be turned off. Definitely NOT a system for the non-techncal car owner.

All Wheel Drive - The system is the current state of the art for modern vehicles and just about every manufacturer offers this in their product line up. The most popular proponents of this technology are Audi with its Quattro models and Subaru with all wheel drive in every vehicle they sell. Their systems, as well as those of other manufacturers, send power to all four wheels all the time. In all vehicles, the manufacturer has a prescribed percentage of where the power goes most of the time. Some send more of the power to the rear wheels and only sends more to the front when slip is sensed by the electronics (this is what BMW's system does). This gives the driver a more sporty feel. Other manufacturers so the reverse - it all depends on the intended use to the vehicle and the type of buyer they are hoping to attract.

in either configuration, the electronic and mechanical components sense slippage and send power (or drive) to the wheel(s) with the most traction. You have, perhaps, heard the slogan, "from the wheels that slip to the wheels that grip…"? That is, in essence, how they all work and their reason for existence.

Not surprisingly, it's not all about economy or safety. Famous race drivers like World Rally Champion, Walter Rohrl, are strong proponents of all wheel drive systems for daily driving, rallying and racing. In fact, Audi was so successful racing their Quattro cars that they were eventually banned from competition. However, these days, all world rally cars have very sophisticated all wheel drive systems in their cars. For that reason, anyone who lives in "snow" country would be wise to consider making a vehicle with all wheel drive their next purchase.

Some thoughts on tires (all season, snow, studded snow and summer tires) in a future article.

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