The IT Nerd

As more cars and SUVs are appearing with one or the other, it’s important to understand the differences between All-Wheel-Drive & 4-Wheel-Drive so you can make an informed purchasing decision. I’ll sum up the differences like this:

4-Wheel-Drive: This is a system where the behavior of the drive train is selected manually by the driver. Under normal driving conditions, a vehicle with this sort of drive train is usually driven in only two wheels being powered (usually the rear wheels). But if the driver chooses to do so, they can activate 4-Wheel-Drive mode which activates a device called a transfer case. The transfer case allows power to be transfered from the engine to the transmission, through its own driveshaft and to the front and rear wheels. These systems also have the ability to select high or low range gearing as well to allow you to scamper over different types of terrain. You’ll often find 4-Wheel-Drive systems in pickup trucks and some SUVs.

All-Wheel-Drive: In it’s most basic form, All-Wheel-Drive operates the same as 4-Wheel-Drive, but a computer backed up with a variety of sensors makes the decision on what wheels to power instead of the driver having to do that. Thus the system is completely automatic. For example, My Toyota Matrix has an All-Wheel-Drive system that will power all four wheels from a standing start. That way it limits torque steer (the tendency for a front wheel drive car to pull to one side from a standing start) and assures that all the power gets to the pavement. Once up to speed, it will deactivate the transfer case and the car is now powered by the front wheels alone. If the car’s sensors and computers detect wheel slippage or any sort of situation where the car can use power from the rear wheels, then it will activate the transfer case to send up to 55% of the engine power to the rear wheels.

When it comes to the latter, car companies have their own tweaks that improve on this:

• Mercedes Benz has an All-Wheel-Drive system called 4Matic that will send about 70% of the power to the rear wheels and 30% to the front wheels. That way you get a rear-wheel-drive feel but still get the benefits of All-Wheel-Drive as all four wheels are powered at all times. Plus this system can transfer any amount of power to the front or rear wheels as required.
• Subaru has an All-Wheel-Drive system called Symmetrical-All-Wheel-Drive that powers all four wheels in a 50-50 ratio and can transfer power to the front or rear wheels as required. Their rationale is that because all four wheels are being powered at all times, you get more predictable handling as a result.
• The Volvo S60 T6 R-Design that I drove recently is similar to my Matrix in that it will power all four wheels off the line and then be primarily a front wheel drive car after that. Plus it can shift power to the rear wheels as required. But it’s system which is called Instant Traction All-Wheel-Drive has an additional trick called Torque Vectoring up it’s sleeve. The All-Wheel-Drive system can route power to the left or right side of the car to reduce under steer. For example, if you’re taking a sharp left turn, the system detects this and shunts power to the right side to help “push” you through the turn. The net result is that you don’t have to change your line if you took the turn too fast.

Now there are some systems out there that bring the best of All-Wheel-Drive & 4-Wheel-Drive to the table. The Jeep Grand Cherokee Summit that I drove recently blends both the benefits of All-Wheel-Drive and 4-Wheel-Drive by giving you the capabilities of 4-Wheel-Drive with the ease of use of All-Wheel-Drive by packaging it in a way that all a user has to do is use a knob on the dash to select the type of terrain that the Jeep is being driven on. Alternately they can just leave it in auto and let the Jeep figure it out.

So which system do you need? It depends on your driving habits. In my opinion, most people will find that All-Wheel-Drive will work just fine for their needs. If you work in construction or you need to go off road in a serious way on a frequent basis, then 4-Wheel-Drive might be a better choice. Just make sure that you quiz your dealer as to what the All-Wheel-Drive or 4-Wheel-Drive system is capable of before you drive off the lot.

This entry was posted on August 5, 2013 at 10:19 am and is filed under Tips. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.