4WD: Not just for offroading


Apr 24, 2000 (Updated May 15, 2000)




Given the plethora of SUVs out there, one has to wonder what has caused this demand for them. Was it the fear of the next ice age? Or was it the increase in gas prices that forced people to buy 30 gallons of cheap gas, store it in their SUV's gas tank and later siphon it out to their econoboxes... In either case, for everyone interested in buying an SUV, the question of 2WD or 4WD has definitely come across one way or another.

As an owner of multiple SUVs (1998 and 1999 Chevrolet Tahoe), I have had many people come up to me and ask if 4WD is really worth it for on-road driving only (as MOST of today's SUVs do...). The fact of the matter is that the question of 2WD vs. 4WD is not really what matters. The real question that should be considered is curb weight vs. torque and their relation to road conditions, suspension and tires. Lost? Let me explain.

Curb weight is the total vehicle weight with a full tank of gas (Some manufacturers also may include passengers but the added weight of one or two people is negligible). Torque is the rotational power that is applied to the road through your tire's contact point (patch) from the drivetrain (engine). The combination of these two contribute to the way the vehicle responds to road conditions and the tires you use.

For example, in a light 2WD vehicle with LOTS of torque (I use torque NOT horsepower because it is the TORQUE that gets a tire spinning... NOT horsepower - a common misunderstanding), if one were to slam on the accelerator on wet pavement you will DEFINITELY get wheelspin and most probably instead of moving forward, you'll just make a lot of noise and smoke. There are two non-computer based (i.e. traction control) solutions: Get wider tires to provide a bigger contact patch, thus increasing the friction which gives the vehicle a lesser chance of wheel spin - that is why high performance cars have such ridiculously wide rear tires - Dodge Viper for instance has rear tires that are about one foot wide. Or get 4WD, which in terms of frictional surface area, is equivalent to the first solution... essentially, with all four wheels spinning, the total surface area of friction is increased. The main point is that to prevent sliding, skidding, and wheel spin, you want to be riding on as much surface area as you possibly could.

OK, so you think that your bloated 2.5 ton 2WD SUV has no problem with the weight but why could you still get wheel spin on wet roads with the huge tires that come standard (if you drive 2WD and never slam on the accelerator like an indy racer after a light turns green, try stopping on a hill in the rain and accelerate... you will get wheel spin - even better are snow covered roads). Gravity. Your truck is now too heavy. :) The force required to "push" your car up the hill is not enough so the wheels break loose and spin. (For those of you with front wheel drive, the situation can be equated to driving in reverse up a hill - essentially making your car "push" also, as opposed to "pulling"). Here is where 4WD comes into play. With the front and rear wheels spinning, more power is transferred to the ground via a greater surface area. Also, your car hunts for friction spots by allowing all four wheels to contribute to the total forward momentum.

OK... enough of this techno-physics lesson. How does this relate to you? Well, if you live in a relatively level area with dry weather 365 days a year (i.e Florida), then get 2WD. Otherwise consider getting really wide sticky tires (Note: high friction tires wear faster than regular tires), a limited slip differential, and/or 4WD - I have all three... winters are so crazy here in upstate NY.

With the experience of driving an SUV for about 11 years now, I try to stay in 2WD as much as possible to save gas but during wet driving conditions and snow, 4WD is a must to keep control of such an unforgiving vehicle... even when driving slow. Also, it is rare that a 4WD SUV gets stuck in anything... it is very common for a 2WD SUV to get stuck in everything. One thing to add is that if you are over-confident with your 4WD driving skills, you can and will get stuck too... possibly getting yourself into an accident as well.

To end, 2WD vs. 4WD is a matter of convenience and safety... NOT luxury. Before 4WD, people still got around with 2WD SUVs in rain, snow and mud... just not as good and fast. But as with the extreme... I'd hate to get stranded in a foot high pile of snow in 0 degree weather. Also, if you look carefully, there are a lot of 4WD cars too... just incase you thought that 4WD is only for trucks that climb rocks or go offroad.

**Updated May 14, 2000**
It has come to my attention that there is some confusion by my definition of 4WD for cars and trucks. After doing a little more research, I realized that 4WD and AWD are slightly different but still essentially function the same way.

Well, what are the differences? Well there are 4 types of 4WD systems on the market... Part-time 4WD, Automatic 4WD, Permanent 4WD, and All-Wheel drive. I have found a webpage that describes each system in detail... this is to save us both time as the explanations are quite long.

Just cut and paste the following link in your browser (The link is broken into two lines because it won't fit. Please make sure you copy the whole thing without any spaces.):

http://carpoint.msn.com/Advice/default.asp?contentid=9783&type=6

Before you go there though let me explain what a differential is, in detail, as CarPoint makes several vague references to it... A differential is the gearing, on the axle, that allows for the left wheel spin at a different speed than the right. The main purpose of the differential is to prevent excess gear and tire wear while driving. How? It all comes down to physics and the mechanics involved when driving.

For instance, when you are making a turn (left or right), the wheel that is on the inside track, will spin slower than the wheel that is on the outer track. Huh? Well, let's say you are making a right turn... all the wheels on the right side of the vehicle will traverse a smaller "circle" than the wheels on the left side. In essence, if we were to make a complete circle with the vehicle, the wheels make two circles (think of the vehicle in snow and you can see the path the tires have travelled). Using basic math, it is obvious that the inner circle has a smaller circumference than that of the outer circle. This visualization can be helped if you think about Olympic runners starting at offset positions on the racetrack. If all of them started at the same point, the one on the outer most lane, would have to run a greater distance than the one on the inner lane.

If the differential did not exist, think of the excess force that is applied to the inner wheels... not healthy for gears or tires in any car or truck. So what is the point of this rambling? Well, in certain situations, a fully open diff (open meaning that each wheel is free to spin on its own) will hurt your vehicles ability to get out of low traction areas. Just think of the times when there were ice patches on your driveway. While attempting to drive on an ice patch, one of your rear wheels starts to spin... the other wheel, although on dry pavement, will have trouble spinning up because the wheel that is spinning is basically taking all the power applied to the axle (an inherent design of an open diff).

So what can you do? When buying your car or truck you may have been presented with a "locking-differential" option. (On some vehicles, locking diffs may be standard...) A locking diff, if faced with the ice situation described, will automatically detect wheel spin and force the differential to close. By closing, the power is forced to the wheel on dry pavement so that, it is forced to turn despite the other wheel spinning out of control.

For 4WD and AWD vehicles, there are diffs in the front and rear axle. Which ever one you have, while in 4WD or AWD, you will be faced with the same problems above if you have open differentials. On my Tahoe, the front differential is locked by default, while the rear diff, locks only when wheel spin is detected. So to clarify, unless your vehicle specifically has locked or locking differentials in 4WD, all the wheels, respective to their axle, may experience wheel spin problems.

To end, I would like to apologize about any confusion that I may have caused regarding 4WD... next time I will be more specific in my explanations. Thanks to "cleanshaven" for helping to point out the flaws in my original epinion.


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