Is X-Wheel Drive a Scam? - Mercedes-Benz Forum

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post #1 of 19 (permalink) Old 02-23-2010, 02:15 PM Thread Starter
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Is X-Wheel Drive a Scam?

First of all, let me give some background information on where I'm coming from. I'm a junior in high school and have only had my driver's license for about a year. Last year, my parents decided it was time to get a new car, and my mom fell in love with the Benz she saw on eBay. My dad was very reluctant at first because he saw that it was RWD and thought it would be a hassle driving it around in the dreadful, Ohio winters. After a few weeks of debate, they finally decided to buy it.

This brings me to today. Winter is about two-thirds over, and from the driving that I've done in it, I can attest to the fact that my dad was right–this Benz is completely hopeless in the snow. What I have seen, however, is that there seems to be a fair amount of misinformation about FWD/RWD/AWD/4WD and snow. You see, this year is the first year where I've had some exposure to a physics course in school, and pretty much everything that I've heard regarding these systems makes no sense at all when put into the context of that what I've learned in said physics class.

For example, a few weeks ago, while driving the car, my dad stated that "it's terrible in the snow because it's RWD." This didn't make sense to me, so I asked him to explain, and he said that it's better in the snow for the front wheels to offer a "pull," rather than the back wheels a "push." Not satisfied with this answer, I asked my physics teacher to elaborate, and she offered nearly the same appallingly nontechnical explanation. This got me thinking about the topic, and here's what I've come up with.

What's most important to this discussion is friction. There are two types: static and kinetic. Static friction is basically the force that keeps an object from sliding relative to another (wheels on a car rolling normally or not moving at all), and kinetic friction is the force on an object that is already in motion relative to another object (wheels skidding). Static friction is usually greater than kinetic friction, and it's the type of friction that people talk about when describing their cars, usually relating kinetic friction to being frictionless. Both types of friction are defined by the equation:



where µ is the coefficient of friction (static for our purposes), and Fn is the normal force (the mass of an object multiplied by the acceleration due to gravity).

When analyzing this equation, you can clearly see that the number of wheels on one's car is not accounted for, with the only things affecting friction being weight of the car and quality of your tires/slipperiness of the road. This is basically what annoys me. People say "my Hummer is good in winter weather because it has 4WD," and as far as I can tell they are completely wrong. "No, your Hummer is not good in the winter because it has 4WD, it's good because it's heavier than an African Elephant."

The same thing goes for FWD vs RWD. Since all the cars major components are located in the front of the car (including the very heavy engine and transmission), there's way more weight on the front axle, making the normal force higher. Doesn't it only make sense that FWD would be better in the snow on most cars?

I can think of only one reason why 4WD/AWD would be better on snow, and that would be in a situation where the road is very unevenly covered (the left side of the lane is concrete, while the right is snow). How often is this really the case, though? I've never seen such a situation, but then again, I've only been driving for a year.

Needless to say, with all of this in mind, we will be buying winter tires and lead weights for the trunk next year, but this also brings me back to my post's title. Isn't AWD/4WD just a big scam in regards to winter driving? Also, am I right, or am I completely naïve?


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post #2 of 19 (permalink) Old 02-23-2010, 02:34 PM
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You are forgetting one huge variable. You need to multiply your equation by the number of driving wheels. aka...friction surface areas.
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post #3 of 19 (permalink) Old 02-23-2010, 02:40 PM Thread Starter
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I thought surface area didn't affect friction?


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post #4 of 19 (permalink) Old 02-23-2010, 03:03 PM
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Quote:
Originally Posted by Edfrommars View Post
I thought surface area didn't affect friction?
(Think drive pavement for simplicity for a moment)
Think of friction as the force working in opposition to the torque applied to the driving wheel(s). As you apply torque the wheel will resist slipping due to the force of friction of the surface in contact with the pavement. The car will move forward with the force available from that driving wheel. If you put narrow tires on the car the wheel will break traction sooner as the torque applied will exceed the friction available. If you put wider tires on the car it will require more torque to overcome the friction.
The variable is the coefficient of friction which is affected by the properties of the two surfaces. It's the "u" in your equation. Research "contact area" and "saturation" in the context of friction.
Then simply imagine that one wheel has a specific "u" value. Two wheels have 2xu, three wheels have 3xu.......

A higher coefficient of friction = more traction = more go.

Any doubts......go out and clip three brake lines and see how fast your fathers car will stop.
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post #5 of 19 (permalink) Old 02-23-2010, 03:06 PM
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It is way more complicated that you present above.
You are not calculating inertia. Take drag racers for example - front wheels on them play minimal role and usually are in bicycle size
Than in snow driving you have 2 major factor what are traction and control.
While FWD cars are very forgiving for driving errors and you can control the direction of the pulling force -they are helpless pulling up on slippery ramp.
RWD has better traction, but than with both driving wheels spinning on snow -the side movements control require lot of skills and advanced reaction.
I drove lot of FWD cars in deep snow at the time where snow chains were not available. They clearly have advantage. But than taking off at green light during a rain will leave the car way behind RWD.
To add to above -the surface area doesn't matter when you have unlimited friction, like car with good tires on dry concrete. Each wheel is this situation can transfer +- 150 hp to the pavement, so only few cars can have a problem.
On ice each wheel for example can have friction equal to 2 hp. Meaning any 2WD will have 4 hp of pulling force, while 4WD will have 8 hp.
I have another puzzle for you. In the past when I was practicing ice driving I had car rolling straight on neutral. Pushing the brakes gave sense me sense of accelerating. Can you explain it?
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post #6 of 19 (permalink) Old 02-23-2010, 03:09 PM
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FWD vs. RWD

On a FWD car, the front wheels have to do double duty. They must provide traction for driving and they must provide friction for turning. The rear wheels mainly keep the bumper from dragging on the ground. There is only so much friction available for any given combination of tire and road surface. If that friction has to be split between motive force and turning, neither will be optimal. That is the reason race cars are almost universally rear wheel drive. The main advantage of FWD in the snow is that the front wheels always pull in the direction they are pointed. On a RWD car, as soon as your front wheels start to "plow" you are finished. Good snow tires, NOT all-season tires, on all four wheels are essential to getting the most out of your car in the slipperies.

IRT to weight distribution, any worthwhile car today strives to attain as close to a 50-50 weight distribution so the "all the weight on the front wheels" theory is bogus.

IRT weights in the trunk; careful here. Weight in the trunk tends to also unload the front wheels making turning control more problematic on a RWD car unless you can get the weight well forward of the rear axle. A better place would be on the floor behind the front seat.

Regardless of FWD, RWD or 4WD, once you take your foot off of the accelerator and apply the brake, they are all the same. Keep that in mind and try to survive your most dangerous driving period, the first few years. Hope to hear much more from you.
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post #7 of 19 (permalink) Old 02-23-2010, 03:18 PM
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When applying your formula to driving on less then ideal dry conditions you need to also consider another factor. Lubrication. In the case of ice and snow these materials will act as a lubricant. This is why a narrower tire is recommended for winter driving. Less surface area = less lubricant.

As for the Hummer...... relatively narrow tires (for its weight), knobby tread, high ground clearance, and locking differentials, all contribute to their prowess in the ice and snow. Weight helps, but to a lesser extent.....because of that little thing called inertia.
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post #8 of 19 (permalink) Old 02-23-2010, 03:38 PM
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Quote:
Originally Posted by Kajtek1 View Post
It is way more complicated that you present above.
You are not calculating inertia. Take drag racers for example - front wheels on them play minimal role and usually are in bicycle size
Than in snow driving you have 2 major factor what are traction and control.
While FWD cars are very forgiving for driving errors and you can control the direction of the pulling force -they are helpless pulling up on slippery ramp.
RWD has better traction, but than with both driving wheels spinning on snow -the side movements control require lot of skills and advanced reaction.
I drove lot of FWD cars in deep snow at the time where snow chains were not available. They clearly have advantage. But than taking off at green light during a rain will leave the car way behind RWD.
To add to above -the surface area doesn't matter when you have unlimited friction, like car with good tires on dry concrete. Each wheel is this situation can transfer +- 150 hp to the pavement, so only few cars can have a problem.
On ice each wheel for example can have friction equal to 2 hp. Meaning any 2WD will have 4 hp of pulling force, while 4WD will have 8 hp.
I have another puzzle for you. In the past when I was practicing ice driving I had car rolling straight on neutral. Pushing the brakes gave sense me sense of accelerating. Can you explain it?
Actually it's quite simple...no more complicated then using a spring guage to measure the coefficient of static friction between any two surfaces. The complicated part is all the variables...e.g. materials involved, atomic level friction, speed, temperature, angles involved, on and on and on.

The 150hp car does not have unlimited friction...it does have limited torque that can't overcome the available friction.

As far as the puzzle....I'll bet its a case of expectations..... like that sensation of rolling forward at a stop light when in fact it's the car next to you rolling backward. That always freaks me out.
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post #9 of 19 (permalink) Old 02-23-2010, 03:49 PM
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Boy I wish our W210s had 50/50 weight distribution. As far as why RWD for most race cars... I would add the effects of weight transfer during acceleration, easier to design a 50/50 weight distribution, the ability to use over steer to advantage on dirt and short tracks, inherently more robust, etc..
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post #10 of 19 (permalink) Old 02-23-2010, 03:50 PM Thread Starter
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Quote:
Originally Posted by Figuero View Post
As far as the puzzle....I'll bet its a case of expectations..... like that sensation of rolling forward at a stop light when in fact it's the car next to you rolling backward. That always freaks me out.
Actually, I think he was referring to inertia and how the car is decelerating while his speed remains constant, causing a feeling of acceleration. That always freaks me out too, though lol.

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Originally Posted by rocky raccoon View Post
IRT to weight distribution, any worthwhile car today strives to attain as close to a 50-50 weight distribution so the "all the weight on the front wheels" theory is bogus.
Yeah, I had a feeling this might be the case. I didn't really feel it was a convincing argument either :P I was going to check the distribution on my own car, but I was sick today and didn't want to go into the freezing cold garage.

Quote:
Originally Posted by rocky raccoon View Post
IRT weights in the trunk; careful here. Weight in the trunk tends to also unload the front wheels making turning control more problematic on a RWD car unless you can get the weight well forward of the rear axle. A better place would be on the floor behind the front seat.
Very good call. I hadn't even thought of that.

Quote:
Originally Posted by rocky raccoon View Post
Regardless of FWD, RWD or 4WD, once you take your foot off of the accelerator and apply the brake, they are all the same. Keep that in mind and try to survive your most dangerous driving period, the first few years. Hope to hear much more from you.
Thanks So far so good.


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