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50K for a luxury/sport car or SUV – what to get?! - Page 8

post #106 of 130
Quote:
Originally Posted by henrikc View Post
Yes, proper AWD systems like Audi's Quattro are great, but if you know how to drive, you really don't need it. It's a nice thing to have, but there's no guarantee that it will keep you from getting stuck.
It's like people only read every second word you post:
Quote:
Originally Posted by Piobaire View Post
I grew up driving old, RWD drive cars in the Canada. Can you get around in snow like this? Sure thing. *** Nothing is going to stop all weather related accidents and I am sure a highly skilled driver in a RWD can outperform a bad/low skill driver in an AWD,
post #107 of 130
Quote:
Originally Posted by Piobaire View Post
I grew up driving old, RWD drive cars in the Canada. Can you get around in snow like this? Sure thing. However, to think that FWD and AWD do not add significant improved handling vs. RWD is just specious. To begin with, think of the physics. In RWD the motive force is on the light end of the car and it needs to push the heavy (and steering) end. In FWD, the engine rests over the motive axle which is also the steering axle.
If you have the tires on a single axle serving both drive and steering functions, the total traction available to them is split between those two functions. From the perspective of maximizing traction, it's better to have the rear wheels drive and the front wheels steer. Also, the weight distribution of the vehicle is not determined by its drive configuration. There are plenty of 50/50 RWD and AWD vehicles. As Andre said, AWD provides only slight advantages over RWD in acceleration in on-road slippery conditions.
post #108 of 130
Quote:
Originally Posted by dah328 View Post
If you have the tires on a single axle serving both drive and steering functions, the total traction available to them is split between those two functions. From the perspective of maximizing traction, it's better to have the rear wheels drive and the front wheels steer. Also, the weight distribution of the vehicle is not determined by its drive configuration. There are plenty of 50/50 RWD and AWD vehicles. As Andre said, AWD provides only slight advantages over RWD in acceleration in on-road slippery conditions.

So it is your position that RWD actually are superior to FWD? Um, okay.

Is not a "slight" advantage (which I am not stipulating is the case) still better than a deficit? When it's my ass on the line, and those of my loved ones, I'd say it surely is.

I will cede the weight distribution as my last experience with a RWD vehicle for daily use was a 1972 Cutlass and I am sure current RWD vehicles have far superior weight distribution.
post #109 of 130
Quote:
Originally Posted by Piobaire View Post
So it is your position that RWD actually are superior to FWD? Um, okay.

Is not a "slight" advantage (which I am not stipulating is the case) still better than a deficit? When it's my ass on the line, and those of my loved ones, I'd say it surely is.

I will cede the weight distribution as my last experience with a RWD vehicle for daily use was a 1972 Cutlass and I am sure current RWD vehicles have far superior weight distribution.
It is my position that RWD is nearly always superior to FWD, but the point of my post was that your analysis of the physics of FWD vs. RWD was wrong.

The slight advantage of AWD, and only under certain conditions, is in acceleration only. Steering and braking performance remain the same (or slightly worse given the general weight disadvantages of AWD vehicles). Because steering and braking performance are far more important for safety, an AWD doesn't really buy you anything from a safety perspective.

The one area in which FWD may be superior to RWD is vehicle dynamics for the unskilled driver. Handling a RWD car in slippery conditions involves techniques that are generally not instinctive so the unskilled or untrained driver is likely to exacerbate a bad situation. A FWD car may be more forgiving to an unskilled driver, but in the hands of a skilled driver, the RWD is superior.
post #110 of 130
Quote:
Originally Posted by dah328 View Post
The slight advantage of AWD, and only under certain conditions, is in acceleration only. Steering and braking performance remain the same (or slightly worse given the general weight disadvantages of AWD vehicles). Because steering and braking performance are far more important for safety, an AWD doesn't really buy you anything from a safety perspective.

Braking performance is a red herring in this conversation, so we can safely set that aside in this discussion. If you think that traction, which is needed for steering, is not superior in AWD we have nothing further to discuss. You cannot steer if you have lack traction.

Quote:
Originally Posted by dah328 View Post
The one area in which FWD may be superior to RWD is vehicle dynamics for the unskilled driver. Handling a RWD car in slippery conditions involves techniques that are generally not instinctive so the unskilled or untrained driver is likely to exacerbate a bad situation. A FWD car may be more forgiving to an unskilled driver, but in the hands of a skilled driver, the RWD is superior.

You are framing this poorly. It is not an "unskilled driver" thing but rather what one is accustomed to. As someone that grew up in the snow belt driving 60s and 70s RWD tanks, I actually found my first foul weather FWD situation dodgy. It's merely what one is used to, not an automatic lack of skill

Anyway, I know how big my dick is. Out of this convo.
post #111 of 130
Quote:
Originally Posted by Piobaire View Post
Braking performance is a red herring in this conversation, so we can safely set that aside in this discussion. If you think that traction, which is needed for steering, is not superior in AWD we have nothing further to discuss. You cannot steer if you have lack traction.
This was addressed in my first post. Wheels on a single axle performing two functions split the available traction of the wheel between those two functions. The physics of it are quite straightforward.

Quote:
You are framing this poorly. It is not an "unskilled driver" thing but rather what one is accustomed to. As someone that grew up in the snow belt driving 60s and 70s RWD tanks, I actually found my first foul weather FWD situation dodgy. It's merely what one is used to, not an automatic lack of skill.
You have a point here in that one can be a skilled FWD driver. That said, RWD would be the choice of a driver skilled in both FWD and RWD whose criterion was maximum potential performance in slippery conditions.
post #112 of 130
Quote:
Originally Posted by dah328 View Post
As Andre said, AWD provides only slight advantages over RWD in acceleration in on-road slippery conditions.

To be accurate, I didn't say it had "slight advantages" only that it helped in acceleration. Since you need to also brake and turn, AWD in and of itself is not a panacea for slippery conditions. It all starts with having the right tires, which is by far the most important thing.

BTW, traction is determined by your tires and suspension. AWD cannot add traction. A good AWD system can make the most out of the available traction under acceleration.

--Andre
post #113 of 130
Quote:
Originally Posted by A Y View Post
To be accurate, I didn't say it had "slight advantages" only that it helped in acceleration. Since you need to also brake and turn, AWD in and of itself is not a panacea for slippery conditions. It all starts with having the right tires, which is by far the most important thing.

BTW, traction is determined by your tires and suspension. AWD cannot add traction. A good AWD system can make the most out of the available traction under acceleration.

--Andre

I'm sorry, but this is incorrect. Let's start off with some definitions of traction. Feel free to add your own, but look at the second definition in this list:

http://www.bing.com/Dictionary/searc...on&FORM=DTPDIA

Quote:
friction allowing movement: the adhesive friction between a moving object and the surface on which it is moving, e.g. between a tire and the ground, without which the object cannot move

Friction, allowing movement. Basically, a counter force to torque. Now, if you have the same torque going to two wheels on one vehicle and four on another, the AWD vehicle has exactly 100% more traction.

If the counter force traction is less than the amount of torque created by the driver's right foot - the wheels break lose and spin. Forward movement will cease. This makes AWD superior and less prown to breaking lose.

/shrug

Edit: I apologize for posting again when I had exited the conversation but felt the need for accuracy was the greater calling.
post #114 of 130
Quote:
Now, if you have the same torque going to two wheels on one vehicle and four on another, the AWD vehicle has exactly 100% more traction.

You are confusing total traction with usable traction. The tires determine your total traction. In traction-limited situations, AWD may let you to use a bigger fraction of that total traction than a 2WD.

--Andre
post #115 of 130
I don't think I am contributing much but in real Canada (where road is made of ice because socialists cannot afford salt), people put sandbags in their RWD cars during the winter.
post #116 of 130
Quote:
Originally Posted by A Y View Post
You are confusing total traction with usable traction. The tires determine your total traction. In traction-limited situations, AWD may let you to use a bigger fraction of that total traction than a 2WD.

--Andre

We are probably saying about the same thing, just not agreeing with how each other are saying it.

I agree, total potential traction is determined, in large part, by your tires. If you want to say that your initial statements were incomplete, and you meant to differentiate between total potential traction vs. usuable traction, I'll agree. However, it remains merely potential until the counter force of torque is applied. This way it can be seen that an AWD vehicle will have 100% more traction available than a 2WD vehicle, all other things being equal.

Also, it is not just your tires, but the weight of the vehicle that contribute to traction, as well as the road surface itself. Traction is made up of more than just your tires, but they are one of three major variables and the one you can control the easiest.

Some engineering defs I pulled from Wiki, but well annotated:

http://en.wikipedia.org/wiki/Traction_(engineering)

Quote:
Traction is defined as:

..a physical process in which a tangential force is transmitted across an interface between two bodies through dry friction or an intervening fluid film resulting in motion, stoppage or the tranmission of power[4] (Copyright: "Mechanical Wear Fundamentals and Testing" by Raymond George Bayer)

Note the three things, two related, one the opposite. The middle one, stoppage, is about braking, which as I've already pointed out is a red herring in this conversation over transmission type efficacy. "Traction" has as a component of its definition the ability to transfer power that causes movement, in the relevant portion of this discussion.

Further, all situations are "traction-limited situations" since the total amount of traction is always going to be finite.
post #117 of 130
Quote:
Originally Posted by A Y View Post
You are confusing total traction with usable traction. The tires determine your total traction. In traction-limited situations, AWD may let you to use a bigger fraction of that total traction than a 2WD.
Equally importantly, whatever greater traction AWD may allow you to use in such a situation, it's strictly for acceleration, not steering or braking. As such, it's of limited utility for safety purposes.
post #118 of 130
Quote:
Originally Posted by Piobaire View Post
If you want to say that your initial statements were incomplete, and you meant to differentiate between total potential traction vs. usuable traction, I'll agree.
Sorry about that. It's easy to fall into shorthand.
Quote:
This way it can be seen that an AWD vehicle will have 100% more traction available than a 2WD vehicle, all other things being equal.
It isn't clear what you mean by all other things being equal. To me, it means you are placing two identical cars on identical road surfaces, and the only thing you're changing is the drive system, in which case what you say is false. Consider 3 cases: 1. The trivial one: your front tires have a smaller contact patch than the rears. 2. Your rear tires are on ice, and your front tires are on dry asphalt. Consider the reverse too. 3. One side of your car is on ice while the other side is on dry asphalt. In none of those cases will AWD have 100 percent more traction than 2WD. In all cases, your traction is limited by your tires.
Quote:
Note the three things, two related, one the opposite. The middle one, stoppage, is about braking, which as I've already pointed out is a red herring in this conversation over transmission type efficacy. "Traction" has as a component of its definition the ability to transfer power that causes movement, in the relevant portion of this discussion.
Tires transmit force to the ground by their traction. "Force" is used in the physics, F=ma sense. That force can be effected by three things in a car: 1. Pressing the gas pedal. 2. Pressing the brake pedal. 3. Turning the steering wheel. From a physical standpoint, it's basically the same phenomena, just in different directions. There's nothing particularly special about one or the other. It is the ground's reaction to that force (Newton's 3rd law) that causes a car to move forward, slow down, or rotate. Having said that, I still don't understand why braking is a red herring. It was claimed that AWD is better and safer in snow, and all else being equal it is, but I wanted to remind everyone not to forget about the other things you have to do while driving. Too often, people think that AWD is the most important thing to get when driving in snow, when really it's the tires, and the reason is because AWD can't help you brake better or turn in better.
Quote:
Further, all situations are "traction-limited situations" since the total amount of traction is always going to be finite.
Sorry about the shorthand again. When I say "traction-limited" I mean those situations in which you are operating close to the traction limits of the tires --- when you have more power than traction, for example. --Andre
post #119 of 130
Quote:
Originally Posted by dah328 View Post
Equally importantly, whatever greater traction AWD may allow you to use in such a situation, it's strictly for acceleration, not steering or braking. As such, it's of limited utility for safety purposes.

I would agree in general, but for the skilled driver or with the appropriate computer controls, you can do a lot with AWD, especially if you have a system like Honda's SH-AWD or the newest BMW xDrive where the computer can not only transmit torque, but speed up a wheel independent of the traction on the other tires.

--Andre
post #120 of 130
I guess you do not figure we are basically saying the same thing.

http://auto.howstuffworks.com/four-wheel-drive1.htm

http://www.4x4abc.com/4WD101/abc.html

Quote:
The engine creates the rotational force "torque" to move the vehicle. The counter force "traction" allows torque to create forward movement.

If the counter force traction is less than the amount of torque created by the driver's right foot - the wheels break lose and spin. Forward movement will cease.

If two wheels are used to bring torque and traction together the available traction at the two wheels will limit the amount of torque than can be applied.

Since heavy trucks will need more torque to be moved, they sometimes require more traction than 2 wheels can provide.

That lead to the creation of 4WD. Torque is now sent to 2 additional wheels, which provide 100% more traction. 100% more torque can be applied without slipping and spinning wheels.

/shrug
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