So Much for AWD - PeachParts Mercedes-Benz Forum

cjlipps · #1 03-30-2013, 10:35 AM

Quote:

Originally Posted by Brian Carlton

Show me some evidence............any evidence...........that a 4wd vehicle can perform better than a Rwd vehicle at the cornering limit.

I define the cornering limit at the precise speed and angle of turn whereby any additional applied torque to the axle or any further increase in the angle of turn or any additional speed will cause the tire to slide on the surface rather than roll.

You cannot provide any evidence because there isn't any. The only possibility of a 4wd vehicle "cornering better" than a 2wd vehicle is when the vehicle is below the cornering limit and the tire has the capability of accepting torque without sliding. I deliberately wrote "cornering better" in quotation marks because most folks, such as yourself, believe the vehicle is cornering better, when, in reality, it's simply going faster through the turn than an equivalent 2wd vehicle because it is not at its cornering limit.

Isn't "going faster through the turn than an equivalent 2wd vehicle" the very definition of better handling?
I maintain that all else being equal, ie: tires, vehicle weight, driver competence, horsepower, road surface...the 2wd vehicle will reach it's cornering limit at a lower speed than the awd vehicle. I agree that in the mitts of a blundering cretin awd merely serves to increase the speed at which the vehicle gets into trouble. But in the hands of a competent driver awd provides better handling and stability than an equivalent 2wd vehicle on virtually any surface.

Brian Carlton · #2 03-30-2013, 10:48 AM

Quote:

Originally Posted by cjlipps

I maintain that all else being equal, ie: tires, vehicle weight, driver competence, horsepower, road surface...the 2wd vehicle will reach it's cornering limit at a lower speed than the awd vehicle.

This is the fallacy that you and Mr. W will cling to, despite the fact that it is patently false.

The cornering limit of a tire has nothing to do with the torque applied to that tire. In fact, the cornering limit of a 4wd tire can be LOWER than the cornering limit of a 2wd tire because if the tire begins to spin due to the applied torque, its cornering limit, as defined by the force it can withstand laterally (lateral g force) is reduced.

All of the benefits of a driven tire occur BELOW the cornering limit where the applied torque can be useful.

cjlipps · #3 03-30-2013, 10:59 AM

Why haven't you answered my or Mr. W's questions?

t walgamuth · #4 03-30-2013, 12:24 PM

Quote:

Originally Posted by Brian Carlton

This is the fallacy that you and Mr. W will cling to, despite the fact that it is patently false.

The cornering limit of a tire has nothing to do with the torque applied to that tire. In fact, the cornering limit of a 4wd tire can be LOWER than the cornering limit of a 2wd tire because if the tire begins to spin due to the applied torque, its cornering limit, as defined by the force it can withstand laterally (lateral g force) is reduced.

All of the benefits of a driven tire occur BELOW the cornering limit where the applied torque can be useful.

now you have made a statement about a single tire. There are four tires in a car or truck. What you have described can be applied in what instance, please? Entering a turn, exiting a turn, solid state turning, or some other instance?

As for qualifications do you have advanced education in vehicle dynamics?

t walgamuth · #5 03-30-2013, 03:10 PM

Quote:

Originally Posted by Brian Carlton

This is the fallacy that you and Mr. W will cling to, despite the fact that it is patently false.

The cornering limit of a tire has nothing to do with the torque applied to that tire. In fact, the cornering limit of a 4wd tire can be LOWER than the cornering limit of a 2wd tire because if the tire begins to spin due to the applied torque, its cornering limit, as defined by the force it can withstand laterally (lateral g force) is reduced.

All of the benefits of a driven tire occur BELOW the cornering limit where the applied torque can be useful.

Alright I will examine your second paragraph. You say that the cornering ability of the tire has nothing to do with the torque applied to it but then say that if too much torque is applied it will lose traction. I agree with the second portion not the first. Having accelerative forces in addition to cornering forces will diminish the cornering ability of the one tire (but not necessarily of the entire vehicle when what is happening to the other three wheels) because some accelerative force is being applied to either:
1. maintain the speed already traveled or
2. accelerate out of the corner.
If neither is true then the vehicle is by definition, losing speed, so if it is on the limit of adhesion it will soon be well under it. Unless it is being coasted down a large hill.

I will not deal with the no power applied because we seldom coast through corners and if we do it will not matter if the car is 2wd, 4wd or unpowered.

If the car applying accelerative force is 2wd then when the powered wheels are applying the accelerative force the other two are going to be well below their cornering limit and in essence, coasting.

If all four wheels are receiving power then all will be equally (theoretically) loaded and each can approach the limit of adhesion at the same time, resulting in faster corner speeds.

In a snowy situation, if a 2wd car can take a corner at 20 mph does this mean a 4wd can take it at 40? Of course not, no matter how many doofuses think so. Can a 4wd car take it at 21, 22 or 23, perhaps.

The reason doofus gets in trouble with the 4wd vehicle on snow is not because it has less cornering power than the 2wd vehicle it is because it can accelerate much better in a straight line than the 2wd vehicle thus instilling in his tiny brain the idea that it will have superior traction in all conditions and it will not be true (for the most part) when it comes to cornering or braking.

Brian Carlton · #6 03-30-2013, 03:32 PM

Quote:

Originally Posted by t walgamuth

Alright I will examine your second paragraph. You say that the cornering ability of the tire has nothing to do with the torque applied to it but then say that if too much torque is applied it will lose traction. I agree with the second portion not the first. Having accelerative forces in addition to cornering forces will diminish the cornering ability of the one tire (but not necessarily of the entire vehicle when what is happening to the other three wheels) because some accelerative force is being applied to either:
1. maintain the speed already traveled or
2. accelerate out of the corner.
If neither is true then the vehicle is by definition, losing speed, so if it is on the limit of adhesion it will soon be well under it. Unless it is being coasted down a large hill.

I will not deal with the no power applied because we seldom coast through corners and if we do it will not matter if the car is 2wd, 4wd or unpowered.

If the car applying accelerative force is 2wd then when the powered wheels are applying the accelerative force the other two are going to be well below their cornering limit and in essence, coasting.

If all four wheels are receiving power then all will be equally (theoretically) loaded and each can approach the limit of adhesion at the same time, resulting in faster corner speeds.

In a snowy situation, if a 2wd car can take a corner at 20 mph does this mean a 4wd can take it at 40? Of course not, no matter how many doofuses think so. Can a 4wd car take it at 21, 22 or 23, perhaps.

The reason doofus gets in trouble with the 4wd vehicle on snow is not because it has less cornering power than the 2wd vehicle it is because it can accelerate much better in a straight line than the 2wd vehicle thus instilling in his tiny brain the idea that it will have superior traction in all conditions and it will not be true (for the most part) when it comes to cornering or braking.

I agree with most of the above.

My statement that "the cornering ability of the tire has nothing to do with the torque applied to it" is technically incorrect. If a tire tire has no applied torque, it can achieve a specific lateral g force before it slips. Once you add torque, the tire needs to perform two functions at once (accept the lateral force and transmit the traction force). Asking it to do both, simultaneously, will diminish it's capability of accepting the lateral force.

However, if the vehicle is not quite at the cornering limit, the traction forces can serve to change the direction of travel and may be more effective than the loss of lateral force capability. This is the likely reason that a 4wd vehicle can turn faster in marginal conditions than a 2wd vehicle. The addition of the traction force is much more effective than the loss of the lateral force.

This might allow you to understand why a 4wd vehicle will outperform a 2wd vehicle when neither of them are at the cornering limit.

Brian Carlton · #7 03-30-2013, 03:37 PM

Quote:

Originally Posted by t walgamuth

If the car applying accelerative force is 2wd then when the powered wheels are applying the accelerative force the other two are going to be well below their cornering limit and in essence, coasting.

This needs a bit of correction.

If a 2wd vehicle is in a turn, the other two might be close to or at the cornering limit........depending on the radius of the turn and the speed of the vehicle.

Your statement would be absolutely true if the vehicle is traveling in a straight line, but, seriously, that's not what the discussion is about.

Quote:

Originally Posted by t walgamuth

If all four wheels are receiving power then all will be equally (theoretically) loaded and each can approach the limit of adhesion at the same time, resulting in faster corner speeds.

This also needs a bit of correction. Say four tires are equally loaded to the limits of traction while traveling in a straight line. If power is maintained and the front wheels are turned, you will absolutely lose control of the 4wd vehicle because the front tires instantly exceed the cornering limit. The "faster speed" that you anticipated has now taken you right off the road.

The only benefit of the 4wd vehicle can offer is faster speeds in a turn when operated below the cornering limit of the front tires. Naturally, very few owners of such vehicle know where the limit lies.

t walgamuth · #8 03-30-2013, 04:04 PM

Sounds like we are mostly in agreement.

I will mention that a vehicle does not need to be traveling in a straight line to be coasting. so two wheels can be coasting in a turn and well below their limit of adhesion whilst the driven wheels are applying acceleration.

all other things being equal, a rear wheel drive vehicle will out corner a front drive vehicle because the front driver needs to turn and apply power to maintain speed simultaneously.

Brian Carlton · #9 03-30-2013, 04:57 PM

Quote:

Originally Posted by t walgamuth

Sounds like we are mostly in agreement.

I will mention that a vehicle does not need to be traveling in a straight line to be coasting. so two wheels can be coasting in a turn and well below their limit of adhesion whilst the driven wheels are applying acceleration.

all other things being equal, a rear wheel drive vehicle will out corner a front drive vehicle because the front driver needs to turn and apply power to maintain speed simultaneously.

All of this began with post #56 where you associated "cornering" with the capability of a 4wd vehicle to outperform a 2wd vehicle, and, therefore, by definition, it was better at "cornering".

I simply take issue with the word "cornering". I would prefer if you reserved "cornering" for the lateral g force that a given vehicle can withstand under a set of conditions. If this is the accepted definition, the 4wd vehicle does not have an advantage. However, it is agreed that the 4wd vehicle has improved "handling" in the turn when below the "cornering limit" and will certainly outperform the 2wd vehicle because of it.

And, no, I do not have, nor do I need an advanced degree to analyze this relatively simple subject.

t walgamuth · #10 03-30-2013, 08:07 PM

It is actually a lot more complex than we got into.

Skippy · #11 03-30-2013, 08:12 PM

Quote:

Originally Posted by Brian Carlton

I simply take issue with the word "cornering". I would prefer if you reserved "cornering" for the lateral g force that a given vehicle can withstand under a set of conditions. If this is the accepted definition, the 4wd vehicle does not have an advantage. However, it is agreed that the 4wd vehicle has improved "handling" in the turn when below the "cornering limit" and will certainly outperform the 2wd vehicle because of it.

How about we use the term cornering to denote the activity of going through corners? If we use that, then a 4wd vehicle will very often have an advantage over a 2wd vehicle for reasons already discussed.