FWD or AWD?

[craigo]

Honda's SH-AWD is interesting: http://en.wikipedia.org/wiki/SH-AWD

 

[callphy]

for moving: AWD+ST >>> FWD+ST >>> AWD+AST >>> FWD+AST

for stopping: Snow Tire >>> All Season Tire ( no matter what ***drive u have.)



so FWD+ snow tire is always a cheaper and better option than AWD without snow tire

but something like mitsubishi awc or suzuki iawd (with drive selector are much cheaper and better option)

 

[Beefy]

Well, being a theretical physicist (I hate throwing around credentials on forums, I know they mean very little, just to add context...), I certtainly know that there is very often a difference between theory and practice. In thery, an object in a curve does not loose any energy as long as there is only a latteral force acting upon it. So it does not require any additional power to maintain speed. Now in practice, there might be some slight energy loss from friction in the differential or whatever. I don't know since I can barely tell a wrench from a screw driver. But I suspect that the energy loss (and thus additional power required) would be too insignificant to even be relevant to this discussion.

Forget friction, it is fighting inertia that absolutely requires power to be put down. You will decelerate faster coasting round a curve than you will in a straight line. The tighter the curve, the larger the effect.

 

[CXVille]

Forget friction, it is fighting inertia that absolutely requires power to be put down. You will decelerate faster coasting round a curve than you will in a straight line. The tighter the curve, the larger the effect.

But is irrelevant in this discussion on handling. If you believe otherwise, please provide some facts. I believe the loss is primarily in friction and heating of the tires. Very, very minimal. So little that it doesn't matter in a AWD vs. FWD discussion when driven on the street by ordinary drivers.

 

[Beefy]

But is irrelevant in this discussion on handling. If you believe otherwise, please provide some facts. I believe the loss is primarily in friction and heating of the tires. Very, very minimal. So little that it doesn't matter in a AWD vs. FWD discussion when driven on the street by ordinary drivers.

Force is required to change velocity, acceleration around the curve, this is simple physics. What people often call centrifugal force, being pulled outward in the curve, is better viewed as inertia trying to keep you going in a straight line. You must spend energy to exert centripetal force to overcome this inertia to actually move around the bend.

The tires are where centripetal force is actually applied the road, but this isn't where the major energy loss is encountered. If you imagine a spaceship in vacuum that has no friction it still requires energy expenditure to create the force to make it turn the corner.

 

[MATR6]

for stopping: Snow Tire >>> All Season Tire ( no matter what ***drive u have.)

Not necessarily the case if the temperature is above 40 deg F (5 deg C)
Which is the case in areas that get snow but average temps don't drop below freezing that often.
The past 2 winters here in Chicago have been mild (but we got hit hard this year as so did everyone else)
It may be the norm to get snow tires up in Canada, but it is not necessary in the States if you live in a metropolis area that plow within hours of snow fall.

This shall be the last discussion about tires. I think we have everything covered.

 

[vroom]

Forget friction, it is fighting inertia that absolutely requires power to be put down. You will decelerate faster coasting round a curve than you will in a straight line. The tighter the curve, the larger the effect.

...You must spend energy to exert centripetal force to overcome this inertia to actually move around the bend

... If you imagine a spaceship in vacuum that has no friction it still requires energy expenditure to create the force to make it turn the corner.

OMG, and all this time I thought Newton was correct! That means the earth going around the sun is loosing energy all the time untill we fall into it. Oh the humanity!

Obviously, I'm not going to go into this any more than I have already, since I agree with Chris_Top_Her that this has gotten way off topic. Those interested can brush up on their physics 101 manuals on their own. I find the link posted by craigo interesting and on topic. I think the Honda system acts with similar principles than the DSC, whereas the DSC controls torque on each individual wheel by using brakes, the Honda AWD does it by increasing power to the required wheel (I know I'm over simplifying things, just talking about basic principles here). The Honda system is way more efficient since no energy is lost trough the brake pads. So I do agree that such an AWD system becomes a safety feature, just like DSC for the CX-5.

Sadly, most low/mid range cars with AWD don't have such an intelligent AWD system, nor does the CX-5. So the safety margin between FWD and AWD in our case becomes way smaller.

 

[Beefy]

OMG, and all this time I thought Newton was correct! That means the earth going around the sun is loosing energy all the time untill we fall into it. Oh the humanity!

Newton is correct, his 1st law is inertia exactly as I have described it. The car wants to keep moving forward in a straight line. Centripetal force must be applied to make it turn the corner.

In the case of planets orbiting the sun, gravity is exerting the centripetal force. If gravity was suddenly removed, the Earth would shoot off at a tangent.

 

[vroom]

Newton is correct, his 1st law is inertia exactly as I have described it. The car wants to keep moving forward in a straight line. Centripetal force must be applied to make it turn the corner.

In the case of planets orbiting the sun, gravity is exerting the centripetal force. If gravity was suddenly removed, the Earth would shoot off at a tangent.

Yeah, and remove friction (say with ice) from under a car and it shoots off at a tangent into the ditch, there: we have come full circle back to friction, traction and the initial FWD/AWD discussion. Can we get on with it now...

 

[craigo]

OMG, and all this time I thought Newton was correct! That means the earth going around the sun is loosing energy all the time untill we fall into it. Oh the humanity!

Obviously, I'm not going to go into this any more than I have already, since I agree with Chris_Top_Her that this has gotten way off topic. Those interested can brush up on their physics 101 manuals on their own. I find the link posted by craigo interesting and on topic. I think the Honda system acts with similar principles than the DSC, whereas the DSC controls torque on each individual wheel by using brakes, the Honda AWD does it by increasing power to the required wheel (I know I'm over simplifying things, just talking about basic principles here). The Honda system is way more efficient since no energy is lost trough the brake pads. So I do agree that such an AWD system becomes a safety feature, just like DSC for the CX-5.

Sadly, most low/mid range cars with AWD don't have such an intelligent AWD system, nor does the CX-5. So the safety margin between FWD and AWD in our case becomes way smaller.

I like that it has a component simply called "Acceleration Device".

 

[Beefy]

Yeah, and remove friction (say with ice) from under a car and it shoots off at a tangent into the ditch, there: we have come full circle back to friction, traction and the initial FWD/AWD discussion.

Yes, lets come back to traction.

If the traction of AWD helps in linear forward acceleration, where inertia is trying to hold the vehicle at a stop...... then why would you assume that the traction of an AWD does not help in accelerating around a corner (changing velocity, constant speed), where inertia is trying to hold the vehicle in a straight line?

 

[ALafya]

I think that the car *does* slow down while turning essentially because increased tire friction while making a turn.
I know my car on cruise-control needs to apply more power to keep the same constant speed on turns. Similarly, with my bicycles, they will slow down if I make a sharp turn. While doing so, I can hear the tires making more noise because of the increased friction. This slowdown is definitely noticeable and increases with sharper turns.
I think the model for tires meeting the road while they roll is that of static friction (as opposed to dynamic friction when they lock up on hard breaking with no ABS). However, as you may know low-rolling resistance tires can improve gas millage because there is some energy loss while tires roll.

A good AWD system *can* be safer than a FWD when turning *if* it could reduce the effects of under-steer. Under-steer is safer than over-steer for most drivers, but if the system could result in more balanced steering overall and prevent slipping on the front wheels while applying power, it will be safer. However, I think that the CX-5 AWD will still under-steer since it is FWD biased. Having a better suspension in the CX-5 does make it safer, because it behaves better while turning.

 

[vroom]

Yes ALafya, tire friction would probably slow you down a bit. After all the contact patch of the tires are rotating, and they are far from perfect pivot points. These are the type of effects I was referring to in post 79. The effect is insignificant in a discussion on AWD vs FWD.

You are also correct in stating that tires are in static friction while rolling. But locking up is not the only way to change that to the less efficient dynamic friction. Spinning out is another way. If an AWD applies to much torque to the back wheels, the car can fishtail. An extreme example being doing doughnuts with a RWD. This winter I developed the reflexes of giving it a bit of gas when under steering to engage the back wheels. Sure enough the back end would kick out a bit and I would regain control.

Most drivers don't have that reflex. A good AWD would do this automatically in a more controlled and transparent way.

 

[ALafya]

I mostly agree with you.
Note that "fishtail" is over-steer, which is less safe than under-steer for most drivers (except highly skilled drivers, which prefer over-steer). When applying power while turning a balanced AWD system, i.e. one that reduces under-steer without increasing over-steer too much, could prevent the under-steering effect resulting in a safer ride. That is why applying power while turning close to the limit of traction is safer in a balanced AWD. You apply power while turning to counter the not so negligible effect of slow down in a turn.

However, in the grand scheme of things, I don't dispute that snow tires are far more important than anything else AND CX-5 is FWD biased.

I know this video is not really objective, but it does show the effect of under-steer under an emergency maneuver :
https://www.youtube.com/watch?v=P21lwEYY-D0

Also FWD / AWD / RWD:
https://www.youtube.com/watch?v=wdiF-BxPSH0

 

[CXVille]

Physics 101

Force = Mass * Acceleration
A wall exerts a force on something that runs into it it. Or along it. Or you pushing your car.

Work = Force * Distance
The wall doesn't exert any work because it does not move. The force is at a point. You pushing your car for 10 feet exerts work.

Power = Work/Time
The faster you work, the more power. If you pushed your car 10 feet in a minute, you get X power. If you pushed it that far in 10 seconds, you get 6X power expended.

For an object experiencing centripetal acceleration, the force and the application point does not move. It is a difficult to grasp concept. The Force changes - a new force is applied, similar to running along a wall. No work because the force does not move. It is that particular point on the wall.
For more on this see: This Link.

So, a car, in a perfect world, turning a corner, does not lose speed because of the turn. It loses speed because of miscellaneous friction components in the tires and drive trains. Slip angle and displacement and recovery of the tire are important here.

Back to the AWD vs. FWD - This is partially why the argument that AWD is inherently better than FWD on slippery surfaces is not true. Friction circle also becomes involved, but that is a whole 'nother topic.

 

[vroom]

Thanks CXVille, that gave me a good laugh to start my day! Of course, all of what you have stated is completely accurate. I was avoiding going into so much detail (I mean tire slip angle, wow!) , but now that you have, hopefully we can now move on to informative discussions on the real day-to-day benefits and drawbacks of AWD vs FWD!

 

[ALafya]

Does it mean that under/over-steer is not real? Or that that they have no effect on safety?

 

[CXVille]

Does it mean that under/over-steer is not real? Or that that they have no effect on safety?

No. No.

 

[ALafya]

No. No.

Back to the AWD vs. FWD - This is partially why the argument that AWD is inherently better than FWD on slippery surfaces is not true.

So if AWD can reduce the effects of under-steer, it will be safer, in the specific conditions where under-steer is avoided. Example are the YouTube videos. No?

 

[vroom]

So if AWD can reduce the effects of under-steer, it will be safer, in the specific conditions where under-steer is avoided. Example are the YouTube videos. No?

No one is arguing that a high end AWD (which the CX-5 is not) or in the hands of an expert driver (which the vast majority of drivers are not) can change the driving dynamics enough to affect control in a curve. That implies having control and hence traction in the first place.

What CXVille and I are saying is that AWD will not give you any more lateral traction than FWD. Once your center of mass starts moving towards the ditch, you are going there no matter what drive train you have.

So having AWD written on your car does not necessarily (CXVille used the word inherently remember) make it safer. This is very applicable to the CX5 being discussed here.

Besides, most of CXVille's post was meant to explain (very well) the physics of why, apart from minor rolling resistance and tire deformation effects, the car does not significantly slow down in a curve, making it a moot point in this discussion.