Two wheel drive vs 4 wheel drive (Torque, traction?) 1

[TrustButVerify]

Hello folks,

So, just imagen you have two commercial trucks, one with 2-wheel-drive and another with 4-wheel-drive, and their WEIGTH is the same.

Great, they are being manufactured to be able to be driven on a flat surface, both of them.. Like driving on the road.

Now, this is the question... Imagine you start scrapping both trucks the same way, I mean, you start to take weight off the truck, like reducing their self-weight...

Can you find the situation that the 4-wheel-drive truck will NOT go forward due to lack of weight on each axle (and thus less friction), but to start SLIDING due to much torque?

The 2-wheel drive truck will be able to go forward since the two wheels need to overcome their own resistance, plus the resistance of the other two wheels?

Where I can read about this?

Have a nice weekend.

 

[MintJulep]

Yes.
No.
Any dynamics textbook.

 

[TrustButVerify]

Are you sure? In reality, I've seen the opposite.

I want to understand the science behind.. but for sure. I've seen the opposite.

Something I would like to add (just in case it is relevant): the mechanism driving the tires is not mechanic, but hydraulic. There is a driven engine (planetarium), inside each driven tire.

So two trocks like this:

One truck, 4 wheels, two of them has driven engine inside the wheel.

One truck, 4 wheels, four of them has driven engine inside the wheel.

 

[LittleInch]

Are you sure?

If the two trucks are equal and hence need the same force to accelerate or move, then the force for the 4WD on each wheel will be half what it is for the 2WD version.

too many unknowns here to give you a good answer. Formulate your question better.


Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[TrustButVerify]

But I just noticed you changed your answer, first it was No, No.

The trucks are the same.

My question is:

If you start reducing the same weight in each truck, will you reach the situation when the 4 wheel driven wheel will start sliding due to lack of friction force to start moving, AND the 2-wheel drive truck will be able to go forward since the two wheels need to overcome their own resistance, plus the resistance of the other two wheels? Surface for friction remains the same in both trucks. Four wheels always in contact with the road.

 

[MintJulep]

It will always be possible for enough torque to spin the driven wheels.

Any weight.

Any number of driven wheels.

 

[TrustButVerify]

To spin, to slide, or to go forward? I think there is a difference.

The torque will make the tyre spin, but can it make the tyre to go forward, instead of sliding?

Again, I've seen this in reality, but I'm trying to understand the science behind.

I used commercial truck as an example, in reality I saw a heavy-lift equipment being employed to transport heavy-load cargo above them. It is design to carry 1000% more weight than their self weight. Thats why sometimes the four wheels driven engine is needed, but when the equipment is empty, four wheel driven engine start to slide.

 

[CapriRacer]

I am puzzled by your question. You talk about trucks, then mention planetarium.

If we are talking about trucks on the road, you'll get one answer (roads have texture, and therefore grip from the tires).

But if we are talking about smooth, untextured surfaces like a steel plate, then the answer will be different.

So I think you need to give us a better description of what you are trying to understand. Be as specific as possible.

 

[MintJulep]

Draw a FBD of a wheel.

It's a dynamics problem, so you need to include linear and angular inertia forces.

 

[MintJulep]

But if we are talking about smooth, untextured surfaces like a steel plate, then the answer will be different.

Umm. No.

The physics is exactly the same. The equations are exactly the same.

 

[TrustButVerify]

I will try to look for a picture, and the share that later today. I will think more about my wording as well.

 

[CapriRacer]

Thinking about this a bit further, the issue is the amount to torque being supplied to wheels. If the torque is capable of being throttled down to zero, then even at zero load, a tire on a road surface will have enough grip not to spin the wheels=- regardless of whether the truck is 2WD or 4WD.

But if the torque is not capable of being throttled down to zero, there will obviously be a load below which the tires will slip - regardless of whether the vehicle is 2WD or 4WD.

 

[MintJulep]

Spinning = rotation without linear motion.

Sliding = linear motion without rotation.

Of course torque can make a vehicle move forward without spinning the wheels. See most any ordinary car on most any ordinary day.

 

[CapriRacer]

Umm. No.

The physics is exactly the same. The equations are exactly the same.

Unfortunately tires on a road surface don't follow conventional friction theory, so the physics is different.

 

[TrustButVerify]

Hope this is a better explanation of the situation.

 

[3DDave]

This appears to be a misrepresentation of some occurrence.

Can we play the 20 questions game in order to finally discover why this seems to be a reasonable situation?

Are the trucks, tires, CG location, operating slope, driver habit, operating surface, temperature of tires, temperature of surface, contamination of surface - all verified to be identical? If so, list the measurements made to confirm these are true.

11 questions to go.

 

[TugboatEng]

Practically, tractive effort for a 4x4 vehicle on dry pavement is equal to the weight of the vehicle. For a 2 wheel drive vehicle the tractive effort would be equal to the axle load of the driving axle.

 

[TrustButVerify]

All identical

 

[3DDave]

No data = not identical.

 

[TrustButVerify]

It's not a misrepresentation of some ocurrence, this is something that has happened many times in several locations with the facts provided in the paper. You don't need to answer the question you are formulation to come up with a conclusion.