Drive wheels engaged

Back in '72, my high school auto shop teacher taught us the following: In a rear wheel drive truck, 2wd, only one wheel is actually moving the vehicle, usually the RRear. In something like a 4R with part time 4wd, and no lockers, when 4wd is engaged, only two wheels are actually getting power to move the truck, the RR and the FL wheels. A locker on one of the pumpkins would make it a 3wd. A true 4x4 would have have power going to all for wheels. Does this explanation still hold up?

 

^Your shop teacher was wrong both in '72 and in '24. If you're cruising at 60 mph on a dry highway, do you really think only one rear tire is propelling the vehicle? That's contrary to basic logic.

In a RWD open-diff vehicle, if both tires have traction, both tires move the vehicle. If one tire is slipping or in the air, then all the torque goes to that tire and neither tire is moving the vehicle.

 

In 4WD only 2 wheel propel the vehicle?

Oh lord.

So we should call it 1WD and 2WD?

 

Yeah that still holds up. Sounds like the people who posted above this don't know how this works. Your shop teacher is correct. Anything else is arguing semantics.

 

If you have an SR5, yes

 

OP, the answer is it depends.

With equal traction on both sides in 2WD, both rear wheels are going to be powered as the differential splits power evenly.

There might be some small bias to one side or another but I’m going to call it negligible and dependent on specifics outside the scope of this thread. So focus on where the traction is lost rather than where the power is going since the differential divvy’s it up anyway.

Lose traction on one side entirely and the open differential will spin one wheel while the other remains stationary - doesn’t matter which side loses traction, the result will be the same. Hence, the minimum of 1WD in 2H.

Lockers, limited-slip differentials and traction control (using brakes to stop/slow slipping wheels) are different ways to keep at least some power going to the wheel with traction. So a 5th gen in 2H would be more like 1.2WD depending on how aggressive traction control is at braking the spinning wheel. (A-TRAC available in 4L is much more aggressive.)

With open differentials front/rear and none of the three potential mitigations above, at least one wheel on each axle will get power in 4WD, hence the minimum of two driven wheels. But which wheel spins depends on available traction/terrain.

As for “true 4x4” that’s more semantics but of course having mechanical traction aids front and rear would sure be handy.

 

As stated before, it depends on traction. Power will take the path of least resistance, so if one tire has less traction than the other than all the power will flow to the tire without traction. If only one tire was driven at all times only one tire would spin when you lifted the vehicle off the ground but both spin.

 

It's the same since 1972 and the same since 1937

https://youtu.be/yYAw79386WI?si=klgpatlgpLB1yIRx

 

Your teacher was sort of right, some of the time.

 

I like how over almost 100 years, this is still the best explanation for how a differential works.

 

If only one tire was driving the vehicle you'd have a pull everywhere you go. If both tires have traction, they both drive the vehicle. If one does not, neither drives the vehicle because power takes the path of least resistance and will go to the wheel without traction.

What they might have been referring to is that one wheel is always guaranteed torque on each axle as covered in the excellent video above. With a 4wd, since the transfer case is locked, one front and one rear tire will always be guaranteed half the available torque. Lockers just guarantee an even distribution of torque at any given time regardless of traction by taking the differential out of the equation.

 

It's a very common and extremely well used over simplification for a complicated thing which no one here has touched on all aspects of. For the sake of lay conversation the oversimplification is plenty good enough to cover such a complicated thing.

You're over thinking it. 4x4 with lockers you are guaranteed to have 4 wheels spin. 4x4 no lockers you are guaranteed to have 2 wheels spin. 2wd no locker you are guaranteed 1 wheel spin. It summarizes this very well that anyone can understand.

It's not really saying power only goes to 1 wheel. But if we want to get into the weeds, 2wd can be worse than 1wd since with 2wd power now has 2 options and will always pick the path of least resistance meaning it has more probability to find tire spin than 1wd.

So instead of saying all that and more and causing more confusion people shorten it to a sentence that makes enough sense.


Take electricity. This is super complicated and almost no one understands it. I sure don't. When we talk about DC power current flows from positive battery to the thing that is being powered. We use the the length of the wire and the amps that will run through it to determine how thick a wire we need. Pretty simple. We do not take into consideration the electromagnetic and gravitational fields around the wire or inside of the wire which are actually moving electrons from the negative terminal of the battery in a loop over to the positive terminal replenishing the battery of its electrons in the opposite direction of the current which means that the actual power doesn't even come from the electrons....WTF? You could do pages of math on these fields to determine the required thickness of the cable but instead we use ultra simplified versions.

 

This isn't really correct. If both wheels have traction, they're both propelling the vehicle.

In 4wd, there isn't a specific tire on each axle getting power. Both tires on each axle are getting power, unless one loses traction.

 

But that's not what was being said and what you agreed was correct.

 

I’m guessing the people that think only one wheel is being driven at all times in 2wd ruined their oil filter housing, changed their PCV valve as part of maintenance and use blackstone labs for oil testing just to post the results but not understand them.

 

Here’s how it was explained to me. Speaking now of 2 wheel drive with an open differential:

Imagine you hang a shelf from the ceiling on 4 cables. The cables have zero stretch. As hard as you try to make the cables all exactly the same length, there will be some difference, however slight. Think of the cables as pairs - each cable is paired with its diagonal opposite. The pair with the shortest combined length will bear the load.

Applying this to our 2WD axle, each wheel represents one of the diagonal pairs. So only 1 wheel at a time is bearing the load. Where the analogy falls apart is the fact that while the suspended shelf is a static system, the axle is dynamic. So while only one wheel is being driven at a time, which wheel is being driven is constantly changing, perhaps more than once per rotation of the axle, depending on steering input, road surface, and the crown/camber of the road. So while the assertion that only 1 wheel is driven may be technically correct, because it’s constantly changing, in practice it’s irrelevant.

 

You too huh.

 

No craptor lights here.

 

Seems like a lot of people are really over-thinking this. Just because one is carrying more weight, or has more traction than the other at a specific moment, does not mean that it is the only wheel propelling the vehicle.

 

Its called one tire fire

 

I'm saying the old adage is correct.

It's not correct. It's complicated. Most people think what 2WD is, is actually 2WD with a locker or LSD, which it isn't.

The old adage is correct insofar as it gets people to understand 2WD is not 2WD with a locker or LSD....which most people don't know what a locker or LSD is either.

So without going into lengthy monotonous explanation which would bore most people and cause them to stop paying attention, the old adage does a very good job getting them on a more accurate track in just a sentence.

It's correct in what it does.

 

Fang lights would complete your front end.

 

Would it be inappropriate to link the fang light gofundme here?

 

Back to the OP’s question; I don’t know, ask the guy who took an Auto Shop class!