Do I understand 4x4 wrong?

So from what I understand, 2WD works like this:

100% of the torque is given to the back differential, which is then split into 50% torque for each rear wheel. If you lose traction (i.e. one wheel is in the air) then you will be stuck because it bases how much torque it gives (in total) based on the wheel with the LOWER traction. Meaning that even the wheel that has good traction on the ground is only getting the small amount of torque the truck is giving to the airborne wheel (in an attempt to get it to catch).

Then, onto 4WD/4x4:

100% of the torque is split between the front differential and the back. Meaning 50% to each. From there, the torque is split again, 25% per wheel (front and back). This means that all 4 wheels will be getting 25% of the total torque.

If, for example, you lose traction in the rear (i.e. one wheel is in the air), the issue of losing power to the back wheels is compensated by the locking center differential giving 50% of the torque to the front. Meaning that since the front is still getting a sufficient amount of torque, the back will be able to be carried through by the front (even if they both are receiving very low torque due to the low traction wheel) since the open differential allows all of the wheels to spin freely.

Putting on the diff lock included in TRD OR models, you eliminate the issue of an airborne wheel, since locking the diff guarantees that both wheels will spin at the same speed no matter what. So this is good for situations where you may have more than one airborne wheel at a time, because you can rely on the fact that at least one wheel will have power as long as one of the ones touching the ground is in the rear.



I've been writing this for some time and am now on a time crunch, so if some of this sounds stupid or doesn't make sense I'm sorry. Just asking for some clarification on this.

 

2WD:

Basically with an open differential, both wheels receive equal torque all the time. When you get a tire in the air, the torque required to spin that tire in the air is the amount of torque both sides receive via the spider gears. Since spinning a tire in the air requires much less torque than spinning the one on the ground with truck weight on it, you don’t go anywhere, you just spin the one in the air.

Systems like brake actuated limited slip help to combat this by using the brakes to stop the spinning wheel so that the torque required to spin the other wheel can build and move the truck forward on that wheel.

4WD:

The transfer case put in the 4WD modes means the front and rear see equal speeds, similar to your comment about how the locking diff means the locked wheels see equal speeds. This is because the front and rear are locked together via the chain and sprockets.

It’s not equal torque, because it depends on how much resistance there is at the front or rear of the truck. If you could somehow get the truck up on its rear wheels only like a wheelie, then the rear would be receiving basically all the torque. But both diffs would be spinning at the same speed. But since the front wheels would have no load or are not doing any work, there is essentially little to no torque going to those wheels.

 

you're basing this on what appears to be experience with a variable or adjustable driveline, possibly with something that has torque vectoring? the tacoma is not that. it's brutish, basic, and mechanical. there's no fancy schmancy computers mucking up power and deciding anything.

4x4 is a locked mode. 100% of the power is distributed to 100% of the wheels. if any one entire axle is in the air, it doesn't matter, as long as both wheels of the other axle still have contact, that axle is getting 100% power. essentially, the axles are redundant power delivery systems.

but because the axles are locked together, that also means that the system should not be used while turning, as each wheel will travel a different distance, which will cause binding within the drivetrain.

 

Just because the hardware all is locked to the same speed, doesn’t mean it all receives the same torque. No, I am not talking about anything variable or adjustable.

Imagine you’re in 4wd and remove the front driveshaft. All of your torque will be going to the rear despite there being a front tcase flange that’s tied to the rear via the chain. It’s no different when a driveshaft is installed, the front and rear will see different amounts of torque based on how much load the front vs rear diffs and their wheels are seeing.

 

In 2wd power only goes to the rear wheels. But unless you have the rear axle locked power is not equal between the 2. The reason for this is because when you turn a corner the outside wheel has much farther to travel and must be able to turn much faster than the one on the inside. When in low traction situations power typically goes to the wheel with the least traction. It doesn't have to be off the ground but that is one example.

Traction control uses wheel speed sensors to determine that one wheel is turning much faster than the other and applies brake pressure to the spinning wheel tricking the diff to send power to the wheel with traction. An old school trick that was commonly used before computers and traction control was to lightly apply the parking brake to do the same thing.

Locking the rear diff sends power equally to both wheels and will either get you out of a bad situation, or get you stuck twice as bad. If locked where traction is good, and you turn a corner you could damage something. Hopefully a U-Joint will break before the diff.

With true 4X4 systems the transfer case sends power equally to both axles. Each axle then does exactly the same thing as they would do in 2wd. But turning corners is even harder if traction is good. Even with open diff's you will have equal power to one rear, and one front wheel. And there is a much greater speed difference between the inside front wheel and outside rear wheel when cornering. True 4X4 should only be used on surfaces where traction is poor and even then it is best to avoid sharp corners.

Being able to lock the rear axle while in 4X4 is pretty common on newer vehicles. There are only a handful are made that will also allow you to lock both front and rear axles from the factory. That is usually done aftermarket if at all. If not driven carefully you can do serious damage with both front and rear axles locked.

All Wheel drive vehicles use an open transfer case. They will send power to all 4 wheels, but the front and rear axles are not locked together. They are safe to drive on any road surface and are actually great in light snow and wet pavement. AWD isn't nearly as good in harsh off road conditions since the front and rear axles can't be locked together.

Some trucks will allow you to operate in 2wd and with the flip of a switch either AWD or true 4X4. I've never seen that option on a Tacoma and it is usually only put on higher end SUV's and trucks.

 

Agree!

 

i get what you're trying to say, but i feel it's falling off the mark of the original post. there's a difference between an axle receiving the power and an axle putting it down to the ground. if the truck is doing a wheelie in 4x4, technically, only the rear axle is applying torque to the ground to move the truck forward, but the front axle is still receiving enough rotational force to spin the driveline, it just doesn't offer any forward momentum force--the front axle in this case is still receiving 100% torque, but without the ground contact, frictional losses are minimal, the front axle in this case only needs to overcome the rotational losses. when the axles are locked, that's it-- the differentials operate as a singular unit, despite driving 2 independent pairs of tires. at that point, the application of torque really has to do with the grip characteristics underneath each axle/tire, the truck isn't dividing it, it's just outputting the power.

 

The wheelie was just an example, and probably not the best because it’s not realistic. I was just trying to demonstrate for a transfer case. Since 4x4 locks the front/rear together the like a diff-locker locks the left and right wheels together, we can find an similar example/concept in a diff-locked axle. If you had a locker engaged and had one tire on the ground, all of the torque is going to go to that wheel. The wheel in the air is locked via the locker and will go the same speed because of that, but since it’s not on the ground, it will receive very little torque.

Put another way, with that same locked rear axle from above, which rear axle shaft would you expect to break? If they’re both experiencing equal torque, then it’s a 50/50 chance right? Nope….the one on the ground is the one receiving the torque, and it’s the one that will break if it gets to that point.

 

Not sure if this is to bash my understanding or whatever, but was just trying to ask a genuine question because I’ve been getting conflicting answers online. Just bought my first 4x4 truck and want to be informed

 

I see. Thanks for the reply

 

just to confirm, Tacomas have true 4x4 correct?

 

so if you had two wheels (one front and one back) in the air would you be stuck without traction control? (I know this is an unrealistic situation)

 

The best way to answer this is to put your Truck in 4wd and get stuck

 

So is the truck putting the same amount of power/torque to both axles you’re saying? If so, wouldn’t the front wheels be spinning WAY too fast?

 

You have to chuckle at the way people convey their answers/opinion.

The way I look at 4X4 is, when 4H/4L engaged...all 4 wheels are spinning at the same time.

Don't use 4X4 when making turns...that damages the gears!

Get familiar with the owner's manual...if you haven't been directed to do so.

And...definitely, take this platform with a HUGE grain of salt. lol

 

Now you have me curious what a faux by four would be. That's probably what I drive.

Here's are the types of four wheel drive:

• Part time 4x4: there is a two speed selector between 2WD and 4WD. When engaged the front and rear prop shafts are mechanically locked together. The Tacoma goes here.
• Part time 4x4 with AWD: a 2 speed transfer case that can swap between 2WD, 4x4 with open center, or 4x4 with locked center. I have seen exactly one truck that does this.
• Full time 4x4: it's an older code sir, but it checks out. Still a 2 speed transfer case, but it swaps between open center, locked center, and 4Lo locked. Did not have 2WD. Some old 1970s trucks were like this.
• AWD: full time four wheel drive with fully open center. Can electronically shift power between front and rear. Does not possess a mechanical lock or a low range. Think Honda crv.
• Ex4: one axle is driven by gasoline driveshaft, the other is driven by electric motor. Mode shifting is done by hybrid voodoo.

 

I'm Dead...