Do I understand 4x4 wrong?

Atrac, vsc and your abs also come into play.. Tacoma has a basic traction system which will engage the brakes through atrac/vsc to vector any traction loss.. your principle is solid but there are many electronic variables that delineate the overall efficiency of the drive line.

 

I just skimmed over it.
It might give OP a better understanding of how the system works.
Then he can come back here with somewhat better understanding and ask any other questions he has.

 

I thought he demonstrated a very good general understanding (yes, with common misconceptions mixed in) and was looking to get a technically correct understanding of the details.

 

Think of it this way, the contact between tire and ground can be considered equal at all four wheels... up to the point until one tire starts slipping. The moment you have a loss in traction or tire slip is when resistance becomes unequal between wheels.

 

I’ll delete my post with the link it.
You can explain it to him.

 

You might want to delete your post where you quoted me.
As the quote still contains the link to the incorrect information.

 

So:

drivers side front wheel is getting little traction, bringing the amount of torque the truck gives to that axle to a lower amount. This means that the passenger side front wheel is practically not turning (EDIT: not that it's "not turning. In this situation, the truck would realize this and direct more torque (practically 100% at this point) to the rear until the drivers side front tire caught traction? then it would even out again?

EDIT: "even out" implies that once the drivers side tire has traction, both axles are dealing with same resistance.

 

Closer. It's only forcing same revs at each axle input (not the wheels). So the torque front to rear is based on what's available in total at each axle. The open diff plays into what's available at a given axle as seen in my one example.

This torque applied to each axle is then distributed equally to each wheel. The individual wheel speeds may or may not be equal but they just must obey the speed constraints imposed by the open differential.

I don't like using "path of least resistance" because while it helps intuitively it's not what actually happens. Technically it's that a low grip tire just limits the torque an entire axle can receive with an open diff and thus limits torque at the higher traction tire. The excess torque becomes angular acceleration and spin-up of the low grip tire and powertrain. Nothing is following a path of least resistance, it's just obeying the limitations of the mechanics.

 

No.

 

Generally yes, this is sounding correct.

 

the reality is that to be in that situation, your frame is hung up or dragging on something--there's almost no off-camber situation where 2 wheels can leave ground contact and the frame won't drag.

the initial solution here, as BLtheP suggested, is to install locking front/rear differentials. but the reality is that it further stresses the axles, and this is the situation that people generally start snapping driveline parts. by the time you're so buried in something that 4x4 can freespin a front and rear wheel, plain and simple, you need to be winched out of that situation. more importantly, you screwed up, got yourself further in than you intended, and you need to stop, take a breath, and re-assess the situation to properly AND SAFELY get back out.

this is where i was indicating to BLtheP that the conversation is going further than you intended. he's right, but we're getting into more the complex physics of the sitatuation, which involves grip and load transference.

the front wheels can't over-spin the rear wheels, they just provide no forward momentum. they're locked together. think 2-300 pound guys pulling a semi truck-- they both agree to pull the truck, and they both can introduce a full 300 pounds of pulling force onto the truck(realistically more given muscle mass and such, but i'm keeping the maths simple).

but if one of those guys decides to eat a cheese burger while pulling, he can't impose his full 300 pounds onto the rope to pull the truck. so now the other guy might be inputting his full 300 pounds, and Mr. Cheeseburger is pulling with 100 pounds of force. the semi truck will move less because there's less torque being input into the pulling force, despite the 2 guys(engine) being the same size.

 

Open diff output torque is always 50/50.

Transfer case is locked and therefore is capable of 100% torque bias.

These are fundamental, known, tested, and proven properties.

 

Yes, basically. Both completely in the air is kinda uncommon (truck needs to be balancing on the two wheels on the ground or supported at a 3rd point which would be the chassis) but the similar case which is very low grip and loss of traction at both a front and rear wheel is very common. This is exactly the case when you see someone driving up an loose or slick irregular hill in 4wd cease forward progress, lose momentum, and start spinning. With an open diff it only takes one wheel with no traction to limit the total torque to that axle. Have that happen at both ends and now no more torque reaches the ground and it just becomes powertrain spin-up.

Of course there are simple solutions to this, as others are suggesting, but it seems to me the question at hand is understanding the dynamics, not solving the traction issue we've uncovered.

 

so the torque provided to each axle is different because they may require different amounts to force the same revs/axle spinning speeds.

edit: sorry to open this up again

 

Yes, different torques to each axle. It's not that the forces are such to keep the speeds equal, it's really the other way around. The speed constraint requires that an individual axle cannot get excessive torque and spin up until both axles have breached their tractive limit and thus both spin up.

 

The title of this thread should be “do I understand differentials wrong?”. Differentials are complicated. Part time 4wd is simple. The entire discussion could be about a Corolla and lose very little.

 

I found this explanation done well.

That "Spider Gear is what makes the difference!

 

Not to drive this closer to a dumpster fire, but I think there are a lot of misconceptions about torque including herein. To back up to force balances, aside from frictional or inertial losses, the tangental forces occurring where each tire contacts the road must balance (see: free body diagram). That is, the propulsive force on the ground under each tire has to match the force exerted by each tire, which multiplied by the wheel radius gives torque to/from that wheel. Thus torque only transfers to wheels that have mechanical resistance which cause tangental forces that must be balanced. Since an open diff gives a torque matching condition irrespective of speed, if one wheel on an open diff is not exerting force to the ground, then neither wheel on that open diff is transferring torque.

Also keep in mind that power
is torque X RPMs so if no propulsive force and thus no torque is transferred between a wheel and the ground, then neither is power.

force: mass X acceleration
torque: force X wheel radius
power: torque X RPMs

If you want to get into some losses, the spinning wheels store energy as 1/2 Iw^2 where I is the moment of inertia and w is rotational rate. You can find how much torque it takes to spin up the wheel energy by looking at how much the stored energy changes over time which is power and relating that back to torque times RPMs (equals power) and summing up the torque X RPMs over the time you used to look at change of the stored energy in the wheel.

Not sure if this helps, but it seemed like this was a thread approaching kinematics and really needed the equations for force, torque, and power to show how they relate.

 

RWD Benz wears the right rear tire first due to their open diffs having more power go there.
There is a reason one wheel peel exists.

you have what you have. If you have open you have open.
Open is the cheapest diff to exist, and exists to allow turning.
Anything better costs more. Like a Torsen or helical LSD both of which are offered on trucks.
Nobody’s wheeling with their diff set open.

the drawback is having a front diff that is solely open without locking. On a limited traction IFS truck, when one wheel comes off the ground, it renders the whole front of the truck useless.
how much that matters. I don’t know. Because usually such cases happen when the truck is going up at an angle and weight is on the rear anyway.

however given that they make LSD type of upgrades for the front, that would be better than solely open, if not getting air or elocker.

I don’t know if there are workarounds. If the truck applying brake in such a scenario to a spinning left front would allow power to go back to the right front with traction. Which is what an LSD would do by manually touching the brakes in older trucks that don’t have any electronics beyond ABS.