Center differential + rear differential + front differential = confusion!

[Dustinhxc]

This helps me alot! Thanks everyone who explained how it all works. I still can't decide if I want full time or not. But this clears it up of how it works.

 

[1engineer]

Very informative. I was not clear on the differential differences either.

This helps me alot! Thanks everyone who explained how it all works. I still can't decide if I want full time or not. But this clears it up of how it works.

There are literally hundreds of informative threads just like this on most 5th gen 4Runner topics. All you have to do is google your question preceded by 5th Gen 4Runner. From 2010-2013 or so the how-to's, technical explanations and just great posts were being written daily. Now, while occasionally someone writes something good, most of the new threads are just a re-hash.

 

[willie2323]

Locking Axle Differentials. Only the Trail Edition, a part-time 4WD, comes standard with a rear differential locker. Some people install after-market lockers.

Hope this helps.

great info put very well in this thread...
no biggie but the pro also has the rear diff locker.

here is some great reading on our 4wd options, its applicable to the 5th gen units.

http://www.toyotanation.com/forum/6...-2015/300984-clarification-needed-please.html


cheers all,
willie

 

[Roof Shingles]

Okay, I was typing my reply when I saw Ryan S's pop up...so just to add to what he is saying:

I had the same confusion when I bought my first 4x4. I have an 08 limited so it’s not a 5th gen, but the principle is the same. You basically have 3 open differentials. An open differential allows the wheels to spin at different rates so when you are turning the inside wheel spins slower then the outside wheel. This keeps the vehicle from hopping on high traction surfaces like a dry road where the tires don’t slip. In 4 wheel drive mode, or full time 4 wheel drive, the center differential is again an open differential to allows the front wheels to spin at a different rate then the rear wheels. So when you are in 4 wheel drive and turning each of the 4 wheels can spin at a different rate. Without open differentials you would bind the differentials causing damage.

Now when you are in “all time” or “full time” 4 wheel drive, e.g. open differentials, you can still get stuck because power will be sent to the wheel with the least traction. So you could basically have 1wheel out of the 4 spinning, be it a front or rear wheel. This is where locking differentials come into play.

A locking differential is just that. It locks the two drive shafts together such that the drive shafts turn at the same rate. Take the center locking differential. When the center diff is locked, both the rear drive shaft and front drive shaft will spin at the same rate. In this case if you were stuck you would have 1 front wheel spinning and 1 rear wheel spinning. This is because you still have open differentials in the front and rear.

Same goes for the rear locking differential. Locking a rear differential will lock the two rear wheels so they spin at the same rate. If you had your center differential locked and also had the rear differential locked you would effectively be a “3” wheel drive vehicle, the two rear wheels and at least 1 front wheel. Add a locker to the front differential and you would be a “true” 4 wheel drive vehicle.

Remember when you lock a differential: rear, center or front, you take away the ability for the drive shafts to spin at different rates which is needed for turning on high traction surfaces. So you should only lock a differential when your wheels can slip to release the binding.

I hope helps and makes sense.

Thanks... I had a 4WD Escape some months ago and ya I was a bit confused when taking my T4R SR5 on sharp turns with H4 engaged. The Escape never did that jerking motion, but I guess it had opened differentials, right? I understood differentials enough to know what was going on with the T4R, but I certainly didn't like it.

I guess what I'm reading is that the locked differentials of the SR5 is better at getting unstuck. Can someone explain that though? If a wheel is spinning (doesn't have traction), don't you want to give it less energy/power? With locked differentials you are always giving that spinning wheel energy. If you tell me that with locked differentials you are giving each wheel 100% of the power, I could understand that. And in the case of 4WD each wheel gets 25% of the energy, I could understand and get behind that... but is that how it works?

Also, am I to think that we are suppose to keep it on H2 pretty much all the time and H4 should only be engaged when we're stuck in a low traction area?

 

[1engineer]

Thanks... I had a 4WD Escape some months ago and ya I was a bit confused when taking my T4R SR5 on sharp turns with H4 engaged. The Escape never did that jerking motion, but I guess it had opened differentials, right? I understood differentials enough to know what was going on with the T4R, but I certainly didn't like it.

I guess what I'm reading is that the locked differentials of the SR5 is better at getting unstuck. Can someone explain that though? If a wheel is spinning (doesn't have traction), don't you want to give it less energy/power? With locked differentials you are always giving that spinning wheel energy. If you tell me that with locked differentials you are giving each wheel 100% of the power, I could understand that. And in the case of 4WD each wheel gets 25% of the energy, I could understand and get behind that... but is that how it works?

Also, am I to think that we are suppose to keep it on H2 pretty much all the time and H4 should only be engaged when we're stuck in a low traction area?

Do a search for ATRAC...

 

[Roof Shingles]

Sigh... I did... and now I'm confused about that too...

 

[1engineer]

Sigh... I did... and now I'm confused about that too...

Very simply - the spinning wheels you talked about above? It applies the brakes to them so torque is now going to the wheels that need it.

 

[Roof Shingles]

I've got a couple of question...


Firstly, regarding the ATRAC, I can't get my head around the bit where more power is delivered to wheels that have traction. In a locked differential setup as the SR5 has, don't all wheels get the same power regardless if they are slipping or not. That's how understand it, why is that wrong? Like the abs is applied to the slipping wheel, got it, but I don't see how with a locked differential system that gives other wheels more power.


2015 SR5 have locked centre differentials right?

Do they also have locked rear differentials?

When they are in H2, is it the wheels in the front or back that have power (I think front)?

Assuming H2 uses front wheel drive, I can't feel any jerking motion on turns so it has an open differential in the front right or is opened only during H2?

Assuming what I wrote above is correct, does that front differential get locked in H4?

Or maybe a better question (than those above) is, does the 5th gen T4R SR5 have front, back or centre differentials locked, and are they opened depending if H2 is engaged?

And most importantly, I just got my 2015 T4R... How do I find these things out on my own? The owner's manual doesn't have any of this stuff, well it explains but also says 'if equipped', so how do I know if it is equipped?


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[(none)]

I've got a couple of question...


Firstly, regarding the ATRAC, I can't get my head around the bit where more power is delivered to wheels that have traction. In a locked differential setup as the SR5 has, don't all wheels get the same power regardless if they are slipping or not. That's how understand it, why is that wrong? Like the abs is applied to the slipping wheel, got it, but I don't see how with a locked differential system that gives other wheels more power.

The SR5s (all 5th gens, except limiteds) have 2 differentials. One front, one rear. Both are open differentials (the rear is able to locked on TRDs/Trails, will get into that later). Open differentials always send the power to the wheel with the LEAST resistance (just like electricity or water). ATRAC (and all brake based traction control systems) takes advantage of the open diff's tendencies to transfer power. Say your left wheel is on ice and the right wheel on dry pavement. The left wheel slips. The wheel sensors indicate that difference in wheel speed to the vehicle computer. The computer then puts out the signal to apply braking pressure to just that left wheel. That brake pressure makes that spinning wheel more difficult to turn. What happens when a wheel becomes difficult to spin? The differential sends power to the side that is easier to turn (hopefully with more traction). This is a major simplification, but at least gives you an idea. All traction control systems work on the same basic principles.

2015 SR5 have locked centre differentials right?

The transfer case can be locked into 4wd. There is no real center diff, as the power can only ever be sent only to the rear or at a 50/50 split to the front and rear. This is the same on about all TRUE part time 4wd systems

Do they also have locked rear differentials?

The Trails/TRDPro/TRDs all have locking rear diffs. The SR5s and limited models do NOT have locking rear diffs.

When they are in H2, is it the wheels in the front or back that have power (I think front)?

The rear wheels have 100% of the power when in H2

Assuming H2 uses front wheel drive, I can't feel any jerking motion on turns so it has an open differential in the front right or is opened only during H2?

See above, no power up front

Assuming what I wrote above is correct, does that front differential get locked in H4?

No locking front diff. When in H4 (or L4), the transfer case is locked, power is sent to the front differential. The differential then sends the power to the front wheels via the open diff (the wheels are free to spin at different speeds. The wheel with the least traction will receive power)

Or maybe a better question (than those above) is, does the 5th gen T4R SR5 have front, back or centre differentials locked, and are they opened depending if H2 is engaged?

As above. only 2 diffs, both open.

And most importantly, I just got my 2015 T4R... How do I find these things out on my own? The owner's manual doesn't have any of this stuff, well it explains but also says 'if equipped', so how do I know if it is equipped?

Keep researching. There are some good youtube videos explaining 4wd systems. Each manufacturer likes to act as though they have all re-invented it and it sooo much better than everyone elses. Most are nearly all the same except for small differences. Once you can comprehend how a differential works, you'll get a much better understanding.

The fact is: In 2wd, power is being sent to the rear diff. The rear diff sends power to the wheel with the least traction. You can get stuck with one wheel in the mud and only that one will spin.
In 4HI and Lo, power is being send to the front and rear diffs equally, all of the time. This is what causes the binding while turning tightly on high-traction surfaces, all 4 wheels are wanting to turn at slightly different speeds, but 2 (one front and one rear) are being forced to always spin at the exact same speed. Again, power is being send to open diffs (front and rear). At any time, only 2 wheels (one front and one rear) are required to get power. If you go over an obstacle which causes 1 front and 1 rear wheel to come off the ground, you aren't moving any more.
That is the purpose of the traction control (ATRAC, TRAC, whatever), to help assist in those situations. It works ok most of the time.
A locked diff would make sure that power is equally split between the both sides 50/50. So, with a locked rear diff in 4wd, you are guaranteed to have at least 3 wheels (one front and both rear) will have power and will be turning the exact same speed at all times.

 

[Toffees]

Thanks... I had a 4WD Escape some months ago and ya I was a bit confused when taking my T4R SR5 on sharp turns with H4 engaged. The Escape never did that jerking motion, but I guess it had opened differentials, right? I understood differentials enough to know what was going on with the T4R, but I certainly didn't like it.

I guess what I'm reading is that the locked differentials of the SR5 is better at getting unstuck. Can someone explain that though? If a wheel is spinning (doesn't have traction), don't you want to give it less energy/power? With locked differentials you are always giving that spinning wheel energy. If you tell me that with locked differentials you are giving each wheel 100% of the power, I could understand that. And in the case of 4WD each wheel gets 25% of the energy, I could understand and get behind that... but is that how it works?

Also, am I to think that we are suppose to keep it on H2 pretty much all the time and H4 should only be engaged when we're stuck in a low traction area?

Not that I've looked into this, but I'd guess your escape was AWD. Would be useful to learn these basic differences between AWD and 4WD as well. That might help give some perspective on the particulars in this thread. (Which has some good, helpful responses throughout.)

 

[1engineer]

The SR5s (all 5th gens, except limiteds) have 2 differentials. One front, one rear. Both are open differentials (the rear is able to locked on TRDs/Trails, will get into that later). Open differentials always send the power to the wheel with the LEAST resistance (just like electricity or water). ATRAC (and all brake based traction control systems) takes advantage of the open diff's tendencies to transfer power. Say your left wheel is on ice and the right wheel on dry pavement. The left wheel slips. The wheel sensors indicate that difference in wheel speed to the vehicle computer. The computer then puts out the signal to apply braking pressure to just that left wheel. That brake pressure makes that spinning wheel more difficult to turn. What happens when a wheel becomes difficult to spin? The differential sends power to the side that is easier to turn (hopefully with more traction). This is a major simplification, but at least gives you an idea. All traction control systems work on the same basic principles.




The transfer case can be locked into 4wd. There is no real center diff, as the power can only ever be sent only to the rear or at a 50/50 split to the front and rear. This is the same on about all TRUE part time 4wd systems


The Trails/TRDPro/TRDs all have locking rear diffs. The SR5s and limited models do NOT have locking rear diffs.


The rear wheels have 100% of the power when in H2


See above, no power up front


No locking front diff. When in H4 (or L4), the transfer case is locked, power is sent to the front differential. The differential then sends the power to the front wheels via the open diff (the wheels are free to spin at different speeds. The wheel with the least traction will receive power)



As above. only 2 diffs, both open.




Keep researching. There are some good youtube videos explaining 4wd systems. Each manufacturer likes to act as though they have all re-invented it and it sooo much better than everyone elses. Most are nearly all the same except for small differences. Once you can comprehend how a differential works, you'll get a much better understanding.

The fact is: In 2wd, power is being sent to the rear diff. The rear diff sends power to the wheel with the least traction. You can get stuck with one wheel in the mud and only that one will spin.
In 4HI and Lo, power is being send to the front and rear diffs equally, all of the time. This is what causes the binding while turning tightly on high-traction surfaces, all 4 wheels are wanting to turn at slightly different speeds, but 2 (one front and one rear) are being forced to always spin at the exact same speed. Again, power is being send to open diffs (front and rear). At any time, only 2 wheels (one front and one rear) are required to get power. If you go over an obstacle which causes 1 front and 1 rear wheel to come off the ground, you aren't moving any more.
That is the purpose of the traction control (ATRAC, TRAC, whatever), to help assist in those situations. It works ok most of the time.
A locked diff would make sure that power is equally split between the both sides 50/50. So, with a locked rear diff in 4wd, you are guaranteed to have at least 3 wheels (one front and both rear) will have power and will be turning the exact same speed at all times.

Bravo!

 

[wfo9]

I agree with most of the points in the post above except the statement about how open diffs work.

Open diffs do not send all the power to the wheel with the least resistane... This is often (not always) the end result in slick conditions, but it is not technically how they work. And it is good to know how it actually works because it gives the driver better insight about how to apply throttle in tricky situations.

Open diffs equalize torque output. If both wheels have equal resistance.. both wheels get the same power. If one wheel has 20% more resistance/traction.. it will get 20% less power (resulting in equal torque if it was instantaneously measured across the axle).

Now what often happens is that one wheel starts to spin (the torque being applied overcomes the static friction coefficient between the tire and the ground).. In most offroad situations (unlike dry pavement) there is a significant difference between static and kinetic friction. Once the friction drops to near 0.... almost all the power is applied to the wheel in that state (again to equalize torque across the axle) and the result is one spinning wheel. And it is easy to see how this condition quickly cascades once traction becomes limited.

ATRAC attempts overcome all this by using the ABS system to apply braking force to a wheel on the same axle with a higher RPM/power. It attempts to simulate equal traction... allowing the open diff to give power to both wheels under many different conditions. Unfortunately, its not perfect and is a bit unpredictable at times. Much better than a pure open diff though. Sometimes you will see ATRAC cycling the power back and fourth between the wheels (sort of chasing itself...)

Locked differentials allow for unequal torque (measured across the axle). If one wheel is in the air and the other is on the ground... All the torque goes to the axle with the wheel on the ground. An open diff would do the opposite and put all the power to the wheel in the air (resulting in very little measured torque, but it would be equal).

Torque is often a tricky thing to conceptualize.. because it takes resistance to apply it.

 

[Roof Shingles]

Alright, I'm starting to understand. So why are 4WD preferred for off roading over AWD? Is it only because 4WD has the low gear? Im guessing AWD is preferred in the city cause no binding happens.

Yes, I am a 4WD NOOB


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[Synister]

I have a 4th gen and this discussion has raged over there for a while. I have always been fascinated with the inner workings of our drive systems regardless of generation. I found this video while cruising around on youtube, its a looonnng watch (41min) but I have yet to find anything that literally shows you how it all fits together. They just so happen to use a 4runner transfer case with a Torsen diff for the AWD/Full time 4wd discussion which is the second half of the video. First half, thats your SR5, Trail, TRD Pro type transfer case. Enjoy, I know I did!!

Center Differential Transfer Cases

 

[proimion]

First half, thats your SR5, Trail, TRD Pro type transfer case. Enjoy, I know I did!!

Center Differential Transfer Cases

Thanks for posting that. I really learned a ton watching it and have a much better understanding of what is going on in the transfer case. And also gained a big appreciation for the beautiful engineering that makes it all possible.

This one shows the shift forks in action on that Toyota 4WD case: https://www.youtube.com/watch?v=4UoBCIjWIoU

 

[Scott4runner]

That you for a very educational post

The SR5s (all 5th gens, except limiteds) have 2 differentials. One front, one rear. Both are open differentials (the rear is able to locked on TRDs/Trails, will get into that later). Open differentials always send the power to the wheel with the LEAST resistance (just like electricity or water). ATRAC (and all brake based traction control systems) takes advantage of the open diff's tendencies to transfer power. Say your left wheel is on ice and the right wheel on dry pavement. The left wheel slips. The wheel sensors indicate that difference in wheel speed to the vehicle computer. The computer then puts out the signal to apply braking pressure to just that left wheel. That brake pressure makes that spinning wheel more difficult to turn. What happens when a wheel becomes difficult to spin? The differential sends power to the side that is easier to turn (hopefully with more traction). This is a major simplification, but at least gives you an idea. All traction control systems work on the same basic principles.




The transfer case can be locked into 4wd. There is no real center diff, as the power can only ever be sent only to the rear or at a 50/50 split to the front and rear. This is the same on about all TRUE part time 4wd systems


The Trails/TRDPro/TRDs all have locking rear diffs. The SR5s and limited models do NOT have locking rear diffs.


The rear wheels have 100% of the power when in H2


See above, no power up front


No locking front diff. When in H4 (or L4), the transfer case is locked, power is sent to the front differential. The differential then sends the power to the front wheels via the open diff (the wheels are free to spin at different speeds. The wheel with the least traction will receive power)



As above. only 2 diffs, both open.




Keep researching. There are some good youtube videos explaining 4wd systems. Each manufacturer likes to act as though they have all re-invented it and it sooo much better than everyone elses. Most are nearly all the same except for small differences. Once you can comprehend how a differential works, you'll get a much better understanding.

The fact is: In 2wd, power is being sent to the rear diff. The rear diff sends power to the wheel with the least traction. You can get stuck with one wheel in the mud and only that one will spin.
In 4HI and Lo, power is being send to the front and rear diffs equally, all of the time. This is what causes the binding while turning tightly on high-traction surfaces, all 4 wheels are wanting to turn at slightly different speeds, but 2 (one front and one rear) are being forced to always spin at the exact same speed. Again, power is being send to open diffs (front and rear). At any time, only 2 wheels (one front and one rear) are required to get power. If you go over an obstacle which causes 1 front and 1 rear wheel to come off the ground, you aren't moving any more.
That is the purpose of the traction control (ATRAC, TRAC, whatever), to help assist in those situations. It works ok most of the time.
A locked diff would make sure that power is equally split between the both sides 50/50. So, with a locked rear diff in 4wd, you are guaranteed to have at least 3 wheels (one front and both rear) will have power and will be turning the exact same speed at all times.

Dude you just nailed it thanks for the education lesson

 

[james.adam.asaro]

Rear Locker

Guys I a new to this thread. I have made the transition from my JK to a SR5 4Runner. I am interested in finding details about putting an Eaton locking diff in the rear end, and I wanted to see if anyone has done this. I have a 2019 SR5. Thanks.

 

[ryencoke]

No flames. I'll talk about the full-time 4WD with the Torsen differential. About whether full- or part-time is better, you'll get lots of opinions.

The Center Differential. When unlocked, the Torsen center differential sends more power to the axle with traction.

Actually, two things happen with the Torsen center diff. When unlocked, more power is sent to the axle with traction and the front and rear drive shafts are allowed to rotate a different rates. When locked, 50% of the power is sent to each axle and the front and rear drive shafts rotate at the same speed (just like a part-time when 4WD is engaged).

The unlocked Torsen center diff will never send all the power to one axle. The front drive shaft can vary from 53% down to 30%. The default is 40% to the front axle. The ability of the unlocked center diff to allow different drive shaft rotational speeds is what makes it possible for a 4WD vehicle to turn tight corners on dry pavement.

The Front and Rear axle differentials. In 4WD-HI, the 4Runner has the TRAC system for the front and rear axles. This is similar to a limited slip (LSD) system. According to Toyota, "Toyota’s TRAC sensors are activated when one of the drive wheels starts to slip. TRAC limits engine output and applies the brakes to the spinning wheel. This transfers power to the wheels that still have traction to help keep you on track."

In 4WD-LOW, the 4Runner has the ATRAC system, which functions in a similar fashion. Both full-and part-time 4WD 4Runners have TRAC and ATRAC.

Locking Axle Differentials. Only the Trail Edition, a part-time 4WD, comes standard with a rear differential locker. Some people install after-market lockers.

Hope this helps.

Thanks! This is a great explanation, a lot of people looking at 5th Gens should understand the difference between the trims, the Torsen should be standard imo.