Center diff vs. rear diff
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SeattleMike
New member
"Torsen" is short for Torque Sensing, meaning that the shaft getting better traction shows as more torque, which then results in even more power being fed to the shaft with traction. Specifically in the current 4Runner, the type of Torsen used in between the front and rear drive shafts employs planetary gearing in a ratio that supplies more power to the rear wheels than to the front, until such time as the front wheels gain more torque than the rear and the Torsen sends more power to the front wheels.
The TE/SR5 have "transfer cases" that split the power in a static ratio regardless of torque or traction. Wheels with no traction simply get the brake applied to it via the traction control, while the other wheel remains turning under power.
In the real world, this means that a Torsen center differential provides you will fulltime 4WD, and it will determine not only which wheels get power, but when and how much, too. If you find your rear wheels have lost traction (say in mud or loose gravel or snow), your front wheels will get more of the power without you even knowing it happened. A TE/SR5 driver would have to manually select 4WD to provide power to the front wheels.
Obviously, some prefer the Torsen for convenience but to me it felt heavy and sluggish in low speed driving and parking. That may have been due to the transmission TSB issue, or not. I'm used to rear wheel drive so I stuck with the old school part-time system.
Regarding the locking rear differential of the TE... they are great to have when you need them, but if you have never used one before, you probably won't use it in the future, either. In simple terms, a locking diff forces both wheels to spin at the same speed.
Cliffs Notes version: The Limited's center differential (which also locks) is better for the street since it is basically a full-time 4wd set-up. The Trail's lockable rear differential is better for serious offroading.
The 4th gen has basically the same set-up as the 5th Limited, with a lockable center differential. However, in 2009 (last year of the 4th gen) a Trail Edition was available that had both lockable center differential and lockable rear differential. In my opinion, Toyota should have kept that 4wd system for the 5th gen because it was the best of both worlds.
The center diff controls how torque is split front to rear. The rear locking diff on the TE controls how torque is split left to right on the rear axle.
The SR5 and TE have a part-time 4WD system that should not be used on dry pavement. The Limited's full-time system can be used on dry pavement. The benefit of the Limited's system is that when it is snowy and the road changes from snow covered to wet to dry and back, you don't have to do anything -- you just leave it in 4WD. In contrast, with a part-time system you would have to be shifting in and out of 4WD.
Offroad, once you lock the Torsen center diff, the Limited acts just like an SR5 or TE in 4WD. The only difference is that the TE has a lockable rear diff that can force both rear wheels to turn at the same speed.
Is the TE better offroad than the Limited? Sure. Will you need that much capability?
The SR5 and TE have a part-time 4WD system that should not be used on dry pavement. The Limited's full-time system can be used on dry pavement. The benefit of the Limited's system is that when it is snowy and the road changes from snow covered to wet to dry and back, you don't have to do anything -- you just leave it in 4WD. In contrast, with a part-time system you would have to be shifting in and out of 4WD.
Offroad, once you lock the Torsen center diff, the Limited acts just like an SR5 or TE in 4WD. The only difference is that the TE has a lockable rear diff that can force both rear wheels to turn at the same speed.
Is the TE better offroad than the Limited? Sure. Will you need that much capability?
Is having a locking rear diff better than having a center diff for off road/snowy roads or vice versa? Why is one better than the other? I would think that having a locking center diff is better but all the more rugged off road trucks have locking rear diff...I mean when you lock the center diff doesn't power go to all 4 wheels and both front and back wheels/axel rotate at the same rpm?
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JB.
New member
It's not usually a choice of one or the other. A locking center is more versatile so it usually starts there. Adding a locking rear helps off road.
See above.
When only the center is locked, only the front and rear drive shafts have to rotate at the same rate. Having all open diffs -- front, center, and rear -- sends equal power to all 4 wheels if they all have equal traction. It's when wheels lose traction that the differences start to show up. Instead of where power goes, it might be easier to think of where it doesn't go. With three open diffs, you could have one wheel lose traction and the truck going nowhere because no power (not enough) was going to the other three. Lock the center, then it would take two wheels losing traction -- one front, one back. Locked center and locked rear, it would take three wheels -- two rear, one front. Three locked diffs and it would take all 4 wheels having little or no traction (spinning) to leave the truck going nowhere. This is mechanically speaking, no ATRAC.
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A locked center diff behaves exactly the same as a part-time 4WD system in 4WD. That is, when you lock the center diff, both the front and rear driveshafts turn at the same speed. That is exactly what happens with a part-time 4WD system when you put it in 4WD.
The difference between the two is that with an unlocked center diff, you can drive in 4WD on dry pavement. So, if it snowed during the day, the streets are plowed and mostly dry, but your driveway is completely snow covered, that's not a problem for a truck with center diff.
Not better, different. And it isn't an either-or.
1) lockable Torsen center diffs are expensive and most people don't know what they do.
2) very few off road trucks have locking rear diffs.
Not quite.
When you lock the center diff, the front and rear driveshafts turn at the same speed. The same is true when you put a part-time 4WD system in 4WD.
But that doesn't mean that all four wheels turn at the same speed. Your front and rear diffs allow your left and right wheels to turn at different speeds. So if you didn't have any traction control system (or locking rear diff or rear limited slip) and both right side tires were on ice, you wouldn't go anywhere. That's what TRAC/ATRAC is for -- to transfer torque right to left.
Titanpat57
Senior Member
Great explanations and thanks to everyone...it certainly has "helped" clear up my lack of understanding with the two different type systems.
:cheers:
:cheers: