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Old 01-28-2024, 09:56 PM #31
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Thanks. That answers the question!
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Old 01-31-2024, 03:01 PM #32
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Um, yeah. A delightful discussion of physics and engineering principles, but they don’t answer my question, which is: if the transfer case splits the torque 50/50 front/rear, how can does 100% of the torque get to a single rear wheel?
I think the answer is: It doesn't. If only one side of any differential has traction but the differential can bias the torque to move the vehicle - 100% of the torque is going to the side of the differential with resistance (traction in this case). The 50:50 split in the marketing material is really a (inarticulate) reference to the ratio of the gears driving the front and rear axles. They are often not the same. Often in RWD biased designs they'd be designed to send 70% or so of the torque to the rear axle. In FWD it would be reversed. The torque split value is relevant to how the car feels on dry pavement. In any other condition - the stated torque bias is basically meaningless.

There is also often a torque limit to one axle or the other. In the Lexus IS for example it's 30:70 Front:Rear in normal driving but can send as much as 50% to the front and IIRC 100% to the rear. What that means in real terms is that the design will only send 150 Ft/lbs forward if the rear tires have no traction. That could be both correctly stated as of the applied torque to the front and also correctly stated as 50% of the input torque being applied to the front. Both are correctly describing two different things using the same words that make it confusing. The torque limit is likely an inherent property of the center differential gear ratios and helical gear helix angle, not an electronic limit. It's designed so that the maximum force applied to the front is half of input torque due to a friction limit in the engineering of the helical mesh.

In the case of the IS awd - if you were on dry pavement full throttle and the engine has 300 ft lbs of torque (it's a lot more due to transmission gearing, but I'm ignoring that for now), 100 ft lbs goes to the front, 200 goes to the rear. If the rear is on rollers, 100% goes to the front, but it would not exceed 150 ft lbs of torque going to the front. So, it would be both 100% relative to the torque out of the transfer case, but 50% relative to the input.

Last edited by Jetboy; 01-31-2024 at 03:07 PM.
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Old 01-31-2024, 11:08 PM #33
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There is also often a torque limit to one axle or the other. In the Lexus IS for example it's 30:70 Front:Rear in normal driving but can send as much as 50% to the front and IIRC 100% to the rear Ö The torque limit is likely an inherent property of the center differential gear ratios and helical gear helix angle, not an electronic limit. It's designed so that the maximum force applied to the front is half of input torque due to a friction limit in the engineering of the helical mesh.
Interesting. I seem to remember reading that in the case of a Limited with an unlocked center differential, the explanation for the asymmetrical torque split was because of the difference in size between the ring gear and the sun gear (itís my understanding that the Limitedís center diff, though made by Torsen, uses a planetary gear set, rather than the classic Torsen diff that uses three pairs of helical gears rotating around center gears.)
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Old 02-01-2024, 02:39 PM #34
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Interesting. I seem to remember reading that in the case of a Limited with an unlocked center differential, the explanation for the asymmetrical torque split was because of the difference in size between the ring gear and the sun gear (itís my understanding that the Limitedís center diff, though made by Torsen, uses a planetary gear set, rather than the classic Torsen diff that uses three pairs of helical gears rotating around center gears.)
I haven't looked at one in a while. But my recollection is that the 4Runner Limited uses the VF4BM transfer case. It is both a planetary center differential AND a Torsen limited slip. Here's a link to more detailed info on it. IIRC it has a 50/50 torque split default gear ratio.

https://www.clublexus.com/forums/att...case_vf4bm.pdf
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Old 02-01-2024, 08:30 PM #35
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I haven't looked at one in a while. But my recollection is that the 4Runner Limited uses the VF4BM transfer case. It is both a planetary center differential AND a Torsen limited slip. Here's a link to more detailed info on it. IIRC it has a 50/50 torque split default gear ratio.
That link is a great resource. Thanks!
Yes, the the t-case has a planetary gear for the high/low gearing, and a Torsen differential, which isbased on a second planetary gear set. I think itís the gear ratio within that second planetary gear set that is responsible for the torque split.

Per the linked document:
ďiii. The torque distribution during straightline driving is 40/60 (front/rear), which is helpful for an appropriate steering response during the initial stage of a turn. During the acceleration stage of a turn, the torque distribution increases in the rear wheels.

iv. This center differential consists of a center differential case, sun gear, ring gear, coupling, 8 pinion gears, clutch plates, and planetary carrier.Ē
So I guess itís 60/40. With an open differential.
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Old 02-01-2024, 09:30 PM #36
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That link is a great resource. Thanks!
Yes, the the t-case has a planetary gear for the high/low gearing, and a Torsen differential, which isbased on a second planetary gear set. I think itís the gear ratio within that second planetary gear set that is responsible for the torque split.

Per the linked document:
ďiii. The torque distribution during straightline driving is 40/60 (front/rear), which is helpful for an appropriate steering response during the initial stage of a turn. During the acceleration stage of a turn, the torque distribution increases in the rear wheels.

iv. This center differential consists of a center differential case, sun gear, ring gear, coupling, 8 pinion gears, clutch plates, and planetary carrier.Ē
So I guess itís 60/40. With an open differential.
I'm fairly confident that it is a limited slip differential rather than an open differential. The differential assembly starting at about 24:00 mark in this video is *I think* a VF4BM or very similar Aisin center torsion differential. might be a VF3AM or other similar model. But they're all very close.

Center Differential Transfer Cases - YouTube
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Old 02-02-2024, 01:59 PM #37
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Haha I just rewatched that video again last night. You are correct. I misspoke. Another semantic issue. In my mind any differential that isnít locked is open, and a limited slip diff is a subset of the open diff.

John Kelly may be a little monotonous, but heís meticulous and thorough in his instruction. Heís got other videos of the workings if the transfer case in situ, and of the ADD. And a great one explaining planetary gears.
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