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Discussion Starter · #1 ·
Hi Guys,

I hope someone can straighten out my thinking.

Every 4x4 I've had, including my 230GE (460) has had a transfer case that is normally in rear wheel drive only and when engaged, "locks" the front and rear axles together. I know the 463s have a transfer case more like many newer 4x4s in which the vehicle is always in AWD (correct term?) and locking the center diff produces a 4 wheel drive system like all my older 4x4s.

What I dont understand is - when the center diff is not locked, it seems to me like it would result in a drive system that is actually inferior to a 2-wheel drive vehicle. Assuming open axle differencials, when a 2-wheel drive vehicle has one rear wheel (assuming rear wheel drive) in poor traction, it is effectively stuck since that wheel will turn and the other will not. It would seem that with a vehicle in AWD (center diff open) losing traction on any one of 4 wheels would result in the same loss of traction. Since it is easier to find a slick spot with one out of 4 wheels than one out of 2, it seems that it would be easier to stick.

What is wrong with my thinking? The reason I ask is that a friend has a 95 Toyota LC that you can only lock the center diff in low range. That would seem to me to make it a poor 4x4 for off-roading in high range. But, he claims it goes good and knowing Toyota I cant believe they would make the LC inferior to a regular 2 wheel drive truck when the LC is in high range.

TIA for an explanation of 4x4 systems with a center diff like the 463s.

Bruce
 

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'82 300GD TD (Sold), '02 G500, '09 B200
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In theory you are right in many ways. Except, in a full-time 4wd system like the 463s (pre-2001) torque is divided up between 4 wheels instead of 2 like in an old part-time system. This makes each wheel get 25% of the torque instead of 50%. 25% of the total torque makes it half as difficult for a given tire to break traction with a given surface. It is too late and I am too tired to explain this properly.

Read the 4wd101 section of Harald's site, it is excellent and will answer all your questions well, I would think. He may even chime in here and rip my (basic) knowledge of this principle apart and explain it correctly. I understand the concept enough, but to explain it is another story entirely.

Steve

http://www.4x4abc.com
 

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Nothing wrong with your thinking.

To follow up on Steve's suggestion, here is what Harald has to say on the subject (I hope I'm not committing cut-and-paste copyright infringement here.)

Full time systems allow all 4 tires to push or pull at maximum rate. In part time this is not the case. This makes full time systems superior for on-road and off-road use.

That's where the name "full time" 4WD is coming from - it can be used all the time. No need to shift back into 2WD like in part time systems.

Full time 4WD: No wind up, no binding, no understeer, no skidding - always maximum pulling power. Also important: ABS as well ESP work the way they were designed. In part time they don't. . .

However, all 4WD systems (part time or full time) only work best on level surfaces with equal traction on each tire. Once the terrain turns uneven (bumpy, deep ruts or holes) full time sytems lose all of their advantage. One spinning tire could render the entire vehicle motionless. In that case the center differential needs to be locked (disabled). Locking the center differential in effect creates a part time 4WD system where both drive shafts are forced to rotate at equal speeds.

So, for off-road use the center diff on a full time system needs to be locked. . .
The preceding can be found from Harald's website, here:
http://www.4x4abc.com/4WD101/def_turnfull.html

Among those pages are also definitions of the terms AWD, Part-time 4wd, Full-time 4wd, etc. If you've never been there, Harald's site is a trove of off-road theory and practice.

I don't think there's anything wrong with your thinking. My supposition is that Toyota set up the 95 LC that way as an idiot proofing measure, so that an uninitiated driver would be less likely to drive with the center diff locked on pavement, thereby causing driveline wind-up and ultimately damage to the driveline. Just a guess.

— Spalding
 

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RE: Nothing wrong with your thinking.

same reason why the electronic diff locks are tied into a specific order (front last) rather than let the driver chose them as needed.
 

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Bruce,

Steve got it right on - the simple fact that all wheels are powered at all times gives a vehicle 100% more traction than a 2WD - that makes it much less likely that any of the wheels loses grip.

However, we all know that wheels can spin. That's where traction control comes in. And traction control has nothing to do with 4WD - it is simply an added management tool. Could be done (and is done) on 2WD as well.

In a part time system traction between front and rear does not need to be managed. Both driveshafts can't move independently since they are coupled together. But the axles need to be managed to avoid or limit wheel speed differences between left and right. Can be done with a variety of limited slip applications (preloaded clutches or reactive clutch packs or proactive/reactive Torsen gears), reactive wheel speed limiters (ETS) or proactive/reactive diff locks.

In a full time system traction between front and rear does need to be managed since the center differential allows independent drive shaft speeds. Can be done with a variety of limited slip applications (preloaded and reactive viscous clutches or reactive clutch packs (Haldex, Gerodisc), proactive/reactive Torsen gears or proactive (center)-diff locks. Wheel speed management on the axles is done the same as in part time systems.

All those features of traction management can be combined. In fact, in the US are right now 28 different setups available. Confusing to say the least. Especially when you add the popular reactive/automatic AWD systems that use the same devices to engage AWD that the others use to manage traction.

For the difference of 4WD and AWD check here: http://4x4abc.com/4WD101/difference_4WD_awd.html

So, the 460 should be easy: Part time 4WD. No center diff, no traction control needed. For wheel spin control they have the best of all systems - manual lockers. Unbeatable off-road. Limited use on-road.

463 - more complex. Full time 4WD. Both high range and low range can be locked manually to prevent drive shaft speed differences. For proactive wheel spin control they also have the best of all systems - manual lockers. And to save the ignorant driver's ass there is ETS - automatic and transparent wheel spin control (reactive). At this point the 463 has the best combination of features on the planet. Nobody else offers it.

1995 Toyota LC. Full time 4WD. High range drive shaft speed differences are limited by a viscous coupling (acts like a limited slip), but high range can not be locked. Shifting into low range, the center diff is automatically locked. Driver has no influence. This is bad, because you cant use low range for towing.
Both front and rear axles can be locked manually. Good. No automatic traction control (wheel speed) available. Bad.

The LC does pretty good off-road with the limited slip viscous coupling in high range. Not as good as the G though. But we knew that already.

Harald
 

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463's & ETS

4x4abc - 2/15/2005 1:19 AM


463 - more complex. Full time 4WD. Both high range and low range can be locked manually to prevent drive shaft speed differences. For proactive wheel spin control they also have the best of all systems - manual lockers. And to save the ignorant driver's ass there is ETS - automatic and transparent wheel spin control (reactive). At this point the 463 has the best combination of features on the planet. Nobody else offers it.
ETS is only present on 2001+ (2002+ in the US) models.
 

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Discussion Starter · #7 ·
Thanks guys! I knew I must be missing something. I think I understand it better now, but I'm sure I'll never know all the various iterations like those that Harald lists.

Bruce
 

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I was trying to think of something to add to this discussion. The issue of what is "better" on slick stuff is what i wanted to comment on. I like the full time 4wd of the 463 for the fact that IF you hit a patch of ice under acceleration only one tire will spin generally. This leaves three tires with some amount of traction and keeps the vehicle stable. You may lose speed but hopefully not control. With a part time system half of the torque is forced to each axle. Depending on the conditions I think there exists a greater chance to lose control because one front and one rear can/will spin. There is a reason I see so many 4wd trucks in the ditches, overconfidence is one cetainly but part time 4wd I think contributes. I've been driving both in the snow for many years and without a traction/stability control system on board, I think full time like the 463 has is the best set up.
 

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i think (if i remember correctly) harald has an interesting story on his site about driving down a slippery hill in 2wd in his 460 while having control, but as soon as shifting to 4wd (locking the center) he immediately lost control and did a donut.

About why the full time 4wd doesn't spin wheels as easily as the 2wd, i think the key is in what somebody said about the reduction in power to the from 50% to two wheels to 25% to 4 wheels. Technically, with 4wd you should be able to reach a given speed twice as fast becuase you can have 4 wheels pulling at the max traction instead of 2 wheels. Or if your rock crawling, the tire with the least contact pressure only needs to have half the traction as a 2 wheel drive becuase it only need to push 25% of the vehicles weight instead of 50%.

brent, I think your point about how many wheels you can spin has some truth to it, becuase it is pretty hard to do donuts with full time 4wd.... at least compared to after locking up differentials. With one spinning, the other three (damned them) keep you on track pretty well.

all this talk about donuts is making me hungry.
 
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