All Wheel Drive Class 101

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XXXR

XXXR

Joined
May 1, 2003
Threads
34
Messages
221
Location
Santa Cruz, California
Okay - I get the idea that a centre-diff' lock does not lock the axle diff's. Well I came across Christo's diff' lock switch. http://www.sleeoffroad.com/products/products_switches.htm

This means that I'd have two wheels (one front and one rear) running with the switch as compared to a normal 80/20, 50/50 on my unlocked FJ80?

I read this https://forum.ih8mud.com/showthread.php?t=12828
but didn't quite get it... :doh:

"To override the default behavior of the center diff lock, you need a center diff lock switch. The wiring harness for the switch is already present in the US 80 Series, and all that is required is to remove some dash components and plug the switch into the harness.
The automatic behavior is that the center diff will be engaged (and ABS disengaged) when Low Range is engaged. With the center diff lock switch installed, you will be able to engage and disengage the center differential in high or low range."
 
OK I decided my long-winded answer need to go. I hope this pic helps out instead. Note this illustration is for a situation where the tires have different amounts of traction. The yellow indicates "less than equal" turning energy at driveshafts and axles and the blue arrows show the amount of energy at the tire. I don't like the use of percentages like 80/20 or 50/50 so this is designed to show proportions not amounts.
AWD.jpg

Remember that any differential (center or axle) delivers the same torgue to both axles (or driveshafts) until the differential action is required for a turn or when the surface provide variable traction for each tire(snow ice etc) and slipping occurs.
 
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To put it simply, With open diff's and an open center, one single wheel has to lose traction to stop the truck. You effectively have a one-wheel drive. You are on a par with the Chevy S-10 Blazer. But on the other hand, you drive a Land Cruiser which is a LOT more sturdy than anything that Chevy ever built.
To up the ante a lot, you can lock the center diff as mentioned.
Here's how I did mine. Prepare for a bit of hillbilly:

First, I pulled the emergency flasher switch out of the dash on my 80. I noted the # on the top of the switch. I wrote the # on a sticky note paper. Then I went into a salvage yard that let me walk around. I took several relays and a bunch of fuses out of cars. Then I found a Camry or Tercel or the like that had a hazard switch that looked just like mine. I checked and the number matched!
Damm! I took that switch and put it in my pocket!
I went to the desk and paid the man for all the fuses that I had got. I gave him $5.00.
I took that little switch and cut out the matters in the way and plugged it into the wire that was left for the CDL. Dammed if it didn't work jist right!
OK, All the hillbilly crap aside, that was almost the way it went. I paid my pilfering rights to the yard and got two CDL switches. They cost me $20 more than nothing, but they were more than worth it! I gave one to my brother and put the other in my truck. CDL magic!!
 
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Brian Macgyver said:
With open diff's and an open center, one single wheel has to lose traction to stop the truck. You effectively have a one-wheel drive. You are on a par with the Chevy S-10 Blazer.
Click to expand...

Except that we have full time 4wd, and the center diff has a bit of an LSD aspect to it, so that as one wheel starts to slip power will transfer to the other axle, until you have a 50-50 split.

Locking the center diff just eliminates the need for 'spool up' of the c-diff.
 
Later 80 Series equipped with ABS came with a viscous coupled center diff. This is not an LSD. It will not "spool up until you have a 50-50 split". Have a look at http://www.lcool.org/technical/diffs/diffs.html for a great series of diagrams and poke around for a wealth of information.
 
That doc doesn't seem updated. The newer center diffs with the viscous coupler should transfer torque more smoothly and provide a bias even when cocking one wheel.
 
It is a bit old and crusty (like me) but the information is still valid. IIRC the addition of the VC had someting to do with getting the ABS to function correctly. No matter. It is a VC not an LSD device. If you remove a driveshaft the truck will attempt to move but you will be putting a far greater load on the VC than it was designed for. Several folks on the 80s list have had to replace their VCs due to this.
 
jonheld said:
It is a bit old and crusty (like me) but the information is still valid. IIRC the addition of the VC had someting to do with getting the ABS to function correctly. No matter. It is a VC not an LSD device. If you remove a driveshaft the truck will attempt to move but you will be putting a far greater load on the VC than it was designed for. Several folks on the 80s list have had to replace their VCs due to this.
Click to expand...

I am unclear as to why the VC has a greater load with a driveshaft removed. My understanding is you need to lock the center diff, thus eliminating stress on the VC, to drive around with one drive shaft. If this is incorrect I would appreciate being corrected.

Buck Buchanan
 
Brian Macgyver said:
You are on a par with the Chevy S-10 Blazer.
Click to expand...
This is not true. With (unlocked) AWD, you are *much* less likely to get that one wheel to spin on gravel or packed snow than with a 2WD vehicle.
Oh, and hi Jon! Curtis here from 3FE.
 
Buckru said:
I am unclear as to why the VC has a greater load with a driveshaft removed. My understanding is you need to lock the center diff, thus eliminating stress on the VC, to drive around with one drive shaft. If this is incorrect I would appreciate being corrected.

Buck Buchanan
Click to expand...
He's just saying if you removed one driveshaft but didn't lock the center diff., then the VC would have a greater load (than it's designed to handle).
 
Okay thanks guys for dealing with me on this. So essentially, I'm running one wheel with varied power to the front until slippage occurs. With a Slee switch, I'm essentially running a 50/50 power ratio with the front and rear wheels, but since my rig is UNLOCKED, I'm spinning 2 wheels, not 4. (1 front and 1 rear.) This is the same effect in "low" but Christo's switch allows for high gear locking. Cool, thank you!

By the way, the doc might be old, (http://www.lcool.org/technical/diffs/diffs.html) but at least it's written at a 5 grade level.. which is exactly what a college grad like me needs to understand this stuff! ; )

Another stupid question: So do you think that a locked diff in snow would be better than just running the AWD in regular mode? I mean hauling on a fireroad, rather than crawling. Just trying to figure out if the switch is worth the cash.
 
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The switch is absolutely worth the cash. I can't answer to the benefits of having the center locked on snow and ice (I live in Houston) but, I can attest to the benefits of driving on washboard roads in high with the center locked. I feel I have much better brake pedal feel with the abs disengaged in this scenario. Also, you can run in low without the center locked. This works well in high traction areas where a locked center creates tire scrub but low end torque is needed.

Buck Buchanan
 
XXXR said:
Another stupid question: So do you think that a locked diff in snow would be better than just running the AWD in regular mode? I mean hauling on a fireroad, rather than crawling. Just trying to figure out if the switch is worth the cash.
Click to expand...

The switch is worth the money for sure - it let's you run AWD in low range. That is a huge feature.

As for any higher speed use, AWD is your best mode. You really don't want the center locked as it will force slip around turns - the tires on the front and rear axles can turn at different speeds if you are turning sharply and with the center locked that means you must have slippage on one end. You don't want this type of traction breakage at any kind of speed. This is a reason AWD cars like Sube's and Audi's are such good performers on the road in moderate snow (but as discussed above are essentially one wheel drive if one tire loses traction completely).

The switch isn't a must have. If you need slow speed snow traction, then low range will lock the center diff and off you go. But it does give a lot of versatility that people who own other rigs, such as Jeeps, pay a lot of money to get, e.g. twin stick 2WD low t-cases and such.

Nay
 
I don't know if this was mentioned before, but there is a nifty mod (the pin 7 mod) that enables you to have full manual control of the center diff once the switch is installed. On the older FJ80s you simply need to unplug the part of the harness that auto-locks the CD in low and we already have the dash switch, but the newer FZJ80s are slightly more complicated.

I find that having the CD unlocked in low range is great for slow speed trail work in the tight, twisty woods here in the Northeast. When you have to make that 5 point hard left around the pine trees with a truck the size of a motorhome it helps not to have the driveline binding under you.

Folks on the 80s list have also used this when loading/unloading boats on boat ramps, moving trailers around the yard, etc.
 
Nay said:
As for any higher speed use, AWD is your best mode. You really don't want the center locked as it will force slip around turns - the tires on the front and rear axles can turn at different speeds if you are turning sharply and with the center locked that means you must have slippage on one end. You don't want this type of traction breakage at any kind of speed. Nay
Click to expand...

I disagree! I run my CDL locked on I-70 at 40-60mph and notice much more advantage of it than opened unless you are making a 3point turn or a turn of 90*. IMO I do not see any traction loss and tires breaking loose from the CDL being locked on highway because the turns are not sharp enough. I am not a fan of ABS, I have never had it,:D and it keeps the tires from locking up in emergency braking situations from retards on the roads.

J

It is easier to keep the truck strait with it locked in snow, however if you do get a$$ sideways, yes it is much more diffucult to get it staightened out.
 
Just a shot in the dark here, but if you are able to run 40-60 mph then chances are you have pretty good traction to begin with.

That said, there is no set "rule" for driving in low traction situations as they will all differ. The key is to be able to make your own choices as to lock or unlock any of the 3 diffs based on changing driving conditions.
 
Hitting a beehive with a short stick.

Jason Andrews said:
I disagree! I run my CDL locked on I-70 at 40-60mph...
Click to expand...

There seem to be two camps here.

There are some who insist that it is beneficial to run with the center and eve the front/rear lockers engaged on dry pavement and even at high speeds.

There is a second camp that considers the first camp to be a few apples short. (See :censor:ing nuts.)

I fall in with the second. Drivelines build tension when locked. When that tension releases there is a push & skid resulting in less traction momentarily. 99% of the time driving on the highway you won't notice.

If a truck slams on the brakes and jack knifes covering the 3 lanes in front of you causing you to pile on the brakes and swerve at speed while CDL locked... with the driveline tension built up one of the tires will have to break free. That unloaded back tire on the inside of your emergency maneuver will likely be it. You are now doing a pirouette down an interstate headed for the side of a semi.

I think this is why Toyota left the CDL switch off a lot of the 80s. US drivers weren't smart enough to turn them off.
 
XXXR said:
Okay thanks guys for dealing with me on this. So essentially, I'm running one wheel with varied power to the front until slippage occurs. With a Slee switch, I'm essentially running a 50/50 power ratio with the front and rear wheels, but since my rig is UNLOCKED, I'm spinning 2 wheels, not 4. (1 front and 1 rear.) This is the same effect in "low" but Christo's switch allows for high gear locking. Cool, thank you!
Click to expand...

Lots of debate on the subject in the archives. With an open diff on the 96 you own, you have full time awd. This means you always have 50/50 f/r torque split under normal conditions. When a driveshaft spins, the Viscous coupling starts to lock up, transfering engine torque from the spinning driveshaft to the one with more traction. At full VC lockup (humping) you have unlimited torque distribution f/r from 100-0-100. In non-humping mode the VC is considered a 80% LSD, which means either driveshaft is capable of supporting 80% of engine torque.

When you lock the center diff (alieviating the stress and heat failure potential of VC humping), you effectively have 2 drive wheels with unlimited and varying torque capability, either capable of supporting 100% of engine torque. In a straight line, a locked center diff torque distribution follows exactly weight transfer.

Another stupid question: So do you think that a locked diff in snow would be better than just running the AWD in regular mode? I mean hauling on a fireroad, rather than crawling. Just trying to figure out if the switch is worth the cash.
Click to expand...

Absolutely. ABS in snow, ice and gravel is not as effective as a locked diff in terms of stopping distance. Locking the center diff at high speeds can cause increase understeer, but I personally consider it most predictable (see video link of my truck at Steamboat ice track www.gruppe-q.com - shameless plug, still a few spots open for those looking to do some low cf experimenting). A lot of debate in the archives here regarding 'control' in a locked center diff scenario, most of which I find to be misunderstanding chassis dynamics.

Bottom Line: Put the switch in. If nothing else, it gives you the ability to evaluate how changes in f/r driveshaft speeds affects your driving style and comfort zone.

HTH

ST
Eventmaster Greuppe-q Steamboat Ice Track Event 2007
94 FZJ80 supercharged and CDL-pin 7mmod
 
Grench said:
I fall in with the second. Drivelines build tension when locked. When that tension releases there is a push & skid resulting in less traction momentarily. 99% of the time driving on the highway you won't notice.
Click to expand...

Not true as stated. There is equal driveshaft speeds at all times with the CDL locked. This means there is forced slip causing increased understeer during turns. It's not inherently dangerous, I claim it's inherently more predictable.

If a truck slams on the brakes and jack knifes covering the 3 lanes in front of you causing you to pile on the brakes and swerve at speed while CDL locked... with the driveline tension built up one of the tires will have to break free. That unloaded back tire on the inside of your emergency maneuver will likely be it. You are now doing a pirouette down an interstate headed for the side of a semi.

I think this is why Toyota left the CDL switch off a lot of the 80s. US drivers weren't smart enough to turn them off.
Click to expand...

ABS has the advantage of control only during impending lockup. However, one of the issues most don't understand about ABS is that all rear circuits use what is referred to as Select Low Principle. This means that the rear wheel with the greatest amount of tracion (least slip) generates only the same amount of braking force of the wheel with the least traction. So your overall effective braking force is reduced, and the steering wheels now are generating all the available braking force. So a lot more factors dictate which system the advantage in an awd chassis, especially one with a locked center diff. Stopping distances with ABS increase on ice, snow and gravel vs a locked center diff.

Predicting an advantage of ABS in the above scenario would depend on the definition of impending lockup. I advocate driver training, not ABS will be the deciding factor here. The maximum braking force is in a straight line. Brake modulation is best performed with the heel on the floor. A locked center diff in a straight line has ideal front to rear brake force distribution (brake torque follows exactly weight distribution). Rear braking during wheel lift on an ABS vehicle will be non existent.

With 15 years of awd chassis dynamics instruction, I see the CDL vs ABS debate from a much different perspective. Chassis predictability and driver skill is what will make you respond and survive potential life threatening situations. In low cf conditions, there is no doubt that ABS has the disadvantage to CDL locked. ABS vs CDL braking in awd chassis dynamics doesn't carry the same advantages as ABS in a front or rear wheel drive vehicle.

ST
 
Poking the beehive some more...

SUMOTOY said:
Stopping distances with ABS increase on ice, snow and gravel vs a locked center diff.
Click to expand...

For snow and gravel with the mound buildup in front of the locked tires, yes.

On ice, professional drivers with LOTS of practice can outbrake the ABS.

On ice, the average person who doesn't drive on ice daily can't come within 10% of the ABS braking distance.

Stoping on Ice to beat the ABS is all about modulating the brakes in a manner finner than the ABS computer is capable of. You may be able to do this since you spend a lot of time racing on ice. The other 99% of the population will not be able to beat the ABS on the FIRST run. Give them 10 runs and they MIGHT beat it by 2%.

Then you get into the whole control issue. Professional ice racers used to sliding about aren't as likely to panic. You're taking advice and techniques used by a VERY small set of elite drivers and making broad statements to a general population that frankly doesn' thave the time to practice or the skillset or interest to do it.

Now, this discussion was not about the ABS/no ABS debate. It was about driving with a truck locked/unlocked at highway speeds on normal varying condition pavement. On ice you have no windup. On clean, dry pavement that high tire may or may not break free depending on how much windup is in the driveline at the time of the maneuver. This makes it unpredictable and dangerous.
 
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