Lockers, I understand. What is not clear is open diffs and LSDs. I have a rear TrueTrac that seems to work well, but I don't know how. I understand that when it slips, it causes a gear driven limit to the slip and this causes the slipping wheel to turn with the other wheel. How long does this remain engaged? Just until it's back in sync? Until you let off of the gas? Are both rear tires pulling when no slip is detected, or do I just pegleg around all the time? Does the TrueTrac act any different when I'm pulling 12,000 lbs?
Same issue with my open front diff. The guy who did the rear diff said the front pulls with both front tires until one slips, then just the non-slip side is pulling. If true, there must be some limit to that or no one would add a front LSD. How would adding a front LSD affect pulling ability (not capacity, I know you can't change that) as to starting a heavy load, pulling up hills, etc? I've seen the videos of 4wd with open diffs spinning on rollers. I now avoid rollers, but there may be a few hidden ones I don't know about. Just trying to get a deeper understanding of how all this works. Thanks.

 

Open differential: power is evenly divided only if traction is evenly divided and both tires can handle the power. Once one wheel slips, most of the power goes to the freely spinning wheel, and the dead wheel looses most of its power. If one wheel is on ice (forget tires, think absolute zero traction) and one wheel is on gravel and you push the gas pedal,since most of the power goes to the spinning wheel on ice, the little power that goes to the dead wheel won't be enough to move the truck.

(From now on I'll say all and none and powered and dead a lot, it doesn't mean absolutely one or the other, just relatively, causing enough power to be lost on the slipping wheel that the wheel with traction isn't getting enough torque to move the weight of the vehicle.)

Limited slip: when a wheel slips, the uneven distribution of power across an open differential causes a torque bias to the powered wheel. In a limited slip, that bias causes either a clutch or a gear to engage, that once engaged lifts power off the free wheel and pushes it over to the dead wheel. If one wheel is on ice and one wheel is on gravel and you push the gas pedal, all the power goes to the spinning wheel on ice, but the torque bias engages the clutch/gear, and the wheel on gravel will receive power and move the truck forward. Clutch based units can be burned out if the powered freewheeling tire overpowers the clutch, and the dead wheel stays dead. Unless there is a power bias, the differential acts open normally.

Detroit locker: both wheels are tied to the carrier and always share power. Uneven power distribution only happens when a wheel is precessed in front of the powered wheel, meaning the slowest wheel is always powered. If one wheel is on ice and one wheel is on gravel and you push the gas pedal, power is distributed evenly across both wheels, and the truck moves forward (with the tire on gravel doing to moving). In a turn, the inner wheel is powered and the outer wheel ratchets forward; once the wheel isn't be drug ahead of the inner wheel, it takes power again. If the inner wheel slips, then the carrier catches the precessing wheel and gives it power automatically.

Spool locker: both rear wheels are powered full time, with no differentiation in wheel speed at all. Not typically useful for anything but dedicated mud runners, because in a turn one tire has to scuff, meaning there is a ton of strain on the axle shaft, the spool, and the carrier to overcome whatever traction the tire has with the surface.

 

Nice overview, thanks. The LSD is what interests me the most, as mine is a daily drive and needs to have nice road manners. When does the LSD shift back out of the limited slip mode? Is it a second to second thing, meaning when on ice or gravel, it goes in and out with each slip and resolution?

A few years ago, I was looking to buy a tractor and bush hog. I pick out the bush hog I wanted and then looked for the tractor. The one I picked could be pulled with a 90 horse, 2wd tractor or a 60 horse 4wd! The front wheels pulling made that much difference. Since I pull really heavy on loose gravel, would putting a front LSD be that much better than my current set up?

 

I'd assume this means that the engagement of the unpowered wheel is progressive and that the differential adapts itself almost instantly to changing power distribution. So yes, I'd say it's a second to second thing, with power transfer being incremental and not all at once.

I honestly don't know about putting in an LS or a selectable locker for towing, I'd guess the first thing to break would be a hub so the downside is relatively small for whatever gains it gets you.

 

I would think that a slow application of power, even with 12k of added resistance would in no way compare to some of the forces that get applied when mudding at break neck speed, but I don't know either. I have started from a dead stop on flat, wet grass and not slipped a wheel in 4wd BEFORE the rear TrueTrac, but I also don't know how that compares to climbing a hill on the gravel drive at the farm.

 

It just occurred to me that there might be some truck pullers out there who could shed some light onto how a front LSD or the like handles heavy pulling situations. Anyone?