First off, I am not a man of my word. Several of you advised me NOT to put in a front TrueTrac, and I said I would not. Last week, I did. In another thread I asked how it would react to heavy towing and if it would react any differently than it normally does and got little to no response. Not sure if no one knew or if it was just a dumb question, but I think I can answer it now.
Pulling 14k both days this weekend was VERY different than before the new front diff. While no noticable wheel slip occured, the steering wheel kept trying to pull back to straight ahead and actually required effort to keep in in the turn. This would go on as long as I was pulling a hard hill and would let off on the level parts. I left the load there overnight last night, drove out in 4wd and noticed no difference from driving in 4wd from BEFORE the TrueTrac. This says to me that even though no noticable wheel slippage had occured, the TrueTrac was engaging, I assume just due to the amount of weight it was pulling. Any thoughts?

 

i'm confused...

were you pulling in 4wd when the wheels were trying to re-center? if so, that makes perfect sense to me. the inside wheel would be seeing a multiple of the torque applied to the outside wheel, which would naturally want to drive the wheels back to "straight"

if not, then i'd be very confused, as the true trac should have nothing to do unless the pinion is trying to drive the wheels.

i would also be aware that the true trac will affect your steering while decelerating as well. it will try to engage if the wheels are driving the pinion as well, which might make it hard to steer going downhill as well.

 

Yes, all towing was in 4wd. Glad you agree, since there was such a strong pull that it either had to be the new diff or something going wrong.

 

Were you on a slick or soft surface that required 4x4? If not, I recommend you disengage the front drive before you break an axle shaft or u-joint.

If the true trac works the way I think it does, on a hard surface it deliveres 100% of the torque to the inside wheel on a turn while the outside freewheels faster. Driving the inside wheel on a turn leads to understeer and the vehicle trying to straighten its path.

 

Yeah, this all happened on the 1 mile, hilly, gravel drive at my farm. The truck WILL NOT pull that load up the hills in 2wd. I've tested that idea with near disastrous results.

 

i don't think the truetrac drives off the inside wheel the same way that the lunchbox lockers do.

IIRC, it allow the wheels to differentiate, but when one wheel tries to turn faster (outside wheel), it will apply torque to the slower wheel (inside) through the built in multiplier allowed by the helical gears. it will do this up to the point where the inside wheel breaks traction and catches up to the other.

this will make it FEEL like it is driving the inside wheel only, but it is actually just driving the inside wheel more, thus making it harder to keep the wheel turned.

 

What I think is interesting is that it appears that no actual slip is needed to engage the TrueTrac, and that it seemed to stay engaged as long as it stays under the heavy load. As I said, steering was easy on the flats. At any rate, with front and rear TrueTracs, this truck seems to be one sure footed mountain goat and now easily able to do what I need it to do.

 

There IS slip when you're turning, at least as far as the diff knows. When you turn the inside wheel goes slower than the outside wheel. The diff sees that as slip and sends torque to the slower (inside) wheel.

 

Are there degrees of it engaging or is an all or none situation? As I mentioned, steering was easy on the flats loaded and I noticed no wheel pull to center at all in 4wd and unloaded.

 

the true trac applies torque to the wheel that is not slipping (inside or slower) through an internal multiplier (internal geometry of the helical gears)

anytime one wheel starts to spin faster than the other, the internal gears transfer a multiple of the torque on the faster wheel to the slower one. so they do need a differential wheel speed in order to transfer torque.

quick example:
lets pretend that your internal multiplier is 4 (might be less for diff intended for a front axle)
lets say that in the flats, you were applying 50lb-ft to each wheel to drive the load (200lb-ft total). when you turn the wheel, the trutrac can apply up to 200lb-ft of torque to the inside front wheel, up to the point where it breaks traction.

lets say in the hill, you are applying 100lb-ft to each wheel to drive the load (400lb-ft total). the trutrac can now apply 400lb-ft to the inside front wheel, up until it breaks traction.

this might explain why it felt more noticable in the turns.

the downside to the trutrac is the same.... on ice, what is 4 times zero? on any low friction surface, the multiplier has a very small number to work with, so you get very little transfer, and it can actualy work not much better than an open diff. .... that is, unless you know how to modulate the wheel spin with the brakes ..... but that's another story.

 

Teach me, Obiwan Kenobi...seriously. I heard the if you apply power and brake simultaneously, you can regain traction. Is there any real trick to that or a practical way to practice it? Any other tips?

 

i'm far from an expert, and i've never owned one. but keep in mind that the truetrac is the same type of differential that the Hum-Vee's use, and they are able to scale vertical walls, with one or two wheels completely off the ground.

the basic principle is pretty easy to understand, but i'm pretty sure there is a fair degree of "art" and "feel" that comes with experience using it.

it works because of the internal torque multiplying effect.

again, let's use 4 as a multiplier.

lets say you are parked on a steep driveway with one wheel is on dry pavement and the other on ice. Ice has very little friction, so the truetrac can only bias 4 times whatever is available to the spinning wheel.

lets say that the ice offers 5 lb-ft of resistance, so that wheel spins and the truetrac does it's thing and applys 4 times that amount of torque to the other wheel. so we have 20 lb-ft of torque being applied to the tire on pavement. that's still not very much, and may not be enough to gain forward momentum. BUT!!!

we can use the brakes to help us out.

let's apply the foot brake. now we have applied 100 lb-ft of resistance to BOTH wheels on that differential. now once the wheel on ice begins to spin, it does so with 100 lb-ft of resistance. so the trutrac does it's thing and applies up to 400 lb-ft of torque to the wheel on pavement. but remember, there is also 100 lb-ft of resistance on that wheel. so we have upto 300 lb-ft of torque available to the wheel on pavement. THAT aughta be enough to get things moving.

realistically, this is only needed in very extreme manuvers where there is a drastic difference in traction from one side of the vehicle to the other. ..... like, say, when you accidentally hang one wheel off the edge of the road becaue you slid off. with carefull application of the brakes, you stand a fair chance of being able to grab some traction, where a normal LS diff would litterally leave you hanging.

i'm a big fan of the helical gear type LS's ... the shortcomings inherent to its design are very easily managed with a little basic understanding of how they work.

here's a good read:
http://www.houseofthud.com/differentials.htm#quattro

 

Nice explanation, thanks. Another thing that I'm uncertain of is what happens with good traction to all 4 wheels, in 4wd, in a straight line. With front and rear TrueTracs, are all 4 wheels then pulling and doing so equally, is it like an open diff and the right rear (and I suppose the right front?) is doing alll the work?

 

If you are straight and level with good traction, a TrueTrac or open differential, or even factory limited slip, all work the same. Torque is divided evenly between all drive wheels.

An open differential sends more power to the wheel spinning faster (usually the one with the least traction). This works well on dry pavement while turning, the outer wheel is the faster. It is not good in mud, snow or limited traction.

Factory LS works the same as an open differential but the clutch packs try to limit the slip and keep the power equal between the tires as much as possible.

The TrueTrac works totally opposite of the open differential, sending more power to the wheel spinning slower.

A locker prevents unequal power distribution. A locked differential does not allow slip.

 

everything redford said ...

your trutrac will function as an open diff until there is a torque bias or speed differential across the diff.

everything you said is essentiall correct, except that in a conventional 4wd, the suspension tries to unload the RR tire, and the FL tire, which is why they want to spin first. it's an equal and opposite thing. the suspension will still unload those tires, but the LS will now give the truck the means to transfer power to the wheel with the most weight on it (LR and FR)

something else to take note: the trutrac will bias torque anytime the helical gears are driven and there is a differential in wheel speed. so it will engage both in acceleration and deceleration. this is good for extreme traction, but it can cause some funky handling, particularly on very slick surfaces (think mud or pack snow) it will be much easier to slide straight sideways. you may find that you have to do some quick thinking and engage or disengage depending on what the truck is doing.

keep in mind that disengaging the front hubs or transfer case will keep the front from engaging, and a bump to neutral will keep them both from engaging if needed. you may find that going down slick hills a bump to neutral may provide the best stability and will allow the ABS to function the best.