Helical style / Truetrac rear differential function check

[davisjstone]

Amateur hour over here and I need a sanity check. This is all new to me.

Not on the road yet. 4R70W/D20 with twin sticks.

I installed a new diff from Quick performance with a Truetrac helical style LSD and new 35 spline axles.

Sanity check - With both rear tires off the ground, in neutral - When I try to spin a wheel there is quite a bit of resistance, it doesn’t spin easily. It’s not the brakes grabbing because I tried with the drum off. When I spin one wheel, the other does not spin.

I also put the car in park and spun the wheel (which took a lot of force) and the opposite wheel spun in the other direction.

Is this right? Is there a function check I can do?

 

[Yeller]

Yes, for all new stuff sounds right. Should turn, take some force but not crazy hard like all you can muster. The true Trac will spin like a stiff open differential, one tire forward and one backward, apply a load and that resistance increases, apply wheel speed to a single tire and it will exponentially increase.

 

[Broncobowsher]

Some of that sounds right. The one wheel forward and the other back when in park, that is the differential action. It doesn't lock up until there is a load on it. But isn't super free like an open diff either. Trying to remember details of a Truetrac, it's been ~5 years since I dealed with one. I recall it being like a stiff open diff, but not as bad as a clutch type.

The both tires in the air and in neutral, difficult to turn, doesn't sound ideal. Do both tires turn the same direction? That would be a turning driveshaft. Or opposite directions, driveshaft stays still? it might just be the new pinion preload magnified by the gear ratio. The lower the ratio, the more leverage the engine has on the tires. But backwards with the tires driving the driveshaft.

 

[Nothing Special]

It's been a while since I played with the TrueTrac in my old pickup like that, so my memory might be faulty. But what I do remember is that it acted just like an open diff with both tires in the air.

 

[Broncobowsher]

It's been a while since I played with the TrueTrac in my old pickup like that, so my memory might be faulty. But what I do remember is that it acted just like an open diff with both tires in the air.

That's what I know them as well. Just a little stiffer than a true open diff, nowhere near as stiff as a friction based limited slip.
I know some of the helical limited slips (Torsen?) have designs that may use a preload spring. Don't know if Detroit has done that or not.

 

[davisjstone]

Some of that sounds right. The one wheel forward and the other back when in park, that is the differential action. It doesn't lock up until there is a load on it. But isn't super free like an open diff either. Trying to remember details of a Truetrac, it's been ~5 years since I dealed with one. I recall it being like a stiff open diff, but not as bad as a clutch type.

The both tires in the air and in neutral, difficult to turn, doesn't sound ideal. Do both tires turn the same direction? That would be a turning driveshaft. Or opposite directions, driveshaft stays still? it might just be the new pinion preload magnified by the gear ratio. The lower the ratio, the more leverage the engine has on the tires. But backwards with the tires driving the driveshaft.

In neutral with both rear tires elevated, if I spin one wheel, the driveshaft spins and the other rear wheel does not move. Same results if I spin the opposite wheel.

 

[serial car restorer]

As I recall, helical diffs don't do much from the tire end. They distribute power to both wheels when accelerating, but on over-run they are like an open diff. With both wheels in the air, try spinning the driveshaft rather than a single wheel. Both wheels should turn the same direction, IIRC. If you hold one wheel from turning, the diff should act like a limited slip and resist even if the other wheel is free to spin.

 

[davisjstone]

As I recall, helical diffs don't do much from the tire end. They distribute power to both wheels when accelerating, but on over-run they are like an open diff. With both wheels in the air, try spinning the driveshaft rather than a single wheel. Both wheels should turn the same direction, IIRC. If you hold one wheel from turning, the diff should act like a limited slip and resist even if the other wheel is free to spin.

Making progress. In neutral, spinning the driveshaft (which took some effort) both wheels spin in the same direction. Seems like thats as it should be?

 

[serial car restorer]

Making progress. In neutral, spinning the driveshaft (which took some effort) both wheels spin in the same direction. Seems like thats as it should be?

Yes.

 

[Broncobowsher]

In neutral with both rear tires elevated, if I spin one wheel, the driveshaft spins and the other rear wheel does not move. Same results if I spin the opposite wheel.

That sounds like brake drag.

 

[Nothing Special]

.... With both wheels in the air, try spinning the driveshaft rather than a single wheel. Both wheels should turn the same direction, IIRC. If you hold one wheel from turning, the diff should act like a limited slip and resist even if the other wheel is free to spin.

If you do this with a TrueTrac, when you hold one tire still while turning the driveshaft it will act just like an open diff. The tire with no resistance will spin freely while you hold the other still. Both tires need to have some resistance for a TrueTrac to work.

If you jack one tire up and try to drive forward it will just spin the tire that's in the air. But if you start to apply the brake (parking brake is best since it doesn't apply all 4 brakes) it will eventually send enough torque to the tire that's on the ground to drive you off the jack.

 

[rjrobin2002]

Those Tru tracks are bad ass, I ran one for 10 years in a 75 Bronco with a hot 347 stroker and 5 spd. I did lots of random drag racing on pavement with 35s and never broke it, but always left 2 black marks on pavement. That bronco would lift a front tire in a race.