Help understanding viscous couplings

[lmabadie]

I am a little unclear about how the viscous coupling in a JGC Limited 5.9 (model 98) with Quadratrac works and how it will react to tires with uneven wear.
1) Does this Jeep normally run in pavement as a 2WD (rear) and only when the coupling senses slippage connects the front axle? How fast does it happen since it needs to warm up? Or is it always connected transferring about 50% torque to each shaft?
2) Is the VC bidirectional, that is, will it also connect if the back wheels are running FASTER than the front ones, as it would happen if worn tires are installed in the back and new in the front?
3) If one pair of tires is more worn than the other one, does it make it any difference if they are installed in the rear or in the front axle?
4) Tires are never identical. I read that some cars indicate 1/4" maximum difference in the circunference of the tires to avoid damage. So, in dry pavement, what does compensate for that difference? Is it that with that difference the heat generated in the VC is not enough to couple it?
5) I am having a hard time understanding the benefits of the Quadratac if it rarely works in pavement, I understand it is not of much help off road, and forces owner to constantly check tire wear and pressure or risk damage to the transmission / differential.....

Any help welcome.

[88fadedglory]

Originally Posted by lmabadie

I am a little unclear about how the viscous coupling in a JGC Limited 5.9 (model 98) with Quadratrac works and how it will react to tires with uneven wear.
1) Does this Jeep normally run in pavement as a 2WD (rear) and only when the coupling senses slippage connects the front axle? How fast does it happen since it needs to warm up?
2) Is the VC bidirectional, that is, will it also connect if the back wheels are running FASTER than the front ones, as it would happen if worn tires are installed in the back and new in the front?
3) If one pair of tires is more worn than the other one, does it make it any difference if they are installed in the rear or in the front axle?
4) Tires are never identical. I read that some cars indicate 1/4" maximum difference in the circunference of the tires to avoid damage. So, in dry pavement, what does compensate for that difference? Is it that with that difference the heat generated in the VC is not enough to couple it?
5) I am having a hard time understanding the benefits of the Quadratac if it rarely works in pavement, I understand it is not of much help off road, and forces owner to constantly check tire wear and pressure or risk damage to the transmission / differential.....

Any help welcome.

A vicous coupling engages when there's sufficient difference between front and rear driveshaft speeds to cause the silicone based fluid to heat up and get thicker. In theory, it could get almost solid.

In normal driving, most of your torque is routed to the rear wheels then if they slip / spin, the front wheels connect thru the coupling.

If there is enough difference in tire sizes, then yes, it could connect the axles but I think it would take much more than a 1/4" difference in tire diameter because just making a sharp turn would cause the coupling to engage if it were true that you have to be under 1/4" difference in diameters.

[96sport]

Here's the solution, junk the the NP249 and install a NP242 or NP231. Sooner or later the VC will wear out and it doesn't make sense to replace it because of what it costs and all the headaches it causes.

On the other hand, 88fadedglory explained it pretty well. Think of it as limited slip differential, it feels the slippage and transfers power to the other end so you can hopefully gain traction. If your tires are smaller on the front than the rear, that means that the front end driveshaft is spinning faster than the rear and the VC is going to heat up and lock, which will then cause transfer case to prematurely wear out (especially the VC) and could cause differential damage. Hence the reason for the tires needing to be as close to the same size as possible.

There is nothing wrong with that type of transfer case if you have the money to replace all 4 tires at the same time if something happens to one of them. They're just a PITA if you don't because that's when things start to go wrong.

Ohh, they work just fine offroad, my buddy has the 318 the NP249 and it works fine for him, just higher maintence costs.