Stupid beginner question: PT 4WD vs AWD for offroad, solid axles...

[jlg]

So, can someone point me to a good explanation FOR Offroad why the PT 4wd in the Jeep is better than what could be done with AWD?

I would think they could have AWD and allow all the differentials to be locked/normal electronically, no?(which I would think would then give 4WD equivalent to the PT 4wd when engaged?)
Also, why are solid axles better than independent suspensions for offroad? Better articulation, or????

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[Rock Krawler Suspension]

So, can someone point me to a good explanation FOR Offroad why the PT 4wd in the Jeep is better than what could be done with AWD?

I would think they could have AWD and allow all the differentials to be locked/normal electronically, no?(which I would think would then give 4WD equivalent to the PT 4wd when engaged?)
Also, why are solid axles better than independent suspensions for offroad? Better articulation, or????

There's a lot of questions to answer there. The biggest of which are 4x4 vs AWD and IFS vs Solid Axle.

4x4 vs AWD:
AWD generally uses a viscous coupler to drive all 4 wheels, it allows the front and rear wheels to spin at different speeds whereas a conventional transfer case is typically chain driven locking the front and rear drive shafts to the same speed. An AWD case with a locking center diff has proven unreliable in most offroad applications over the years though it does work very well in street and rally applications.

IFS vs Solid Axle:
It is tough to compare apples to apples here but we'll try. A solid axle generally has a much lower spring rate, operating in a 1:1 Motion ratio meaning that the wheel movement is nearly directly proportionate to the shock movement, its generally accepted that a solid axle works better in rocks as the light spring rates and 1:1 motion ratio alows the axle to work easily. IFS runs much heavier springs and much shorter shocks, operating on motion ratios typically in the 1.4-1.7:1 range but packaging a long travel shock into the average IFS vehicle is challenging. IFS is generally accepted as much better in fast paced open offroad, low articulation scenarios. All of those rules go out the window when getting into legitimate off road race applications where many of the most extreme Ultra 4 cars are running IFS, some even IRS, but in packaging that would never fit in a production IFS vehicle.

Anything can be done in either one but these are the most basic of generalizations. Hope that helps.

 

[jlg]

PERFECT -- exactly what I was looking for.

One small question, what about the AWD center diff that causes it to be unreliable? I would have thought that, with enough engineering, they should be able to have brought that up to snuff.

 

[ShadowsPapa]

One of the best examples of solid axles and SLA/IFS differences is shown when a person takes an already lifted SX4 rock crawling and maxes it out, then goes home to their shop and puts a Jeep solid axle under the front and goes back out.......

As the control arms swing up and down on a SLA (short/long arm suspension) they also swing in. That restricts travel but also means the wheels camber badly (tip in at the top) and you have limited axle travel up and down- - and must use CV joints at the steering knuckles because of the up and down plus left to right motion of the spindle. I have replaced axles in my SX4 - at about 160,000 miles)

You have two joints in each axle with a SLA or IFS vehicle, one at each axle for a solid axle and you restrict the angle of that joint to only turns, not terrain movement.

Note near the end of the video below how the one SX4 has a right wheel up off the ground - limited motion due to the spring and control arms being restricted to a small arc.
The arc of a long solid axle is at least triple that. (of course only with sway bar disconnected)
(I have an SX4 but have never tried it like these guys - just in snow, mud and ditches and it's a BLAST to drive. And, it sits higher STOCK than many modern SUVs.)

 

[ShadowsPapa]

PERFECT -- exactly what I was looking for.

One small question, what about the AWD center diff that causes it to be unreliable? I would have thought that, with enough engineering, they should be able to have brought that up to snuff.

They MAY be referring to the coupling inside that makes it act like "limited slip" between the axles - the viscous coupling in them goes bad now and then, not as strong as a direct connection.
I can leave my little Eagle in 4x4 mode all year all the time, no trouble and yet if that coupling goes bad it's an open differential in that transfer case.
I had to replace my t-case due to a blown coupling. Get one wheel on slick surface and it was not fun. Luckily I was able to find a brand new old stock, still in the crate, transfer case. It's almost unstoppable now - until that coupling blows in 100,000 miles.

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