Middle axle only use in rear

Good luck with it and I might use Lucas gear oil stabilizer with the heavy weight gear oil to keep temps. down.

update: Generally speaking... she is back to driving on both axles...

The air solenoid Piston is being held in place by taking an old air fitting drilled and tapped; then an all-thread rod w/lock nut cranked inward to push the piston home. This will get updated though. It is semi temporary. Before making final decision on what will hold it home permanent am going to check with Meritor to see if they will clue me in on make up of piston (Cast hollow or solid milled, steel or aluminum, etc.) Steve6x6x6?? or anyone else ever seen the piston inside the solenoid? If so speak up your input would be appreciated.

Drive shaft shortened by about 2 inch if I recall right. Rebalanced in process.... though it was already a new shaft (NOT.... NOS but brand newly constructed)

Still got little more to do....
. Airline brackets for all the airlines
. Sway bar brackets as well as make up some bushings for sway bar affix to axle. (nylon rod stock milled out is the plan). Have new sway bar bushings for other points.
. Breather line- This one has slightly different location for axle breather so need to make new line for that.

It seems to never stop lol.... but then again all these little things would be same no matter how did this install.. be it a flipped 4x4 axle housing or a new one like done here. Flipped 4x4 axle housing still think would be harder cause would have to make a completely new spring perch pads. Anway.... some of the little things still do go....

. Making up bracket and hooking up the brake load adjuster is another task to do as well.
. Final shim to go under leaf springs to rotate axle to match axle angle to tranny output angle (seeking perfect parallel between two). Right now we are little over 1-ish degree off. This will be made after the Ambo box is installed and she sits closer to normal weight thus normal squashing of the suspension. Will then see where we are and how much shim to make.

PS .... third is older than rest of axle which is new. Tan Third contains the R&P gear upgrade and Detroit locker. Yes it will get painted. Maybe paint them Sport Car Red since they are "high speed axle ratios" LOL.

Am also going to 49" tires FWIW. Delivery all new tires coming sometime in next couple months. Was not in a rush to do tire change...... but two deals just matched out to a **** good combined price (5 NEW tires around 2500 delivered) Will be selling 4 **** good MTV's

Good progress!

Roughly what did your driveline angle come out to be?

It’s been along time since I’ve set any up, but if memory still serves, within a degree difference end to end was about as good as you realistically get and would typically run out smooth. Also for the life of me I can’t remember the theory why, but I think I remember you want to avoid a “perfectly” straight dive line, and wanted a least 2-3 degrees angle on the u-joints.

Jbulach said:

... Roughly what did your driveline angle come out to be?

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Not sure exactly.. the fellow with the knowledge doing that end of the work (Chris Jones) knows. I'll ask him and report back...

Jbulach said:

but I think I remember you want to avoid a “perfectly” straight dive line, and wanted a least 2-3 degrees angle on the u-joints.

Click to expand...

Yes in my conversations with Chris he did say typical down slope desired is a line thru the crankshaft right to axle yoke. As discussed earlier in this thread; when we discovered this was way off on my truck.... we thought something was fubared. Asked about that in thread linked and it was brought up that on these trucks the line thru crankshaft to tranny output is actually an up slope purposefully. Designed that way because the 3 degree down would have put the front driveshaft output way too steep for such a short drive line. So it was done with the down slope instead being toward front to give the front drive line that nice 3 slope. The rear tranny output though now being up sloped means the rear driveshaft has to pitch down further from ideal ..... but it was decided that this was the lesser of the devil than having the front being way to off from ideal. With this higher pinion conversion this devil has been converted to a "nicer guy" because it helps push things much closer to an ideal design. Maybe that is why the military went to all of them being high pinion with later A1's?

coachgeo said:

Designed that way because the 3 degree down would have put the front driveshaft output way too steep for such a short drive line. So it was done with the down slope instead being toward front to give the front drive line that nice 3 slope.

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As I mentioned in that thread, the Army documentation claims it was done because of lubrication requirements of the engine.

ADA416765 said:

The vehicle is required to operate on a 60% grade. Tilting the engine slightly off horizontal helped the oil lubrication in this condition. This tilt also improved ground clearance for the [24"] step test and off-road performance. This tilt, however, increased the rear driveshaft angle.

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Also, they originally had double-cardan joints to help deal with the angle, but they were weaker.

ADA416765 said:

The original design of the FMTV as provided by Steyr at the beginning of the FMTV program used double cardan constant velocity joints in the front positions on all vehicles and on the LMTV rear position. The design experienced durability issues associated with these driveshafts on test vehicles. This prompted the change to single cardan joint driveshafts.

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The reason to be clear about the cause is that if you don't care about operating on a 60% grade (31°), you could adjust the engine tilt if you wanted to. You could also try to go back to a double-cardan joint, possibly with a larger u-joint, or other design to improve robustness. Or you could try the Rzeppa-style CV, which the study seemed to show was superior in all tests except water intrusion.

One of the other factors these studies talk about is how close to critical speed these driveshafts are operating. Yes, switching to the 3.07:1 gears helps, unless you then still just drive it at its new max speed (~70MPH). One study also points out that on a downhill, especially loaded, it's easy to exceed the normal engine-governed top speed of the truck, and reach or exceed the driveshaft critical speed. (That's bad.)

Awesomeness said:

As I mentioned in that thread, the Army documentation claims it was done because of lubrication requirements of the engine....

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have heard both.... yet do not know if the design of slope done to help front drive line angle was mentioned earlier cause it was documented as well....... or not. Curious if any other CAT applications also slant up instead of down??? IF CAT's with this engine design is are also up slopped.. that would certainly support what you saw in the documentation.

Either way the High Pinon seems to be working well for the military since they switched to that design.

coachgeo said:

have heard both.... yet do not know if the design of slope done to help front drive line angle was mentioned earlier cause it was documented as well

Click to expand...

I haven't seen an actual Army document that mentions anything about it having been done to improve the front driveline angle. I generally read everything very carefully, Army documents and forum posts alike, and the only mention of the front driveline angle theory that I've seen has been here on SS in posts. In other words, I've only "heard both" on here, nowhere else yet. That doesn't prove a source doesn't exist, but personally, I'm beginning to find it increasingly unlikely. It would be good to see if other machines/trucks that use the engine tilt it forward too, especially 4WD things.

coachgeo said:

Either way the High Pinon seems to be working well for the military since they switched to that design.

Click to expand...

I would certainly like the ground clearance gains too. I wonder if there were other reasons they switched to it, because it seems like a big/complex/expensive change to make when they could have just used a Rzeppa-joint shaft with a better dust seal to solve the angle issues.

How much did you have to shorten the rear driveshaft with this axle? I wonder if that is a benefit (or possibly a defecit, for some other reason)?

Awesomeness said:

I haven't seen an actual Army document that mentions anything about it having been done to improve the front driveline angle. I generally read everything very carefully, Army documents and forum posts alike, and the only mention of the front driveline angle theory that I've seen has been here on SS in posts. In other words, I've only "heard both" on here, nowhere else yet. That doesn't prove a source doesn't exist, but personally, I'm beginning to find it increasingly unlikely. It would be good to see if other machines/trucks that use the engine tilt it forward too, especially 4WD things.
I would certainly like the ground clearance gains too. I wonder if there were other reasons they switched to it, because it seems like a big/complex/expensive change to make when they could have just used a Rzeppa-joint shaft with a better dust seal to solve the angle issues.
How much did you have to shorten the rear driveshaft with this axle? I wonder if that is a benefit (or possibly a defecit, for some other reason)?

Click to expand...

you got a way better detail memory than I in your doc. reading. Good quality to have. Thanks for adding your insights from this.

Only had to shorten like 1.5" -2"

Awesomeness said:

I haven't seen an actual Army document that mentions anything about it having been done to improve the front driveline angle. I generally read everything very carefully, Army documents and forum posts alike, and the only mention of the front driveline angle theory that I've seen has been here on SS in posts. In other words, I've only "heard both" on here, nowhere else yet. That doesn't prove a source doesn't exist, but personally, I'm beginning to find it increasingly unlikely. It would be good to see if other machines/trucks that use the engine tilt it forward too, especially 4WD things.
How much did you have to shorten the rear driveshaft with this axle? I wonder if that is a benefit (or possibly a defecit, for some other reason)?

Click to expand...

I agree with you and wasnt going to weigh in about it until I read your post. Anyone can spend as much money as they like on their truck and do 'upgrades' in any way they wish, as long as it isn't aimed to influence others and make false scientific claims.

Engine lube or driveline angle. wellll... logic seems to fit driveline angle unless there is more to the lube thing..... Just cause lube is documented does not mean it is correct information. While the research about the driveshaft angles via physical measurements is probably very accurate (math does not lie ... though it can trick you sometimes) that does not mean the mention in the study on "why" the engine is sloped up is accurate. The researchers doing the study; to them the "why" the CAT sat at an angle; may have been of little consequence in their minds and put little effort into documenting the "why" accurately. They were instructed to do a driveline test.... so they did. Obviously the actual angle measurement of the slope the CAT sat at was of consequence to their study... but the "why".... Not really. Now if there is an annotation in the study about the "why" being engine Lube .... then obviously that increases the the accuracy of their statement. Next time someones reading the study...... see if it is annotated. For all we know they miss wrote the reason... maybe it should have read for "Transmission Lubrication" reasons

Doing some quick research... found one thing that shows CAT 3116 to 3126 allowed for various mount angles from horizontal to front of engine raised up as far as 15 degrees and down as far as 5 degrees.
see ( http://www.oya.com/service/mechanical/3116_Maintenance.pdf ) but that is just one paper (written by CAT) though these are marine engines..... not sure if that makes a difference when it comes to lube. Yes for cooling systems but lube?

Anyway though..... biggest mitigating factor on the decision to go this route was the logic that less angle from tranny output the better when it comes to transmitting vibration thru the driveshaft to other components. Noting that the military changed to the high pinion.... just sorta pushed it over the edge. True....... for all we know though this decision by Military builder may have been entirely to protect the soldier...... aka a sloped up driveshaft might turn into a missle thru the floor of the truck when it rolls over an IED. Orrr..... it could have been as surmised by a few folk (mostly me lol) a combination of... reduced driveline vibration transmitting to tranny , tucked up higher out of way reducing shaft damage... all resulting in reduction of maintenance.

. Expense...... really has not been to much in all honesty. Not much more than someone getting and installing the upgraded gears.
. Not to much of a pain to do the change either.... real pain has only having about 4 hours a week to work on her and being put on hold for 3 months to get it put in a big milling machine. Honestly that though is my fault..... should have just skipped that.... grinder with many cut off wheels and or a portable bandsaw would have worked. Milling was a bit of an overkill.. but so is the whole axle swap anyway.

I find the documented explanation credible and logical. Even a very experienced driveline tech won't have much insight into the strange things done by engineers on military programs, to meet the often bizarre and demanding specifications the Army requires. In any case, my point was merely that if you choose to believe that the engine is tilted because of lubrication on steep grades, you could change the angle if you want.

My comment about the expense wasn't about YOUR expense, it was the Army's. During the engineering of the A1's, the engineers could have picked any changes they wished... different CVs, different shafts, different engine/transmission/transfer case, different axles, etc. It's a curious choice to use the intermediate axle, which is the most expensive of the 3 axles previously used. They also still didn't change the engine angle or front driveline. None of this stuff happens by accident, so there is a very intentional reason out there for it.

Don't get me wrong (which I think you are), I like the idea of switching to the intermediate axle. I just think there are much more complicated and plausible explanations behind some of the driveline choices that were made.