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Front Axle Disconnect Discussion


Torq_Shep
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I was considering putting my front disco axle in my truck, I read a bunch of forums, and finally searched the SAE papers... I found one published last year by FCA, GM, etc "Axle Efficiency Comparison Method and Spin Loss Benefit of Front Axle Disconnect Systems"

 

I am pretty sure I cannot share the entire paper, and most of it is test methodology (interesting to me as it aligns with what I do for work but useless to most). However, the results were interesting. 

image.png.f201248b5ab4108c2fea6064f137a73d.png

 

image.png.2a5308a8fed21d96b0f7b9af2de7eb41.png

 

What these two graphs represent is the "On Throttle Loss" and the "Off Throttle Loss". So if you are trying to get more power from your rig, the FAD won't impact anything, however, if you want to improve fuel economy, the FAD will decrease drivetrain losses while going down a hill or slowing down fairly significantly. The axle disconnect style is of the same type (although electronically controlled) as the ones in our old D30s. 

 

Will I swap back to disco axle with a posi-lock? I am not sure yet =P

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still just a tiny fraction of the power needed to move a truck around.  you'd never get your money back from buying a selectable CAD.  :(  you can get 2wd LOW, which could be useful for certain things, but don't do it for saving money. 

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4 minutes ago, Pete M said:

still just a tiny fraction of the power needed to move a truck around.  you'd never get your money back from buying a selectable CAD.  :(  you can get 2wd LOW, which could be useful for certain things, but don't do it for saving money. 

I would only do it because I already have a CAD axle with the right gears. I am pretty sure I am not going to but still =P

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27 minutes ago, Pete M said:

2wd LOW

I've said it in other threads, but its worth repeating: I LOVE having 2wd Low. Getting into tight spots, backing trailers, going through rough fields, etc: I just pop it in 2wd low, select a gear, let the clutch out and forget about the throttle. My truck can idle along in 5th gear and 2wd low. Its awesome and well worth it IMO. 

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Yeah, 10 Nm isn't much.  The 80 MPH will have a much bigger negative impact than that.

 

Still, at reasonable speeds that little bit of reduced effort will add up.

 

I wouldn't go to any length to add it.  But if it's there I wouldn't put effort to remove it either.

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Just now, derf said:

But if it's there I wouldn't put effort to remove it either.

 

I would absolutely remove it.  the factory CAD is a headache waiting to happen. :( 

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15 minutes ago, Pete M said:

 

I would absolutely remove it.  the factory CAD is a headache waiting to happen. :( 

Oh, I'd remove/bypass it once it broke.  But if it ain't broke yet...

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On 6/29/2021 at 11:26 AM, JustEmptyEveryPocket said:

I've said it in other threads, but its worth repeating: I LOVE having 2wd Low. Getting into tight spots, backing trailers, going through rough fields, etc: I just pop it in 2wd low, select a gear.... Its awesome and well worth it IMO. 

This  has been on my bucket list, I saw it years ago but haven't for a while. Who did you buy yours from? How was the install? I need to rebuild by t-case and thought I would add it them.

 

A few month ago I had inadvertently popped the vacuum tip off the CAD. Thought it was broke, but enjoyed 2 low for a few weeks when I needed it. great on hills and picking up debris.

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On 6/29/2021 at 11:49 AM, derf said:

Yeah, 10 Nm isn't much.  The 80 MPH will have a much bigger negative impact than that.

 

Still, at reasonable speeds that little bit of reduced effort will add up.

 

I wouldn't go to any length to add it.  But if it's there I wouldn't put effort to remove it either.

 

I would (go to the effort to remove it).

 

As Pete already commented, the increased friction/drag loss of the engaged front axle compared to the unlocked front axle may look significant in that graph, but that's ONLY the front axle. In the macrocosm of the entire vehicle, it doesn't amount to anything. And don't forget, that's an SAE article, it's not specific to Jeep with the XJ/MJ half-baked disconnect that still leaves the left side axle connected to the differential, so there's still some friction loss there, plus the internals of the front diff are still churning around in there. So we're not getting the reduction that a vehicle with Warn hubs would see.

 

Beyond that, it (the front axle disconnect) is just one more thing to break. My original '88 Cherokee has it. When do you suppose it decided to fail? On my way to work one morning, in a blizzard, when I was wearing a suit. And I didn't even know that it had failed, because the dash light that said I was in 4WD came on even though the vacuum line at the CAD had rotted and fallen off. So I was going up a hill in 4-Hi ... and I got about halfway up and that was as far as I was going to go. I tried probably half a dozen times. Even with a functional Trac-Lok in the rear axle, I could ... not ... get ... up ... that ... hill.

 

So I gave up and took a round-about route that didn't involve steep hills. I didn't find out the CAD had failed until I was underneath the Jeep doing the next oil change.

 

It all depends on your usage. If you often find yourself in situations where you can really benefit from 2WD-Low, then install a Posi-Lock. For most of us, I doubt we'll need 2WD-Low often enough to justify the cost of converting to a Posi-Lock. For the rest of us, just open up the CAD, slide the collar over and lock it in place, and button it up again. No more worries about the vacuum lines rotting, swelling, or splitting.

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I also wonder about how it applies to the Jeep CAD, although most of the axle disconnects I’ve seen rely on a similar setup, a sliding collar on the axle shaft. The one I’d most be interested in knowing about the effects on MPG are the automatic locking hubs, like the Ford in the mid 2000’s (and probably other years, I’m just not interested enough in Ford trucks to know). Just stop everything from spinning at the spindle. Mind you, that particular system is pretty trash if you don’t want to ever have to get out of the vehicle, considering they like to seize up if you get anywhere near road salt, but at least there’s a visual indicator (assuming the auto hub also spins the dial over to the locked position) and a manual override in the event the actuation system fails.

I will add that my fuel economy numbers did drop after I swapped in the one-piece shaft. It’s hardly a scientific study with actual control over driving speeds, weather conditions, fuel pump calibrations, or vehicle deterioration, etc., but with CAD unlocked I have a few fill-ups where I got between 19 and 19.3mpg, and after the one-piece swap I’ve never touched 19 again. I think I’ve seen 18.7 on a road trip that didn’t get recorded, but mostly it’s been below 18.5. Again it’s not a proper scientific study and only anecdotal, and I made no efforts to properly compare before and after or ever to achieve consistent mpg numbers at all, so take it how you will. But while the difference between 18.7 and 19.3 may seem small at only 13 miles further across a 23-gal tank, but that 3% difference does add up over time. 

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11 hours ago, gogmorgo said:

I also wonder about how it applies to the Jeep CAD, although most of the axle disconnects I’ve seen rely on a similar setup, a sliding collar on the axle shaft. The one I’d most be interested in knowing about the effects on MPG are the automatic locking hubs, like the Ford in the mid 2000’s (and probably other years, I’m just not interested enough in Ford trucks to know). Just stop everything from spinning at the spindle. Mind you, that particular system is pretty trash if you don’t want to ever have to get out of the vehicle, considering they like to seize up if you get anywhere near road salt, but at least there’s a visual indicator (assuming the auto hub also spins the dial over to the locked position) and a manual override in the event the actuation system fails.

I will add that my fuel economy numbers did drop after I swapped in the one-piece shaft. It’s hardly a scientific study with actual control over driving speeds, weather conditions, fuel pump calibrations, or vehicle deterioration, etc., but with CAD unlocked I have a few fill-ups where I got between 19 and 19.3mpg, and after the one-piece swap I’ve never touched 19 again. I think I’ve seen 18.7 on a road trip that didn’t get recorded, but mostly it’s been below 18.5. Again it’s not a proper scientific study and only anecdotal, and I made no efforts to properly compare before and after or ever to achieve consistent mpg numbers at all, so take it how you will. But while the difference between 18.7 and 19.3 may seem small at only 13 miles further across a 23-gal tank, but that 3% difference does add up over time. 

The paper was based on an IFS one side shaft electronic disconnect. Mechanically should be fairly similar to our trucks but fuel economy is extremely difficult to measure. Torque is fairly easy to measure =P

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I had a Posi Lock on mine before I swapped to the 1 piece axle. 2wd low was nicer the one time I used it. The strength and simplicity of the one piece seems worth the loss of fuel mileage.

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On 7/2/2021 at 8:53 AM, War_Pony89 said:

This  has been on my bucket list, I saw it years ago but haven't for a while. Who did you buy yours from? How was the install?

Well I started with THIS kit. Install was fairly straightforward, and took me an afternoon. I did not drill through the firewall, instead I found a hole already in place and bought a grommet for it. Another change I did was mount the cable through the dash so that the button stuck out through the hole for the transmission power/comfort plate (plate was missing in my truck already so this was free real estate). After some adjustments it worked flawlessly for ~3 years. 

 

Eventually the outer sheath worth through and the inner cable popped out. So I removed the whole thing and went back to the vacuum operated CAD housing. However, there was no way in hell I was giving up on 2WD Low after having had the good life. To make it all work I used some vacuum hose I had, found a relay kicking around, some wire, and purchased two vacuum solenoids. Everything mounted where the 4WD indicator switch sits, pulled vacuum from the reservoir ball I relocated there, and I wired it all so that when the switch in my cab is off vacuum is applied to the unlocked direction of the CAD. When the switch it flipped it grounds the relay and activates the solenoids so that vacuum is applied to the locking side of the CAD. Again, works flawlessly (so far), has short easy to replace runs of vacuum hose, and is very easy to diagnose if something does go wrong.

 

I understand this is not the norm and won't be the best choice for everyone. But for how I use my truck its easily worth it for 2WD Low, which I use way more often than 4WD. YMMV. 

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2 hours ago, JustEmptyEveryPocket said:

 

I understand this is not the norm and won't be the best choice for everyone. But for how I use my truck its easily worth it for 2WD Low, which I use way more often than 4WD. YMMV. 

I have to ask what you use 2wd low for? Maybe for maneuvering trailers? I can also see it being useful in stop and go traffic. I got stuck behind a collision a few weeks back and even with 4.10’s and stock tires I couldn’t go as slowly in 1st as the line was creeping along. 

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9 hours ago, gogmorgo said:

I have to ask what you use 2wd low for

My truck is a daily driver as well as a farm truck on my 30+ acres. Its spends 90% of its life in regular 2WD High going down the road. 9% of its life is in 2WD Low to back my small trailers around, drive over the uneven ground around my land, putting the truck in tight areas to unload grain or hay, bucking haybales out of the field, traversing the back county roads around where I live that try to rattle your fillings out (hence it really nice to pop it in a gear and forget about anything except the steering wheel and not dying), and probably other things that I am not thinking about right now. The remaining 1% of its life is is 4WD of some sort, but I only use that if 2WD Low failed me.

 

While I could use 4WD Low for most of the above things (and therefore just lock the CAD) there would be alot of stress on the transfercase. Some of the above operations require lots of steering movement afterall. So I opted for max utility.

 

This leads me to wonder: does anyone else use their MJs like I do?

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Mine is pulling daily duty mostly city at the moment. About 12 miles/day. Once I sell my other vehicles and buy another van it will probably be a mostly weekend/nice weather daily. 

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