• NAXJA is having its 18th annual March Membership Drive!!!
    Everyone who joins or renews during March will be entered into a drawing!
    More Information - Join/Renew
  • Welcome to the new NAXJA Forum! If your password does not work, please use "Forgot your password?" link on the log-in page. Please feel free to reach out to [email protected] if we can provide any assistance.

Joel's multipurpose XJ build (rocks/boulevard)

These fuel economy guys need to come back to earth. A stock xj with the straight 6 auto and 4wd was rated at 18 highway. Lifts and bigger tires do not increase fuel economy period. 13 combined sounds about right if you drive it. Hyper mile your ass off, following big rigs at 60 and you might see 20.

Really ?? Very Nice Cherokee BTW !!

My 96, which is stock except for a 3" lift (still has stock tires on it), and 158k on the ticker, knocks down 21-22 GPS mpg on the highway regularly. I do drive the speed limit, but still...

My 88 with 350+ k on the ticker, locked rear, 2" lift, and 31's gets 15-16 GPS mpg highway, and would do better if the AW4 wasn't on its way out.

BTW, both are 4.0, AW4, 4x4 cherokees...........So I'd say something ain't right with your MPG.
 
Last edited:
Really ??

My 96, which is stock except for a 3" lift (still has stock tires on it), and 158k on the ticker, knocks down 21-22 GPS mpg on the highway regularly. I do drive the speed limit, but still...

My 88 with 350+ k on the ticker, locked rear, 2" lift, and 31's gets 15-16 GPS mpg highway.


So I'd say something ain't right with your MPG.
my jeep runs fine, and 10-12mpg on 31's is average. i dont believe anyone who claims higher than the epa ratings, which are typically pretty liberal as it is.
 
Really ?? Very Nice Cherokee BTW !!

My 96, which is stock except for a 3" lift (still has stock tires on it), and 158k on the ticker, knocks down 21-22 GPS mpg on the highway regularly. I do drive the speed limit, but still...

My 88 with 350+ k on the ticker, locked rear, 2" lift, and 31's gets 15-16 GPS mpg highway, and would do better if the AW4 wasn't on its way out.

BTW, both are 4.0, AW4, 4x4 cherokees...........So I'd say something ain't right with your MPG.


Yup. You're the norm here and anyone who claims to be getting less than 20 mpg is the exception. You solved it.
 
The nice thing about treatting this as a secondary vehicle is that mileage wasn't the primary concern. Thus far as an all arounder, I've been really happy. I'm might try having the injectors cleaned at some point, but improving that is more of a bonus than a real goal.

To be fair that 13 mpg I found on the big drive back to California was doing 80-85, the whole way. I pretty much think a lifted XJ has all the aerodynamic properties of a brick. But at least it's an attractive brick!
 
Thought I’d give this one away with the flexing shot above but seems like no one picked up on it…

Gas Tank Skid:

Having done a few fast car projects I’m not used to the concept that weight doesn’t matter. Between that and the fact that a car fire usually ruins your day (even if you get it put out), I wanted to build a gas tank skid so I’d have one less thing to worry about under the vehicle when I bang it on something. Plus the bottom side of this jeep was just about as clean as the top and I’d like to keep it presentable (the boulevard part of the build, right?)

I read that there was an OEM skid but it was only 1/8” steel and still weighed 30+ lbs. There’s also the problem that most of the OEM skids seem to have rusted their way to dust by now. You can buy an aftermarket skid (usually made from the 3/16” steel that the custom fab folks seems to love) but you put about 70 lbs under the back of your vehicle and that seemed a little nuts to me just for some gas tank protection.

Instead I did a little bit of math and figured out I could make this out of 1/4” aluminum plate and it’d still have greater bending stiffness than the steel. I did go with 5052 aluminum rather than 6061 because it’ll take an impact better. Since I’m welding it (but didn’t plan to heat treat) I’d lose a lot of the properties of the stronger alloy anyways.


Didn’t take any pictures of the cardboard mock-up but it looked exactly like what I built (only made of cardboard and tape). Didn’t both mocking up the gussets.

5DSC_0314Custom.JPG


Note I added 7/8” drainage holes in all four low corners in case I go wading as I’d rather carry back memories of a pretty stream, rather than the stream itself. ;)

The rear bumper had a few supports built in that ran along the rear unibody rails so I replicated those into my skids. Getting the fit correct was a little challenging but I’m happy with the end result. It’s well gusseted and 100% welded inside and out.

5DSC_0541Custom.JPG


5DSC_0539Custom.JPG


Oh and it weighs one lb more than the stock skid while being a hell of a lot stronger (less than stock if you include the supports I removed). Looks gangster from the back too.

5DSC_0540Custom.JPG
 
That was about $150 in aluminum and maybe 20 hours labor, 15 or so being the actual fab. I don't know how many sticks of filler rod are in this thing but it was something like 40 linear feet of weld. We fixtured the heck out of it to keep it from moving around too much and it still warped a bit. It was mostly just the flanges along the edges and thankfully it pulled back down flat when we bolted it in.

It's pretty easy to lift the rear of the jeep via the skid and a regular mechanic's jack now (which I guess was kind of the point if a rock would be doing the same thing for me). The skid spreads the load out to the frame rails nicely.
 
Last edited:
For those folks referencing the dual shear track bar... Where's the failure point with the cantilevered setup I have now? Is it the bracket that lets go or the stud? Hey Luke, if you're reading this, do you remember if the stud is OEM or part of the BDS kit?

Since my track bar itself is already a pretty nice piece (adjustable and included with the BDS long arms), I might try to retrofit something to support that a bit more while trying to avoid replacing the whole bar. FWIW that stud on the driver side frame rail has a little play in it. The BDS folks tell me it's rebuildable though.
 
Last edited:
I would definitely pay top dollar for a skid like that. The weight of my JCR is what's preventing me from mounting it permanently to my Jeep.
 
For those folks referencing the dual shear track bar... Where's the failure point with the cantilevered setup I have now? Is it the bracket that lets go or the stud? Hey Luke, if you're reading this, do you remember if the stud is OEM or part of the BDS kit?

Since my track bar itself is already a pretty nice piece (adjustable and included with the BDS long arms), I might try to retrofit something to support that a bit more while trying to avoid replacing the whole bar. FWIW that stud on the driver side frame rail has a little play in it. The BDS folks tell me it's rebuildable though.
single shear track bars loosen up constantly with any wheeling. i couldnt keep mine tight to save my life, and i only had 31's. even when it is tight, you can see the bolt flexing a lot when someone turns the wheel while you watch. i got a $60 rubicon express double shear track bar bracket and it fixed that, stiffened it up completely.


nice work on the tank skid, i too had wondered about making armor out of aluminum since it's lighter and typically has a higher yield strength than mild steels.
 
For those folks referencing the dual shear track bar... Where's the failure point with the cantilevered setup I have now? Is it the bracket that lets go or the stud? Hey Luke, if you're reading this, do you remember if the stud is OEM or part of the BDS kit?

Since my track bar itself is already a pretty nice piece (adjustable and included with the BDS long arms), I might try to retrofit something to support that a bit more while trying to avoid replacing the whole bar. FWIW that stud on the driver side frame rail has a little play in it. The BDS folks tell me it's rebuildable though.

I noticed that skid, looks really good. I had steel one but didn't put it on because I decided I didn't need the extra weight for not ever wheeling more than I did.

The stud is BDS. The double sheer mount everybody is talking about replaces the entire mount that is bolted to the frame on the side and bottom. It will end up looking similar to the mount on the axle side. You can also reinforce that mount by tieing it into the passenger side by going under the oil pan.

If I planned on wheeling it hard I would have replaced it with a different mount. I wanted to test out the complete BDS kit in stock form for myself. It's obviously held up great for years now if you have never had a problem.
 
Interesting... Thanks for the link, I hadn't seen that one. The track bar bracket at the frame rail is definitely different than what I have yet the overall shape of the bar's connection point is similar (round bore with some kind of bushing, rather than a heim joint).
 
Luke, that's food for thought for sure... Thanks for the link, I hadn't seen that one. The track bar bracket at the frame rail is definitely different than what I have yet the overall shape of the bar's connection point is similar (round bore with some kind of bushing, rather than a heim joint).

Vanimal, thanks for the Rubicon Express mention too. I'll look into that as well. As to your implied question:

nice work on the tank skid, i too had wondered about making armor out of aluminum since it's lighter and typically has a higher yield strength than mild steels.

Working with aluminum is all about picking the right alloy for the job. First and foremost unless you REALLY know what the heck you're doing don't use it for structural parts if you can help it (IE I won't be making a track bar mount from the stuff) since it basically has no fatigue limit. The upside as you mention, is the weight. The density tends to run about 1/3 that of steels so you can go heavier wall (which helps bending moment of inertia) while still saving weight. The other downside of aluminum is that’s it’s expensive… I usually pay about $3.29 per lb for alum plates where steels are often only $0.89 per lb.

If you compare strengths you find:
-A36 mild steel: 36 ksi yield, 60 ksi ultimate
-5052-H32: 28 ksi yield, 33 ksi ultimate
-6061-T6: 37 ksi yield, 42 ksi ultimate
-7075-T6: 67 ksi yield, 76 ksi ultimate

If you look closely the ratio of yield (at what point we have permanent deformation) to the point it finally tears apart (ultimate strength), the mild steel will outlast the aluminum by miles. To put in another way, in an impact we want the ability to dissipate energy and the ability to stretch without breaking is great for that.

Minimum elongation at break:
-A36 mild steel: 20%-23%
-5052-H32: 12%-18%
-6061-T6: 12%
-7075-T6: can be as small at 3%, best case is 11% (varies with thickness)

This basically means that 7075 is quite strong but also super brittle. It’s a lot more likely to crack than to deform when push comes to shove.

See also the minimum bending radius (shown as a function of material thickness)

alumbendrad.jpg


For the alloys I mentioned if I were using 1/4” plate I could bend the 5052 at 3/8” radius, 6061 I could use a 1” bend radius, but I’d have to use a whopping 2.5” radius on the 7075. Bend any sharper and it’ll just crack on you.

The other catch is welding… Once welded, aluminum is in the annealed state (meaning weakest but most flexible state possible state for the chemistry of that alloy). This is the “T0” state referenced in the chart. For the 5052 I used here, the T0 state has an yield strength of 13 ksi and an ultimate of 28 ksi. Not terrible, but still WAY down for the hardened numbers above.

In my case, I choose 5052-H32 for my gas tank skid because I thought it was more likely to need the impact resistance, but I went 6061-T6 for my belly pan because I wanted the stiffness and strength for the larger area and thought it was less likely to see blows and more likely to see scraping. I also used ribbed and riveted construction on the belly pan to avoid the heat affected zone issues, but now I’m getting ahead of myself and have a few other things I need to write up first.
 
Last edited:
Luke, that's food for thought for sure... Thanks for the link, I hadn't seen that one. The track bar bracket at the frame rail is definitely different than what I have yet the overall shape of the bar's connection point is similar (round bore with some kind of bushing, rather than a heim joint).

Vanimal, thanks for the Rubicon Express mention too. I'll look into that as well. As to your implied question:



Working with aluminum is all about picking the right alloy for the job. First and foremost unless you REALLY know what the heck you're doing don't use it for structural parts if you can help it (IE I won't be making a track bar mount from the stuff) since it basically has no fatigue limit. The upside as you mention, is the weight. The density tends to run about 1/3 that of steels so you can go heavier wall (which helps bending moment of inertia) while still saving weight. The other downside of aluminum is that’s it’s expensive… I usually pay about $3.29 per lb for alum plates where steels are often only $0.89 per lb.

If you compare strengths you find:
-A36 mild steel: 36 ksi yield, 60 ksi ultimate
-5052-H32: 28 ksi yield, 33 ksi ultimate
-6061-T6: 37 ksi yield, 42 ksi ultimate
-7075-T6: 67 ksi yield, 76 ksi ultimate

If you look closely the ratio of yield (at what point we have permanent deformation) to the point it finally tears apart (ultimate strength), the mild steel will outlast the aluminum by miles. To put in another way, in an impact we want the ability to dissipate energy and the ability to stretch without breaking is great for that.

Minimum elongation at break:
-A36 mild steel: 20%-23%
-5052-H32: 12%-18%
-6061-T6: 12%
-7075-T6: can be as small at 3%, best case is 11% (varies with thickness)

This basically means that 7075 is quite strong but also super brittle. It’s a lot more likely to crack than to deform when push comes to shove.

See also the minimum bending radius (shown as a function of material thickness)

alumbendrad.jpg


For the alloys I mentioned if I were using 1/4” plate I could bend the 5052 at 3/8” radius, 6061 I could use a 1” bend radius, but I’d have to use a whopping 2.5” radius on the 7075. Bend any sharper and it’ll just crack on you.

The other catch is welding… Once welded, aluminum is in the annealed state (meaning weakest but most flexible state possible state for the chemistry of that alloy). This is the “T0” state referenced in the chart. For the 5052 I used here, the T0 state has an yield strength of 13 ksi and an ultimate of 28 ksi. Not terrible, but still WAY down for the hardened numbers above.

In my case, I choose 5052-H32 for my gas tank skid because I thought it was more likely to need the impact resistance, but I went 6061-T6 for my belly pan because I wanted the stiffness and strength for the larger area and thought it was less likely to see blows and more likely to see scraping. I also used ribbed and riveted construction on the belly pan to avoid the heat affected zone issues, but now I’m getting ahead of myself and have a few other things I need to write up first.
you're not really supposed to weld 7075 though, it's no bueno. sure does machine well though. i use it for the bicycle parts i machine and sell since it wears much better than any other aluminum i've found.

ps, i was an engineer too so i'm fairly familiar with the properties of metals. manufacturing engineer though.
 
Pardon me, you probably knew most of the above. Hopefully the information helps someone. Now that you mention it, I've heard the no welding rule about 7075. Always wondered how they make mtn bike frames from the stuff. I work in the theme park industry which is about as general an engineering role as you can find (but we certainly know how to analyze fatigue!)
 
Exhaust (out with the old):

I was on a trail not terribly long ago and I watched an H2 Hummer high center itself on a rock shelf that was about 30” high but shorter than the wheelbase. There was no way around this obstacle on the trail. They then proceeded to hook up a snatch strap and had a Jeep ahead yank on the strap harder and harder to drag it off the rock. To get it free, the lead Jeep was hitting that strap so hard it was yanking his back tires off the ground. I was waiting for someone’s bumper to get torn off.

That poor Hummer got dragged across several feet of rock on its under belly and was a lot worse for the wear by the end of the day (it broke an axle later too). After looking at the bottom side on my XJ I realized that if I ever hung up in similar fashion I would instantly destroy my exhaust and could easily wedge something relatively hard into something relatively tender.

That and the fact that the stock exhaust is easily 5” below the frame rails means there’s definitely potential for improvement.

5DSC_0362Custom.JPG


5DSC_0363Custom.JPG


5DSC_0364Custom.JPG



I’m trying to make this rig even more capable than it was as purchased but I don’t really intend to destroy it. I’d like to keep it shiny as long as I can and I think I can still have a lot of fun and go a lot of places without flopping it.

That said, my exhaust was pretty rusty (just about the only rust on the rig actually) and one spot on the muffler was soft like the rust might be about to break through. As such, I decided to replace it and see if I couldn’t tuck things a little higher to add in ground clearance.

Once it was all down and off the car I was in for a surprise. I’d read about the factory dent to allow pinion clearance just off the header, but I was totally unprepared for what I found. You’re kidding me, 4.0L of exhaust displacement is supposed to flow through THIS!

5DSC_0367Custom.JPG


5DSC_0368Custom.JPG


It looks like about the cross sectional area of a 1.5" pipe. Yes I understand you can tune a motor with a little back pressure, but there’s also something to be said for maintaining exhaust velocity and smoothing the flow. I could be wrong but XJ’s are the only Jeep I know of that comes with this “feature” (can anyone confirm?)

So opinion time: anyone know anything about this? Anyone have articles or references to bring to the table of whether this serves any purpose beyond clearancing? I already made my call and built the new system but I’m curious what y’all think.

Discuss.
 
well i know i welded a 90 in mine and up a little higher. It feels alot more torquey, but that might have something to do with the high flow cat and glasspack i added at the same time

also added a hanger to the body and removed the stock hanger on the crossmember. Now my exhaust only hangs about 1/4 of and inch below the framerails
 
Back
Top