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  #31  
Old March 30th, 2012, 08:58
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Vanimal Vanimal is offline
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Re: Joel's multipurpose XJ build (rocks/boulevard)

Quote:
Originally Posted by frijolee View Post
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.
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  #32  
Old March 30th, 2012, 09:11
Luke Graham Luke Graham is offline
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Re: Joel's multipurpose XJ build (rocks/boulevard)

Quote:
Originally Posted by frijolee View Post
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.
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  #33  
Old March 30th, 2012, 09:13
Luke Graham Luke Graham is offline
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Re: Joel's multipurpose XJ build (rocks/boulevard)

You might be able to just replace that threaded end on the track bar and get a new bracket. Or just build it.

Here's some pictures.

http://bds-suspension.com/instructions/124611.pdf

Only problem with that bracket is it's dropped lower to match up with a drop pitman arm.
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  #34  
Old March 30th, 2012, 10:18
frijolee frijolee is offline
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Re: Joel's multipurpose XJ build (rocks/boulevard)

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).
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  #35  
Old March 30th, 2012, 10:30
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Vanimal Vanimal is offline
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Re: Joel's multipurpose XJ build (rocks/boulevard)

here's the one i have: http://www.rubiconexpress.com/Produc...px?part=RE1665
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  #36  
Old March 30th, 2012, 11:09
frijolee frijolee is offline
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Re: Joel's multipurpose XJ build (rocks/boulevard)

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:

Quote:
Originally Posted by Vanimal View Post
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)



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 by frijolee; March 30th, 2012 at 11:16.
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  #37  
Old March 30th, 2012, 12:49
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Vanimal Vanimal is offline
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Re: Joel's multipurpose XJ build (rocks/boulevard)

Quote:
Originally Posted by frijolee View Post
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)



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.
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  #38  
Old March 30th, 2012, 13:30
frijolee frijolee is offline
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Re: Joel's multipurpose XJ build (rocks/boulevard)

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!)
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  #39  
Old March 30th, 2012, 13:45
frijolee frijolee is offline
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Re: Joel's multipurpose XJ build (rocks/boulevard)

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.








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!





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.
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  #40  
Old March 30th, 2012, 13:57
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Tacedaddy Tacedaddy is offline
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Re: Joel's multipurpose XJ build (rocks/boulevard)

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
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  #41  
Old March 30th, 2012, 14:08
frijolee frijolee is offline
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Re: Joel's multipurpose XJ build (rocks/boulevard)

Tace... I think yours was one of the builds I stumbled across in my digging as your description is very similar to a few pictures I saw. I think I've searched every variaion of "low profile", "exhaust routing", "belly pan", and "tummy tuck" I could think of.

I've been reading a great deal in an effort to keep the basic questions to a minimum.
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  #42  
Old March 30th, 2012, 14:09
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Vanimal Vanimal is offline
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Re: Joel's multipurpose XJ build (rocks/boulevard)

Quote:
Originally Posted by frijolee View Post
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!)
no worries, it is great info, thanks for posting it for everyone.
bike frames dont use 7075, they usually use 7005, which is weldable. similar in strength as well.
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  #43  
Old March 30th, 2012, 14:14
frijolee frijolee is offline
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Re: Joel's multipurpose XJ build (rocks/boulevard)

Hmmm, likely my mistake then. I'm not a materials engineer, but I try to at least make informed decisions for what I use where. If anyone wants a basic materials overview applicable to the automotive world I highly recommend:

"Engineer to Win" by Carroll Smith
http://www.carrollsmith.com/books/ngner2win.html

Actually his whole series of "... to win" books are brilliant, particularly if you care about the implications of go-fast projects.

EDIT:
I was curious so I looked it up and they do make 7075 frames (glad I wasn't entirely delusional), but they tend to be glued/bonded construction. Mystery solved.

Last edited by frijolee; March 30th, 2012 at 14:42.
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  #44  
Old March 30th, 2012, 14:24
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TRCM TRCM is offline
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Re: Joel's multipurpose XJ build (rocks/boulevard)

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Originally Posted by Vanimal View Post
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.
Quote:
Originally Posted by DeftwillP View Post
Yup. You're the norm here and anyone who claims to be getting less than 20 mpg is the exception. You solved it.


I getcha....sarcasm.....I didn't believe it either when I first got the jeep, but it has proven real time and time again. The trip to my hunting property and back is 198 miles. I have made this trip now over 10 times since I got the 96, and each time, it takes 9-10 gals to fill it back up, depending on how much I drive around on the property. In the 88, I have made the same trip over 100 times, and it takes ~ 15 gals to fill it back up (it varies as I never just drive out and back in it).

Don't believe me, come on out, we'll go for a drive and I will prove it to ya. I do consider myself lucky in that it does get as good a mpg as it does.

I wasn't trying to say everyone gets that kind of mileage (or even should, as I see how most people drive), but that to get as low as the OP was getting, he might have some problems to find. I know my 88 was in the 12-13 mpg range when I found out my TC wasn't locking up. Put in a switch, and got my mpg back.

Last edited by TRCM; March 30th, 2012 at 14:29.
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  #45  
Old March 30th, 2012, 14:31
frijolee frijolee is offline
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Re: Joel's multipurpose XJ build (rocks/boulevard)

Hey TRCM,

Any chance you have a write-up on the converter switch? How did you prove it wasn't locking up? This sounds like yet one more thing for me to learn about, but you might save me some searching if you have the info handy. To be honest, this is the first automatic vehicle I've ever purchased. I just bought a new Mazda5 to be the family hauler and I got it with a 6 speed (only van in the USA currently sold that comes with a manual).

My wife prefers the stick as well but 90% of the guys playing in rocks swear by the auto.

Last edited by frijolee; March 30th, 2012 at 14:47.
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