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post #1 of (permalink) Old 08-30-2003, 05:03 AM
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Truss of choice?

What are the best kind of trusss'<--- (SP???)
Ones that support above, below or horizontal to the Axle?

Do you have a favorite?

This one seems pretty beefy on a D30.

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post #2 of (permalink) Old 09-02-2003, 09:37 AM
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Re: Truss of choice?

If it is mounted below the axle, the truss will be in tension and the axle will be in compression. If mounted above the axle, the truss will be in compression and the axle in tension.

In compression, the truss must be bigger to resist buckling. In tension, it just needs enough cross section to carry the tension load and can be much smaller. Unless you plan to jump it, I don't think you'll need a truss on the front axle. I haven't seen or heard of any shortwheelbase jeeps bending axles, just fullsize trucks and fullsize jeeps when jumping as in tough truck competitions.
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post #3 of (permalink) Old 09-02-2003, 01:30 PM
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Re: Truss of choice?

What you have linked to is not really a truss, it's a brace. Your comment about it being "beefy' is a giveaway. Trusses are designed to be strong and light by proper placement of minimal size material.

Will,
Not exactly. No matter whether the truss is above or below, some members are in tension while other members are in compression. That's what makes a truss design work.

So why do you think you need a truss anyway. That pumpkin in the center looks big but it's not that heavy so it doesn't need a lot of support. The weight of the Jeep is supported pretty near the ends of the axles and near the tires so unless you are carrying a lot of weight and/or doing Evel Knievel style jumps or land really hard coming off of a boulder, you probably don't need much.

Well, there is one other consideration, torque. With an NV4500 and a Tera-Low or Atlas case, you would have a gear ratio of 25:1 multiplying the engine torque into the differential trying to twist it about the pinion axis. Drag racers used to have a strange looking brace for this that was heavy on the top of the axle on one side and on the bottom on the other side but until they started doing the burnouts, they never backed up.

Now for the design, first, it's not easy to weld to cast so to avoid that, assuming a rear cover diff., I'd cut and drill a plate to fit under the cover. This would also add strength to the case. The plate would have tabs bent over at the top and bottom to weld to the truss members. Also note that the one you linked to has coils. If your application doesn't, you need to end the truss before the U-bolts or do something special to accommodate them.

Here's a design I came up with a few years ago. I never built it so it's untested.


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post #4 of (permalink) Old 09-02-2003, 02:31 PM
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Re: Truss of choice?

</font><blockquote><font class="small">In reply to:</font><hr />
Will,
Not exactly. No matter whether the truss is above or below, some members are in tension while other members are in compression. That's what makes a truss design work.


[/ QUOTE ]

Try rereading my post.

Truss above, truss in compression, axle in tension.
Truss below, truss in tension, axle in compression.
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post #5 of (permalink) Old 09-02-2003, 03:47 PM
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Re: Truss of choice?

</font><blockquote><font class="small">In reply to:</font><hr />
Try rereading my post.

Truss above, truss in compression, axle in tension.
Truss below, truss in tension, axle in compression.

[/ QUOTE ]

That's fine for the main members, but you disregarded the uprights and diagonals if any. If as you say, truss below, truss in tension, axle in compression, then the connecting verticals are in compression. For truss above, truss in compression, axle in tension and the ends are diagonal, the end uprights must be in tension. If you have no others, the pumpkin serves as an upright.

The whole thing is based on triangulation and I believe that if two legs are in tension, the third must be in compression and vice versa.
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post #6 of (permalink) Old 09-03-2003, 09:55 AM
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Re: Truss of choice?

</font><blockquote><font class="small">In reply to:</font><hr />
That's fine for the main members,

[/ QUOTE ]

I was talking about the main members. The in between pieces are to reduce the unbraced length. You also forgot to mention that in the picture of the truss above the dana 30 in the top post that the web is in shear.
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post #7 of (permalink) Old 09-03-2003, 11:16 AM
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Re: Truss of choice?

[img]images/graemlins/laugh.gif[/img] [img]images/graemlins/laugh.gif[/img] [img]images/graemlins/laugh.gif[/img] [img]images/graemlins/laugh.gif[/img] [img]images/graemlins/laugh.gif[/img] [img]images/graemlins/laugh.gif[/img] [img]images/graemlins/laugh.gif[/img]

Good one Will. You had me going there for a minute but then I looked at your profile and knew no engineer would be making such comments in earnest. That web in shear was the tip off. It was just too over the top. Funny stuff.
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