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Old 07-02-2011, 07:19 PM   #1 (permalink)
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Question Calling PBB engineers! F.S. for upper link?

What are you folks calculating for a factor of safety for your upper links? I'm designing the 4 link for my '96 4 door XJ which currently has a shitty 4.5" lift where the short arms have a F.S. of close to 10:1 since they're so short at 1.750 0.125wall.

I'm having trouble exceeding a F.S. of 2.12:1 by using 1.5" 0.250" wall for uppers.

In school I've usually stuck to a F.S. of 2 for most tests and homework problems so is 2 ok for your typical weekend warrior and daily driven XJ? 35's are all this thing will see.
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Old 07-03-2011, 06:44 AM   #2 (permalink)
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I think you're over thinking this. 1.5 x .250 DOM is plenty strong for upper links. I'm running the same DOM with 7/8s heims for my front link, and it's pretty long. In my rear I'm running 1.75 x .120 DOM with 7/8s heims.

Uppers will see 0 rock contact, unless you're doing something really stupid. Use the DOM you have, and forget about it.
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Old 07-03-2011, 07:29 AM   #3 (permalink)
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Originally Posted by Twisty View Post
I think you're over thinking this. 1.5 x .250 DOM is plenty strong for upper links. I'm running the same DOM with 7/8s heims for my front link, and it's pretty long. In my rear I'm running 1.75 x .120 DOM with 7/8s heims.

Uppers will see 0 rock contact, unless you're doing something really stupid. Use the DOM you have, and forget about it.
Agreed and me too. I have 6 total links thats are 1.5x.250. Front upper, rear uppers, drag link and tie rod, and the panhard.

Your definatly over thinking this.
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Old 07-03-2011, 07:59 AM   #4 (permalink)
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7/8" hiems and 1.75 OD .25 wall uppers and 2.25" OD .375 wall lowers with 1.25 hiems.

Done.

3 races and hundreds of miles of hard west coast wheeling and thousands of miles of east coast wheeling. They are perfectly fine. Pretty sure I abuse them 10 times harder then what you will do.

Material matters. Get DOM and be done with it.
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Old 07-03-2011, 08:27 PM   #5 (permalink)
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"Better to overthink than not use any sort of brains at all", one professor always used to say in class.

This 4 link is almost parallel, and with the links being 37" long I've got space issues to deal with on the uppers...no shock there. All the links come back to the crossmember after a 2" front stretch to put the front coils on top of the D30/D44 (haven't decided yet which axle).
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Old 07-03-2011, 08:39 PM   #6 (permalink)
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My 3 link front upper has a section of 1.75x.120

My rear 4 links uppers are 1.75x.120

I'm not concerned about them.

You can reduce the forces on the links by getting more separation at the frame and the axle... Of course, that puts more of a moment onto the mounts / tabs on the axle as they get more and more cantilever off...
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Old 07-03-2011, 08:56 PM   #7 (permalink)
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Like so. Critique? (I understand this is a "rear" 4 link but the numbers stay similar for front applications). If anyone knows of a front 4 link calculator with a pan hard bar option please point me in the right direction.

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Old 07-04-2011, 07:44 AM   #8 (permalink)
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My 3 link front upper has a section of 1.75x.120
Same here.
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Old 07-05-2011, 09:54 AM   #9 (permalink)
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Anyone have a comment with the design? I sat under the Jeep for awhile the other day and just figured where I had the most space.

Part of the reason why you guys are ok with .120 wall uppers is due to the large OD of your tube. On the driver side my upper is going above the drive shaft, so at full bump I need to keep the tube size to less than 1.25 OD.
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Old 07-05-2011, 10:21 AM   #10 (permalink)
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I wheeled for 2 years on 1 tons (37s, then 42s) with a 3 link which used a 1.25x.120 wall upper. Same material for the panhard. No issues. BUT I am upgrading them both to 1.5x.250 DOM.
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Old 07-05-2011, 11:28 AM   #11 (permalink)
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You don't have enough triangulation for a 4 link. Design a 3 link.

And you will have a crazy amount of steering feedback if you don't do full hydro with a 4 link. If you have a normal drag link steering setup with the 4 link, as the suspension droops and compresses your wheels will turn because you axle is not moving in an arch like your steering does. With a parallel draglink and track bar you will have little to no feed back or turning as the suspension moves.

4 link + steering box/drag link = deadly
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Old 07-05-2011, 11:39 AM   #12 (permalink)
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Quote:
Originally Posted by Xjcrawler736 View Post
You don't have enough triangulation for a 4 link. Design a 3 link.

And you will have a crazy amount of steering feedback if you don't do full hydro with a 4 link. If you have a normal drag link steering setup with the 4 link, as the suspension droops and compresses your wheels will turn because you axle is not moving in an arch like your steering does. With a parallel draglink and track bar you will have little to no feed back or turning as the suspension moves.

4 link + steering box/drag link = deadly
I think it's just supposed to be a parallel 4 link he designed, he was asking about a link calculator that takes into account the panhard bar in the same post as the pics haha.
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Old 07-05-2011, 12:16 PM   #13 (permalink)
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I think it's just supposed to be a parallel 4 link he designed, he was asking about a link calculator that takes into account the panhard bar in the same post as the pics haha.


I'm having a rough day to say the least. But still, there's no point in doing a long arm 4 link when a 3 link will work just fine.
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Old 07-05-2011, 01:02 PM   #14 (permalink)
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I'm having a rough day to say the least. But still, there's no point in doing a long arm 4 link when a 3 link will work just fine.
Redundancy....2 uppers=less chance of having a top loading axle if you suck at building, under-engineer, or drive stupid and snap joints.
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Old 07-05-2011, 01:31 PM   #15 (permalink)
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Redundancy....2 uppers=less chance of having a top loading axle if you suck at building, under-engineer, or drive stupid and snap joints.
Or you just build it right with 7/8" chromo joints and 1/4" wall up top and 1.25" down bottom with some strong mounts.

Countless racers with front 3 links, 40' tires, stupid amounts of HP and thousands of miles of beating their rigs with no failures.

If you build a parallel 4 link long arm up front you are going to have one hell of a time with your exhaust and driveshaft. Ask all the people here that have done 3 links on XJ's. It sucks trying to fit everything. Then add another link in the equation. What a nightmare.
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Old 07-05-2011, 02:08 PM   #16 (permalink)
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My upper is 1.5 .120 with 7/8" hiems. I beat the shit outa it and hasn't broke yet. I DD it too.
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Old 07-05-2011, 06:00 PM   #17 (permalink)
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1" or 3/4" 0.12 wall will work
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Old 07-05-2011, 06:19 PM   #18 (permalink)
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1" or 3/4" 0.12 wall will work
Yeah, stamped sheet metal will work too

Its interesting OEM actually designs those control arms to crumple in a front collision. Take a look at a wrecked cherokee in the bone yard. I once seen a wrecked cherokee with aftermarket arms and a lift. I don't know what they hit or how fast they were going but the god damn control arm went right into the drivers seat. The guy driving was surly fucked up. It's why most of your short arm lifts sell the upper arms separately. LIABILITY. I got one hell of a deal on the arms and still run them to this day, only needed bushings. (5 years ago) They are Rough Counrty super flex 1-1/4 OD not sure the wall thickness, .25 I think.

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Old 07-05-2011, 06:39 PM   #19 (permalink)
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Redundancy....2 uppers=less chance of having a top loading axle if you suck at building, under-engineer, or drive stupid and snap joints.
if that is a valid concern for someone, they should probably take a step back and NOT build a custom suspension as a first project.
I am by no means a "skilled" craftsman when it comes to this stuff, but I have enough faith in what I can do, to build a 3link and leave it as such, without the need for "redundancy, just in case"

like XJCrawler said, build it right, and don't worry about it.

2"x3/8wall lowers
1.75"x3/8wall upper
with 1 1/4" heims here
only went that stout because a buddy already bought X amount of DOM for his heavy buggy.
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Old 07-06-2011, 07:01 AM   #20 (permalink)
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Wow 10:1? We design industrial lifting devices and are only reccommended to designto a 3:1 safety factor.

When I designed mine I didnt worry about the FOS so much, just did a lot of research saw what did and did not work and then added beef where I knew beef was needed.

1.38" Dom .38 Wall with Johnny Joints
Panhard is 1.5" .25" wall

I had some limitations because of running mod stock class in RRock, but here is a screen shot of my design minus the panhard bar some gusseting, the hydraulic assist, etc etc. Cant find a decent picture of it.

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Old 07-06-2011, 07:47 AM   #21 (permalink)
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What are you folks calculating for a factor of safety for your upper links?
I have to ask: What forces are you using in these calcs? How did you determine them? I'm not asking because I know the answer, I'm asking because I don't even know how you would get them in the first place.

You could probably back some things out from braking and acceration performance, but all the dynamic loads it really needs to withstand.... that'd be a tough one for me to figure out, short of just reverse engineering something that I know is proven/works.

BTW, great sig.
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Old 07-11-2011, 09:25 AM   #22 (permalink)
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Been out of town for awhile for my dad's wedding...Let me try to catch up.

My design is a 'parallel' 4-link for the most part, with very little triangulation. The lowers have the most triangulation, the uppers are almost straight. The only reason the lowers are triangulated slightly is to clear the tires at full lock. If the "rear 4 link" calculator had a panhard option, you'd see it in my design. This will be my 4th linked suspension this year, and for DD status this is going to be modeled very similar to a Dodge front parallel 4 link w/panhard.

The steering is already up to snuff on this XJ. It's an inverted-T setup with GM 1 ton TRE's. No more inverted Y. During the swap the front is getting a Wagoneer D44 front, and the under-the-knuckle current D30 steering will be moved to over-the-knuckle D44 steering, with the drag link going to a high steer arm. I'll have to build the frame-side panhard bar stronger but no lower hanging down. The axle-side panhard bar mount will be in front of the passenger coil, on the same horizontal plane as the steering.

Why 4 link and not 3? Because I'm ok with putting forth the extra effort to make 4 work. This thing is mostly highway driven, I just hate the short arms and I'm getting paid to do this with all materials expenses covered.

TabooCustoms, I was curious as to what the F.S. was on the first set of short arms I built for the 4.5" rusty's coils. When I designed them I based my tube selection on a previous truck I worked on which had a short arm Fabtech lift...so I used the same material on this XJ. For shits and giggles to compare current F.S. to what I'm designing, I calculated both and the short arm lowers I built are 10x stronger than what they need to be. They've been beat to hell and are still straight, so it makes sense.

jalbrecht42, the math involved isn't hard but it requires a background knowledge in this type of field. I don't have time at this moment to take you through step by step but maybe later if I get a wild hair up my ass I'll come back to detail my calculations for ya. BTW, if you were commenting on my sig, which would you rather have? 10hp or 10lb-ft of torque?
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Old 07-11-2011, 09:59 AM   #23 (permalink)
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BTW, if you were commenting on my sig, which would you rather have? 10hp or 10lb-ft of torque?
Extra 10hp FTW.

If you get a chance to go through your calcs, or even just the assumptions/conditions I'd like to see 'em...
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Old 07-12-2011, 07:19 PM   #24 (permalink)
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Extra 10hp FTW.

If you get a chance to go through your calcs, or even just the assumptions/conditions I'd like to see 'em...
Same here. I've always thought about calculating the forces on the various components of a rig's suspension, I just have no idea where to get the forces acting on the rig itself
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