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post #1 of 13 Old 02-03-2019, 05:01 PM Thread Starter
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3 link help

Hey guys, I'm not sure if this is the correct place for this post but I was just looking for some advice on my 3 link setup. To give yall a little info, I have a 2011 jku that I'm building a 05 superduty 60 and 10.5 sterling to go under it. Currently I have finished building a custom truss with spring perches and a cross member that sits just forward of the transmission crossmember and exhaust. I have yet to install and lca,uca,or track bar brackets becuase I'm still fighting with my roll axis numbers and that's why I have came to this forum to pick a few of y'alls minds. Oh a few other details are a stretched front 1.5", truss raised coil perch by 1 3/8, 1" body lift, and currently have 3 1/2 springs but I've mocked my setup to 5 1/2 springs and like the looks alot better so I plan to stay at the 5 1/2 for a total of 8" of lift for my 42" mtrkevlars. Yes I know that's alot of lift but I live on the gulf coast so I'm only on mild trails and mud so a low cog isnt as helpful down here as up north in the rocks. Plus I'm only running 4 3/4 back spacing so I dont look like a spider running down the highway but it does require a little bit of french​ing to the frame. Ok sorry about the rambling, my biggest problem is my roll axis and I will post a pic of the calculations I have at this time.
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post #2 of 13 Old 02-03-2019, 06:17 PM
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I think you must have put in an incorrect number. Frame end, lower link, Y being 4 makes no sense. That's what you'd do in a double-triangulated suspension. Doing this with a track-bar is not a good plan. It'd also be really, really hard to accomplish this in the front of a JK so I assume it's in error.

You have a very-negative roll axis angle because the lower links converge at the frame. If you change frame end lower link Y to match the axle side Y, your roll axis angle basically goes to zero.

Other than that one number, this looks like a very good suspension design to me. Instant center is at the other end of the vehicle, lower links are flat, anti-squat is a pretty good value, and track bar looks about as high as you could probably reasonably get it. Only concern I'd have is that 6" of separation at the axle is not much especially running 42" tires. Might break stuff with such a low axle separation.

Last edited by christensent; 02-03-2019 at 06:19 PM.
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post #3 of 13 Old 02-03-2019, 07:26 PM Thread Starter
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I just noticed the separation problem, I'm glad you pointed that out. With the new separation numbers, it will be a pain to put the bolt in my frame side upper but its doable. I'm trying to keep my lowers up as high and tight as possible so to get to the 9.5" I'm just going to see if I notice my frame side lowers being 1" higher than the axle sides. With the new upper configuration, I will have about 5 1/2 up travel. Oh and I was able to get the 4" off center to my lowers because of the cross member I built. The one thing I forget to check with the new frame side lower being at 15" is how much clearance I have at full lock now. I dont own a bender but I may be able to find one if I have clearance issues. The site will not let me upload another pic but I'm now at -1.35 and anti squat of 67.7 with a frame side upper 28 1/2 and axle 30. Frame side will be tucked between the frame and cat on passenger side and axle side will come up beside the oil filter

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post #4 of 13 Old 02-03-2019, 08:51 PM
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You should raise the frame side upper or lower it on the axle. Same with the lower links. This will get your anti squat number up.

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post #5 of 13 Old 02-04-2019, 01:45 AM Thread Starter
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How much anti squat should I be looking for

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post #6 of 13 Old 02-04-2019, 05:03 AM
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Quote:
Originally Posted by byrd View Post
I'm just going to see if I notice my frame side lowers being 1" higher than the axle sides.

...

The site will not let me upload another pic but I'm now at -1.35 and anti squat of 67.7 with a frame side upper 28 1/2 and axle 30. Frame side will be tucked between the frame and cat on passenger side and axle side will come up beside the oil filter
1" of slope on the lowers is not a big deal at all, that's totally fine. It's when you get up to 3-6" of axle vs. frame separation at the lowers that things get a bit off (still not the end of the world, but does start to really affect the performance).

-1.35 roll axis angle is alright. When possible it's good to get it positive but honestly unless you're going for the absolute best suspension possible, I'd be totally happy rolling with that number. Ultimately you have to build what fits and can't always get the very best numbers. I agree with Dirtman, try to get higher on anti-squat. There's huge variation in preference but a lot of people like 80-100.
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post #7 of 13 Old 02-04-2019, 08:56 AM Thread Starter
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When I get off work I'm going to play with locations a little but I wont have any problems with adjusting the upper. A few issues I'm seeing is: sway bar my hit coils, track bar will be tight between pitman arm and truss, and I dont like the angle of my drag link and track bar so I may opt out for high steer arm on driver side. Below are the numbers I tried to post last night.
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post #8 of 13 Old 02-04-2019, 04:07 PM
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If you can, put in brackets for the uppers at the frame with multiple mounting points, some closer to the ground than you have in your last numbers. That'll give you adjustability to increase the anti-squat.

Also I seriously doubt your sprung mass CG is at 31". It's probably more like 45-50". That changes things a ton, makes your anti-squat figures much lower than you currently are calculating increasing the need for further compensation.
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post #9 of 13 Old 02-04-2019, 06:52 PM Thread Starter
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Is there a "too much" frame to axle separation for the upper I should stay away from or do I just pay attention to the amount of pinion angle in articulation?

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post #10 of 13 Old 02-05-2019, 07:32 PM
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Is there a "too much" frame to axle separation for the upper I should stay away from or do I just pay attention to the amount of pinion angle in articulation?
What exactly do you mean by "frame to axle separation for the upper"? Do you mean the difference in height off the ground for the upper link joints at the axle vs. frame? You cannot have too much or too little of that, you set that to set your anti-squat and instant-center position. It's the lower that matters on keeping it flat. That is, assuming the lower link is near the center of the axle tube. If it were the other way around with the uppers near the tube and the lowers hanging 8" below the tube, then we'd be concerned with keeping the uppers flat. That is obviously not how the 4x4 world works, though, as that'd be a dumb setup.
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post #11 of 13 Old 02-06-2019, 05:40 AM
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Not sure where the flat lower link is coming from but that is not right. The lower link should not be flat, it should angle up some towards the center of the jeep. It really does not matter if it were center of the tube or hanging 3" below You also don't want a lot of slope to the lower link to reduce axle shift and movement during articulation.

And since you mentioned pinion angle are you building your axle for caster correction at 8" of lift? If not you will likely bind your front driveline out with only a couple inches of droop.

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post #12 of 13 Old 02-06-2019, 11:08 AM
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Originally Posted by thedirtman View Post
Not sure where the flat lower link is coming from but that is not right. The lower link should not be flat, it should angle up some towards the center of the jeep.
Why is that? It usually happens to end up that way as a compromise due to a desire for more clearance at the frame-side mount, but I was always under the impression that there's nothing particularly good about it and the goal should be to have them flat at ride height for optimal geometry. Making them flat makes rear-steer as low as possible (which basically means roll axis angle is minimized in terms of that spreadsheet), and if the lower arm mount is near the axle tube center line then it means all the thrust force is pushing the vehicle forward rather than some pushing it up as you get with an angled lower link (which can be mostly cancelled through anti-squat setting in a lot of scenarios, but better to not have the force there in the first place especially for rock crawling and low-speed high-torque stuff).
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post #13 of 13 Old 02-19-2019, 08:10 AM
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You want to make them as flat as you can to minimize axle shift but you need some slope in the arms to get the triangulation needed to get decent geometry. A flat lower will put the convergence below the engine and will force the weight shifts into the coils vs the chassis. This will result in rear squat when acceleration while lifting the front and will do the opposite on braking. The rear will raise and the front suspension will compress.

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