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Unread 05-01-2010, 03:16 PM   #1
Dogman
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4 Link Geometry Question

I'm learning more about suspension geometry and I'm designing a 4 link rear with triangulated uppers. I'm just to the point where I'm charting the uppers and I am unsure about the angles.

My uppers are 45* relative to the frame rail. I'm not sure if that's too much. I know that if it's too shallow of an angle you would get less lateral control.

My other issue is that the uppers are longer than expected to reach the frame mounting points (because of the triangulation). That throws the .7:1 ratio off. I would be at about 88%.

I'm shooting for 30" lowers.

I decided to graph this out just so I could learn more and actually measure my rig to find out what's possible. I'm just looking for any advice from others who have taken on the challenge.

I have put a link to my mock-up below (didn't want to link directly because it's big). The grids on my graph represent 2".

4 Link Mock-up Image

I'm working through this article: Truck Suspensions, Four-Link Suspensions, and Suspension Installs - Part 2 - 4 Wheel & Off Road

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Unread 05-01-2010, 03:39 PM   #2
never monday
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Quote:
Originally Posted by Dogman View Post
your going to get some pinion twist with the two different circles of the arms. Try to get the frame side pick up points near each other.
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Unread 05-01-2010, 05:00 PM   #3
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Originally Posted by never monday View Post
your going to get some pinion twist with the two different circles of the arms. Try to get the frame side pick up points near each other.
You mean separation between the frame side mounts? They are 12.5% of the tire diameter right now. Of course when I actually do this, the upper mounts would have multiple mounting points for adjustment.

Or do you mean moving the upper frame mounts inboard somehow?
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Unread 05-01-2010, 05:21 PM   #4
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Quote:
Originally Posted by Dogman View Post
You mean separation between the frame side mounts? They are 12.5% of the tire diameter right now. Of course when I actually do this, the upper mounts would have multiple mounting points for adjustment.

Or do you mean moving the upper frame mounts inboard somehow?
move the upper frame side pivot point forward. This will decrease your 45* angle some, but having the pinion track the DS is more important IMHO.
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Unread 05-01-2010, 05:28 PM   #5
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Ok, that's where I'm confused. I thought the uppers were supposed to be 60-80% the length of the lowers? I'll have to put a compass on this to see the arcs of the uppers and lowers with different arm lengths.
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Unread 05-01-2010, 05:32 PM   #6
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Originally Posted by Dogman View Post
Ok, that's where I'm confused. I thought the uppers were supposed to be 60-80% the length of the lowers? I'll have to put a compass on this to see the arcs of the uppers and lowers with different arm lengths.
in a short travel race car/hot rod set up I can go for that. With the long travel we demand. I think arc of travel is more important.
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Unread 05-01-2010, 06:46 PM   #7
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Here's Claytons 4 Link kit, it almost appears that the uppers are longer and mount at the same point on the frame as the lowers. I guess I don't have to worry about it It actually makes things easier.

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Unread 05-01-2010, 11:01 PM   #8
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I'm interested as well. I have heard that the lowers should be longer than the uppers. Clayton's kit is fine, but I wish it was adjustable.

I had their arms, still using their front arms, but I am re using their mounts with my own rear arms. I really like the Poly Performance bracket system because it allows for the uppers to be adjusted in height.

Ballistic also makes a separate mount which would work well. Angled lower mount so you don't have the hang ups like the clayton mounts cause (trust me, I know.) and you can use their separate upper because it's adjustable.

I also bent one of the clayton frame mounts.

lower



upper


poly mount

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Unread 05-02-2010, 12:21 AM   #9
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Quote:
Originally Posted by Dogman View Post
Ok, that's where I'm confused. I thought the uppers were supposed to be 60-80% the length of the lowers? I'll have to put a compass on this to see the arcs of the uppers and lowers with different arm lengths.
60-80 Effective length. As in draw a line from the top of your diff down the centerline of the rig. Now measure along that line until you arrive at 80 percent of the lower length and mark a point. From that point draw a perpindicular line across the frame and that's about where you would put the link mounts.

Of course if you're like the rest of us working on TJ's. We just shove the uppers as high as we can get them somewhere right over the mounting point of the lowers, get roughly 8" of vertical separation at the axle and call it good.

That assumes the lowers aren't below the axle tube and the frame side of the lowers are flush to the frame with the control arm joint.
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Unread 05-02-2010, 07:58 AM   #10
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Ok, I made some adjustments and it looks much better.

4 Link Mock-up V2

Quote:
Originally Posted by mrblaine
Of course if you're like the rest of us working on TJ's. We just shove the uppers as high as we can get them somewhere right over the mounting point of the lowers, get roughly 8" of vertical separation at the axle and call it good.
Yep, I've got (on paper ) 8" of separation at the axle. I plan on the lower axle end links mounting in front of the axle tub. I was actually unsure about how that would affect axle wrap, but it seems like it's ok?

Quote:
Originally Posted by JeepRocks86
I really like the Poly Performance bracket system...
I've been looking at brackets for a long time! I would love to get the PP brackets, but they're spendy. We'll see. It all comes down to funds. I may end up making my own.

I definitely plan on using JJs on both ends of the arms.
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Unread 05-02-2010, 08:14 AM   #11
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Quote:
Originally Posted by Dogman View Post
Ok, I made some adjustments and it looks much better.

4 Link Mock-up V2
why are your lowers crossing under the frame?
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Unread 05-02-2010, 08:45 AM   #12
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Quote:
Originally Posted by never monday View Post
why are your lowers crossing under the frame?
No particular reason - I just picked an axle side mounting point for the mock-up. During the build it would probably be based off of where the frame side mounts are. If I go with the Poly style brackets, the frame side would mount a little inboard, Clayton/Ballistic style would be directly below the frame rail. From the frame, they will probably angle out about 10*.

Does that sound like the way to go?
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Unread 05-02-2010, 08:52 AM   #13
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Quote:
Originally Posted by Dogman View Post
No particular reason - I just picked an axle side mounting point for the mock-up. During the build it would probably be based off of where the frame side mounts are. If I go with the Poly style brackets, the frame side would mount a little inboard, Clayton/Ballistic style would be directly below the frame rail. From the frame, they will probably angle out about 10*.

Does that sound like the way to go?
Just watch out for the lower links crashing into the frame. A triangulated axel moves very differently than what you're used to seeing, so what looks like it may clear at static ride height often runs into stuff when you cycle it.

I personally run mine very similar to yours as drawn. I like the lowers to be as close to the tires as I can get them and keep them out of trouble.
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Unread 05-02-2010, 10:07 AM   #14
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I appreciate the input I'm getting!

Quote:
Originally Posted by mrblaine View Post
Just watch out for the lower links crashing into the frame. A triangulated axel moves very differently than what you're used to seeing, so what looks like it may clear at static ride height often runs into stuff when you cycle it.

I personally run mine very similar to yours as drawn. I like the lowers to be as close to the tires as I can get them and keep them out of trouble.
That's something that I wondered about. Of course things will be tacked in place and the suspension cycled before burning anything in. I'm planning on 4" Currie springs and I'll probably use their bumpstop kit as well.

Those Poly brackets are so sweet...they would save a lot of fab time.

Some questions rolling around in my head:
1. Does anyone have experience with the PP brackets?
2. What about the uppers hitting the tub?
3. How much up travel should I be shooting for? I imagine the truss would be a major factor in that.
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Unread 05-02-2010, 12:03 PM   #15
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Dogman, do you have the 4Link Calculator spreadsheet? It's quite convenient for quickly getting a glance at arm lengths at stuff and droop and graphing out things without using an eraser and a ruler. Just a thought.

Stolen from POR
http://highvibesoffroad.com/drawings/4BarLinkV3.0a.zip
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