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Somebody School Me on Link Suspension Design

MountaineerGreen

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2012 F150 4x4
I'm trying to do a low COG SAS on my Mounty- LINK

I really want to build my own suspension, possibly a 3 link or a modified 4 link like the Wrangler long arm kits use. I plan to use coilovers, DOM tubing and Johnny joints, probably some tabs from Ruff Stuff. All this to hang a waggy axle under my Mountaineer.

I've found a few link suspension kits, but they are all vehicle specific and over $1000. With 2" DOM at around $12 a foot, johnny joints and bungs around $50 for a pair, I can build my own link suspension for ~$500.

But, I don't know jack about link suspension. I can't find any resource explaining the basics, like the importance of link length, link end placements, link geometry, etc. I've looked for books, (I'm willing to buy a moderately priced book) searched the webbernet- came up with nothing. I can't even find good pictures of a link suspension setup.

So, someone with some PS or MS Paint skills, draw some pictures, explain theories in detail- we can make this a good learning thread for everyone.
 



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Some bookmarks from my first SAS swap (in no particular order):

http://ausjeepoffroad.com/forum/showthread.php?t=53367&highlight=build+control
http://www.4wheeloffroad.com/techar...link_suspension_part_2/link_measurements.html
http://www.pirate4x4.com/forum/showthread.php?t=254876&highlight=build+control+arms
http://rockymountainextreme.com/showthread.php?t=16355
http://www.pirate4x4.com/forum/showthread.php?t=307322
http://www.pirate4x4.com/forum/showthread.php?t=204893&page=1&pp=25

But I dont think anyone can "school" anyone on link suspensions because there are just too many variables. So I think a better approach would be to talk about it piece by piece. So what would you like to talk about first? :)
 






I am far from an expert, but I have picked up a few things along the way. Here goes...

For a 4-link, you want the arms (upper and lower) to stay parallel throughout their cycle (or as close as possible.) If they move from parallel during the cycle, your caster will change due to the axle housing rotating.

This is not always possible, so, with some tricky engineering, you can use different length arms not moving in parallel, and end up with a minimal caster change.

Did you see this article? Not really in depth, but a good starting point. There are some book recommendations in there, also.

I think I have some articles from years past on 4-links. I will look tomorrow and see what I can find.
 






Ahh, I'm glad I refreshed before I responded- ye ole king of the quick edit.

The last Pirate link has a good calculator download I think I can use, the other links have some good reading- I've got a whole lot of research to do- I've searched on pirate and other sites, just didn't put in the right keywords I guess. Heck, I've known the tittle/content of threads here, searched for it and couldn't find it.

It's been a long day for me, I'll have to dive into the reading tomorrow.
 






I've searched on pirate and other sites, just didn't put in the right keywords I guess. Heck, I've known the tittle/content of threads here, searched for it and couldn't find it.
I totally understand -- sometimes when I'm searching for bolt sheer strengths, I accidentally type "banana" :)

But just in case you do need bolt shear strength and so you dont have to search Google for "banana":
http://www.rockcrawler.com/techreports/fasteners/index.asp


This is not always possible, so, with some tricky engineering, you can use different length arms not moving in parallel, and end up with a minimal caster change.
It is prefered that the caster changes because then you'll start sheering pinion shafts and U-joint bodies if you max out the U-joint's spec angle. A double cardan driveshaft for example, should be pointing at the driveshaft and in order for it to do that, it must change caster throughout its suspension cycle. Caster is really only important when using a steering box (non hydo cylinder steering) and travelling on normal roads as caster keeps the wheels to 'straight' -- because caster "drags" the wheel's contact patch behind the knuckle's axis of rotation.
 






Here's the things I learned doing my rear 4link...

1) The calculator is great for getting in the ballpark, but reality sucks..
2) Build it to fit what you have, worry about other things later..

In the rear, the problem is the location of the gas tank. It totally blocks one set of link mounts, unless you want to do a 'M' type setup, and not a 'XX'. I did a 'M' on the zuk rear, and it really worked great.
 






DB1 did some mighty fine work on his, but ended up with a modified 4 link- I don't think I have room for a a bridge over my diffy like he did.

http://www.explorerforum.com/forums/showthread.php?t=148354

I've been reading the links posted above- This is going to take some serious engineering. The worst part about it is that I am going to have to remove all my stock suspension to mock this up. My shop is only one bay, I hate to have the mounty sitting there for months getting it figured out. Other things may break or need oil changes in those months. :dunno:

Locating the upper link on the drivers side frame is going to be tough. RockRanger hasn't gotten that far yet, and at the rate he is going, I may beat him to it. :p:
 






This is going to take some serious engineering. The worst part about it is that I am going to have to remove all my stock suspension to mock this up.
Don't think about it too much -- the Explorer's chassis and body forces you to make comprimises so dont worry if the calculations dont come out even close to perfect.


My shop is only one bay, I hate to have the mounty sitting there for months getting it figured out. Other things may break or need oil changes in those months. :dunno:
Linking the rear is actualy a bit easier than linking the front. Once the gas tank and the leaves are out, you can knock out the linked suspension in a few days -- maybe a weekend (minus mounting the coil overs or whatever you're running but still even thats just figuring out where the coil overs are going to tie into). You dont have to worry about steering geometry and you dont have a massive cross member that needs to be cut out.

Locating the upper link on the drivers side frame is going to be tough.
One of the Ostman brother's and I are running basically the same top link mount at the chassis end -- but its pretty simple to do, just welds to the top and bottom of the chasssis and mine has held up so far.
 






Linking the rear is actualy a bit easier than linking the front. Once the gas tank and the leaves are out, you can knock out the linked suspension in a few days -- maybe a weekend (minus mounting the coil overs or whatever you're running but still even thats just figuring out where the coil overs are going to tie into). You dont have to worry about steering geometry and you dont have a massive cross member that needs to be cut out.

I only plan to link the front for now- the rear leaves are fine with me. I'm not quite ready to link it front and rear just yet- it's a progressive build much like yours has been and will be.

Is it possible to run a 3 link + panhard bar up front?
 






AHah oh I thought you were doing the back! My bad!

Yes it is very much possible to run a parallel 3 link + 1 panahard at the front -- that is what I was running with the D44 and that is what I will also be running with the new chassis and axle combo (parallel 3 link + panhard on both the front and back). The only difference in your setup from my D44 setup is you have a V8 so you will have to see where your upper link mounts and make sure it doesnt get in the way of your wider transfer case (I think its wider).
 






I'm having trouble with the upper link thing- I suppose I am going to have to truss over my differential to locate the top link?

My low COG build puts a kink in that plan, I don't know that there will be room for a big honkin truss under there?

Now, that being said, can I do a Y link on the drivers side like the Jeep long arm suspension kits do and a single link on the other side? I suspect there may be an issue with that, but what would it be?
 






I'm having trouble with the upper link thing- I suppose I am going to have to truss over my differential to locate the top link?
My low COG build puts a kink in that plan, I don't know that there will be room for a big honkin truss under there?
Compromises -- you might be forced to have a bit more lift than you expected. Either that or you can put the upper link mount not directly on top of the housing, but off to the driver's side (assuming it still clears the driver's side frame rail).

Now, that being said, can I do a Y link on the drivers side like the Jeep long arm suspension kits do and a single link on the other side? I suspect there may be an issue with that, but what would it be?
Two foreseeable problems with this:

1) there will be bind in the system during "flex". But then again, so does the "long arm" kit -- the uppers fight each other as each applies opposite torque vectors to the axle.

2) the upper link that ties to the chassis should be on the driver's side because (as Kirby N pointed out in my first SAS thread), the pinion will dive opposite of what you want during "flex" (because the pinion is on the driver's side).
 






1) there will be bind in the system during "flex". But then again, so does the "long arm" kit -- the uppers fight each other as each applies opposite torque vectors to the axle.

This bind is also what makes a Jeep style front link system so streetable - it creates a 'natural' sway bar effect.
 






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