Link suspension questions

[Tacoclaw]

Hey, everyone, I've been slowly gathering parts here and there to put a linked, airbag suspension on the front of my truck. After lots of planning and brainstorming, I've settled on a radius arm setup being the simplest to fab up.

I bought some rear spring hangers for an F350, and plan to run a 3" poly bushing at the frame connection. Lower bar length will be in the 48-55" range, with the upper link coming into it at about 1/3-1/2 it's length. It's not a flex monster, so I'm just going to run poly bushings at all of my joints. Heims seem to be a waste of $$, but my poly bushings do have threads for adjustability.

I was wondering if anyone had an idea on what stock I should use for the lower link. I've seen plenty of link calculators, but they're usually for 4-links and I'm not sure the math will apply here.

It'll have to be square tubing, probably A500B grade, and 1.5" inner diameter. Truck weight sits at 6600, obviously heavier over the front axle. I can get a front axle weight tomorrow. Truck is a DD, no plans to pull but it is due for a blast down the strip one of these days. Getrag will keep those pretty infrequent though.

What are your thoughts on the wall thickness I should be shooting for for the main link?

 

[adamsmarshall]

0.25" wall 1.75-2" OD square tubing should be plenty strong (1.5" OD if using round). Any particular reason why the bars would be 48-55" long? Just seems MUCH longer than necessary. Traditional 4-link setups tend to run about 18-24" at the longest and even the longest link in factory 3rd Gen is 17 and some change if I remember right and that allows for plenty of travel. Ladder bar setups tend to run slightly longer but even then most aren't over about 28".

Probably don't need double adjustable bars as single should do the trick just fine for your application I'd think. I would however mount the adjusters on the axle end (lower mount) to be able to fine tune pinion angle.

As for a traditional 4-link calculator they would apply just the same here as that's exactly what you're building here even though it's essentially backwards ladder bars since there's only a single frame mount on a radius arm as you well know. Play with the upper and lower bar lengths on the calculator to figure your general lengths for point angle and then you can allow for available adjustment length for fine tuning. For center of gravity on the truck I've been told that the top of the bellhousing is a good general height.

Poly bushings will tend to be a lot more street friendly due to the cushion of the poly and they don't require the maintenance that heim joints do in general. If you've already purchased the bushings the 3" will be fine, but 2.5" is also more than enough and may be easier to mount on the frame due to frame width.

For additional parts from bag mounts to brackets to lower bars that you could add an upper to, these guys sell most if not all that you'd ever need.

http://thorbros.com/catalog/4-link-parts

Now I'm sure there will end up being plenty other options thrown out here from those who have done the same before but that should help get you started. I have not however done one of these before so take anything above with a grain of salt as needed or double check please beforehand.

Also keep us posted on the project, it sounds like a fun one.

 

[Tacoclaw]

Awesome, thanks!

2-.250" square is what I was planning on, I just didn't know if it would be enough. I figured the longer the better with the links, since it's a radius arm it would keep caster changes in check and make it a little less headache to make a panhard. My airbags are double convolute, so total travel will only be around 7", but I did get them with 1/2" ports to try to negate their ride habits somewhat. I may change to a sleeve style for travel once I figure this out.

I just got 3" bushings to match the hangers, everyone sells those Ford ones for cheap, the rest are 2.5". I actually got my stuff from Thorbros, they're very nice pieces.

I'll be sure to keep you guys updated, I've got to order the steel. I'll probably throw together a traction bar for the rear while I'm in there too, it's still a little hoppy.

 

[Tacoclaw]

Very slight update, but I figured I'd throw it up in an attempt to prove I'm not wasting Adam's time. :lolly:

Gotta drill out the plates to 9/16" to match the bushings, but I'm otherwise ready to start laying out my link(s). I've got a 20' stick of tubing waiting for me on the rack.

 

[whitneyj]

Word of advice for your axle side joints: Bushings flex like crazy. If you're in 4wd often you'll crumble the poly urethane from the sleeves fairly quickly. For traction bars (same concept for torque as your radius arms) poly bushings get destroyed quickly and people switch to either UHMW plastic or swap in heim joints.

Also, the longer your arms are the more force you're applying on the link. (think leverage). I wouldn't make them any longer than 36" total. Your best bet is to mount the frame side on the bottom of the frame directly after it's made the end of it's turn down under the cab. Like adammarshal said, stock trucks run a fraction of the length you plan on and run far smaller material. Plus with only 7" of travel you won't be utilizing any advantages of having longer radius arms.

As far as the mounting goes, just make sure the arms are parallel to the ground when they're mounted. That's your best setup to help nuetrilize any brake dive while maintaining good ride characteristics.

Also, with radius arms that aren't triangulated at the frame end: your track bar will see a ton of force, make sure both mounting ends are stout.

 

[Tacoclaw]

I'm rarely in 4wd in this truck, and if I am it's generally to try to keep from destroying someone's yard that I'm trying to get out of. It's no racer or off-roader, just a DD. Max time it'll see 4wd is if there's more than 6" of show on the road.

I will keep an eye on them though, thanks for the heads up.

The thought had crossed my mind to use polys in the traction bar but I figured that may be a bit torquey for them. That's an area where I figured the cushioning would work against their intended purpose. I'll just bite the bullet and get some Heims for it. (Or maybe tractor joints :kick: )

As for the rest of the stuff, I'll try out a couple mounting positions and post them up for perusal. I wasn't aware that having them angle downward would cause it to dive though, I was hoping to keep their mount somewhat tucked up. I was hoping the longer arms would alleviate some of the angle that would cause, but if they need to be somewhat parallel then I guess I can really shorten them up.

Thanks a bunch for the insight, hopefully I can get some locations sorted and a few pics up for everyone to chew on.

 

[whitneyj]

Tucked up will fight antidive/brake dive, but then you don't have a vertical plane for your suspension to travel when you hit a bump (makes it a more harsh ride) and on hard acceleration in 4WD you have to worry about the front wanting to walk under the truck versus pull forward (not an issue on flat ground, but it's there and you won't remember it until you're at Johnson Valley wheeling the hammers

You won't notice a little downward angle, but rule of thumb is parallel to the ground. mounting them to the bottom of the frame rail makes it far easier for both mounting and angles. The issue with that then is with ridiculously long links (I mean no offense) you run into uptravel issues with the links contacting the frame rails.

 

[Tacoclaw]

None taken, feel free to call me on whatever ridiculous crap I'm cooking up here. I'm just stabbing in the dark.

 

[Ghostman]

I used 2" and 0.25" wall round DOM tubing when I built my long arms.


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[Tacoclaw]

Here's my mockup. I would say some mockup, but due to the pumpkin on this axle it's crowding my setup clear over to the end so this is about all I can do. I don't really mind, as I figure the wider it is the less leverage the axle will have over it, but it makes it impossible for me to get the links under the frame without bending them. I don't want to bend them since I really don't have the tools to do it correctly, so I'll either drop the brackets down on the outside or run them at this angle.

It's not an exact representation due to the shocks interfering with the bushing some, but it's actually pushing it to look worse so it should actually lose a couple degrees of angle. I intend to just weld a stud onto the link mount and mount the shock there. I don't think the links need the extra stress of the shock being mounted to them anywhere.

These things are gonna be around 3ft long it looks to me, I could shorten them up some but there's a body mount in the way up there so it'll probably just be easier for me to build heavy for the stresses.

I've got the angle measured out to ~15*, I can probably get it down to around 12* by cutting all the interferences out of the way and putting stuff where it'll actually be, but to get it any flatter I'll have to hang the mount down under the frame.

Thoughts? Should I just stick to welding crap onto the side of my engine? :lolly:

 

[Tacoclaw]

Spent a few minutes in the thinking room, then grabbed the torch and told my shocks to get ****ed. No use screwing around with this thing if I'm looking for actual input.

Now it's sitting tight against the axle, with about a 10* angle at the lowest point I could get it to hold itself on the frame. I'm an idiot for not bringing one of my hangers, but it didn't hit me as I was on my way out the door.

I'm betting between 7-10* for a final angle if I mount my hangers as high up as I can. I can drop them, but parallel to the ground is hanging waaaay down under the truck. IMO.

I will be rerouting the brake lines, but they're not in the way of fab so they're safe for now.

 

[dvst8r]

I'm not up on my front suspension geometry, but it looks similar to a few long arm "kits" I have seen.

 

[Tacoclaw]

Doh!!! I put crossover steering on my truck and cranked the stops in to just before the u-joints bound up. Looks like they'll be coming back out some.

Also looks like my hangers are worthless, I'll just have to make something that tucks the mounts right up to and under the frame. The Ford ones hang too far off the side. Also turns out the steel we had was 1/8" wall. **** day for fabrication, I'm going home and drinking. :bang