Link Suspension Basics

[bbaxter51]

The trend these days is to ditch the leaf sprung setups in favor for a more up to date setup. Whether you're wanting to run coil's, coil over shocks, or air shocks; you're going to need to a link setup. In this tech article we will talk about the most basic and popular link style setups; how to choose what's right for you, and where to start. We'll also talk about vocabulary you'll need and the cost involved with converting over to a link setup. Sound good? Good. If at anytime you've got questions, comments or opinions, throw them out there we'll discuss.

We're only going to discuss 2 of the most popular types of link setups in this writeup purely due to the depth and breath of the subject.

Part 1.

4 Link. This setup if the easiest for people to understand and wrap their heads around the concept of. It's exactly what the name implies. There are 4 link bars mounted from the axle to the frame holding the axle in place both forward, back, and side to side. The traditional way the 4 link is setup on off road rigs is to have 2 links holding the axle from moving front to back and 2 links mounted in a triangle shape, used to locate the axle from side to side. This is an example of a typical 4 link setup in the rear of a rig.

At first glance, it doesn't look like 4 bars would be able to keep an axle in place under a vehicle with all the stress the power of the engine and the size of the axle and tires; but you have to know, that strength is in shapes, not in size. One of the strongest shapes is the triangle, and that's what we're trying to accomplish with the 2 upper bars in the V pattern. In order to gain enough strength of the the triangle shape, there's a magical number we're searching for and that is 40 degrees. Meaning add up the angles from the upper and lower links, it needs to be 40 degrees or more in total to properly hold the axle in place under your rig. Think how a scissors lift works, same concept here when we're talking strength in holding the axle in place.


Up next, 3 links.

 

[bbaxter51]

3 link setups

Part 2. The 3 link

The 3 link is also just like it sounds, sort of. There's actually still 4 link bars holding the axle in place, but in completely different orientations. You've still got got the lower bars generally in the same place holding the axle from moving front to back. The difference in the 3 link is you've got only 1 bar holding the rotational forces of the axle in place instead of two like in the 4 link setup. How do you hold the axle from side to side motion you ask? Ah, here's where the 3 link setup really takes a turn. There's another link bar called a "pan-hard bar" mounted parallel to the axle, up to the frame. Here's 2 pictures I nabbed from "Stinkbug" on his build on Pirate. It's a perfect example of a proper 3 link setup.

There's a couple reasons why you would choose a 3 link setup vs a 4 link with the main reason being packaging. It' a pain in the ass to fit a fully triangulated 4 link setup under the front of your rig which is why you see a lot of people running a 4 link in the rear and a 3 link setup in the front. Case in point is bradzilla's willys. (local MN forum)

One think to note about the 3 link is when the axle drops out of the rig while articulating, it actually swings to the side a bit. If your link setup is not done properly, this can be a major issue. What you're looking for on this setup is make sure the pan-hard bar is sitting as level as possible with the rig sitting at ride height, this will ensure the least amount of side swing your axle will encounter while articulating through the trails.

Next up, the 4 link calculator, and the vocabulary you'll need to understand how it works.

 

[bbaxter51]

Math + Vocab = Sweet handling rig

Part 3. Back to English class.

When you start talking link setups it can seem like people start talking a different language; let's arm you with knowledge to know what their talking about.

1. Anti-squat. Anti-squat is expressed in terms of percentage and refer to the rear of your rig squatting under acceleration.

2. Roll Axis. The roll axis is the imaginary line through the vehicle from front to rear that the vehicle rotates about during cornering.

3. Roll Center Height. The point in the transverse vertical plane through any pair of wheel centers at where side forces can be applied to the sprung mass without producing suspension roll. This one is solely effected by the suspension geometry. (this one can get a little technical, but it's very necessary to understand how it effects your rig off road)

4. Sprung Mass. The portion of the vehicle weight that is supported by the suspension. (pretty much everything north of the axles)

5. Unsprung Mass. The portion of the vehicle that is NOT supported by the suspension. (axles, wheels, tires, brakes... you get the idea)

6. Vehicle Center of Gravity Height or "CG". Depending on who you ask, this measurement can be guessed at by measuring the height of your camshaft from the floor in your rig. But know all your other measurements are pulled from this. You're going this far with your rig; do it right. here's how.

**Find Your Center of Gravity

Now that you've got your terminology down, let's dig into that 4 link calculator you downloaded from the "tech" section of this site. You remember, the one you opened and got confused by then promptly closed it.

It's not as scary as it looks. The boxes in light blue are the only ones we'll be messing with. Let's fill out the ones that are obvious first. The boxes that are confusing, just hold our mouse over the box and a description will pop up and tell you what to do and what measurement you need to enter. Here's some rough estimates I just threw in from the best I could remember from my rig.

Dont be afraid of this little spreadsheet, play with your numbers and see how you changing them affects the links in the picture. The numbers you're trying to chase down are 80-100% anti squat and 0 degrees roll axis. That will give you a very predictable and stable rig on the trails we like to wheel.

4 link calculator link. You'll need Excel for this to run.

 

[bbaxter51]

**Disclaimer** these prices are from when I built my rig back in 08'. Obviously they've changed some, but you're smart. You get the picture.

What's this mess going to cost you? Well, I'll start out by saying it's not cheap; at least not for me. Depending on your fabrication ability and tools on hand, you can get by for around $1,000.00 in materials. Here's the breakdown of costs for an average 4 link setup.

48" x 4 of 2.0" .375 wall DOM tubing. $12/ft. Total $192.00
8 Rod threaded rod ends. $35 each Total $280.00
8 weld in adapters for rod ends. $10 each. Total $80.00
Axle Truss Total $220.00
2 Axle side lower link bracket $18 each Total $36.00
2 Frame side lower link bracket $20 each Total $40.00
2 Axle side upper link bracket tabs $10 each Total $20.00
2 Frame side upper link bracket $30 each Total $60.00

Grand total cost of materials only... $928.00

Now, since we'll need some sort of suspension, we'll need to add that into the equation too. Let's throw out the most common types of suspension used so you can make choose for yourself which best fits your rig and your budget.

2 Coil springs $40 each Total $80
2 emulsion shocks $100 each Total $200
2 Air Shocks $280 each Total $560
2 Coil over shocks $500 each Total $1000

While we're adding this up, let's not forget that we're only adding up materials for the rear of the rig. So Let's assume we're going to convert your rig to a 4 link with air shocks, you're looking at a grand total of ~$1,600.00. Plan on linking both the front and rear and you're looking at roughly $3,200 in parts and materials. While you're working up your budget, go ahead and add in another $500 for shipping cost, welding wire/gas, grinding wheels, safety glasses, energy drinks, beef jerky, and other hidden costs when tackling a project of this size.

Scared? Surprised? Excited to get started? You can do this, it's not rocket science. just don't go cutting up your rig without a plan of attack. All this takes is some planning, measuring, patience and confidence. The last thing we all want to see is a rig half done because someone got in over their head.

 

[MarsFab]

This is a decent write-up but there are a few very important things you should add. I think leaving them out could cause some pretty big issues.

1. You do want the panhard bar as close to parallel with the ground as possible at ride height, but for front suspension that is not always the case. You need the panhard bar to be parallel with the drag link and as close to the same length as possible. If not you will have terrible bump steer and body roll will will fight the steering.

2. Packaging 4 links can be a bit harder than 3 links. But the main reason for which one to select in the front should be what steering you run ie. steering box with drag link with or without hydro assist HAS to have a 3 link w/panhard or a parellel 4 link w/panhard, If you run full hydro THEN you can run a single or dual tri. 4 link. With mechanical steering the axle needs to swing in the same radius side to side as the drag link. A triangulated 4 link makes the axle travel vertically.

3. If you have 100% antisquat that means that when you accelerate the rear will not go up or down, it 100% counteracts the the forces applied on the axle. 80 - 100 percent may be a good number for a rig with coil springs or coil-overs but with airshocks less may be better because airshocks unload or push the truck up more than the other. Also depending on what the rig is used for may determine the antisquat. A hill shooter may actually want more than 100% to help plant the rear tires. A go-fast race buggy like ultra 4 may want less to to get the buggy to fly level when it takes off of a jump.

 

[bbaxter51]

Perfect! Very solid points. I was hoping some more people would add to my very basic writeup and this would be an active topic.

 

[nctom]

This is a decent write-up but there are a few very important things you should add. I think leaving them out could cause some pretty big issues.
1. You do want the panhard bar as close to parallel with the ground as possible at ride height, but for front suspension that is not always the case. You need the panhard bar to be parallel with the drag link and as close to the same length as possible. If not you will have terrible bump steer and body roll will will fight the steering.
2. Packaging 4 links can be a bit harder than 3 links. But the main reason for which one to select in the front should be what steering you run ie. steering box with drag link with or without hydro assist HAS to have a 3 link w/panhard or a parellel 4 link w/panhard, If you run full hydro THEN you can run a single or dual tri. 4 link. With mechanical steering the axle needs to swing in the same radius side to side as the drag link. A triangulated 4 link makes the axle travel vertically.
3. If you have 100% antisquat that means that when you accelerate the rear will not go up or down, it 100% counteracts the the forces applied on the axle. 80 - 100 percent may be a good number for a rig with coil springs or coil-overs but with airshocks less may be better because airshocks unload or push the truck up more than the other. Also depending on what the rig is used for may determine the antisquat. A hill shooter may actually want more than 100% to help plant the rear tires. A go-fast race buggy like ultra 4 may want less to to get the buggy to fly level when it takes off of a jump.

Well said, Chris. There is no one perfect set up on antisquat. You just pick your most likely senario and build for it.

 

[nctom]

Good write up Baxter!

 

[Jeff B]

 

[paradisePWoffrd]

This is a decent write-up but there are a few very important things you should add. I think leaving them out could cause some pretty big issues.

1. You do want the panhard bar as close to parallel with the ground as possible at ride height, but for front suspension that is not always the case. You need the panhard bar to be parallel with the drag link and as close to the same length as possible. If not you will have terrible bump steer and body roll will will fight the steering.

2. Packaging 4 links can be a bit harder than 3 links. But the main reason for which one to select in the front should be what steering you run ie. steering box with drag link with or without hydro assist HAS to have a 3 link w/panhard or a parellel 4 link w/panhard, If you run full hydro THEN you can run a single or dual tri. 4 link. With mechanical steering the axle needs to swing in the same radius side to side as the drag link. A triangulated 4 link makes the axle travel vertically.

3. If you have 100% antisquat that means that when you accelerate the rear will not go up or down, it 100% counteracts the the forces applied on the axle. 80 - 100 percent may be a good number for a rig with coil springs or coil-overs but with airshocks less may be better because airshocks unload or push the truck up more than the other. Also depending on what the rig is used for may determine the antisquat. A hill shooter may actually want more than 100% to help plant the rear tires. A go-fast race buggy like ultra 4 may want less to to get the buggy to fly level when it takes off of a jump.

X2...

When running a panhard in the rear, you want to design the panhard to be parallel with the axle/ground halfway through the suspension cycle, and as long as possible... this will allow for full travel of the suspension with the least amount of flex steering...

This means that for most rockcrawlers that are running more droop then uptravel, the panhard will not be horizontal at ride height, but will be lower on the frame side, then the axles side...




Also, to add... When putting bends in link bars, the geometric forces are still going to work in the strait line between the joints... bent control arms are very common, but need to be strengthened to compensate for the weakness in the design...

 

[Kap10merica]

I have read on some other sites on this subject that there is a ratio for your link length. The lower links should be a certain percent longer than your uppers but I'm not sure what that ratio is or if it makes a difference. So my question is how much longer do my lower links have to be than my uppers. I hope this makes sense.

 

[rockmafia]

I have read on some other sites on this subject that there is a ratio for your link length. The lower links should be a certain percent longer than your uppers but I'm not sure what that ratio is or if it makes a difference. So my question is how much longer do my lower links have to be than my uppers. I hope this makes sense.

For some reason they throw around that the uppers should be 60% as long as the lowers. I don't know where it came from, but it's not good for a long travel suspension. just makes the pinon move in the wrong direction. I make them all about the same length.

 

[paradisePWoffrd]

For some reason they throw around that the uppers should be 60% as long as the lowers. I don't know where it came from, but it's not good for a long travel suspension. just makes the pinon move in the wrong direction. I make them all about the same length.

X2.... If your lowers are longer then your uppers, it will cause the pinion to rotate towards the ground under droop. If the lowers are shorter then the uppers it will cause the pinion to rotate up during droop. Equal length uppers & lowers will keep the pinion at approx the same angle during travel.

 

[Kap10merica]

ok that makes sense. Is a dual tri 4 link better than a single or is it jus up to ur prefernce.

 

[bbaxter51]

Personally I prefer dual tri if you have the room.

I've found it's easier to reign in your roll axis numbers with that setup and you don't get the "rear stear" if your links are on the shorter side with the dual tri setup in the rear.

edit: Plus you're not relying on the misalignment in your joints to take up the articulation, they move in line as the axle moves.

 

[Kap10merica]

How do you determine the length of your links is judged by wheel base?

 

[bbaxter51]

Correct. Do yourself a favor and leave enough room between the front and rear lower links to build a skid. 30-40" long has worked well on the last few rigs I've setup.

On my personal rig, they're 45" long with a 106" wb and building a skid was a pita.

 

[Kap10merica]

any pics of your setup?

 

[bbaxter51]

Here's a pic of the rear of mine when I built it. I can take more if there's something specific you're looking for.

Here's the front.

 

[bbaxter51]

You'll have to excuse the poser pic, but this is the best shot I have on my computer showing what I had to do to build a skid with the lower links so close. I ended up mounting the skid to the bottom of the upper link mount brackets. It works good, but I'll do it differently next time.

 

[Kap10merica]

looks great. I was mainly interested in ur skid but that is some awesome fab youd. Is that a kit or did you build all the mounts and brackets.

 

[Steve97TJ]

I never saw you mention anything about seperation. I've always heard that is a big part of building a link suspension. How do you measure it and what do you usually shoot for?

 

[bbaxter51]

For separation at the axle end, I've used the 25% rule.

Meaning take 25% of the tire size you intend to run and use that as your goal for vertical separation.

42" tire X .25= 10.5" of vertical separation.

edit: You measure for vertical separation by measuring the distance from the lower link mounting hole to the upper link mounting hole.

 

[Hurley]

For separation at the axle end, I've used the 25% rule.

Meaning take 25% of the tire size you intend to run and use that as your goal for vertical separation.

what's the thought process behind this?

 

[bbaxter51]

Thought process is the bigger the tire, the greater the forces are on the upper link bars. If the vertical separation is isn't enough with a large tire, you'll rip the links off.

Just like all of this, there's no hard fast rules, only guidelines. I'm posting what has worked on the setups I have built and the 25% rule has worked well for me.