Dispelling 4-Link Myths
IN THIS WORLD OF BEING ABLE TO BUY A KIT FOR PRACTICALLY EVERY ASPECT OF A TRUCK BUILD, YOU’D THINK A TRUCK COULD ALMOST BUILD ITSELF. Kits definitely make things tons easier, but there is still a basic level of knowledge necessary to properly install these parts. A perfect example is 4-link kits. There are many bolt-on solutions, but if you don’t know what you’re looking for, you won’t know how a certain kit will affect your truck’s ride and handling. If it was up to Max Fish, owner of Bio Kustumz and author of Air Suspension Design, Volume 1, everyone would have a one-off suspension tailored to their exact needs, but with the realization that a lot of people buy pre-made, “universal” parts, Max obliged us and shared some of his vast knowledge to help us better understand how to properly set up a 4-link. We came up with a few common questions, and Max came through with his thoughts on 4-link design. He even hooked us up with the helpful illustrations seen here, which can also be found (along with many others) in his book. We have no doubt that you’ll find this information useful.
We see a lot of people set up their triangulated links at 45-degree angles, others at 30-ish degrees. Does it matter one way or the other?
Max: The amount of triangulation necessary to limit lateral movement of the axle has a lot to do with the vehicle’s intended purpose and the compliance [the amount of flex] of the bushings. For example, a vehicle that will spend its life railing corners around a track will need more triangulation than a dailydriven commuter. And as far as bushings go, a more compliant bushing, like a rubber bushing, will need more triangulation. While
a less compliant joint, like a heim joint, won’t need as much triangulation. However, it still isn’t possible to apply a specific number to the acceptable angle for a particular vehicle. Ideally, 90 degrees between the triangulated bars is considered optimum, but that isn’t always possible to fit into the confines of a rear suspension.
What about upper bar lengths? Lately, we’ve seen you post that it might not be as crucial to have the upper bars super long. Can you elaborate, and is it different for triangulated and parallel 4-links?
I see a lot of people who like to apply rules to upper bar lengths with no real reason why. The only theory that I will give credit to is the concept of equal length bars for both the uppers and lowers. This is generally accepted as the best way to keep the pinion angle perfect throughout travel, but again, it’s not always possible to fit that into the confines of your vehicle. The reality is that there is no rule for how long the upper bars need to be in relation to the lowers. I’ve actually talked to road-race guys who ran longer upper bars to get more traction during cornering. Personally, I like to run the upper bars shorter than the lowers to help with packaging. It also makes higher triangulation angles a lot easier to produce as well.
Can you elaborate on the difference you’ll see between setting up a 4-link at ‘ride height’ versus setting one at mid-travel?
This one is tough because my half-travel theory only works if the suspension is designed properly. If your suspension geometry is screwed off in such a way that it is only drivable at the design height, then you have no choice but to use ride-height as the set-up height. However, if your suspension is set up properly, it should be useable at any point of travel. So with that, to minimize geometry change throughout the travel, designing it at the half travel point is the easiest way to do that. This video does a pretty decent job of explaining the concept: http://bit.ly/2o6lbla.
Some parallel 4-links have upper and lower bars that are completely parallel to each other, while others converge slightly on one end and may actually have many adjustment points built in. Is there a place for both, or is all of that adjustment overkill for most people?
Again, the concept of equal-length bars that are parallel to each other is a great way to keep the pinion angle perfect, but do you really think that’s what suspension designers are using as their primary focus for design: to first and foremost keep the pinion angle happy? On a driven axle, the energy is transferred through the axle housing into the suspension components and then to the chassis. The orientation of the link bars and how they are attached to the axle and chassis will determine how that energy is transferred. By designing a suspension with multiple points for the bars, the suspension can be tuned to have more or less traction during acceleration, or in the case of a dirt track car, during cornering. I have heard a few different theories as to how the concept works and even what it is called, but it is most commonly called instant center or instantaneous center. The theory is outside of the scope of this article, but this video may help make some sense of things: http://bit.ly/2o6lbla.
What is the biggest misconception people seem to have when setting up a 4-link? On a related note, what’s the most common mistake you see regarding 4-links when someone brings their truck into Bio Kustumz for repair?
Biggest misconception? That’s a tough one … probably that they have it figured out. Most people think they know what they are doing because they looked at a ton of photos online, read a few articles, and know how to weld, but no one pays attention to the subtleties. This is where the most common mistake comes in. A large majority of people don’t realize that roll-steer is a thing. And more importantly, it’s a scary thing when you have too much of it. The limitations of most vehicles that are trying to lay frame on a big wheel only compound this issue. It’s unfortunate that poor geometry isn’t as ugly as runs in a paint job or steel belts hanging out of a tire, because more people would probably be changing their habits really quickly. The halftravel versus ride-height video addresses roll-steer a bit as well.
Why does everyone think link bars need to be so long?
Because most people have no idea how geometry works, so they push things to the extreme to ‘minimize front-to-back movement.’ Look at a ’64 Impala and tell me how long those link bars are.
Why don’t people like pan-hard bars?
I blame poor designs being accepted as correct simply because they haven’t failed yet. There’s a big difference between ‘strong enough’ and ‘properly designed.’ A properly designed pan-hard bar will keep the side-to-side movement very minimal and is far easier to design than a Watts-link.
Thanks once again to Max for his amazing insight on properly setting up suspensions, and be sure to check out his book, Air Suspension Design, Volume 1.
ABOVE. ON THE RIGHT IS A SUSPENSION THAT HAS NOT BEEN SET UP AT HALFTRAVEL. YOU CAN SEE HOW MUCH FRONT-TO-REAR MOVEMENT THERE IS COMPARED TO THE LEFT, WHICH HAS BEEN SET UP AT HALF-TRAVEL.
RIGHT. THIS EXAMPLE SHOWS HOW LITTLE DIFFERENCE THERE CAN ACTUALLY BE WITH SHORTER LINK BARS, ASSUMING THE SAME RANGE OF TRAVEL. A 12-INCH DIFFERENCE YIELDS LESS THAN 3/16-INCH OF DIFFERENCE.
THIS IS AN EXAMPLE OF 4-LINK BARS THAT HAVE BEEN SET UP TO HAVE AN INSTANT CENTER JUST BEYOND THE FRONT BUMPER AT HALF-TRAVEL.
AS ONE SIDE OF THE SUSPENSION COMPRESSES IN A TURN, YOU CAN GET A LOT OF REAR STEERING INPUT IF YOUR 4-LINK IS NOT SET UP CORRECTLY. THIS CAN CAUSE UNDESIRABLE, AND POSSIBLY DANGEROUS, EFFECTS WHILE DRIVING.