Custom Link Suspension Rules
General guidelines for custom suspension setup.
Dear Four Wheeler,
Hi, I am working on building a custom three-link front suspension for my very custom 1948 Willys CJ-2A. The tub is stretched, and it's on a custom frame. I mainly use it for trail riding and rock crawling, but I do drive it to and from the trails, so road manners are important. Tires are 39s, but I may go bigger. My axles are Dana 60s, and I will be using 12-inch travel shocks. Are there any hard and fast rules to follow when considering link suspension geometry? I've done a ton of reading on the subject, but it's honestly all a little overwhelming.
Thanks, Mike Lee Austin
Hi Mike, the truth is there is a ton of good information on the internet about link suspensions, but it is hard to wade through. Experience counts, and books have been written on this subject. Still, some of the info available isn't that great, and other sources have good info that is just hard to understand. Much of the geometry people talk about online is aimed at road cars that have independent suspensions with very little travel (compared to off-road rigs) and need to only handle in the turns. That just doesn't translate into a solid axle suspension that works well off-road and has decent on-road manners.
Also, much of the information online, as it relates to off-road performance, is aimed at a specific off-road activity (mud racing, rock crawling, rock bouncing, go-fast desert racing, rally racing, etc.) or people's experience and their opinion. Opinions are subject to issues with trusting the source. You don't want to build a suspension that would work well for a road racing car, or a mud race truck if you are going rock crawling. People (ourselves included) also tend to overthink suspension geometry. It's a good idea to focus on it and understand what you are doing without getting lost in the details, and remember that compromises will have to be made, especially when adding link suspension to a pre-existing frame with a pre-existing steering system, engine location, exhaust, and more. That's why full tube chassis buggies work so well. You can more easily control placement of all the parts to optimize function.
To simplify things, here are some guidelines that we try to follow when building a link suspension. You are headed down the right path, though. When building link suspensions for a general off-roader that will see road time, we shoot for a front three-link with a trackbar and some sort of triangulated rear four-link. Here are our rules or guidelines.
1) Try to keep the links as level to the ground at ride height as possible.
2) Try to keep the upper and lower links as close to the same length as possible.
3) Longer links are better.
These rules help mitigate major caster and pinion changes when the axle cycles, generally helps to keep the roll center of the vehicle low, and can prevent a harsh ride on bumps. Still, sometimes these rules will need to be broken or pushed. You don't want links so long that they hit the ground on every breakover, or link mounts so low that they hit the ground on every obstacle. Also, packaging issues can limit link length and link placement.
Next, know it's important to understand anti-squat, anti-dive and roll center/roll axis, but don't stress out too much about any of these parameters. There are several online websites that do a good job of explaining roll center/roll axis and roll steer, all of which are complex and have to do with your vehicle's center of gravity and how it will react in a turn or side hill. Also, info on squat, anti-squat, and dive and anti-dive abounds. All these parameters are affected by your link mounting points relative to each other on the frame and axles, both side to side and up and down.
Here's what you should know. You want the roll center/roll axis to be low. That will keep the vehicle stable in turns and side hills. The best way to do this is by avoiding building a super tall vehicle. Keep the rig low, and the vehicle will be stable. That's a bit of an oversimplification, but it's true. Same can be said for building a rig with gobs of wheel travel. 8, 10, 12 inches of travel (with 4 inches of up travel and 6 inches of down travel, or 3 up and 7 down) will work way better than a suspension system with 16-18 inches of travel that binds and breaks when flexed. The fact is, suspension systems with more travel are harder to engineer, and droop travel is more important than many realize. Keep it real, and don't forget droop.
Anti-squat and anti-dive are also important and directly linked to what you want to do with the vehicle. For the rear we shoot for an anti-squat number between 70 percent and 120 percent. The lower numbers will be better for go-fast dirt work, while higher numbers will work well in rock crawling and climbing. Anti-dive is similar as it affects the front suspension with 100 percent being neutral. Over 100 percent will cause the front to stiffen during braking and squat during acceleration. That can be beneficial during a climb but can also compromise suspension function when braking. Anti-dive numbers below 100 percent will cause the front to squat during braking (absorbing bumps a bit better, but also shifting the vehicle's weight forward) and will jack or raise the front of the vehicle during acceleration. For a rock crawler, we would shoot for an anti-dive number between 100 and 70. Still, we've seen suspensions well outside these parameters that have worked maybe they don't work well, but they work including many cheaper lift systems.
Track bars and link placement are also important in a link suspension. You want your track bar to be 1) as level as possible at ride height. The angle also needs to match your drag link angle on a front or other steering axle, or you will have bumpsteer. 2) You want your track bar (and drag link on a steering axle) to be as long as possible. As your suspension loads and unloads, the track bar will force it side to side. If the track bar is at an angle at ride height it will probably force the axle to only move one way. A flat track bar should allow the axle to move from one side to the center and then to the other side as the suspension cycles. Shorter track bars will cause this movement to be more extreme, moving the axle side to side faster relative to the amount of suspension up and down movement.
We will also say that with a three-link front suspension, you want to do everything that you can to keep the upper third link on the same side of the axle as the driveshaft. That isn't necessarily easiest, from a packaging standpoint, but it's the best. Having the arm on the other side of the axle can work but will cause your driveshaft to change length rapidly and change your pinion angle rapidly. This can cause a bind and or damage. Ideally your upper and lower links are about the same length as your driveshaft and are on the same side of the axle.
Lastly, when you have all this figured out, you'll have to test it out to make sure it will all work. That means building the suspension as you think it will work (build at full bump for clearance but keep measurement parameters at ride height in sight). As it comes together you will have to cycle the suspension from full bump to full droop and flex both sides up and down (each side fully compressed with the other side fully drooped and vice versa). Test the steering and check the driveshaft length and pinion angle in all four positions. This is why you should take your time and heavy tack-weld parts in place until you are sure things will work the way you want them to.