The Nitty Gritty Confusion of Four-link Suspensions
Last month we started an in-depth look at the benefits and detriments of a four-link suspension. We touched on how a four-link will reduce the number of variables down to just the spring rate and shock valving. In addition, a four-link is expensive to do right, and this second installment will hopefully take you from the drawing board to the garage floor.
There are many different link configuration possibilities, but for this discussion we'll stick to a basic four-link where the upper two links start at the frame and converge at the top center of the rear axle. The lower two links will also run from the frame to the outer ends of the axletubes. A three-link is similar, but the upper links are replaced by an A-arm with a single joint at the top of the axle. The three-link setup puts that upper axle joint under greater side loads than the upper two links of a four-link, but it is a viable alternative. Also, suspension builders will argue till the cows come home about what works best, but what we have done is discuss with some of the top desert-race suspension builders how to get you started on a four-link. This design is just a launching pad, and you will need to spend a fair bit of time dialing everything in.
In addition there are many excellent books available to learn more about suspension design. We would recommend:
Chassis Engineering by Herb Adams
Fundamentals of Vehicle Dynamics by Thomas D. Gillespie
Race Car Vehicle Dynamics by Milliken and Milliken.
Though some of these books are pretty heavy, they do help explain the theories behind four-link suspensions, but mostly when applied to street cars and not off-road vehicles. To truly explain a four-link, we would need this entire magazine and a few engineering degrees, and even then there would be things that would be missed. This, however, should be enough to get you started. Just take your time and enjoy the process, because if you don't have the patience to adjust and rebuild your suspension until it works just right, then you should stick to leaf springs.
The first step in building a four-link involves a tape measure and some graph paper. What you are going to do is figure out the angle of the links and their mounting locations. This will in turn give you an idea of where to start building your four-link. From there you can fine-tune it. Park the truck on flat ground and measure your wheelbase and the tire size you will be running. Plot the axle centerline points on the bottom half of the graph paper as if you were looking at the side of the truck. Now draw the framerail as it sits above the axle centerlines. This should be where you expect the frame to sit above the axle if you have not yet lifted it. If you know the height and location of your center of gravity of the sprung weight, plot that as well. If not, estimate it by measuring from the top center bolt of the bellhousing to the ground. You may need to add the height of the expected lift if the truck is still stock.
Now plot a point on the front center of the rear axletube. This will be your lower link mount. Some people mount this above or below the axletube, but we have found that the important part is more the difference in height from the upper-link mount. If your truck is going to be very tall, you may want to put these links on the top of the axletube. To find the upper-link axle-mount point, multiply the tire diameter by 0.25 (25 percent). Use that number as the distance in inches that the upper link will be above the lower link at the axle. If you were running 36-inch tires, you would want the upper links to be mounted 9 inches above the lower-link mount. You will most likely be mounting the links 8 to 11 inches apart. The farther apart you can get them right now, the better, as this will help control the leverage of the tires and fight axlewrap. The limiting factor will most likely be the bed of the truck. Continue by plotting the upper- and lower-link axle-mount points. If this is getting confusing, then you are normal; if it's clear as a bell, you may be a bit too smart for your own good.
Since you have a rough idea of where your axle-mounting points will be, it's time to move onto the frame mounts. The first point to plot is the lower-link frame mount. To determine this, draw a link with a 5 to 10 degree angle up from the axle mount to the frame in the sideview drawing. Watch where the link intersects with the frame; this point will most likely be near the transfer-case rear output. It should also be as high as possible for ground clearance, but low on the frame to keep the link as level as possible. If you cannot get the link to intersect the frame at 5 to 10 degrees, you may need to move the lower-link axle mount up on the axletube. If so, you will also need to move the upper-link axle mount as well to keep the predetermined 8- to 11-inch vertical spacing between the links at the axle.
Another option is to consider building a crossmember mount below the framerails. At this point you should be realizing that a four-link involves tons of variables and compromises, and we haven't even gotten to actually looking under the truck yet! Now take the horizontal distance from the lower-link frame mount to the lower-link axle mount and multiply that number by 0.7 (70 percent). This is a good horizontal length of the upper links. The distance apart that you mount the upper and lower links on the frame should be about half the vertical distance apart of the link's axle mounts. Again, try to keep the links as level as possible.