4Link Tech Measurements

7/31/2019 4 Link Tech Measurements

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4Link Tech Measurements & Material Selection

Four Link Tech

Part 1

What Is a Four link? And Is It For You?By Fred Williams at 4Wheeloffroad Magazine

A four-link suspension uses links to locate the axle from moving side-to-side and front to back, while allowing it to travel up anddown and articulate. We must agree with the current majority that a four-link suspension with coils, Coilovers, quarterelliptics, or airsprings is definitely cool, and that is the most common argument for building your own setup. The problem arises when you thinkyou know what you are doing and just start putting bars and links under your truck. Just because you saw it on some race truck orrock crawler doesnt mean you need it for your weekend wheeler, though we have no problem with building one just because youwant to try it.

There are some definite benefits to running a four link, but to do it right takes time, money, and some more time and money. Wehope to give you a realistic overview of a simple rear four-link suspension, but first, the pros and cons of building one. The choice isyours, but please consider everything before you get started. The fact remains that a well designed and tested four link will provide asuperior translation of power to the ground and higher ride quality than a leafsprung suspension. The secret is really in the testingportion. If you build a four link on your rig then be prepared to finetune it and tear it apart quite a few times before it works right. Andduring this testing stage we would not recommend driving it to work on the highway at 60 mph. You may get lucky the first time, but ifnot, remember that tearing your truck apart and rebuilding it is fun.

Cons id

erations

The biggest question with building a four link is how long should the links be and where should they attach to the frame and axle.This alone will determine how the axle pushes the vehicle, if the rear of the vehicle lifts or squats under acceleration, if wheelarticulation causes the rear axle to pivot and steer, and how the body rolls in turns and over obstacles. The desired amount thevehicle does each of these things is different depending on what the vehicle is designed to do (go fast, corner, crawl, articulate) andhow the driver desires the vehicle to respond on different terrain. There is no one right way to build a four link the same as there isno one perfect off-road vehicle, but it can be tuned to do certain things better than others.

For most truck owners an all-around four link is the desire, but that will not necessarily be the best rock crawler, desert jumper, andmud bogger suspension. In addition to all the geometry of designing a four link there is also the problem of what will actually fit onthe vehicle you are building. Will the frame support the links where you want them? Will the fuel tank, exhaust, cross members, anddriveshafts all fit with the links and allow for proper articulation? Unless you are building a truck or buggy around the suspension,plan on doing some compromising to get the best setup you can. If you are starting to like the idea of keeping the leafspringsuspension, we dont blame you. If you are up for the challenge, stay tuned for next month where we start getting into the technicalpart of the buildup.

Till then you have a bit of homework. Youll need to round up a tape measure, a calculator, graph paper, and a pencil. Now gomeasure your wheelbase and decide on the height of the tires you want to run on your rig. Follow that by measuring the rear axlewidth just inboard of the brakemounting plates and the height of the frame at various points between the axles along the frame railswhile the truck is on level ground. Next find the height of the top center bolt of your bell housing to the ground. Plus start doingresearch of where you can buy the materials we mentioned below. Just remember you will want to wait until you have read thesecond part of this story next month before you attack the four-link issue under your truck.

The benefits of a four link over a simple leafspring suspension include controlling axle wrap, better departure angles, controlling axlepath, and reducing the uncontrolled variables of axle movement down to just spring rate and shock valving. In addition, a four linkcan also allow for more travel and articulation that can provide more traction, though we feel that too much of both can causeproblems. Weight is also a concern of the modern-day wheeler since excessive weight eats power. Though a coil spring is lighterthan a leaf spring, when you consider the weight of the links and mounts and everything else, the gains in weight are minimal.

The major benefit of a leafspring suspension over a four link is cost and maintenance. It will take more time and money to removeleaf springs and design, build, test, and rebuild your four link than it would to just put on a good leaf suspension, and this is if you doit yourself. With shop rates ranging from $25 to $75 per hour, a professionally fabricated four link is gonna take a serious bite out ofyour wallet. Plus we have seen some very impressive leafsprung suspensions that allow plenty of travel and articulation.

Material is an important factor and concern for strength and safety. Your lower links could be hitting trail obstacles depending onhow low you mount them to the axle, so we would recommend no less than 1 3/4inch DOM tubing with 0.25inch wall thickness. Ifyour truck is a full-size or extremely heavy, or if you are planning on mounting your shocks on the lower arms like some race trucks,then you will want to go to an even larger tubing size, or better yet, sleeve the 1 3/4 x 0.25 with a slightly larger piece of tube. Theupper links are less likely to be hit by rocks and such, but we still do not recommend anything less than 0.120 wall, 1 3/4inch DOMtubing.


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As for the joints at the ends of the links,get the best you can afford. The linkswill last longer and be stronger if youspend the extra dimes, plus if they fail it

can be tragic. Everything from JohnnyJoints to Heims is applicable. ManyCompanies like Poly Performance andAvalanche Engineering can supply youwith these joints and the proper weldingbungs to

screw them into the links. Andremember that the mounting boltsshould be as close to perpendicular tothe link as possible when installed at

ride height to get the maximum strengthfrom the joints.

For the mounts we will steer youtoward 0.25inchorlarger material. Many

shops like A&A Manufacturing,McKenzies Performance Products, andcircle track race shops offer tabs andbrackets that can be welded to framesfor link mounts if you dont have thefacilities or tools to cut and build your

own. As for the axle mounts, your upperlink mount will most likely be a bracketthat bridges over your differentialhousing. Welding directly to the casthousing is tricky and if you dont knowhow then plan on building a bridge out

of 0.188 wall 1 3/4inch or larger DOMor square tubing.

Grade 8 hardware is the best bet forattaching all the links to the brackets,and should be at least 7/16 inch if notlarger. Remember when you haveeverything together that, like everythingelse on your off-road machine, you

should check for loose or broken partsbefore trail runs.

Four Link

Tech Part 2The Nitty Gritty Confusion of Four link Suspensions By Fred Williams at 4Wheeloffroad Magazine Last month we started an in-depth look atthe benefits and detriments of a four-link suspension. We touched on how a four link will reduce the number of variables down to just thespring rate and shock valving. In addition, a four linkis expensive to do right, and this second installment will hopefully take you from the drawing board to the garage floor. There are manydifferent link configuration possibilities, but for this discussion well 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 axle tubes. A three link issimilar, but the upper links are replaced by an Arm with a single joint at the top of the axle. The three-link setup puts that upper axle jointunder 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 cowscome home about what works best, but what we have done is discuss with some of the top desert race suspension builders how to get youstarted 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 AdamsFundamentals of Vehicle Dynamics by Thomas D. GillespieRace 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 tostreetcars and not off-road vehicles. To truly explain a four link, we would need this entire magazine and a fewengineering degrees, and even then there would be things that would be missed. This, however, should be enough to get you started. Just takeyour time and enjoy the process, because if you dont have the patience to adjust and rebuild your suspension until it works just right, then youshould stick to leaf springs.


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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 linksand their mounting Locations. This will in turn give you an idea of where to start building your four link. From there you can finetune it. Parkthe 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 ofthe graph paper as if you were looking at the side of the truck. Now draw the frame rail 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 thesprung weight, plot that as well. If not, estimate it by measuring from the top center bolt of the bell housing to the ground. You may need to addthe height of the expected lift if the truck is still stock.

Now plot a point on the front center of the rear axle tube. 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 verytall, you may want to put these links on the top of the axle tube. To find the upper link axle mount point, multiply the tire diameter by 0.25 (25percent). 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 36inch

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 11inches apart. The farther apart you can get them right now, the better, as this will help control the leverage of the tires and fight axle wrap. Thelimiting factor will most likely be the bed of the truck. Continue by plotting the upper and lower link axle mount points. If this is gettingconfusing, then you are normal if its 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, its time to move onto the frame mounts. The first po int to plot is thelower 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 side viewdrawing. Watch where the link intersects with the frame this point will most likely be near the transfer case rear output. It should also be ashigh 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 frameat 5 to 10 degrees, you may need to move the lower link axle mount up on the axle tube. If so, you will also need to move the upper link axlemount as well to keep the predetermined 8 to 11inch vertical spacing between the links at the axle.

Another option is to consider building a cross member mount below the frame rails. At this point you should be realizing that a four linkinvolves tons of variables and compromises, and we havent even gotten to actually looking under the truck yet! Now take the horizontaldistance from the lower link frame mount to the lower linkaxle mounts and multiply that number by 0.7 (70 percent). This is a good horizontal length of the upper links. The distance apart that youmount the upper and lower links on the frame should be about half the vertical distance apart of the links axle mounts. Agai n, try to keep the

links as level as possible.

Start looking at the width of the frame at the point where the upper links attach, and write this measurement down. Subsequently measure thedistance along the axle tube from just shy of one brake mounting plate to the other. This will be the distance apart of your lower link axlemounting points. If at this point you are starting to really like the idea of leaf springs, then we congratulate you for having some commonsense. If you are still thinking that youll be the talk of the town with your new super four-link suspension, then dig out that piece of graphpaper and sharpen your pencil, because there is more work to do.

If you have a pile of graph paper crumpled into balls and a headache from thinking too hard, then you are right on track. The tricky part comes

when you take your drawing and see if you can actually attach the links you drew on the trucks frame and axles. This is wher e compromisecomes in. You may need to move the fuel tank, exhaust, or various other low hanging parts of your truck. There is always a bit of adjustmentavailable. The upper links can be slightly longer or shorter than 0.7 (70 percent) of the lower links, but try not to pass 0.6 (60 percent) or 0.8(80 percent).


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On the graph paper draw lines up to the top of the paper from the front and rear axle centerline, upper and lower link axle, and framemounting points. Next, in the upper space draw the rear axle from the top view with the lower link axle mounts plotted at the distance apartyou measured from the actual rear axle. Follow that by drawing in the frame rails from the top view with the center of the frame over thecenter of the rear axle. But wait, theres more.


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Draw in the upper links first. Remember to have them start from the frame, but at the axle keep them slightly separate to allow access to thenuts that will go on the ends of the bolts running through your rod ends. Now grab an angle finder. It is very important that the angle of thetwo upper links be no less than 40 degrees. This angle is what locates the axle laterally or side to side. The smaller or more shallow the angle,the weaker the lateral control. This again may mean shortening the upper links, but try not to make them less than 70 percent of the horizontallength of the lower links. If need be, you may need to shorten the lower links horizontal length as well, but try to keep them as long and aslevel as possible. If you are wondering when you get to start installing your really cool new Coilovers shocks, then you might need a lesson inpatience. Get another cup of java and keep studying.


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squat you will be aiming for. This will be discussed further in Figure 9.

Next draw a vertical line down from the LCPs to the side view drawing. Extend lines from the upper links back and lower links forward until

they cross the lines you brought down from the LCPs. This will show you the heights of the LCPs. Now when you connect the two LCPs on thelower drawing with a straight line, you will get the roll axis. This is the imaginary line perpendicular to which the axle will articulate. As such,you want the front of the roll axis slightly lower than the rear. This will give better handling and less rear over steer as the axle articulates. Ifyour roll axis leans toward the back of the truck, you may need to lower the upper links frame mount or increase the height of the axle bridge.Just dont let the upper links frame mounting point get lower than the lower links frame mounting pointbetter yet, keep them apart. As youcan see, there will be many opportunities to adjust and diverge from the original design to get the geometry correct.


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4Link Tech Measurements