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4x4 Suspension Basics - Common Knowledge

Posted in How To on February 1, 2011 Comment (0)
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Photographers: Kevin Blumer

You didn't learn to drive without some basic fundamental knowledge, so why would you attempt to lift your 4x4 without at least knowing some 4x4 suspension basics? Take a peek at the following information to increase your suspension knowledge base, so you can weigh your options and lift your 4x4 with confidence.

Entry-level coil-spring suspension kits range from spacer and shock kits designed for larger tire fitment and a limited amount of increased flex to full-bore long-arm and coilover shock systems that eliminate the factory coil spring entirely.

Coil Springs
Coil springs will offer the best pavement ride when set up properly and can also handle trail use with ease. Coil-spring suspensions use a coil spring at each corner of a vehicle with link arms in place to act as pivots. Most production setups use two-to-four link arms, not including the track bar, which is often included in the count.

Made from a round bar of steel that is twisted in to a spiral shape, coil springs can be made with a constant or variable spring rate. The strength of a coil spring depends on diameter and length of the round steel bar, the number of coils, and the diameter of the finished coil. While coil springs are inexpensive to manufacture compared to leaf springs, their set up requires more labor and parts since they do not incorporate attachment points. Coil springs also feature different types of ends depending on their application.

One of the first 4x4s to use a coil-spring suspension was the '66-to-'77 Ford Bronco. Its front radius arm arrangement used coil springs and bent radius arms with the lower front shock mounts securing to the arms. With its front track bar helping to locate the axle it can be considered a three-link radius arm system. Many aftermarket kits are available to help early radius arm Broncos fit larger tires and increase flex.

In 1984 Jeep incorporated a similar design for the Cherokee, with its front four-link coil system. The Jeep Grand Cherokee and Wrangler TJ took the concept to the next level by adding the same coil setup to the front and rear. Jeep called it Quadra-Coil Suspension and it worked pretty darn well. Some of the available aftermarket long-arm lift kits for TJs, such as the Rubicon Express system, are actually radius arm systems since the upper front arms secure to the lower arms.

Combined with an independent front suspension many Toyota 4x4s, such as early '90s 4Runners, employ use of a rear four-link coil-suspension arrangement. This setup replaced the leaf spring rear that was previously in place on IFS Toyotas.

Leaf Springs
Leaf-spring suspension is the easy-peasy of the suspension world. It's easy to set up, it's easy to maintain, it's easy to lift, it's durable...it just requires less parts and less worry than other types of suspension systems. It's also one of the oldest suspension styles, having been put to use on wagons and farming and battle implements dating back to medieval times.

This Jeep Wrangler YJ suspension remains largely stock except for the addition of a set of extended shackles. Extended shackles are a cheap and easy way to lift a 4x4 with a leaf spring suspension. Be careful how tall you go with a shackle lift as steering and handling get worse as you go. This is a reasonable degree of lift shown on this YJ, enough to fit 33-inch tires. Remember, a shackle lift is still a lift and can require additional suspension, steering and drivetrain modifications in order to operate most efficiently.

As seemingly easy as they are, however, leaf-spring suspension systems can still be set up incorrectly if the spring isn't matched to the weight of the vehicle or if too tall or too short a spring is used. Remember, not all leaf springs are created equal. There are many styles and options to choose from beyond lift size and spring rate, including spring eye type (standard, Berlin, reverse, reverse military), bushing type (steel-encased rubber or urethane), and how the leaf ends are finished (square, diamond point, or rolled).

Multi-leaf spring packs, also called semi-elliptical spring packs, are the most popular style of leaf spring. It's what you see on most 4x4s that don't use coil springs. Multi-leaf spring packs aren't just a bunch of spring leafs from a scrap pile clamped to a main leaf. A lot goes in to the design of a leaf spring and every facet of its design can affect a spring's performance. The number of spring leaves, types of spring leaves, length and rate of the springs, how they are stacked, the placement of rebound clips, and type of spring eye all come in to play in the design of a leaf spring. A well-designed leaf spring will not only function properly but it will last a while.

Mono-leaf springs are simply a single leaf spring that is thick in the center and tapers down to the spring eyes. It mounts the same way a multi-leaf pack does with spring eyes to the frame and a spring plate and U-bolts to the axle. Mono-leaf springs can work well for dedicated trail machines, but we're leery of their safety for highway use. If you break a leaf with a multi-leaf pack you can typically drive home on it. With a broken mono-leaf you have nothing left to support the weight of the vehicle and could be looking at some serious damage, especially at highway speed.

Quarter-elliptical spring packs look like a leaf spring cut in half. The spring eye end typically mounts to the axle while the leaf pack end is fixed to the frame. It works kind of like a swimming pool diving board allowing the axle to pivot up and down as the leaf spring compresses and rebounds. While quarter-elliptic leaf spring setups can work very well mounting is more difficult than a standard semi-elliptical arrangement. Most quarter-elliptical setups we see are on more custom 4x4s.

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IFS
Independent Front Suspension (IFS) is found on almost every new production 4x4. IFS consists of an A-arm arrangement used in conjunction with coilover shocks or torsion bars. The most common set ups we see in use on trails are those found in Toyota trucks, 4Runners, and FJ Cruisers, as well the Nissan Xterra, Jeep Liberty, and late-model Grand Cherokees. Available A-arm lift kits range from basic spacer kits designed for slight tire size increases to complete systems that include new machined A-arms, new control arms, new coilover shocks, and more. Lift availability varies by vehicle make. Toyota models seem to have the best selection.

TTB
Used in production Ford trucks from 1980 to 1997, the Twin Traction Beam (TTB) front axle came in two varieties: a coil-spring version, which is the most popular, and a leaf-spring version. It mimics an A-arm-style suspension in that each front wheel is able to move independently of one another on a center pivot but it still offers comparable strength to a solid front axle and uses 'shafts with U-joints and traditional locking hubs. As a result the coil-spring version is widely used in desert racing and prerunner truck applications.

Spring Rate
Spring rate is the amount of weight required to deflect a spring 1-inch. A lower rate creates a softer spring; a higher rate a stiffer spring. Some other terms that you'll hear in conjunction with spring rate are spring load and load rate. Spring load and load rate are not the same. Load rate is the amount of weight a spring can support at a certain height. Spring load is the amount of weight on the spring.

Shock Size And Selection
There are two situations you want to avoid when selecting shocks for your 4x4: too much damping and not enough. With most off the shelf suspension kits the manufacturer has done the homework for you and will package the correct application shock with the lift you purchased. While this works for most new install situations, what if you just want some replacement shocks to soften or stiffen a seasoned suspension?

The easiest method to gauging the proper size and length shock is to measure your existing shocks - but that's only if your existing suspension is functioning to your needs. Have you experienced any shock bind, over-extension, or over-compression that have damaged the shocks or other suspension components? If bind exists, your shocks may be too short and limiting spring flex. Over-extension and over-compression are results of too long a shock and can lead to shock failure and can damage many other components. Over extension can be controlled by limiting straps to limit shock extension and over compression can sometimes be controlled by bumpstops (more on limiting straps and bumpstops to come).

In coil-spring vehicles, such as Jeep TJs, a too long of shock can cause the coil spring to fall out of place during shock extension (a spring retainer clip is helpful in this situation). Both coil-spring and leaf-spring 4x4s are also at risk of ripped brake lines should a shock overextend, as well as causing damage to steering components.

Running too short of a shock won't do the damage of too long a shock but it will limit suspension flex by binding with or fighting the spring and won't offer the optimum ride. In order to get the most out of a shock you also need to consider shock placement. For example, a shock mounted perpendicularly will be most effective with its damping ability waning the greater the mounting angle becomes. Mounting shocks at an angle is typically reserved for situations where a longer shock is desired for greater articulation but space isn't available. There are some advantages to mounting shocks at a slight angle in leaf spring 4x4s so the shock will arc in the same direction as the spring. In leaf-spring 4x4s with a front-mounted shackle the shock should lean forward a couple degrees. A rear-mounted shackle calls for rearward lean.

Measuring for new shocks isn't too difficult but it takes time to do it right. You'll need three figures to help you determine the length; collapsed or compressed length, ride height, and extended length. First you want to determine your compression travel by measuring the distance between the bumpstop and the bumpstop pad (contact point) at loaded static height, adding a rough 1/2-inch for bumpstop compression (more or less depending on the condition of your bumpstops ). Then measure the shock mounts eye-to-eye. So, lets say the bumpstop distance measures 6 1/2 inches, giving you a figure of 7 inches between the bumpstop and pad, and the eye-to-eye mounting distance is 16 inches. The difference between these figures is 9 inches; it is essentially the compressed/collapsed length of the shock and also the required length of shock body needed to contain the shock shaft when compressed.

To determine extended length you'll need to flex the suspension. This is best done on a RTI ramp but can also be completed on an obstacle. It's best to extend and measure each shock on the vehicle to rule out inconsistencies. Once you have your three figures on hand you can start shopping for shocks. Most shock manufacturers offer a catalog of available shocks with compressed and collapsed lengths listed. Make sure you get a shock with the correct mounting ends (i.e. eye, stud, etc.) for your application.

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Bumpstops And Limit Straps
The purpose of bumpstops and limit straps is to control and/or limit suspension flex. Without bumpstops or using poorly located bumpstops a suspension can over-compress and cause damage to shocks and other suspension components, as well as drivetrain and body parts. Proper placement and length of the bumpstops are crucial. Bumpstop length is determined by compressed shock length, lift height, tire size, and even wheel size and back spacing. While you could scribble equations all day trying to gauge the best mathematical location and length of bumpstops, the best way to figure it out is to slowly flex the suspension and watch carefully.

Look for contact areas between steering and suspension components, around the engine oil pan and fan/shroud, and everywhere else. Also, watch and feel for tire rub at the body and under the fenders and make sure that you do not over compress the shock. Once you've determined how much flex needs to be controlled (i.e. stopped) you can establish bumpstop length.

Location largely comes down to available space. In leaf spring applications the bumpstop is typically on the frame with the lower pad placed on the spring U-bolt plate or axle tube. Coil setups usually have the stop inside the spring with the pad in the center of the lower spring bucket. In coilover shock setups a separate hydraulic bumpstop is incorporated that can be welded to or frenched-in to the frame.

Limit straps are like bumpstops for the other end of the suspension travel spectrum. While bumpstops limit compression travel limit straps are designed to limit extension travel. Limit straps are available in all varieties of lengths. They are used in situations where you want the suspension to extend only to a certain length. For instance one might install a 14-inch-travel shock for cooler rapid cycling but only desire 12 inches of shock extension.

Tire Size
What's considered big? Back in the day we yearned for a set of "big" 31-inch Armstrong Norsemen tires for our '71 Ford Bronco. Before long, 33s were all the rage and actually enjoyed a pretty good run as the go-to tire size for trail use. While 33s certainly were and still are popular, Interco Tire constantly pushed the envelope, offering its bias-ply Super Swamper tires in gargantuan sizes to fit fullsize, mud-bogger 4x4s and highly-modified Jeeps and Toyotas. With the ever-expanding suspension aftermarket, however, demand called for larger diameter radial tires that could maintain daily driver ability with ease. With that, a handful of major tire manufacturers such as BFGoodrich, Goodyear, and Mickey Thompson, among others, began offering a 35-inch-diameter tire that replaced the 33 as the ultimate go-to size. Lift manufacturers set their sights on this tire size and ran with it, offering lift kits galore to stuff 35s under almost any set of fenders.

The larger-diameter trend continues today, with 37-inch tires nearly established as "the new 35" and 40-inch-and-larger sizes becoming more and more common on every trail. BFGoodrich even has a 42-inch Krawler tire available for those seeking something even bigger.

When, Why, And How
This is a question we get more than any other: when should I, why should I, and/or how should I lift my 4WD? Our top reasons to lift: increase ground clearance, improve ride, and larger tire fitment. Only you can determine when and why you may need this. How you lift (type of lift and tire size) also depends on you and your 4x4, but it can usually be summed up by answering a few easy questions.

First off, which is more important to you: highway performance or trail performance? Is your 4x4 a daily driver? How frequently do you tackle trails and what is the difficulty of said trails? What size lift and tires do you want? The last question is a trick but if you answered it, you know how you'll lift your 4x4.

Lift Considerations
Installation of a suspension lift can be somewhat easy (leaf-spring lift), or much more complicated (long-arm coil-spring system). Can you do it yourself or should you go to a pro? If you have the tools, time, and skills, then go for it. If not, look to those who do and pay out of pocket.

Sometimes increasing tire size means making more room under the fenders than a new set of springs provides. Are you prepared to alter the stance of your new 4x4 and possibly cut away sections of factory body panels to achieve proper tire fitment? You also need to expect changes in the vehicle's center of gravity and overall vehicle weight, both of which will affect vehicle handling.

Is your 4x4 a daily driver? If so you'll want to check your local bumper height regulations to ensure that you aren't making yourself and your rig a target for tickets from your local PD.

What else is needed? Depending on your chosen lift and tire size, you may need to continue the modifications to ensure that your new lift and tires aren't damaging other vehicle components. Components to consider include but are not limited to axle gear ratio change, steering upgrades, track bar upgrades (if not included with chosen lift kit), driveshaft and transfer case output shaft modifications, brake line and brake system upgrades, wheel spacers (for proper wheel fitment), body mods (for extra tire clearance), fender flares and/or mud flaps (for legality).

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