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Suspension Advisor

by Jim Allen

Tires, wheels and suspension will likely be your biggest investments in a Jeep buildup and you want that money to pay interest in the performance department. The starting point for decisions on the suspension is tire size. Everything else revolves around it. Tire size will dictate the cost and complexity of the job, the street driveability characteristics and the level of trail performance.

Once you’ve made the tire choice, you can start shopping for a suspension to match. Lift kit manufacturers will offer a “recipe” that matches a lift kit with a particular tire size. Sounds easy… and most of the time it is… but not always, because every Jeep is a little different.

Back to TopLift Numbers 101

The lift numbers and tire fit recommendations you see advertised are averages. The manufacturer builds a kit around what they think is an “average” Jeep. You may get more or less lift than the kit manufacturer lists because vehicle weight varies. A heavier vehicle will compress the springs more than a lighter one and end up with less lift, or vice versa. It all has to do with spring rate.

Spring rates are measured in pounds per inch of travel. A 400 lb/in. spring takes 400 pound to compress one inch, 800 to compress two, and so on. The kit maker plays the law of averages so if your Jeep is lighter than average, you end up with a little more lift and perhaps a stiffer ride. If you are heavier, then your ride will be smoother and your lift less.

You need to be aware of this particularly if your Jeep has (orwill later have) an engine swap, a heavywinch up front or lots of accessories. If it does, you may not have the clearance for the tires you have selected. In some cases, youmay find a selection of spring rates available from a kit maker to suit your heavy Jeep better. If not, you have several choices, from adding small corrective measures (read on) to adjusting your tires size choice smaller to suit the available space. One way to be sure is to weigh your Jeep on a commercial scale and get front and rearweights.With that info, the lift kitmanufacturer can predict how their kit will react to those weights and help you make a better selection.

Back to TopLeaf Spring Lift 101

The best way to lift a leaf spring Jeep is via new springs. Lifted springs have more arch and are slightly longer. Up to about 4-inches of lift, you can count on a good ride and good travel but above that point, the amount of arch tends to interfere with ride quality and limit uptravel more severely.

There are other lift methods you have heard about, including add-a-leafs and shackle lifts. Add-a-leafs usually add an inch or two of lift at the cost of some ride quality, assuming the vehicle weight remains the same. They are best used if an increase in spring rate is needed for a heavy Jeep or to bolster sagging springs.

Longer shackles are great for correcting pinion angles but not so good as a lift device. The longer shacklewill tend to tilt the nose of the diff pinion up on both the axles. That’s assuming the front shackles are in the stock location at the front of the springs. On Jeeps with reversed shackles, a longer shackle will tilt the nose of the diff down. Remember that whatever lift a longer shackle offers, is half of the increased length fromstock. A shackle that is two inches longer than stock will only offer an inch of lift. If the pinion ends up in a useful position, the extra lift from a longer shackle can be used for ride height correction as long as the angle on the shackle remains tilted back from vertical. Inmost cases, pinion angles are corrected with the use of tapered wedges that are sandwiched between the axle and the spring. They often are included in a lift kit but also come separately in various angles.

Track bars on the ’87-96Wrangler YJ offer a special complication. Lifting a YJ with a stock bar tries to pull the axle towards the driver’s side because the eye-to-eyemounted distance is decreasedwith angularity. There are several cures, the usual is either relocation brackets to drop one end or longer bars, either of which is included in a lift kit. There are also adjustable track bars. Track bars definitely improve road handling and should be kept in place, but they also restrict off highway articulation. Quick release track bars are offered to give you the best of both worlds.

Back to TopCoil Spring Lift 101

Coil lifts come in two basic forms, spacer lifts and spring lifts. Spacer lifts are inexpensive, easy to install and effective up to about two inches of lift. After that, the uncorrected changes in suspension geometry make them iffy in the driveability department. Brick-simple, they consist of a spacer mounted between the coil spring and the upper spring mount. It can be polyurethane, metal or even an adjustable spacer. A spacer lift is a great choice for people who need the clearance for a mild bump in tire size from stock. Spacers can also be used to level or correct ride height in combination with another type of lift. A spacer lift comes with either shock extensions or longer shocks. Spring lifts come in many varieties. They start with a simple spring swap alone, which is just a longer spring that comes with either longer shocks or shock extensions.Without correction, the spring lift has essentially the same limitations as a spacer lift, namely about two inches of lift.

Two more spring lift categories are the short arm and long arm. The short arm type uses control arms of a similar length to the original. These are corrected lifts, meaning the geometry has been corrected according to the amount of lift. That take several forms, most commonly longer lower arms and adjustable upper arms. It may also use drop brackets. Speaking of upper arms, kits come with or without. A basic lift doesn’t necessarily need new uppers for correction purposes, but the aftermarket arms are much stronger than the OE. The OE upper arms sometimes fail in rough service when subjected to the extra angularity of a lift.

Short arm lifts should be kept to about three inches, four at the outside, for the best driveability. The more the lift, the sharper the angle on the control arms and the more adverse street symptoms will be noticed. These include a harsher ride, more rapid control arm bushing wear, a jacking effect on acceleration, roll steer in turns and so on.

There are exception to the 3-inch rule, such as dropbracket kits that move the chassis end of the arms lower to reduce angularity. These are very viable alternatives that offer very good trail performance and good street manners.

Long arm kits are the top dogs in the world of coilers. As the name implies, they use longer control arms, or radius arms. This reduces the angularity of the arms, which reduces or eliminates the negative driveability effects and increases travel, even over a short arm dropped kit. They often combine other beneficial modifications, such as a triangulated upper axle link in the rear.

Back to TopThe Dreaded DeathWobble!

The death wobble can occur with any 4x4 mounting big tires. It’s a steering oscillation that comes at speed on the highway. It isn’t quite as dangerous as the name implies…but will make you think it is! The primary cause is usually improperly balanced big tires. It can also be caused, or amplified, by loose steering components. Beyond correcting worn steering and suspension components, a good balance job will go a long way toward correcting the death wobble. A key element in balancing is to use make sure the weights are placed on both sides of the rim, not just the inside. Not as pretty…but a much better balance job with big tires. The final element for death wobble avoidance is a beefy steering damper.

Back to TopShocking Revelations

New shocks often come with lift kits. Shocks are not a “one-size-fits-all.” The first requirement is that extended and collapsed shock length should match suspension travel. Also, leaf and coil sprung Jeeps have somewhat different requirements for valveing, so a good match is type specific. Piston size is an important consideration. Big tires increase unsprung weight and you need a bigger shock to handle it.Within reason, you can’t go “too big” on piston size if the valveing is correct for your general vehicle type.

Differences in shock requirements also come with the type of Jeeping you do. Jeepers in the easy-wheeling category can get by quite well with “regular old” hydraulic shocks. Most rock crawlers can too, but both categories can benefit from low-pressure gas charged shocks. Fastmovers need high pressure gas charged shocks to dealwith oil foaming and heat.

Heat is the primary effect of lots of fast suspension movement. Fast movers often installmultiple shocks to share the load. Dual shocks should be valved softer for use in pairs or you may end up with a rougher ride that you like. Premiumor high performance hocks are so good these days that only a few people really need a dual shock setup.

Back to TopGood Vibrations: Driveshaft Angles

Liftswill increase driveshaft angles and to avoid vibration and shortened ujoint life, these need to be addressed. Many lift kits include some means to deal with this but you are well advised to go beyond. Even a mild two inch kit on a ShortWheel Base (SWB)Wrangler will increase the driveshaft angle to almost 28 degrees from the stock 15 degrees. With about 30 degrees as the maximumangularityonavery short shaft, something has to be done.

The first option is a transfer case lowering kit,which reduces the driveshaft angles by lowering the transfercase/skidplate assembly. This is accomplishedwith spacers, is simple to install and usually included inmost lifts. In leaf spring applications a spring degreewedgemay also be needed.

With higher lifts, a better solution is a fixed yoke, slip-yoke eliminator kit for the transfer case a CV type two-piece rear driveshaft. This shortens the transfer case length by four inches or more, and that extra length in the rear driveshaft significantly reduces rear driveshaft angles and vibrations. These modifications also make the transfer case much stronger as well, and are suggested for serious off-road use. The more expensive kits have stronger rear output shafts and these are necessary if you have a 4:1 transfer case or a big engine.

Back to TopSway Away!

While they do restrict suspension articulation in the dirt, operational sway bars are important to highway safety, handling and driveability, so don’t chuck ‘em. That’s easier to do now that you can get sway bar disconnects for virtually any Jeep. Hooked up on the street for good handling and unhooked on the trail for maximum articulation. Many kits include them. If not, they are a worthwhile supplementary purchase.

Back to TopSmooth Steering

There are several types of steering systems used on Jeeps, all susceptible to bump steer when lifted. Bump steer is when suspension movement introduces unwanted steering input and causes wandering or darting. It’s all about the angle on the drag link, which connects the steering box to the tie rod. Ideally you want that link to be as close as possible to parallel with the axle.When it’s not, the up and down movement of the axle changes the end to end distance and in effect steers the vehicle.

The cure is a dropped Pitman arm, which reduces the angle of the drag link. On YJs with track rods, the ideal is to have the drag link at the same angle as the track rod, especially in those first few inches of axle travel. The same goes for the coil sprung TJ.

Big tires and ‘wheeling puts a lot more stress on steering components than the OE engineers planned for. Two common problems then become the steering box mounting and the drag link and tie rod. Every ‘wheeling Jeep with oversized tires should have a steering box brace. They make it much less likely that the box will tear away from the chassis. TJ steering boxes are vulnerable to rockcrawling trail damage, so a skid plate is in order. Heavy duty tie rods and drag links resist the greatly increased bending forces applied by big tires These HD rods often come with high angle rod ends that can tolerate the extra angularity imparted by lifts. At about 33 inches of tire diameter, a steering linkage upgrade moves more into the “necessary” category.

Back to TopBushings Bared

Polyurethane bushings are the best choice for most rigs, most times. Especially in trail situations, they will outlast OE rubber bushings by massive amounts. In leaf springs, they allow for a little more suspension movement because the center sleeves move freely in the bushing, unlike bonded rubber bushings. Many types are available in greaseable form, which helps them last even longer and avoid the creaky noises Poly is famous for. Ply is suitable for 99 percent of everything on Jeep suspensions. The exception are the coilers. If used on the chassis end of the suspension link, they can transmit a lot of noise and vibration that some find objectionable.