There are a lot of great lift kits out there and these kits are becoming increasingly sophisticated. From your basic coil upgrade for load, to entry level ride height kits, to more advanced performance based kits and beyond, the options in the market are truly stunning. It can also be a topic with a steep learning curve.
To help wade through the mire a little, I’ve compiled a glossary of terms and definitions to help you choose. This will cover basic suspension geometry terms and where they are often overlooked, as well as shock and spring terminology and technology. Hopefully, armed with this information you can make an informed decision. When you are ready, Overlander can provide most options to help you make your dream a reality.
The maximum physical travel of the axle path from top to bottom of the suspension stroke without restriction. This doesn’t take into account tire interference, spring and damper limits, or other stops and limiters.
The actual physical travel of the axle with all components attached and all clearances and stops considered.
Bump stops are soft limiters to the cycle of the suspension. In full bump a suspension is cycled up to its maximum limit minus the small amount of travel left in the bump stop, which is usually a firm piece of rubber with limited travel to prevent metal on metal contact and damage.
Droop is the opposite of full bump; it’s the downward most limit in the travel of the suspension. Unlike full bump, full droop is usually not limited by the cycle, but limited in some other way to prevent the cycle from exceeding a design limit. I.e., if the geometry or joint limit is severely affected after a certain amount of down cycle, your full droop would be where your shock limit or limiting strap needs to be to prevent the cycle from traveling that far.
The travel of the axle path moving into the body, factoring in stops and limits.
The travel of the axle path moving down away from the body. This factors in stops and limits
Stuff is the maximum up travel as limited by the tire/body interface. A tire is fully “stuffed” when it’s either at full stop or the stop is the tire hitting the body.
Sag is the amount of the suspension up travel used relative to the total travel when resting statically. Having the correct amount of sag allows your travel to work appropriately through up and down motions while keeping static geometry correct.
This is the relative amount of travel between diagonal wheels, or from one side of an axle to another. Flex is important because keeping tires on the ground is the oldest and best form of traction control. A balanced flex with sufficient ground pressure will keep you moving.
Moving on to suspension geometry, this is to help clarify that a lift is far more than just being taller. Its also about the effects altering geometry can have on your vehicle in other ways. It’s been said that modifying your vehicle is ruining it in degrees, and in a sense that is true.
The manufacturer has made careful trade-offs and compromises to balance ride, stability, performance, and life cycle of very component. Every time you move the slider one way in a single category you move another slider the other way. Finding that balance is the key to a solid performing overland vehicle that is safe and long lasting.
Camber is the relationship between the distance of the top of 2 tires on the same axle and the bottom of the same tires. Where the top distance is smaller than the bottom, the camber is negative, when the top is larger than the bottom the camber is positive. The camber angle is the relative angle between plum and this difference. Finding the correct static ride height camber with the appropriate resting sag increases on road handling, off-road traction and increases tire life. Camber is dynamic and will change through the travel. Camber is affected through eccentric bolts in lower control arms or hub to strut interfaces with independent suspension. Camber is typically not adjustable with solid axles. A major restriction on independently sprung vehicles is the limit of additional camber you can add. As you raise the suspension you will add positive camber and the stock joints may be insufficient to reset your static camber to acceptable levels, or they may become compromised by their new static angle and have decreased life.
Castor is the angle that is formed between the true vertical of a wheel and the axis that is drawn from the upper suspension mounting points and the axle center. Castor affects straight line stability, anti-dive, compliance, and other drivability issues. Castor is often affected negatively when lifting a solid front axle vehicle and many handling and drivability problems can be traced back to castor. Correcting castor should be a part of any comprehensive lift.
The steering axis is the line that is drawn between the top point of the steering axis and the bottom point. In the case of struts, the top point is at the top of the strut and the bottom at the lower joint. In the case of a double wishbone suspension, it’s the top of the hub location and the bottom of the hub location on the A-arms. With solid axles it's the top of the steering knuckle to the bottom. The axis is almost always angled in towards the body of the car and back.
Scrub radius is where the kingpin axis and the tire contact patch center meet. A zero scrub radius is where the kingpin axis and center meet, a positive radius is where the kingpin axis meets on the inside of the tire centerline, and a negative radius is where the axis meets on the outside of the tire centerline. Scrub radius is affected by changing almost anything on an independent suspension, or by changing wheel offset or diameter. A modified scrub radius will alter turning circle, tire wear, bearing loading and lateral grip. More often than not a scrub radius change happens with new wheels with increased offset to clear body or suspension components. Be mindful of what is required to lift your vehicle outside of a lift kit and what that will affect.
Thrust is the amount of travel from back to front on an axle as it goes through its cycle. As trailing arm suspension is compressed it travels either back or forward and changes the wheelbase of the vehicle slightly in the process. The amount of thrust is the force that is applied when the distance in the wheelbase is changed by this movement. A static change in wheelbase occurs when lifting a vehicle with trailing multi-link solid axle suspension. Some thrust can occur with independent suspensions related to the amount of castor. This effect is most noticeable on short wheelbase vehicles with lots of lift as the additional height pulls in the wheelbase and dramatically increases thrust.
Not strictly a suspension term, changes in pinion angle are a result of a lift that alters the angle the drive pinion coming from the differential has relative to driveshaft angle. A pinion angle that isn’t inline with the driveshaft at static ride height, or one that changes dramatically though relatively small changes in the cycle will create noticeable vibrations and affect noise, vibration and harshness as well as affecting tire wear and even tire balance. Driveshafts with special configurations can alleviate this, such as rzeppa joints (common on modern Jeeps), other constant velocity joints, or by using double cardan type u-joints or canceling the pinion angle through a "broken back" arrangement where the pinion angle is mirrored on both side of the driveshaft.
With leaf springs, axle wrap occurs when torque is applied to the differential and the leafs twist with the axle as it resists the natural tendency of the axle to rotate with the direction of the tires when force is applied. This affects the pinion angle dramatically.
With independent suspension at least 1 arm will attach to the frame and to the hub assembly, and because most of the time it attaches at two frame points it naturally forms an A. MacPherson strut type setups are single A arm (or trailing arm in the rear), while “double wishbone” or double A-arm systems have two. In this case there is an Upper Control Arm (UCA) and a Lower Control Arm (LCA). The UCA locates the top of the hub and is the top pivot for the kingpin axis. The limit to most lift kits' cycle range is often the UCA’s ball joint and its range of motion limitation. The LCA carries the bulk of the weight and is also attached to the hub and forms the other half of the kingpin axis. The LCA is responsible for camber and to a lesser degree, castor. With MacPherson struts, the UCA is replaced with a fixed attachment to the top of the hub, making the effective top of the hub the top of the strut, where there the top of the kingpin axis is located around a joint called the top hat that allows for axial rotation as well as some flexibility for strut movement. For anything other than mild lifts on independent vehicles, control arms will be a necessity to maintain geometry and durability.
Solid axles require location in 3 axes - front to back, up and down, and side to side. With leaf springs you get all three from a single stressed spring. With coil springs the trailing arm locates the axle front to back by holding the axle at a fixed distance in an arc from the mounting location ahead of the axle (thus a link that is trailing), leaving the coil stressed only vertically. With Independent Rear Suspension (IRS), trailing arms can also be used, along with lateral links and struts to save space. Front solid axles use trailing arms as well, but they are actually leading as the axle is in front. In some cases these are just called “arms” and in other cases they have names like "radius link". For 5 link setups, two links locate the bottom of the axle and two locate the top, keeping the axle better aligned through its travel and allowing more flexibility. When you lift a vehicle with trailing arms you decrease the wheelbase of the vehicle slightly because of the fixed length of the arms which move down and in towards the chassis center in their arc. This can be a factor in tire fitment in some cases and it will likely affect pinion angle. You can correct this with different length arms or drop brackets.
Trailing links/arms locate the axle front to back through its cycle and coils support the vertical axis. Panhard rods locate the axle side to side. Because they are single link, they create an arc in their cycle. This arc is called the sagitta arc and it can move the axle laterally by several inches in its travel. When you lift a vehicle you move the sagitta outside of its balanced resting state and move the axle to one side or the other. You can correct for this with a longer panhard, though a drop bracket more correctly re-aligns the sagitta to factory specs.
A shock or a damper is nothing more than a friction-adding component to the suspension that takes a spring's natural oscillation and converts the excess, undesirable motion into heat. Here is a video I made that illustrates the value of a damper.
A shock is just a tube filled with oil that has a plunger or piston inside. The tube is connected to the chassis or body on one end and the piston is connected to the axle or suspension on the other via a rod that travels in the tube. As the piston travels through the highly viscous oil it passes through holes that limit the flow in either direction. The friction generated converts the kinetic motion into heat energy which is dissipated through the fluid and out the shock body into the atmosphere. The tuning of a shock is accomplished by restricting the flow of fluid through the piston through various means like deflecting disks or changing apertures. There are 2 main adjustments - bound and rebound - but those can be divided up into different categories depending on the sophistication of the technology into high speed and low speed for both bound and rebound.
Bound is the resistive force to compression, also commonly called compression damping. The stronger the bound, the faster the forces compressing your suspension are halted. Too strong and your ride is excessively harsh, too soft and you will blow through the travel and bottom out. This tradeoff is largely solved in some newer technologies.
Rebound is the opposite of bound. The rebound characteristics control the suspension unloading. Too much and the springs never have a chance to return to ride height, leaving you with less suspension travel. Too much and you bob up and down as the shock can’t control your motions.
As a piston moves through a fluid its motion can either be linear, progressive or digressive. Here is what that means.
Linear - The damping (bound or rebound) rate is consistent with both stroke and speed. An X amount of travel nets a Y amount of resistance at all points along the travel and through all points in time.
Progressive - Either related to time or travel a progressive curve increases the damping effect as it progresses. A progressive stroke shock will start out slowing the stroke only a little but as it travels down it may become 2-3 times as firm. This is ideal in bound as it means that the mid point in travel is compliant but resistance to bottoming out becomes stronger the deeper into the stroke gradually.
Digressive - Digressive starts out strong and gradually tapers off. This is commonly used in rebound to allow the sharp rebound effects of a spring to be slowed heavily at first, but then allow the stroke to quickly reset to static height.
There are a few other terms you will see when shopping for shocks that denote a technology worth looking into:
As the shock fluid builds up heat it can eventually boil and start to form air bubbles or cavitation. These bubbles dramatically change the viscosity of the fluid and lead to a condition called “shock fade” where the shock loses a lot of its ability to properly damp motion. By pressurizing the shock oil with inert nitrogen you can increase the boiling point of the fluid and delay the onset of fade.
This is a secondary piston inside the shock body that separates the nitrogen gas from the working fluid. This is a form of nitro charging a shock but doesn’t allow the fluid and the gas from contacting. It allows the pressure of the nitrogen to be progressive as the fluid moves through its travel as well as allowing for a larger diameter shock body by moving from twin tube (one tube inside another) to monotube.
A shock's ability to convert motion into heat is limited by the ability for the shock to get rid of that heat. A greater volume of oil is the best way to create additional heat capacity and heat shedding. A remote reservoir allows including more oil and it moves part of the shock assembly - like the IFP - outside the main body to allow the working fluid to have better heat control. As a bonus, the passage from the main body to the reservoir can be used to tune the shock by restricting or allowing flow. Many remote reservoir shocks have adjustability here.
In order to get the ideal progressive damping curve you need a shock that can be mild in one place but strong in another. It’s difficult to manage that flow at the piston so instead a manufacturer can create a piston with a specific valve that is strong, but then allow oil to flow around, or bypass, the piston when that strong rate isn’t desirable. This effectively creates multiple zones of damping in the shock body allowing for a soft, compliant ride zone and one or more levels of increased damping to prevent bottom or top out. This can be done “internally” with passages of oil traveling between in the inside and outside tube, or externally as seen on most race setups.
Note: ICON shocks use a different method of a similar idea in their shocks that have a separate piston and inner tube that create multiple zones of bottom out protection without the “float” that bypass shocks can have in the ride zone. You can read more about that here.
The high performance version of a rubber bump stop is called a hydraulic stop or a hydrostop and it is in effect a completely separate shock absorber with far more travel which allows for a less aggressive bound rate which smoothes out the transition from regular travel to stopped travel.
The opposite of a bumpstop is a limiter or limiting strap. These prevent the suspension from harshly topping out, but are not commonly used except in cases where topout is common like off-road racing.
Lastly, we get to springs - types, applications and rates. There are four basic types of springs you will find in off-roaders today. They are:
These are semi-elliptical arcs of steel that compress and elongate with force. They are typically used on heavy load applications because of their ability to strongly locate an axle laterally as well as their ability to progressively add load more easily than other springs. They are laid up in a specific way to tailor the rate to the need, including overload rates that dramatically increase rate with load.
The most common type of spring found today, a coil is a pre tensile wire wound into a coil shape that can compress and expand. The advantage of a coil is that it offers no friction and is considerably smoother and more consistent compared to leafs. The downsides to coils is that they offer only vertical support and don’t locate the axle in any meaningful way otherwise. Coils also suffer from a condition of bind when all the coils compress into one another that can limit travel with very strong rates. Coils are also not typically positively retained except for coilover or strut installations, and they can deflect laterally or even unseat under extreme circumstances.
This is a twisting spring where the spring force is from trying to rotate a steel bar along its axis, and its resulting desire to return to its static state. These are ideal for packaging reasons because the spring can be put under the chassis and in-line with the frame to make room for more ideal component placement or suspension geometry in the front wheel wells. Torsion springs are also well known for their ability to compensate for load or ride height by cranking additional pre-load into their rotation.
Air is similar in most respects to coil spring applications as they require very similar suspension setups. An air strut replaces a coil strut, for example. The advantage of air is that it’s highly adjustable, and incredibly sensitive to small inputs making it even smoother than coil. By shaping the chamber of the air volume you can give the spring a variety of characteristics and by adding pressure you can vary the load capacity and ride height very easily. This is the preferred choice for high end luxury SUV’s and trucks. Their main disadvantage is their cost, complexity, durability and longevity.
When discussing aftermarket options you are most often confronted by three variables:
Ride height or lift is a function of the first 2 variables. The spring may have a taller free height (that is the height of the spring along without weight) or an increased spring rate, or both. When looking for a lift, its a good idea to try and separate those to your needs. A taller free height allows for greater height and more wheel travel (shock and hard point limited) but will sag considerably more with a load. An increased rate spring will give you the additional lift, but will feel unduly harsh without added weight. Rate curve is similar to damping curve in that its the input and output relationship to a spring.
Linear rate - This spring will deflect X for Y load throughout all points in the travel.
Progressive rate - This spring will decrease its deflection relative to load as it becomes compressed. This is less common than is marketed, and in most cases these are actually dual rate springs.
Dual rate - These are springs that have 2 distinct rates in their configuration. In the case of a leaf pack, a dual rate spring is a spring that acts mostly linearly until a second set of leafs is engaged, at which point the rate is increased dramatically. One example is overload springs on pickup trucks. A dual rate coil spring has a soft rate and a firm rate. It acts in much the same way that a dual rate leaf pack might, where the soft rate is a tiny amount of the total travel and the combined soft and firm rates work past that point. A progressive spring changes the wire diameter and pitch constantly though the wind, and a dual rate spring changes these attributes distinctly at one or more points. Most progressive rate spring kits are in fact dual rate.
Above, you can see an example of a dual rate coil, marketed as progressive. You can see a tight series of coils at the top that is in or near bind most of the time. This allows for a very small soft rate window, and allows the coil to stay seated and applying pressure under full droop while still using an acceptable load rate.
There is a lot of terminology here to cover and I didn't get to all of it, but I hopefully gave you enough to go in to a conversation armed with the appropriate information.
Building a good lift takes time, and it takes an understanding of wholistic vehicle dynamics and not just height and technology. If you, like me, enjoy experimenting there is a lot to work with. If you are looking for a high quality kit that you can go right to playing with, please look seriously at some of the high quality complete kits from ICON, FOX, TERAFLEX, or others who know their stuff.
See you out there.
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