THE ULTIMATE SUSPENSION 411
By Your's Truly
No matter what type of vehicle we drive, the suspension is the single most important element in regards to how it handles. With a poorly designed suspension, even a well-built rig won’t go the places that it should. Yet it wasn’t long ago that leaf springs were high tech. Shackles were mounted in front of the tires, and leaves were under the axle. Trends have evolved to reverse both: conversion kits to spring over our trucks and put the shackle in back for better all-around performance. But we’ve seen further evolution in our vehicles with the advent of linked suspensions. Like our older and sometimes wiser siblings, desert racers have perfected this evolutionary stage and are enlightening us about how to handle the rough stuff better, while drag racers are showing us the art of “The Launch.” Unlike a leaf spring, which locates or holds the axle under a vehicle while suspending it over the tires, a linked suspension only positions the axle under the vehicle, relying entirely on another separate system to hold the body and chassis above the axle. The location, angle, and length of those links in relation to the rest of that vehicle affect how that vehicle will drive down any given road.
There are quite a few companies out there that have some decent leaf springs. And while leaves work great for their cost, they are limited in what they can do for a vehicle. Drag racers, Baja guys, W.E. ROCK, OEM’s and even clumsy monster trucks all use link type suspensions. Even newer daily driven trucks are linked in one way or another. An 80, 100 and 200-series Land Cruisers, FJ Cruisers, 4Runner are linked, sometimes front and back (80-series).
So why are links so awesome? By nature they have the ability to enhance driving characteristics and attributes of your choice. Whether you’re building to drift around corners, drag race or tear through Baja, a linked suspension has the ability to be tailored to the needs of the sport or recreation of your choice. If built with adjustment in mind, minor driving characteristics can be changed with a wrench and a few spare minutes.
Let’s consider short track-type racing, comprised of many sharp turns. CORR trucks are typically set up for more traction off the line and coming out of turns. For this we’d set up our suspension to help launch the vehicle when you hit the gas and maybe even oversteer the rear of the vehicle to help get around corners. Rock crawlers, on the other hand, build theirs to help apply extra traction while going up steep hills. A vehicle that sees a lot of highway use would typically be built with little to no roll steer, so that it travels straight and doesn’t wander in the lanes. It’s this kind of design and forethought that most amateurs don’t think about when building a vehicle. Many Baja designers feel strongly that a neutral suspension is the best all around setup for their application because it’s the most predictable or benign. This allows them to do all of their tuning with shocks, springs, anti-sway bars, wheelbase and track width. In case you haven’t noticed a pattern, there is no such thing as the perfect suspension, meaning no one suspension can be great at everything. Designing a suspension for your needs will be a matter of knowing what you want and knowing want you can live without. You can’t have mad-crazy wheel articulation and still beat a Lexus IS-F around a road course. It is a matter of compromises.
THE SCIENCE AND TERMINOLOGY OF SUSPENSION
Many handling traits can be measured through a traced line running though the links that attach your axle to the vehicle to an imaginary point, through which force is applied. It is the location of these points in space, in relation to the rest of the vehicle, which cause it to react to circumstances. Some of these points are seen when viewing a vehicle from the side and others from above. The Instant Center (IC) for example, is a point in space, when viewed from the side where links placed on top of the axle and links placed at the bottom converge to form a single point where force can be measure from. This point can be anywhere the designer wants it, from twenty feet in front of the vehicle, to the output shaft of the transfer case. Ever notice that some crawlers have a tendency for the rear tires to hop on certain hills? A characteristic called Anti-Squat (AS) is a key player in wheel hop. Maybe you’ve noticed that your own vehicle jacks sideways pretty hard on certain types of bumps while screaming across the desert? It could be that your Roll Center (RC) is a little too high. Let’s look at things a little deeper. INSTANT CENTER When your vehicle is viewed from the side, the IC is a point in space where the upper and lower links converge. It is where we can measure the transferred force from both links and put just a single load point on the vehicle when under acceleration, a point in space where forces from the converging upper and lower links are applied to the chassis. The only concern the Instant Center has on a vehicle is in respect to Anti-Squat. Some people design around the IC by saying the IC should be at the water pump or some other point. The problem is, that only works as a comparison if the comparing is of two similar vehicles. ANTI-SQUAT Anti-Squat is the amount of force applied to the links that will resist the rear squatting under acceleration. When you step on the gas (from either a standstill or while moving forward) to accelerate and the rear of the vehicle either drops down or props upwards. By adjusting the location of the Instant Center you change the amount of AS. Seen another way, Anti-Squat is the amount of force placed on the links from the weight transfer of acceleration. When you accelerate weight is transferred from the front axle to the rear axle. There are two things that will keep the rear suspension from compressing because of the extra load transfer. 1: the springs themselves, 2: Anti Squat. With a 0% anti-squat all of the weight transfer goes to compressing the springs. If the springs are stiff they won’t compress much and if they are soft they will compress a lot. If you have 100% AS all of the weight transfer goes into the links of the vehicle and none of it goes to the springs. Because of this the suspension will neither compress/squat nor extend/raise. If you have 60% AS, then 40% of the weight transfer will go into the springs and 60% will go into the links. If you have less then 0%, aka negative amount of Anti Squat, a.k.a. Pro Squat, the rear suspension will squat more than the spring rate would infer. To measure the AS of any linked vehicle you first need to know the height of the center of gravity followed by the wheelbase. From the side draw a line across the length of the vehicle at the height of the CG. Next draw a vertical line from the contact patch of the front tire up to the line indicating the CG. Now draw a diagonal line from the contact patch of the rear tire to the intersecting point of the CG and the vertical line of the front tire. That line is the 100% AS line. Next draw a line from the contact patch of the rear tire forward through the IC and continuing through the vertical line of the front tire, this would be your AS line. If your AS line intersects the front tires vertical line at 30% of the total distance from the ground to the 100% line, then you have 30% AS. In other words, if your AS line intersect the front tires vertical line at 30” off the ground with a 40” CG, then you have a 75% AS. If the measured distance from the ground to this point is 20” then you have 50% AS. If it’s 10” then you have 25% AS. Many desert guys run around 20-50%. Drag racers have been know to run all kinds of AS numbers from 50 to 140%. As rock crawlers, we’ve slowly been adjusting the amount of AS we use on our vehicles. Just a few years ago we ran numbers above 100% like the drag racers. Today more common numbers are closer to 50-80%. For your home-brewed application an adjustable link bracket will help you dial in what is best for your needs. What’s the magic number? You tell us what works best for you and that’s the magic number. Just remember the three contributing factors: height of center of gravity, tire size and wheelbase. To accurately measure the height of the CG please visit the links below. ROLL AXIS An imaginary line running through intersecting points of a suspension, through which either the front or rear reacts, if the vehicle were held fixed. It is this line between these two points in which the suspension rotates about or moves around. In other words it is the line your suspension swings from.
On a triangulated four-link, assuming both upper and lower links are triangulated, the converging point of the upper links just behind the axle and the converging point of the lowers form the needed two points in space to draw the intersecting line that is the Roll Axis. On a suspension with a panhard bar, this line would be determined by drawing a line though the points that are the center of the panhard bar and the converging point of the lower two links. Generally speaking, a flat or slightly negative angled sloped axis is desired. It is also this line or axis, which determines the amount of oversteer or understeer your axle has. If this slope rolls downhill toward the center of the vehicle you have a negative Roll Axis and your axle will have understeer characteristics. If this slope rolls downhill away from the center of the vehicle then you have a positive angle and therefore oversteer characteristics.
ROLL STEER Roll steer can be felt while traveling down most any road. The roll and swell of most roads can be felt as it forces the body to lean from side to side, even while travelling in a straight line. It is when the body leans that roll steer takes effect, causing the axle’s angle in relation to the frame to change and thus have a steering type effect on the vehicle. Oversteer: your suspension causes your axle to oversteer a turn, meaning you turn more than the input of the steering wheel infers. Example: while in a left-hand turn, the body will roll out of the turn to the right. If your rear axle has oversteer, it will turn outward and cause the rear end to come around quicker than it would have otherwise. When you want it, it’s great, but when you don’t, it’s really annoying. A vehicle with solid axles front and rear will drive much nicer down the road with a little understeer. Vehicles with oversteer typically wander in the lane, which makes for constant steering corrections while driving. If you find yourself with oversteer in your front suspension, then you can compensate by building understeer in the rear. If your Roll Axis is downhill towards the center of the vehicle, regardless of front or rear suspension, then you have a negative angle or understeer. If your Roll Axis points downhill away from the center of your vehicle, then you have a positive angle or oversteer. Many designers try to build a little understeer into their systems, because it seems to be more predictable and benign. It also has a tendency to track straighter and wanders less at speed. However, if you need to get around corners quickly, like a CORR truck, you may find that you need oversteer from the rear suspension to help slide you around corners. ROLL CENTER The Roll Center is the point through which the body wants to rotate about the axle, or the point that the axle supports the body laterally. It is again, an imaginary point directly above your axle that intersects the Roll Axis. In that regard it is just like the IC but from a different perspective. It can also be thought of as Anti-Roll, if the Roll Center is at the Center of Gravity there will be no body roll and if the Roll Center is lower than the CG but higher than the ground, then some load from cornering will go into the springs. If, however the Roll Center is on the ground, then all of the cornering load will go into the springs. In other words the higher the roll center the less body roll you get when on side hills and cornering. However, with a high Roll Center you increase the amount of sideways movement applied to the body from the axle’s movement. This is mostly noticeable at high speeds while traversing cross-grain sections of desert. In other words, a high Roll Center can push your truck sideways when one wheel compresses at higher speeds. In rock crawling, your axle will simply move a bit more to the compressed side then the other when articulated and you’ll never notice any sideways forces due to such a slow overall speed. Desert guys tend to use a lower Roll Center, it may be because it’s easier to package or it may be because it reduces the lateral force on the truck. They tend to compensate for this lower roll center and body roll by adding additional track width via wider axles, anti-sway bars and/or stiffer springs. However at slower speeds like rock crawling, a high Roll Center helps maintain stability without having to add additional components like anti-sway bars and stiffer springs.
VEHICLE ROLL AXIS
In many respects this is similar to Roll Axis, but is measured by the height of the front and rear Roll Centers. It is these two points that form the Vehicle Roll Axis. All though there are many factors in getting a vehicle to handle better, it is generally accepted that a flatter Vehicle Roll Axis will handle more predictably. It is also commonly accepted in the race world that the more equally loaded the tires (same amount of force) both inside and out, the better the traction and handling.
There are ways that handling or the loading of the tires can be improved. One: adding stiffer springs; two: adding an anti-sway bar; three: rebuild and raise your Roll Centers. This topic can get a bit complex, so we’ll leave it at that.
ROLL ANGLE Roll Angle is simply a measurement of how much the body leans to one side while in a turn. A higher roll center can help reduce this number, however so can stiffer springs and or an anti-sway bar. LINK SEPARATION This refers to the vertical distance between link mounts. As a general rule, the vertical separation of the links at the axle should be 25% of the tire diameter. This is to help handle torque from the axle and ensure less axle wrap. As for the separation at the frame, the amount of desired Anti-Squat will determine this number. Think of the separation as leverage on the links and brackets. The closer together they are the more leverage or force is applied to them, which means you need to make them stronger, both link and bracket. Also the stronger your motor is, the more torque it’s applying to those links. LINK LENGTH For the most part, a long link measures at a starting length of 32”. This would be the typical long arm suspension that many mainstream companies speak of. Many people find that they prefer even longer links. One of the advantages of a long arm suspension is that as the suspension cycles the angles of the links change less. This keeps the characteristics of the suspension from changing as much, so that the intended driving characteristics remain more constant though out the suspension’s movement. FINE TUNING LINKS The length of the uppers is also important. Many feel strongly that protecting the angle of the u-joints on the driveshaft is very important, so they build the length of their upper links close to the same as the lowers. The advantage here is you maintain the u-joints angle in relation to the pinion, which helps longevity. The down side is that as the suspension cycles downward the Instant Center changes and the Anti-Squat can increase. By reducing the length of the upper links you have the ability to fine tune where the IC is during the suspension cycle and thus the anti-squat as well. If you have the uppers about 80%-ish of the length of the lower when viewed from the side, you can maintain a steady AS number through the suspension cycle. If you push the uppers back to 70%-ish you can get the AS numbers to reduce as the suspension drops and increase when the suspension compresses. This can help to stabilize your vehicle in steep climbs as well as help rear traction as the wheel compresses while hauling butt through the desert. However the most common agreed upon method of design is to always keep the lower links as flat to the ground as possible. This will help eliminate any strange jacking and unloading of the links while in hairy situations. A steep angled link can have some strange effects on a vehicle; sometimes the axle will walk under the vehicle while on a steep climb. This can be a scary experience, as the axle continues to climb it pushes under the vehicle, propping the rear into the air and making for a very unstable and scary experience. LIMIT STRAPS Limit straps can be used for many things. They can be used to stop your axle from over extending those expensive shocks, or to stop your suspension from moving downward so much that your drive line (u-joints) become bound up. If you have excessive AS numbers a limit strap can be used to limit how much the axle pushes the rear of the truck upward. This is used wisely by many comp guys that desire the instant traction a high AS offers, but don’t want the axle walking under the truck that a high AS can give on a long slow articulating climbs. Some think of it as a Band-Aid, but it works. BUMP STEER When you hit a bump in the road and the steering wheel and tires change direction and cause you to have steering input to stay on course then you have bump steer. Your suspension soaks up those bumps by moving upward, known as cycling. Since your steering is attached to the axle, it too cycles, just like the suspension. However, when your suspension and steering don’t cycle along the same given path, one system will win out over the other. Since the steering system has a pitman arm that also rotates, and the suspension has fixed points, the suspension will win the fight and force the steering system to follow. So your suspension is causing your steering linkage to change in length, by pushing and pulling on it, which is why your steering wheel turns in your hand when you hit a bump. That’s why it’s important to make both the drag link and panhard bar the same length and sit at exactly the same angle. If for some reason, like a packaging constraint, you can’t get both the same length, make sure they are at least sitting at the same angle. The angle is more important than the length. WHEEL RECESSION From a rock crawling perspective, wheel recession is when the front end of your vehicle is forced upward while your front tires are up against a rock. Example: While climbing a small hill one tire meets a ledge. Your rear tires push you forward, but your front tires don’t climb but rather the nose lifts into the air stretching your front suspension until it’s nearly maxed out, only then does the front start to climb. The likely cause of this is? The angle of your lower links is perhaps too steep in relation to the ground. As the rear tires force the front tires forward and into a rock face, the angle of the lower front links forces the front end up, worsening the angle and compounding the problem, and now you’re looking at the sky. There are two ways to combat this. 1: build your lower links closer to parallel with the ground, so that when you hit a rock face the force pushing on that link is closer to a right angle with the truck. 2: use a winch to suck the front of your vehicle down, disallowing the axle to pull away from the vehicle. Comp guys do both. RADIUS ARM TYPE One of the more typical link suspensions for a crawler or daily driver is the Radius Arm system that you would find on something like an 80-series Land Cruiser. This suspension uses two lower arms that attach from the frame down to the axle. This arm at the axle attaches itself both above and below the axle’s centerline or in the case of an FJ80 front and back. These attachment points keep the axle from twisting under the vehicle. In addition to these two links a panhard bar is also used. It attaches from the driver side frame down to the passenger side axle again using misalignment joints and or bushings. Its duty is to keep the axle centered under the vehicle from lateral forces. Traits: Due to the use of a panhard bar a lower roll center is typical of this type of suspension when used in the front. Oversteer is also a common trait among lifted vehicles. The IC is located by the placing of the lower links at the frame; therefore radius arm suspensions typically have very high AS numbers and are none adjustable. Advantage: Provides a very good pinion angle through out its range of travel. Relatively easy to design/build and doesn’t have the complexity of the other systems. Disadvantage: Steering suffers, in that the links don’t allow rotation of the axle in relation to the frame as the suspension cycles up and down, which cause caster changes. Wheel articulation is also limited due to link binding, a problem that is inherent with this system. One wheel pushes up and the other drops, which cause the links to twist the housing along its axis, much like a torsion bar. Options are limited in link placement making it inherently difficult to fine-tune Anti-Squat. THREE-LINK + PANHARD: The Three-Link + Panhard, differs in only one area from the radius arm design. Here two lower links attach from the frame down to the axle housing. The third link attaches from higher up on the frame down to the axle housing. The upper and lowers are not typically parallel to each other, with more separation between them on the axle side than the frame. Like the radius arm system, it too uses a panhard bar to locate the axle from side to side. Traits: Due to the use of a panhard bar a lower roll center is typical of this type of suspension. Oversteer is also a common trait among lifted vehicles. Advantage: In relation to the frame this system allows for minimal caster change while suspension cycles. Desired anti-squat numbers are easier to attain, because the IC is located by two converging links instead of one. The three-link seems to have the least amount of binding, offering the most amount of wheel articulation for a system with a panhard bar. Disadvantage: Pinion angles change due to the links rotating the housing during suspension travel. Playing with link lengths can minimize this. FOUR-LINK + PANHARD The Four-Link + panhard bar which is also referred to as a “Five-Link” by the OEM’s, uses two uppers, two lowers and a panhard bar. Unlike the three-link, which has the upper and lower converging at some point, the four-link’s upper and lowers are of equal length and parallel to each other. Without this key ingredient the four-link is a glorified version of the radius arm system. Traits: Similar to the three-link and radius arm systems, it too has a lower roll center and a bit of oversteer. Advantage: Because links are parallel and equal length steering angles stay consistent throughout wheel articulation. Disadvantage: Pinion angle suffers when steering angles remain the same. Tying to place four links of equal length, which are parallel to each other can be a serious packaging problem. Long travel versions of this system only work if a spherical bearing is used at each end of each link, which gets expensive very quickly. Total of five links. TRIANGULATED FOUR-LINK One of the more designer-friendly suspension systems. It consists of two upper and two lower links. One or both pair of links needs to be triangulated to make it work. No panhard bar is necessary. Typically speaking the upper links are triangulated. They mount above the differential close together and then separate as much as possible at the frame. The greater the angle these links have, the better the lateral control over the axle. Triangulating both will help even more. As a rule of thumb the uppers should be at least a 40-degree angle when viewed from above. Traits: When the uppers are triangulated you get a higher roll center. When the lowers and uppers are triangulated you normally see a flatter roll axis. Advantage: Wide-open design possibilities when used in the rear. If double triangulation is used understeer characteristics can be achieved. This is not to imply that you can’t attain understeer without it, but with a lifted vehicle you start to limit your options as to where you can put mounting brackets that won’t act like rock anchors. Disadvantage: Packaging constraints. Just about every truck-based Toyota has their gas tank located beside the rear driveshaft. This design requires the relocation of the factory gas tank. Because of this many people choose to go with a panhard bar system in the rear. TRIANGULATED THREE-LINK Also called a wishbone suspension. Consists of one upper and two lower links. No panhard bar is necessary. With three attachment points this single link has two at the frame; one on either side and one at the axle housing forming a ‘V’ or wishbone, which acts as the necessary lateral support for this design. This design functions exactly like a triangulated four-link. Traits: When the uppers are triangulated you get a higher roll center. When the lowers and uppers are triangulated you normally see a flatter roll axis. Advantage: If double triangulation is used understeer characteristics can be achieved. This is not to imply that you can’t attain understeer without it, but with a lifted vehicle you start to limit your options as to where you can put mounting brackets that won’t act like rock anchors. Disadvantage: Packaging constraints. This design requires the relocation of the factory gas tank. Typically less favored by designers, all of the torque loads from the upper links are transferred to one single point instead of the two, requiring much stronger link components and brackets. THE INS AND OUTS OF A ROLL AXIS With a Radius Arm, Three-Link+Panhard or Four-Link+Panhard it’s all the same. Your roll axis starts at the middle of your panhard bar, so the higher your panhard is from the ground the better, if a flat RA is desired. Of course this is difficult to do in the front because of the engine, so options will be limited. From there the axis either follows the same angle as the lower two links provided they are not triangulated or runs through the point at which the lower links converge on themselves when viewed from above. The flatter your lower links, the flatter or closer to a negative RA number you will be able to get. Don’t be surprised if in the end you can’t get a negative roll axis. It’s not easy with an oil pan. With a triangulated four-link, things are almost the same. Instead of the panhard being what locates the axle portion of the axis, it’s the converging point of the two upper links. As with a panhard bar system this axis will mirror the angle of the lower links unless the lowers are also triangulated. Again flattening the lowers helps a lot with your roll axis. STEERING Q&A When using a suspension system with a manual steering setup, the suspension must follow the arc or path of the drag Link in order to minimize bump steer. When viewed from the front, a system with a panhard travels in an arc relative to the length and angle of that link. It should be the same length and sit at the same angle as the drag link. If it doesn’t the panhard will push and pull the pitman arm changing the steering input at the wheel. It is highly recommended that if using a manual steering setup, a suspension with a panhard be used. The panhard is a perfect fit for a drag link, it can be built to mimic the length and angle of the drag link and by doing so minimize any bump steer. It also makes for a system that is reliable, predictable and safe to drive in any condition whether it’s street, Baja or crawlin’. With a triangulated four-link front end, it is highly recommended that you use full-hydraulic steering. Also known as full hydro, it has no steering box or drag link only a tie-rod that attaches to a hydraulic ram. Because this suspension has no panhard it does not travel in an arch when viewed from the front. Instead it travels straight up and down. Remember a drag link needs to travel in an arc; a triangulated system doesn’t do that. If you combine conventional steering with a suspension like a triangulated four-link, the results would be drastic amounts of bump steer, to the point that it could be undriveable. However it is frowned upon to use a full hydraulic system while driving on the street. If a hose breaks, or the engine cuts out you are left with no steering and a dead stick, which is extremely dangerous on the street. Please don’t run full hydro on the street.
Don’t settle for, “I put it there because everyone else did.”
You will get better street manners by minimizing your suspension’s oversteer.
Professional builders agree; flatter links perform better in ALL conditions. Example: Baja racers, CORR trucks, monster trucks, drag racers, stock car, OEM’s and competitive rock crawlers. This is one reason so many vehicles handle better with lower ride heights. The links flatten out.
Beware of those that claim they just tossed their links on and it all works great. What’s their definition of great? What yardstick are they using to measure success?
Flex is one of the easiest attributes to come by. Long links+spherical bearings+long shocks = lots “o” flex. Don’t judge the performance of a linked suspension only by the amount of flex it has.
Anti-squat is something that has been debated much in recent years; it too is not the only attribute of a suspension system. Don’t get stuck on it, build adjustment into your design.
Link and angles calculator:
Lots of suspension study:
CG and rollover angle calculator: