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Truck Science
Geometry | Control | Spring Engineering | Springs vs. Grommets
Geometry
Tight Turns: The New Geometry
The term “geometry” refers to the angle of the axis that your truck hangers pivot on. This determines how fast the axles swivel in the horizontal plane as you tilt your deck. The steeper the angle relative to the horizontal, the more the axles swivel per degree of deck tilt, and the quicker and tighter you’ll be able to turn.
With traditional skateboard trucks (based on 1930s roller-skate technology), the geometry you see is NOT the geometry you get. The pivot arm doesn’t rotate precisely in the plastic pivot cup, because the king pin and the grommets don’t support a stable pivot axis. Instead, the pivot arm rocks against the sides of the pivot cup at a shallow, unstable angle, making tight turns difficult or impossible. (This angle, called the “action angle,” is roughly marked by an imaginary line connecting the pivot arm and the center of the grommet collar.)
Many truck manufacturers try to solve this problem by setting the pivot arm in the pivot cup at a very steep incline. This quickens turning response, but it interferes with grommet performance, making steering control go from bad to worse.
The Seismic pivot pin, on the other hand, is solidly fixed in the baseplate at a perfect angle (either 30° or 45°). It’s machined from the highest-quality steel, then burnished and galvanized. The hangers rotate on this rock-steady axis with total precision, so Seismic turning response is absolutely consistent and predictable — quick at lower speeds, yet safe and stable at higher speeds. And because of this steel-solid geometry, angled riser pads have a more radical effect than on the old roller skate model — they can make your Seismic trucks turn more slowly or far tighter than anything else available!
Why the Old Geometry Doesn’t Add Up
Fine control is impossible without a consistent steering geometry. But with the old roller-skate design, there’s no way that plastic grommets can align a steady geometry and form an adjustable suspension system at the same time.
Whenever you loosen or tighten the grommets (or whenever the kingpin nut rotates on its own due to road vibration or a worn-out lock insert), the action angle changes. Grommets shrink and weaken over time, so the action angle gradually degrades even if the kingpin nut stays fixed in one place. Result: you have to constantly re-adjust to an ever-changing steering geometry.
Further, turning forces, body twisting, and deck weighting can flex the grommets and the pivot cup in all directions, thus throwing the hangers out of proper alignment, even when the deck isn’t tilted. This means uncontrolled (and potentially dangerous) turning responses.
In contrast, the Seismic steering geometry is exactly aligned by a precision-ground steel pivot pin fixed in the baseplate. (Although it’s perfectly integrated with the rock-solid geometry, the Seismic spring system isn’t needed to align or stabilize the pivot axis.) Deck tilt alone determines how much the axles swivel, and a given angle of deck tilt always produces the same amount of axle movement, regardless of spring tension or side loads.
Maneuverability with stability and control: the Seismic difference!
Control
What are Stability, Maneuverability, and Control?
Their definitions may seem obvious, but the skate world has always been vague and confused about the terms stability, maneuverability, and control.
You control the angle of the deck’s tilt, and thus the size of your turns, by varying the distance you shift your weight across the width of the deck. These weight shifts create forces which compress the suspension elements (springs or, in other cases, grommets). When the suspension elements flex, the deck tilts, the axle hangers pivot on the baseplates, and the wheels roll through a turn.
Turning stability means trucks that don’t respond much to sideward weight shifts, so you can move your weight from edge to edge without causing the board to tilt or turn very much. Turning ease, or maneuverability, means trucks that respond more to lateral weight shifts, so you can make tight turns with less edge pressure.
Steering control (sometimes called “smoothness”) means the ease with which you can feel how far to shift your weight to make turns of varying sizes… in other words, the balance between stability and maneuverability.
Steering Control Redefined
Geometry alone doesn’t determine how well you can turn. Skateboard trucks need some type of suspension system to resist deck tilting. Ideally, this stabilizes your skateboard when you’re moving straight ahead and gives you precise control over the steering response.
Traditional grommet-based designs force you to trade off control and stability for maneuverability, but Seismic trucks make tight turns with total control and no wobbliness. To put it another way, they’ll steer loosely without feeling loose.
Some skaters can’t believe at first that any truck can make sharp turns while feeling so smooth and stable. If you’re used to squirrelly turning, Seismic trucks may feel a bit tight, but it won’t take long to adjust to the superior handling.
Our custom-made progressive-rate springs are engineered with a detailed understanding of the bio-dynamics of skateboard steering and suspension. When you lean sideways to start a turn, Seismic springs resist deck tilting far more logically than urethane grommets. This means a natural, easy-to-sense relationship between how far your weight shifts and how sharp you steer. No dead or flat spots in this truck’s response, just pure liquid velvety smoothness. Your turns get tighter at the exact same rate that you move your weight towards the edge of the deck, so it’s incredibly easy to feel how far to lean to make turns of varying sizes. You get unprecedented control and a remarkable combination of responsiveness and stability.
Spring Engineering
A New Approach to Springs
In the 1970s, several truck manufacturers briefly tried using coil springs. Trucks like the X-Caliber and the Speed Spring didn’t succeed because they were too heavy, too squirrelly, and/or too complex (and thus vulnerable to damage and breakdown).
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Don’t confuse Seismic with spring-based
trucks from the 1970s! | |
Seismic trucks incorporate springs, too, but the similarities end there. Unlike the poorly-constructed spring-based gizmos seen in the 1970s, they’re light, stable, simple to handle, and durable. We took the time to do things right!
Our custom-made springs lie at a perfect 90° angle to the pivot axis, so they provide direct, precise opposition to the hanger’s motion. To prevent buckling, Seismic springs pivot at both ends on ball-and-socket joints. Small inner springs expand when necessary to keep those joints working smoothly.
Also, Seismic trucks were patented long before scaled-up, stripped-down imitations began appearing on off-road skateboards and surfing cross-trainers. We were first!
Calculating the Spring Rates
Seismic springs are custom-made to the most exacting specifications. The spring rates (the power of the spring throughout its compressive range) are calculated using a set of extremely complex mathematical equations and the most advanced theories of skateboarding bio-dynamics. The inventor, Dan Gesmer, spent over ten years deriving and refining these equations, which can only be efficiently solved with mathematical analysis software.
Click here to see article: Mathmatics steers skateboards into 21st century
Through kinematic and force-vector analysis, Dan determined the precise mathematical relationships amongst every variable relevant to truck performance. These variables include numerous key dimensions of the truck itself; carefully-measured average values for wheel diameter, wheel clearance, wheel base, deck width, and skater size; the tilt angle of the skateboard deck; the lean angle of the skater’s body; the lateral distribution of the skater’s weight; forward velocity; and the coefficient of friction between wheels of average hardness and average riding surfaces. Through careful study and scientific experiment (including video analysis of skaters making turns at varying speeds), Dan developed original theories on the relationship of turning radius with velocity, knee bend, and deck weighting. He also corroborated his research with two American professors of engineering who may be the only ones in the world to have studied the dynamics of skateboard turns.
Springs vs. Grommets
Superior Energy Return
For performance carving, your trucks’ suspension system should snap the axles back to parallel as fast as possible when you unweight the edge of the deck. Otherwise you can’t make quick, powerful turns. Because of internal friction (known as hysteresis), the urethane grommets used in roller skate-type trucks soak up and keep most of the energy you put into them, especially if they’re soft or loosely-set. The result is sluggish or sticky axle rebound.
Seismic metal coil springs pop the axles back with lightning speed, powering you out of turns like never before. Propelling yourself with alternating turns (pumping or gyrating) has never been this easy. You might even find yourself pumping from one maneuver to the next…
No Breaking In and No Wearing Out!
Traditional urethane grommets need to break in before you feel comfortable with them, and you have to break in new ones every few months because they wear out so quickly. Grommets also get softer in hot weather and stiffer when it’s cold, so it can be a constant chore to keep them adjusted the way you like. Seismic springs respond perfectly the first time they’re used, and they’ll continue to respond consistently, regardless of temperature, probably for as long as you own the trucks.
If you steer to one direction more than the other, ordinary trucks sometimes start to feel funny, like they “want” to go in that direction. That’s because the plastic cushions and pivot cups can lose their bounce and get warped. Metal springs stay resilient for a very long time, so Seismic trucks will always respond the same way frontside as they do backside.
Those plastic pivot cups wear out, too, and if you don’t replace them your turning performance will deteriorate. The low-friction Seismic pivot system is based on the steel pivot pin and baseplate insert rotating against the German-made Teflon-coated flange bearing inside the hanger. These components will last the lifetime of the truck, without needing any maintenance. The secondary contact between the hanger and baseplate surfaces is naturally self-lubricating.
Why Traditional Grommets will Never Cut it
The grommet-based truck suspension system developed in the 1930s is simply crude and outdated. The grommet collars swing and twist against small side portions of the cushions, instead of pressing directly against the large, flat end surfaces. Even with advanced urethanes, variable durometers, fancy contours, and careful adjustment, traditional grommets can’t react to your weight shifts with the biomechanical precision needed for today’s high-performance skating. They’ll never give you the edge control you deserve.
Unlike metal springs, grommets rapidly lose stiffness and resiliency. Even if you never adjust the kingpin nut, you have to continually re-learn how much edge pressure it takes to make turns of varying sizes. By losing most of the energy you put into them, grommets steal power from your skating, too.
Ordinary grommet-based trucks also force you to sacrifice stability for turning ease. With soft or less-tightened grommets, standard trucks respond too quickly — they can get squirrelly and hard to control, especially at high speeds. With harder or more-tightened grommets, they respond too slowly, losing most or all of their turnability. Most skaters have adapted to one of these trade-offs, and some have even grown to like wobbly or too-tight trucks. But sacrificing either stability or maneuverability means limiting your skating. To let you move beyond those limits, we took a whole new look at truck design.