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!

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). 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. 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. 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.