Truck geometry refers to the action angle of the truck. If you draw a line from the tip of the pivot stem through the center of the ring, the action angle is the angle between that line and the horizontal (shown in solid red above). The hanger pivots on this line. It's the most important thing that determines turning radius. The larger the action angle, the more the hanger pivots for a given amount of deck tilt and the smaller the turning radius is. Changing the kingpin angle or the pivot stem angle can change this action angle.
So when a truck manufacturer makes a HI version of their LO or MID trucks, all with the same action angle, they will have the same turning radius, right? No, HI trucks almost always have a worse turning radius. This is because most HI trucks are designed with the original baseplate and a modified hanger with the axle is dropped down further away from the ring. The result is that it actually takes less deck tilt for the wheels to hit the bottom of the deck with HI trucks than it does with LO trucks and riser pads. So if you need more clearance between your big wheels and your deck, buy some riser pads instead of HI trucks. You'll save some money and your skateboard will turn better (and it'll also be more stable, click on the Stability link below to learn why). This is why Alpha doesn't make a HI version of any of our trucks.