When you lean to a side of a skateboard, it turns. The deck tilts and causes the trucks to pivot. The wheels on one side of the skateboard move closer together and the wheels on the other side move further apart. If you tilt the deck as far as it will go, and then extend a line through each axle and find where they cross, you have the minimum turning radius of your skateboard. The smaller the turning radius, the sharper turns you can make. For most types of skateboarding, a smaller turning radius is better (but too small can be dangerous). Many things determine this turning radius: truck geometry, wheel base, deck height, wheel size and shape, and so on.
First, even with the best geometry, a truck with crappy bushings won't turn very smoothly (that's not to say that good bushings will fix a bad truck either). Bushings are made of urethane. High quality urethane has a high rebound, which makes your trucks recover quickly after a turn or after landing a trick. But high quality urethane isn't cheap, so some truck companies try to save a few bucks by using low quality urethane to make bushings. These bushings are stiff and don't have much rebound. The result is a truck that feels slow and unresponsive. We use the highest quality American-made urethane bushings in all our trucks to give the best ride possible.
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.
People commonly refer to the distance between the front and rear mounting holes of a deck as a 'wheel base,' but wheel base is really the distance between the axles of the trucks of a complete skateboard. The closer together the axles are, the smaller the wheel base, and consequently the smaller the turning radius is.
Wheel base affects the turning radius more than some people realize. In the old days skateboards used what is called the 'old school mounting hole' pattern. The holes in the baseplate were 2.5 inches apart. Today the 'new school pattern' is used, which has a smaller hole distance of about 2.1 inches (this was done to protect the mounting nuts from being gnarled up during noseslides, etc.). Sadly, though, to save tooling costs a lot of truck manufacturers converted to the new school pattern by moving the holes near the pivot cup back the full amount and leaving the holes near the kingpin where they were. Why is this a mistake? The mounting holes should be centered on the baseplate to make a smaller wheel base. Remember, a smaller wheel base means a tighter turn. Even something as trivial as this makes a difference: a typical street skateboard with a 7-foot turning radius on baseplates with properly centered mounting holes would have a turning radius nearly a quarter foot larger simply if the mounting holes weren't properly centered.
Deck Height, Wheel Size, and Wheel Shape
Low trucks are great because they are more stable and turn better than high trucks, but with big wheels sometimes you simply need more clearance. Use riser pads to raise your deck. This will decrease the turning radius of your skateboard and give you sharper turns, but the higher you go the more unstable your skateboard becomes.
It's pretty obvious that bigger wheels bite with less deck tilt than smaller wheels. The shape of your wheels has a little to do with it too. It takes less deck tilt for wheels with squared-off edges to bite than it does for wheels with rounded edges. This isn't really a concern for street skating because most street wheels have rounded edges. For longboarding, though, a lot of wheels are made with squared edges to increase their grip on the pavement. Keep this in mind when selecting a wheel.