Let's face it, skate trucks take a ton of abuse. They need to be strong enough to withstand some pretty serious forces, especially for skaters who skate gaps and stairs. Trucks also need to be lightweight. For these reasons, the axle and kingpin of a truck are made of steel and the hanger and baseplate are aluminum.
There are easily 100,000 different types of steel and thousands of things that determine it's strength. A simple and fair measure of the strength of steel used for axles and kingpins is their grade. Common grades are 2, 5, and 8. Grade 2 is the weakest, grade 5 is 62% stronger than grade 2, and grade 8 is 41% stronger than grade 5.
In the old days almost all kingpins were grade 5 steel, which have the tendancy to break. If you flip over the baseplate of an older truck you'll see some markings on the head of the kingpin. These markings are used internationally for generic bolts, and tell you its grade. You'll likely see 3 lines in the shape of a Y, telling you it's a grade 5 bolt. Sadly, some truck manufacturers still use grade 5 kingpins to save a few bucks. If you look at the head of an Alpha kingpin, you'll see the ALPHA logo instead of generic strength markings. This is because we custom make our kingpins specifically for our trucks. We use only the highest quality grade 8 steel for our kingpins, so think of the ALHPA logo as the mark of grade 8 strength.
So grade 8 kingpins are 41% stronger than grade 5, what does that really mean? To avoid the statistics, think of it this way. Say you're batting in a baseball field. Most of the time when you hit the ball it lands somewhere in the infield. This is like your everyday skating. Sometimes the ball goes way out to the outfield. This is like skating gaps and stairs. Occasionally you hit the ball so hard that it flies out of the field completely. This is like breaking a grade 5 kingpin--it happens, but not very often. Now, what if the baseball field were 41% longer and wider? In that case it would be almost impossible to hit the ball out of the field. That's about how often you'll break a grade 8 kingin.
For the same reason that grade 5 kingpins are bad, grade 5 axles are bad. Axles bend a little bit under the large impact forces of a skateboarder, which can cause the less flexible aluminum of the hanger to crack. A grade 8 axle is way more rigid than a grade 5 axle, which makes the whole hanger stronger. Alpha axles are made from the highest quality grade 8 steel.
An important part of an axle is the knurl. The knurl is a textured part of the axle that prevents it from spinning when you screw on the bolts. There are many types of knurls. In the old days axles had straight knurls because they were pressed into the hanger after the hanger was cast. This resulted in the axles slipping after skating the trucks for only a few days, making the wheel on one side not rotate and the wheel on the other side super loose. Most truck manufacturers eventually figured out this wasn't the best way to make trucks, so they started casting the axles in place inside the hanger. The knurls changed from straight to diamond shaped to prevent axle slip. But, a lot of truck manufacturers use a shallow diamond knurl that can't really grip the aluminum, so those trucks still get axle slip pretty easily. The axles for all Alpha trucks are custom made with the deepest diamond knurls possible, making axle slip a thing of the past.
Hanger and Baseplate
The hanger and baseplate are made of aluminum (really an aluminum alloy). They are cast, meaning molten aluminum is poured into a mold. There are two ways to cast aluminum trucks: sand casting and permanent (or steel) mold casting. Sand casting is done by using what's called a matchplate to make impressions of the hanger or baseplate in special casting sand, and then pouring the molten aluminum in this 'sand mold.' Permanent mold casting is done by pouring molten aluminum directly into a steel mold.
Sand casted aluminum is typically less dense than permanent-mold casted aluminum, so trucks made by sand casting are weaker. Also, because sand is used as the mold, it is much easier for particles to get in the aluminum and create weak spots. But, since making a matchplate for sand casting is significantly cheaper and easier than making a steel mold for permanent mold casting, a lot of truck companies use sand casting. All Alpha trucks are made using permanent mold casting, making them as strong as possible.
If you turn over an Alpha truck and look under the hanger, you'll see an opening in the aluminum where the axle shows. This is done because the axle is cast in place (to help prevent axle slip). During casting, the molten aluminum is at a temperature of about 1000° F, but the axle is much cooler, even if it's pre-heated. When the hot aluminum and cool axle make contact, air bubbles and stress points can form in the aluminum that make the hanger weak. The openings in the aluminum where the axle shows through provide a route for air to escape, reducing the chances that any air bubbles will form. So beware of trucks with hangers that don't have openings for the axles.