Recently, I had the rear truck on my V1 boosted board fail while a friend was riding. It was very surprising, to say the least, and I believe I am the only one to have snapped a truck on the V1 board. The board had a little over 2000 miles on it when it failed and Boosted covered it like they do all their products. At the time, I was designing my own electric longboard drive around the same Caliber II truck that was used in the V1 boosted board. This failure scratched that part of my brain that I couldn't ignore. I had to know why this failed the way it did, where it did, and when it did. This topic will take a close look at longboard truck failures and how to design to prevent them.
But first, some let's catch up on some things.
Let's Talk Fatigue
Aluminum has a notable property in that it has no fatigue endurance. In other words, with any amount of deflection, it WILL fail after some n number of cycles. Other metals such as steel, have a range of deflection where it can operate indefinitely without ever failing. So just remember next time you're on an airplane and you see the wing bouncing up and down, that it WILL with certainty fail eventually. Luckily, this property of aluminum is well known by engineers, so it is accounted for in designs for airplanes, buildings, and vehicles. Unfortunately, it seems to have been overlooked by some longboard truck manufacturers.
Stress concentrations are areas on a part that cause stresses in the material to build up or concentrate in one area. This is typically caused by sharp, internal corners, sharp grooves, or any feature with a small radius. Going back to airplanes, this is precisely why the windows have rounded corners. In early airliners, the windows were square and crack propagated from the corners in the aircraft's frame due to high-stress concentrations. In fact, a crack itself creates a stress concentration at its end. This is why cracks spread and can be halted by drilling a round hole at their ends.
Boosted Board V1 Truck Failure
What Was the Cause of Failure?
Above you can see the failure of the boosted board truck, which is a modified Caliber II truck, which occurred where the motor mount ends. This is due to a few things. Firstly, it is an area of high stress since this area of the truck has to handle the shear force of the rider's weight, and the torque on the motor mount from the motors. However, this is not why this area failed.
The main cause of failure here was the machining that was done to accommodate the motor mounts. This created an area with a sharp internal corner, which resulted in a stress concentration in perhaps the worst possible spot. In fact, it is precisely this spot that the failure initiate from.
The failure was a result of high cyclic stress creating a stress concentration due to poor geometry leading to a premature fatigue failure. Looking at the images, you can see the classic "clamshell" pattern starting from the bottom right (where the highest stress concentration was) moving up through the rest of the part.
Clamshell-Like Fatigue Crack Propogation
What About the V2?
This same type of failure is becoming an even bigger problem for the V2 boosted board. Boosted designed their own line of trucks with a better bolt on motor mount to interface. With this new design, wheels have been shearing off the last inch of the trucks left and right. Below are some examples:
Now, this is an interesting failure since it does not look like a fatigue failure. Instead, this looks like the aluminum itself was more brittle than it should have been and broke cleanly off along with the steel axle. There are a couple things that could contribute to this. Firstly, the diecast aluminum used (most likely A360) cooled improperly and or was not heat treated after casting. Additionally, the finish on this part of the truck is grooved from the machining process where the bearings sit. Either way, this is a big design flaw that boosted needs to address.
How Many Boards Does this Affect?
Boosted isn't the only one with these problems, in fact, their boards are known to be some of the most reliable. Evolve had motor mounts snapping like crazy on their boards before changing the design and almost all of the hub motor boards have the hubs fall off after some time. One of the scariest designs out of them all is the new Enertion Raptor 2 design. They have two large hub motors on an 8mm shaft which extends the full width of the wheel. Yikes! Not only this, but the axle is made from stainless steel which is significantly weaker than carbon steel. Also, the aluminum portion only has threads about 1/2 inches in with no load distribution collar. The first of those boards are starting to fail. The images below are from a Raptor 2 with only 300 miles on it:
You can see it was the stainless steel axle that failed, right where it threads into the aluminum. This, of course, makes sense with the largest moment occurring there and also sharp threads to create stress concentrations. I hope Enertion addresses this before people get seriously injured, especially since their board is one of the fastest available.
So What Now?
Well, we can only hope that the engineers who designed these trucks will recognize the problem and address it soon. It is pretty unreasonable for this to be something that happens, especially since these boards are treated more as vehicles than toys. I hope that this article gave some insight on things to be aware of for anyone trying to design their own trucks, or those looking to purchase an electric longboard. This should not be a problem that is swept under the rug and forgotten about but should be highlighted and solved for a safer a brighter esk8 future.
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