Hollow Wheel Manufacturing and Shipping Update #12
Thank you for putting your faith in me and I am working hard to get across the finish line.
Cliff Notes: I am feeling confident enough to place the order. I am making a small tool change to strengthen the wheel. I am awaiting a specific timeline and feedback from the manufacturer. See below for details.
Here is a video update for those that want to see more of a behind of the scene look:
In this blog post, I will talk about all the durability testing, small changes to the wheel, and the timeline for production.
I durability tested all the wheels that I had on hand to get as much data to determine if the wheels in the latest configuration are reliable. This means I brought every single wheel that I had to failure to see how they would fail and why and when. Your safety is my ultimate concern.
The bad: I did identify a failure and it is failing on the inside of the wheel during extreme lateral forces (see below to see what "lateral forces" are).
The good: I already came up with a solution and the manufacturer is aware. I tested a bunch of different designs to strengthen the wheel for lateral load conditions and it is much stronger. I hand casted a wheel (see below). The new wheel is a tank even though it is a wheel. Sorry. Bad joke. No failures on the hand casted wheel with lateral load. So far we have 3x the run time over the previous configuration and on the competition. Of course, this is a hand casted wheel and not a production wheel yet, but the production wheel material is better than the hand casted urethane that I use.
The Hollow Wheel stood up against the pot hole test very well, but the lateral test was the true killer. In my durability testing, the Hollow Wheel lasted hundreds of thousands of bumps on the bump test at 30-35mph (See reference section for pot hole testing).
However, when simulating for extended lateral loads of a 600 lb person doing 1G in a corner (what is "G" in a corner? See reference section), the wheel was bending in a certain direction that caused a crack. The reason why I selected 600lbs and 1G corner force is I wanted to get a factor of safety of 2. In other words, there are so many variables in the real-world that I can happen and so I want some extra buffer in case I did not take it into account. I also selected 1G corner force for the same reason. Most standard electric skateboards on urethane won't be able to touch 1G.
That said, this isn't a failure that is very sudden; it doesn't explode. Rather, if you were riding on the wheel, there would be more vibration and you'd probably look to see what the heck is going on. Of course, I am working to completely eliminate this occurrence.
To bench mark the competition, I ran the same lateral test on other wheels to gauge where I was. Here is a picture of another wheel. The main failure here was delamination from the core to the tread.
New Rib Design & Production Mold Modification
The new rib design adds 13% more strength to withstand lateral load testing better. Specifically, the ribs help stiffen the wheel against the bending which the wheel has to experience during extreme lateral forces. Here is a simulation we ran. This picture depicts deflection. The yellow zone is where it bends most post ribs. Now it is away from the critical zone.
I tested multiple rib geometries and found this was the most durable and also doesn't adversely impact comfort. It just strengthens the wheel for lateral load conditions.
Here is a picture of the a wheel that I hand casted so I could slap it on the test rig to see if how it performs on the test rig with lateral force loads - if it would out perform the other design. Hint: it does outperform and is now very durable - we have 3x the run time over the previous configuration and on the competition
Next Steps to SHIPPING
Walking through the timeline, they need about 20 days for production + tooling machining, then it is 2-4 weeks for shipping from China, then I am going to test and inspect many of the wheels.
Then if they check out, then it ships from there.
For those that are not familiar with what "pot hole testing" is, then see this video:
What is lateral G force?
When I say "1G", it refers to the gravitational force equivalent. In other words, when you go through a corner in a car, you can feel a side load on your body. 1G is equivalent to your body weight. In a really fast car, like a Corvette, they can max out at 1G load. Most road cars can only do a maximum of 1G of force in a corner. The race electric skateboards out there maybe could muster 1G in a corner, but most electric skateboards won't come close. Still, I use 1G as a max load for buffer.
Here is a graphical representation of what lateral force is. The driver in this F1 car is experiencing cornering loads of 5G. So this driver is experiencing 5x his body weight in the corner. So if he only weighs 100lbs just standing still, he is experiencing 500lbs of side load through the corner.