Hollow Wheel Manufacturing and Shipping Update #7

January 18, 2021

Hey all,

Got 3 more samples of wheels and I am durability testing the wheels. A lot of progress in the right direction and the durability data so far makes me feel optimistic about a release soon. First, I apologize for the delays as a lot of these small design tweaks eventually add a lot of time. It is hard to foresee, but I am trying to be as transparent as possible through the entire process.

A lot of the material adhesion problems we have had between the core and the tread was an unforeseen issue. A lot of tweaking and back and forth between 4 teams (urethane supplier, injection molder, myself, my team in China) and shipping between China and US adds up on time. However, we are all taking a data driven approach and working as quickly as possible to bring to market the Hollow Wheel.

The wheels are not shipping till after February due to China having Chinese New year, which is typically about a month long holiday. However, there is a lot of good news in this email. I have had a user approximately 1000 to 1500 miles (he said closer to 1500 miles) on a set (the first sample with the harder urethane) and they look in great condition.

We are now on the 5th iteration and the wheels are better than ever, but just need to make sure they don't break.

Here is a quick recap of what was done in the last update and what I am doing now.

Tool Changes. In progress last update: Additional tooling change to the core to help with bonding strength between tread and core. What I am doing now: Got new core design on the 14th of Jan and durability testing now. See last update for more info on what I changed.
Tread and Core Material Changes. In progress last update: Material changes (reduce durometer, increase adhesion between core and thread). What I am doing now: Got 3 different wheels with different materials (all with new core geometry) and durability testing those now. More on that below.
Injection molding process testing. In progress last update: Testing of injection molding parameters to bond strength between thread and the core. What I am doing now: We did a lot of testing and feel we came up with the best bond strength vs. production yield, then shipped from China to me, and now I am durability testing now. More on that below.

Design of Experiment and Durability Testing

Since the last update on the 31st, we wanted to increase durability of the wheel by improving the bond strength between the core and the urethane. As such, our team decided to try out a couple new iterations of the wheel material break down. We think with the new tooling change (see last update for more info on what was done), we decided to do a design of experiment (DOE). This is a testing method to control variables and change only 1 variable at a time. By changing a few variables, we can then see what works and what doesn't.

With the new tooling update, it has increased the mechanical grip between the core and also added more stiffness in the wheel where it can help prevent core and tread delamination.

Based on the last two batches, we applied that learning to change the injection molding parameters (urethane injection pressure, mold temperature, etc.) to get the best bond strength without drastically increasing injection molding defect rate.

Since the new geometry is now a constant, we decided to use the chemical compound to increase the bond strength. We have three new samples and I am beginning to accelerate the durability testing. Also, finally, we were able to get a durometer for the tread in the 75-80A range, which is in spec to the prototypes.

We have three new samples. We changed the chemical composition

Accelerated Durability Testing

The test rig is up and running and beating the crap out of these wheels. This allows me to put on a lot of miles in a short amount of time. It is running off Arduino and has a program that automates full throttle to 50 mph, then hard braking, then just repeats over and over and over.

Here is the rendering: