Hollow Wheel Manufacturing and Shipping Update #16
I have been testing the new wheels. Bad news. There are still some durability issues, but I have solutions and a plan. Sorry for the late update, but I didn't want to just post up a problem without having any solutions. It would have caused hysteria. I had to measure a lot of data and put together a well formulated plan to share.
Cliff Notes: got the wheels (uncolored and small batch instead of the big 200 batch), but there is another heart breaking set back - some material issues - they are over heating. With summer upon us and pavement being hotter than ever, with a 220lb person (with 20-30lbs board) 250lbs+, the wheels can overheat and eventually thermally deform. Something that I didn't see when it was colder and on the test rig. So I am doing 3 things - going back to the vendor to tell them to increase the heat resistance, introducing air flow within the wheel to cool them, and altering the contact patch to reduce the heat generated.
I am embarrassed to say that I didn't see this issue.
Initially, we were going to receive 200 sets, but because we added ribs from last time, it meant that injection temperature and pressure increased a lot. The vendor had some Injection molding tuning difficulties and wanted to verify a couple of uncolored wheel samples before running the production.
New shipping timeline:
The issue and solutions have incurred an additional ~9 weeks from the last estimated shipping from the manufacturer. Keep reading and I'll walk you through what solutions/fixes have been completed and what is in progress.
No one wants to ship more than myself. That said, if you do want a refund, I am happy to give you one. I will not give up on this project and I am confident I'll find all the bugs so that this is the very best product that I can make and am really proud of.
If you're having trouble zooming into the timeline, then click here: https://cdn.shopify.com/s/files/1/0082/2617/7124/files/Timeline_16.png?v=1625727132
The wheel is overheating and melting in some spots, but only on heavier riders (it seems like 220lbs+) that love to carve for miles on end and on very hot days. Riders that are lighter have not reported any issues so far. Nevertheless, this is a big issue. This is something I didn't quite foresee as much of the testing has been done on the test rig and on riders during winter and spring, but not the summer where temperatures of pavement can reach 140-150F, in some cases.
Add the temperature generated from tire scrubbing during carving and some heat generated from wheel deformation, the wheel can easily reach 170F. The melting point of the current urethane is pretty close to this temperature.
Luckily, there are no core to thread delamination; that issue has not been seen since batch 2.
There are three solutions, which are designed to cool the wheel, to increase high temperature heat resistance (increase melting point of urethane), and/or to distribute load so as to not generate as much heat in the wheel.
The easiest one is to reformulate the material by increasing the melting point. The manufacturer is working on this solution right now. Moreover, I have been working with other vendors, which I have identified two other suppliers, that have materials with a higher melting point temperatures. This will help significantly and be the quickest to implement. See the timeline for next steps. More on the progress in the next update. Currently in the buying stage and reformulation of TPU stage for higher melting point.
Another way to limit the amount of heat which is generated in the wheel is to distribute the load of the wheel more evenly. The negative camber on the wheel definitely increases pressure. For a constant load with a given area (aka contact patch), the contact patch is can be quite small. So the obvious thing to do is to increase the area so that pressure is lower. Lower pressure equates to lower heat generation.
As such, I designed another wheel profile but with less negative camber (but still having negative camber) and removed some mass from the wheel to cool of the area of the wheel that traditionally gets hot (see the picture above for the area which I am referring to... inside edge of the wheel). Then put it on the test rig and also went for a ride to measure the top surface of the wheel. Of course, I didn't cut any production tooling so it is currently impossible to measure the temperature delta using the injection molded TPU material. The next best thing was to test the same hand cast material but compare the surface profiles. So I spent a while doing CAD, creating 3D printed molds, and then molding it. See the picture below. The temperature delta is an average 20F lower on my rides and holding as many variables constant, which is very good news. However, this will require another production mold modification.
As a secondary test, I wanted to put the current wheels on a lathe to cut some other profile geometry to distribute the load so that I can get more data on the impact of heat generation based on the current TPU material. I'll be picking up those machined wheels on Friday (7/9), then riding with my fat friend and measuring temperature deltas. More on that in the next update.
I am sharing this data with the injection mold factory to prepare them for an additional mold change for wheel profile. They'll have a look and share any feedback with me as far as cost, timeline, and any difficulties they see with it (shouldn't be much since it is a simple change).
Cool down the wheel:
Other solution is to cool to the wheel by introducing airflow through the wheel - cooling through convection. Though, I suspect this will not be as effective as the two aforementioned solutions only because the ability of Urethane to reject heat is poor and it will require a significant amount of air flow to reject the heat. Nevertheless, I am experimenting with this solution.
Here is a snippet of what the Mercedes F1 team implemented when they had tire wear issues - they introduced air flow through the wheel.
First, I wanted to see if I could introduce air flow through the wheel, so I started by drilling holes through the wheel.
Then, I packed the wheel with glitter and put grease on the cover to capture any glitter floating out of the wheel. To be sure that this was not just a fluke, I repeated this test 3 times. Moreover, I repeated this test on a wheel with no holes. If glitter fell out of the wheel and stuck to the cover on both wheels (holes and no hole wheel) in every run, then I would know that there is no impact of air flow on the wheel. A side note is that there is a big lip inside the wheel so it is impossible for glitter to just fall out of the wheel and stick to the cover. Coupled with high centrifugal force, the glitter would just want to stick to the inside of the wheel unless there is a big gust of wind trying to blow it outside of the wheel...
Here is a comparison of one wheel with grease on a wheel with no holes (no air flow) and the cover of the wheel with holes, indicating air flow going through the wheel.
The next step on this is to measure how much exact air flow is going through, which is very difficult to do empirically, so I am going to try to estimate through simulation, but I will have to try to estimate air flow. The next best thing is just simply to measure temperature delta of the wheel in real life, which I am still currently doing.
This is a picture of what some proposed holes could look like, but there will be some more optimizations here.
When it comes to temperature, I don't think the Hollow Wheel is the only one that is susceptible to degradation due to high temperature; I think there are others. Other wheels can be seen to have delaminated and after seeing these failures myself, it now is pretty clear to me. That said, I am taking a data driven approach to kill this issue.
Sorry for the delays and just know that I am not going to stop working my butt off to ship this. I am extremely frustrated with myself. This is my top priority. As always, you can reach me via email (the best way to contact me at our support email - Hello@momentum-boards.com, on my website chat, or my personal cell 408-203-6130 (just try not to call after midnight pacific or before 830am).