[ailurux]

Note that technically you can use approximately any flat thin nonconductive thing in the right shape as a touchwheel cover. IIRC there were originally plans to use glass that didn't pan out for some reason, and the FR4 ones we got were the backup plan.

It'll probably work fine without a cover too, but if you're using it hard enough to need to replace the stock touchwheel cover, you'll probably also damage the solder mask behind it at some point.

Actually the touchwheel cover plays an important role as the dielectric material for the capacitive sensors, and FR4 has good dielectric properties (as well as being quite easy to source with custom shapes and screenprinting). We did also consider glass but FR4 was always the main plan to go with. Without the dielectric material it does still work somewhat, but it's my understanding it would be a lot less sensitive and more likely to not work with moisture on the hands and I wouldn't recommend it.