I have tried so many methods of getting reliable drive from the motor to the tire. Been down the polyurethane track and it does improve grip over a smooth metal roller but any moisture on the tire, and we are back to square one.
This is a mountain bike so you are going to be very limited to when you can ride if the track is damp.
At this stage the knobby tire has been placed on the backburner due to noise and replaced with the latest version of the Schwalbe Hurricane. the tire needs a bit of rework with a finger belt sander to make the rolling surface completely smooth, but once you do this, the tire is as quiet as any tire I have tested. This latest tire in 2.4" wide and offers a surprising amount of grip out on on the tracks even in damp condition. Trick is to run its minimum pressure of 23psi to get the side knobs more engaged and team it with a really aggressive front tire.
To make the roller work in wet weather, I have gone back to belt sander material but applied it with better moisture protection for durability. What I previously found was that the moisture would get under the edges of the grip tape causing it to de laminate in a short period of time in wet conditions. I have now added adhesive heavy duty heat shink on each side of the roller to stop moisture getting to the edge of the grip tape. With this setup, the grip tape is the part of the roller in contact with the tire.
I have riden this in both damp and really muddy conditions. No surprise, deep mud sucked however packed wet and damp conditions we not a problem at all.
As previous mentioned, this is a low powered setup with 100W used most of the time and boosting to 150W when I really need it. This might not sound like much, and it isn't however it is just enough to make the bike feel light to pedal. Riding with my regular non assisted friends, we usually pretty even in overall speed however with just 100W extra, they had no chance of staying with me through the hard pedaling sections of the ride
One small drawback with the belt sander material is that dirt can stick to it under some conditions but not enough to cause any major issues.
Internal batteries are now completed. These are tiny. 2 x 6S1P 18650 for 3ah per pack. With the 2 packs, this gives me around 120Whrs which gets boosted to around 160Whrs with regen. This is good for around 30km of assisted riding which is plenty for me especially with knowledge that if when completely flat, the drive is fully disengaged from the bike.
Weight penalty is negligible. The drive system including internal batteries adds 1200 grams to the bike. The Schwalbe tire I am now using is 300 grams lighter then the original tire I had on the bike and I have installed a light weight rear cassette for another 300 grams savings. So total added weight to bike is a mere 600 grams which is the the weight of a water bottle. Needless to say their is no weight penalty that you feel on the bike.
An added bonus is that the bike rolls even better then before due to the change of back tire.
I have bought a knurling tool and am keen to still give a knurled roller a go. I am hoping with only 100W, the tire will handle the load.