Slopes wrote:I'm having problems getting the friction drive to engage. On twisting the throttle, the motor moves into contact with the tyre - as it should - but then it immediately drops back a little so that the tyre is no longer being driven. This happens even though the motor continues spinning at the same rate. I have to release the throttle and twist it again to get the motor to reengage - but then the same drop-back happens. I've set the gravity spring as tight against the motor as it can be without pushing it forward, and it stays in contact with the motor throughout its movement up into the tyre.
I'm thinking that maybe the distance between the centre of my bottom bracket and the tyre circumference is marginally too great for the drive to function as it should... and the weight of the motor in its more horizontal (engaged) position is too great for the spring to hold in place ???
A few minor adjustments needed. See if you can pivot the main bracket forward so the motor is driven into the tire further. 1 or 2mm is all it needs. You need to set up your end stop so when the motor disengages, it just clears the tire. Even if the tire just brushes the motor when disengaged isn't a bad thing as it guarantees easy engagement and it doesn't cause any noticeable drag on the bike. Last thing, it looks like you haven't adjusted your gravity spring to make the motor neutrally weighted. Make sure you have this adjusted correctly as this is important for accurate engagement.
Keep in mind if the bike isn't under load, the drive will drop away straight after it starts. It relies on load to ensure the motor climbs the tire. The more load, the harder it will bite into the tire. You can simulate the load on the test stand by appling the rear brake slightly straight after the drive has started.
The bike you have fitted to the drive too has long chainstays to accommodate a kickstand. This means the drive has been set up right at the end of it adjustment range. Give these changes a go and see how you go. If you dont have any luck, I can look at doing a custom swing arm that is 5mm longer for you. I am placing an order with my laser cutters early next week so let me know if you need it. Because it will be a one off cut, cost will be around $15 plus a few dollars postage.