Chain rub

Hello,

I'm building a stretch beach cruiser e bike, and have run into an issue with the chain.

Using a 5kw QS motor with single gear, the chain rubs on the stock fender, and the tire.

Should I just change the single gear cassette? With what? Or use a spacer?

As long as there is sufficient room between the outboard side of the FW and your dropouts/frame, a spacer between the FW and the motor would work, however it will also mean less thread engagement of the FW to the motor.

That would mean less torque-handling capability from the pedal drivetrain to the wheel for those circumstances you might need that in. (without stripping the threads)

It also means higher cantilever force against hte side of the hub when applying torque thru the pedal drivetrain, though it still wouldn't be as high as if you had a regular multispeed freewheel on there and were shifted to the smaller cogs, so should be as safe as anything is with the thin cast covers on these things.

If you can fit a 5 speed (or more) freewheel on there, you can probably remove the inner few sprockets to make a single speed freewheel with a wider chainline. You'll need to take off all the sprockets that are larger than the one you intend to use, so the chain can't catch on one, jam and break stuff.

However far outward you move the rear sprocket, you need to line up the front sprocket with it. Single speed chain drives don't tolerate misalignment between front and rear sprockets. Depending on what crank and bottom bracket you have, this might be a little bit complicated to do.

You should check the frame alignment, or have a bike shop check it, before you set up the sprockets. Chainline is dependent on frame alignment, and if your frame isn't straight, that might be the only problem you have to fix.

as its rubbing the tyre you will need to correct that with spacing. (or get a narrower tyre)

as chalo said, a 5 or 7 speed cassete free wheel is a good option. but make sure you can get one that has removable gears.

you can turn this into a single speed at the correct spacing (you might need to make your own sized spacers out of tube) .....and you also have some options to change gear size to clear your fender.

This is the spacer you need from AliExpress. It comes in two lengths and is metric threaded 35 x 1mm I think. This size is becoming more of a standard nowadays. Check the drawings to see the difference in length. I got the longer and cut it down so I think you’d be ok with the shorter one.