I should like to move the chainring on my single speed bike a little further outward (fig 1). The reason is to have enough clearance between chain ring teeth and frame (fig 2).
At present I have a spacer between the left bottom bracket cup and the bottom bracket shell (fig 3). However, this leaves me with quite a bit less spline engagement of the left crank arm to the spindle (hollowtech 2) than what I should wish for. The stop plate at the crank just clicks into the gap on the spindle with no room to spare.
To summarize, I have to remove or reduce the bottom bracket spacer. This will leave too little clearance near the chain stay. Hence the need for spacers at the chainring.
I can reduce the spacer width as much as i like by grinding it down. Which is how I got that spacer in the first place. At present the spacer is about 1.3 mm to 1.5 mm wide.
The chainring has about 2 mm to 3 mm clearance at it's closest approach. It is an oval ring. The crank is roughly at the 4 or 5 o'clock position then.
The frame is aluminium. I'm a heavy rider (85 - 90 kg) and push quite hard in short bursts.
Chainring and sprocket size get the, so called, magic gear ratio for that frame. That is, the drivetrain length just fits the chain length. The dropouts and bottom bracket of this frame don't leave a degree of freedom for adjustment. In order to reduce chain wear I've chosen the largest ring/sprocket. The smaller mechanical advantage reduces overall drivetrain strain.
The actual question has several aspects:
What should I use for shims between chain ring and cranks? Is a round washer ok?
How thick may such a shim be without running into trouble at that interface?
How close may I get the chain ring teeth to the frame?
Can I get rid of the spacer at the BB, is it worth the bother?