Thanks, Martin, for the kind words and the time and trouble you took to illustrate the throw from spray and where it lands. Very helpful and much appreciated. That red bike is stunning with its matching bags.
And now some details for Mike; if too long for others, just skip...
Making this chainguard was something I've had in mind for some time and so I put a lot of advance planning into it. I think by sharing I can save you some time, so I'll do it in a tabular fashion:
1) When I said "I shamelessly copied a BMX chainguard that was way too short for my needs" I was referring to a broken one I recovered from a trash skip and used for inspiration. By looking closely at it, I was able avoid some initial pitfalls and I was also able to test-fit what was left of it to check basic clearances. It was too short for my chainstays and designed for a 34t chainring, so I needed to scale it up to fit my 38t 'ring and allow clearance for something larger in the future.
2) I made my "buck" (plywood form) from 3/4in labeled plywood that actually measured 11/16in thick. Make the plywood mold as smooth as possible to save yourself polishing later. The outside will take care of itself, but the inside will pick up every little imperfection in the wood mold and those flaws will set in the hardened plastic.
The polycarbonate I used was 2.3mm/0.09in, so pretty thin but sturdy enough, especially when "folded" over the mold (the 3-D shape helps stiffness). I sourced my plastic locally, but this link will give you a general idea as to thicknesses and availability:
https://www.acmeplastics.com/cut-to-size-clear-polycarbonate-sheetI initially set my oven to 175°F and kept the door closed until it was thoroughly warm, then opened it and placed the buck on the rack set on the lowest bracket until it began to soften and then adjusted the temperature. I really have no idea what the final temperature was because I kept opening the door and moving the plywood and plastic so it would form properly. I used leather gloves to handle the plywood buck and later, undersized nitrile gloves (no wrinkles) to do the final shaping outside the oven with the aid of my heat gun and a spackling spatula with a metal blade. If you do a longer chainguard, oven size might be a limitation.
3) I first drew what I wanted in profile using heavy cardstock and then trimmed it until I achieved the desired shape and transferred that to the plywood later. This pattern let me check clearances as well as serving as a final pattern to cut away the excess plastic -- and there was a
lot of excess. It is necessary in order to achieve a good "drape" of the softened plastic over the plywood form. The final chainguard is about 1/3 of the total plastic I shaped over the mold. My initial pattern was a large rectangle of plastic from which I later carved a chainguard using my cardstock pattern as a template. I drilled some small holes so I could secure the rectangle of plastic sheet to the edge of the plywood mold; this kept it from shifting while the sides softened and draped; the nail holes became my oiling ports. A Dremel with a fresh/sharp spiral cutting bit set to medium speed (to prevent melting) did a nice job on the trimming. I polished the cut edge after as I did the whole chainguard. I used Blue Magic polish on a cotton buffing wheel set to low speed to avoid melting the plastic.
4) While it is easy to warp/wrap the softened plastic over the vertical top edge of the plywood mold, it is much more difficult to make the front wraparound for the chainring and this is why I stopped at 4 o'clock instead of 6 o'clock. The greater the curvature, the greater the distortion as you are working with three sides there. The plastic wants to wrinkle where it goes around the bend and more wrap can makes it worse. I used a dual-range heat gun to ease the plastic around and this helped a lot; a spackling spatula with a metal blade helps smooth/press/shape any distorting plastic against the plywood buck.
I too have a tandem and was mightily tempted to also make my first project a timing chain cover. I may yet, but there would need to be chainring curves at each end and a simple chainguard seemed the best first project. Also, making mounts would be more challenging and the longer distance could make it a challenge to fit in the oven unless it was made in two pieces. My tandem has a front eccentric to tension the timing chain so that would need to be taken into account as well.
5) I'm running a 1/8in chain on my Fixie, so it is 1/32in wider than the usual derailleur chain. I would say I have about ~5mm or roughly 3/16in between the chain pins and the inner side of the chainguard. Remember, the chainguard will never be narrower than your plywood buck, so it is best to choose the needed thickness when you make your mold. You will need to check the clearance between the crankset spider/chainring and the back side of the crankarm so you can avoid rubbing the chainguard as you pedal.
6) The mounts must be solid to impart rigidity as the chainguard which won't sag but can twist. As noted, I tied the steel rear mount into my dropout mudguard stay adapter so it is rigid but the front mount took more care. I used a glass-filled nylon reflector mount and used a Dremel to mill a small slot into it, sized to accept an M5 nut and drilled a lateral 5mm hole to accept a screw. I left a little flange on the inside of the chainguard and tapped it to M5x0.8 to accept an allenhead machine screw started from the inside. I used kneaded beeswax on the screw threads to hold the nut in position until it could be clamped in the bracket; in this way, I could adjust the offset and center the chainguard over the chainring simply by adjusting how far the captive nut screwed onto the screw. Mine was spot-on because I calculated the offset needed when I modified the bracket but thin washers could be used as spacers easily enough.
7) Yes, open the windows during the operation. I also wore my brazing mask fitted with charcoal cannister filters so I didn't smell the plastic until I removed it.
8 ) I sourced my plastic locally, but this link will give you a general idea as to thicknesses and availability:
https://www.acmeplastics.com/cut-to-size-clear-polycarbonate-sheet9) As for mudflaps, I prefer the pre-made BuddyFlaps in "Regular" size from Portland, Oregon. They are a really nice size and I often use the longer "rear" flap on the front so the spray zone is kept well below the chain. They are cut from thick virgin vinyl and have enough mass to avoid being blown back at speed. See:
https://buddyflaps.com/I hope this answers your questions, Mike.
Best,
Dan.
