So, next item in the Gravel's drivetrain assembly... the cassette.
Might seems odd at first to dwell on a set of cogs as it's quite straightforward - especially when it's related to a modern cassette, on a modern hub.
Indeed from a gear range and technical assembly there is no issue. No need to look for ages for that cog with the specific tooth count that is needed to bridge the gap between two cogs, and to make sure it has the right diameter to fit onto the freewheel at the specific position, with the right notch pattern or threads, etc... I guess whoever has ever worked on a Maillard freewheel will understand.
However on a project such as the Gravster, there is a different issue.
The target drivetrain is a 10-speed one and the frame was initially designed for a 5-speed one.
One might think that since the rear dropout spacing has been spread from 120mm to 130mm everything should be sorted... Well not exactly.
It is true indeed that the rear wheel can now slide without any effort into the dropout.
But unfortunately increasing the rear dropout spacing is not enough to allow a wider set of cogs to be used correctly. The rear stays - and especially the seat stay, kept their original shape near the hub axle. And in those days the part of the seatstays near axle were relatively straight if not slightly curved inside the space between the rear dropouts - that is closer to the spokes... and the cogs for the DS seatstay.
As a result, the first cog will be very - as in very very very, close to the bottom of the right seatstay. Which means that the chain is very likely to rub against that part of the right seatstay when it is on the first position cog.
Unless you take a few measures...
1. Use the smallest cog available - that's an 11t cog on a 10-speed cassette. That way the first position cog's radius will be minimal and will increase the distance between the top of the teeth and the seatstay.
It might seems a bit cumbersome or even too much of a constraint to use an 11t first positon cog - especially if one is already using a relatively high tooth count chainset, but that's extremely efficient - probably the best and easiest way to prevent the chain from rubbing against the seatstay.
2. Insert a 1mm washer at the end of the hub axle - on the Ds obviously. This will move the dropout - and the attached seatstay further away from the first position cog.
It might seem objectionable to proceed so and generate an off centered wheel but that 1mm imbalance still falls within the tolerance limitations. Not ideal but still OK.
3- use a chain as thin as possible to gain a few tenth of millimeter. Using an 11-speed chain will provide an additional 0.4mm clearance compared to a 10-speed chain.
None of these solutions is a silver bullet for chain rubbing against the DS seatstay. Nor are they fully satisfactory form a gear range usability, or structural or mechanical standpoints.
But the combination of these three simple measures will provide an easy option to run a 10-speed drivetrain on a frame designed and built with a 120mm rear dropout spacing.