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Old 11-27-21, 10:09 AM
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Join Date: Sep 2011
Location: Richmond, Va
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Bikes: Rivendell Sam Hillborne, Peugeot UO8, Trek 820, Motobecane Grand Record + projects

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Originally Posted by dddd View Post
There's going to be about 6-7mm difference between the outer and inner width dimensions, so your 20mm wide rims will probably measure i13mm, which is very near the lower limit of what is possible to mount tires on and have the tube and valve stem fill the space between the rim sidewalls and tire beads.

Back in the 70's, it wasn't possible to make a wider rim without some serious weight gain. Later design development allowed deeper and wider rims extruded with much thinner walls and of stronger alloys like Maxtal, and which defines today's state-of-the-art alloy rims.

30mm (as mounted) tires on such narrow rims invites the possibility of the tube suffering stretching stress concentration as the pressure tries to push the tube down between the tire beads after the tube has already firmed up and started to pin the tube against the inside of the tire. The rubber reinforcing pad surrounding the valve stem, in particular, may not fully descend to the floor of the rim and so may cause tube wall ruptures to either side of said rubber reinforcement (look for tell-tale stretch marks to either side of the rubber valve stem reinforcement).
It is helpful with such combinations of wide tires on narrow rims to tug on the valve stem as the tire is being inflated past 5psi, to prevent such tube ruptures.
It is also helpful to use only tubes having generous width relative to the tire width.
Thanks, good info.
I'll un-mount the tires and check it out. They are presta valves ( there isn't anywhere near enough room for schrader )
I bought this bike as a frame and fork project ( a Raleigh Super Course ) and I wanted aluminum rims for better braking, these were all I could find locally at the time.
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