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Old 11-26-21, 09:02 PM
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dddd
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Originally Posted by Panurgist View Post
I have a set of the Swift Tires ( black wall ). They measure 30mm at 45psi on VERY narrow rims. That works out to just a bit under 1 1/4" . The O.D. on these rims is 20mm sooo maybe 18mm I.D. ? The cheap 1 1/4 tires I took off were measuring around 27mm. I'm sure they would be plumper on normal rims.
I am very happy with them. Smooth, fast, and enough tread to last a while. Definite step up from the Chin Shens they replaced.
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.

Last edited by dddd; 11-26-21 at 09:07 PM.
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