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Old 07-01-18, 09:33 AM
  #19  
fietsbob 
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.... but commonly steel & other, tube is imperial diameters so bike frames express 1.125" as 28.6mm,
1.25" is 31.75 and so rounding up, said as 31.8..
And as you should have memorized; as a conversion factor 1 inch is 25.4mm

Example: Brompton used imperial tube for their seat posts , plastic bushing in frame ..

many seat posts are a result of frame tube wall thickness differences
resulting in various inside diameters ...

Math..


...
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