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a 23mm tire@15c on a 23mm wide rim becomes ((23-15)/2=+4mm) = 29mm
23+4=27
Using your formula:
25+((21-15)/2)=28, but really my 25C GP4000S comes up to 30mm on a 21mm internal Mavic Crossride25+((16.5-15)/2)=25.75, but really my 25C GP4000S comes up to 26.5mm on a 16.5mm internal Mavic MY2016+ Aksium
If I were trying to guess at a rule of thumb for estimating the width of a tyre on a rim bigger than the one on which it manifests its nominal width, I'd divide the difference by π/2 (about 1.6) rather than dividing by 2.
25+(2(21-15)/π)=29
25+(2(16.5-15)/π)=26Because the extra width is actually a chord rather than an arc, this formula will always come up with an answer smaller than the correct result, but it will be closer than your version 🙂
gbj_tester-
Are tyre widths as they are sold usually meant to be correct on a rim with 15mm internal width?
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yes thanks I was trying to make it simple :)
also it was meant for measured width on a given rim. 25c gp4000s count as 26mm or 27mm tires on a 15mm rim -
Maths thread >>>
So, assuming I'm running 25mm+ all the time, can you think of any disadvantage to running one of these sillywide™ Kinlin rims on the back of my ultra bike?
Only thing I can think of is small weight penalty and I'm already a big weight penalty.
SwissChap
svenhöek
hippy
usually tire size on wide rims is: size on a 15mm rim + half the difference between rims:
a measured 23mm tire@15c on a 23mm wide rim becomes ((23-15)/2=+4mm) = 27mm...
edit: tldr: tire size changes with half the rim width difference