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My bike is early 2000s and came with Shimano wh-r540 wheelset and 26mm tires

googling shows ETRTO: 622 x 13C (internal width?) and 19mm (external width?)

I've successfully used 25mm tires but considering going to 28mm since I believe there is clearance everywhere important for a tire that wide

However I am questioning if the rim is too narrow to properly support a 28mm and that it will "balloon" defeating the benefits of 28mm

But I've noticed new bikes that come with 28mm typically have a very wide rim to support the tire so the tire sits more square or inwards and not ballon out

ie. modern rim + 28mm tire profile = /----\

ie. old 540 rim + 28mm tire profile = \----/

does it matter? will it defeat the benefits of 28mm for handling?

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    I do not know what the 'x 13C' part of the spec means, but the 19mm spec is the internal rim width (which is the relevant width for tires, because they fit inside the rim). The Shimano datasheet can be found here: bike.shimano.com/media/techdocs/content/cycle/SI/Wheel/ROAD/… – Argenti Apparatus Jul 25 '17 at 17:27
  • @ArgentiApparatus That's incorrect, the 18.8mm largura do aro (rim width) measurement on that link is referring to the outside dimension. 13mm is the inside. People like sticking the C on there because of confusion about what it actually indicates. – Nathan Knutson Jul 25 '17 at 19:52
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    I mean to paste in a link the sheet in english: fish.shimano-eu.com/media/techdocs/content/cycle/SI/Wheel/ROAD/… – Argenti Apparatus Jul 25 '17 at 20:04
  • The difference between 26mm and 28mm tires is inconsequential, unless the existing tire is already on the hairy edge of fitting the rim. – Daniel R Hicks Jul 26 '17 at 2:49
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    @ArgentiApparatus thank you for that PDF, it actually shows Shimano themselves say the acceptable tire range for the WH-R540 is 700C (19-28C) at the bottom, so I assume 28mm is indeed the maximum acceptable – Crazie Daizee Jul 26 '17 at 21:44
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Wider road rims have become more popular, in part because of the reasons you're talking about, but 28 is far from the reasonable maximum for a more classic 19-20mm road rim, and will be totally fine. Consider how many people race cross on 19-20mm clinchers with 32s and 35s. Where the actual line is of what's reasonable is a hotly debated topic that there are other questions about.

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Thanks to @ArgentiApparatus posting the technical paper from Shimano about the WH-R540 which clearly states that 28C is the maximum recommended width for best performance

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