motorapido

What is the effect of changing rim width on tire diameter? Specifically, if I mount the same tire on a narrower rim and on a wider rim, will the tire diameter be wider on the wider rim or narrower on the wider rim? Something tells me there is an inverse relationship between rim width and tire diameter. I am thinking that if the bead of the tire is squeezed more tightly together in a narrower rim, it will make up for that by bulging more at its widest point in diameter than it would if it were mounted in a wider rim. Am I onto something? And can you tell I am worried that when my new, wider rims arrive, I will no longer be able to fit my beloved 38mm tire between the chainstays?

FBinNY 

because of the nature of internal pressure forces, inflated tires try to assume a circular cross section. There are ways to counter this, which is why they can make low profile oval section car tires, but it requires specific engineering features like belts.

So if the tire will end up trying for a round cross section, the key factor becomes the circumference. On a clinch tire, the rim is part of that with the circumference being equal to the sum of the flat bead to bead width + the rim width (less a bit lost to the overlap from bead to rim edge). So increasing rim width will increase section by an amount approximating the difference divided by 3 (Pi). Since all of the added width will be to the outside, the overall diameter will increase by 2x rim width/3.

These are rough approximations, because there will be some distortion, but the trend is there.

motorapido

FBinNY -- thanks for that helpful reply. Follow-up question: I surmise that the contact patch of the tire will also increase when the tire is mounted on the wider rim. If that is the case in my specific instance, if I can no longer fit a 38mm tire, and instead must fit a 35mm tire, if my assumption that the contact patch increases with rim width, would the resulting feel of the 35mm tire on that wider rim be close to the feel of a 38mm tire? I ride a lot on gravel bike paths and have a return trip planned for the Great Allegheny Passage, which includes the often rough and muddy C&O Canal towpath, and I sized up from a 35mm tire to a 38mm for my last trip and really like the improved ride comfort and enhanced grip in the mushy stuff.

FBinNY 

Again speaking in rough generalities and approximates, the size of the contact patch is always such that the area X the tire pressure = the axle load. This is true because you don't ride on the rubber, but the air within. It isn't precise, because there's some rigidity in tires which can spread load beyond the exact contact area, but we can keep it simple for discussion purposes.

So for any rider and pressure, the contact patch area is fixed. What changes is the shape. Narrower tires produce narrower patches, which therefore need be longer, and wider tires have contact patches that are rounder. You can imagine taking a knife ans slicing off a flat section of the tire having the right area to see what I mean.

Since the tire is round, a longer patch means greater distortion, which is why (speaking strictly in terms of tire/pavement friction) fatter tires can roll easier, and why auto/truck tires have gotten wider over the years (better gas mileage) and are proportioned to the vehicle weight.

Please understand, that I'm not saying, as some do, that fatter is better, just that it may be, and there's a sweet spot, with tires either thinner or fatter having more friction.