The move toward wider wheels continues and there are now examples of 26 and 28mm internal width wheels with outer widths of around 34mm popping up. These are being marketed as suitable for road as well as gravel. in principle something like a 32mm gp5k str would come up as a pretty smooth transition to the rim on these wheels
Yet if you look at continental’s chart it says a 26mm internal requires at least a 35mm tyre while a 28mm internal needs at least a 47mm tyre (which is obviously pure gravel, not road) territory. What’s going on here, wheel manufacturers being dangerous or conti/etrto being ultra conservative/outdated?
They are sharing a standard that is known to work and ensures that your tyre and wheel will play nicely.
Inspection of the chart and basic physics suggests that it’s conservative in some areas, for instance look at the difference in allowable tyre sizes between 15mm and 17mm rim width. So you could push the envelope and be fine, but the question is whether it’s worthwhile e.g. pro teams crashing after using 28mm tyres on too wide road rims.
There’s also the fact that using a reasonably wide rim for a given tyre gives a better tyre profile for aero and lower rolling resistance, so choosing a rim on the wider side usually makes sense.
I get the impression that some of the high profile tyre blow-off incidents in the past couple of years were likely caused (or at least contributed to) by teams/manufacturers pushing wheels with rims too wide for the tyres (e.g. 28mm tyres on 25mm internal rims). (Hookless didn’t help of course)
But equally it seems evident that there must be some conservatism in those tables, given some of the big jumps and groupings. E.g. if a 35mm tyre is fine to use on a 27mm rim, it seems implausible that if you increase the rim width by 1mm to 28mm, you need to go all the way up to a 47mm tyre for it to work.
Part of the issue is that ETRTO doesn’t differentiate between hooked and hookless wheels for tire width. The rule of thumb that I’ve heard is that for hooked wheels the tire needs to be 3mm wider than the interior width, and 5mm for hookless.
I also think it’s telling that World Cup xco setups have mostly stabilized around a 29 x 2.4 tire with 30 ish mm internal rim… yet with gravel there are so many questions about what rims even wider internally than that.
