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Moving to one-by drivetrains on MTBs (which enable lavishly wide tyres of 2½″ to 3″) was motivated by the simplicity of shifting on technical terrain.

But when gravel bikes were introduced, a common refrain was that switching to a one-by drivetrain was necessary to accommodate wider tyres.

Yet at this time some newer endurance road bikes offer clearance for tyres up to 38 mm.

What happened in these few years to make an inner chainring on a two-by drivetrain compatible with a wider rear tyre?

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  • What? Who said "But when gravel bikes were introduced, a common refrain was that switching to a one-by drivetrain was necessary to accommodate wider tyres."? You can buy gravel bikes both as 1x or as 2x, and you always could. With wider tyres than those 38 mm. Just choose which one you want. I am not ever of any incompatibility at any time. Whot is your source for this "common refrain"? Of course, the cassette size and range matters. Dec 20, 2023 at 5:50
  • An indirect datapoint: GRX chainlines were increased by 2.5mm, tire clearance is 45mm (in 2x, with "knobby" tires, with usual Shimano reserve). SRAM gravel 2x has also a 2.5mm increase in chainline compared to 1x, and the same 45mm tire clearance limit. So it's not unreasonable to assume that 40mm is still OK with standard chainlines and slick tires.
    – Rеnаud
    Dec 20, 2023 at 6:58
  • @VladimirFГероямслава gravel is a bit too heterogeneous as a category to make generalizations. In the current ranges, the 2x are now typically the race ones (with thinner tires) and the 1x with the widest tires are using boost MTB wheels and cranks (and for the latest iterations: MTB cassettes as well, if not derailleurs).
    – Rеnаud
    Dec 20, 2023 at 7:03
  • @Renaud But then you are getting into the solid fork MTB with drop handlebars territory. I was really thinking of typical drivetrains like GRX which are available both as 1x and 2x. Dec 20, 2023 at 7:20
  • 1x was motivated by designers wanting more space around the BB area on MTB squishes. They sold it's simplicity to shifting and ignored the lost range and big jumps between gears. The only people who could ride early 1x on technical terrain were the very few who could mush 32-36 at 40rpm, and spin 32/11 at 140 to get places fast. Also suited down hillers who (generally) do not need much range. The wider tire is a red hearing - MTB in the 1980's ran 1.75" (45mm) on 3x. Frame/BB and hub width determine tire width, not number of gears.
    – mattnz
    Dec 20, 2023 at 8:05

1 Answer 1

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I don't have hard data to prove it, but I would not be surprised the tire clearance limit of a 45mm chainline (standard road chainline) is around 40mm, so in other words the chainline is not yet limiting the adoption of wider tires on road bikes, and nothing needed to be changed.

On road bikes, brakes (and performance considerations) were more limiting than the transmission when it comes to tire clearance, until UCI allowed disc brakes.

To look at what has been done on gravel, where there is a clear demand for tires wider than 40mm, GRX chainline was increased by 2.5mm, to allow tires up to 45mm (with knobs and Shimano's reserve — they didn't propose a wider chainline in their 12-speed revision, byt the way). On SRAM side, chainlines are 45mm for 1x and 47.5mm for 2x (and 73mm MTB BB shells). It's then not unreasonable to assume that 40mm slick tires would still be fine with the standard road transmissions.

Otherwise, 2x transmissions exist for modern MTB groupsets, but are not widespread (only Shimano). The principle remains the same: if you increase tire size, you need to increase the chainline, which is itself a function of the crankset and rear hub designs. MTBs with 135mm QR hubs have a chainline of 49mm (not 100% sure, but I think it limits the tire clearance to 55mm), then Boost arrived and it moved to 52mm (with wider hubs, on the crank side, it can be through the cranks or by adding an offset to the chainring).

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  • I would point out, that my gravel frame was originally equipped with a road groupset. Either Claris or 105 depending on the exact model. There is no problem using 40 mm tyres in it, there is still some clearance for even bigger. So far I used 38C G-One Allround in the back and I am going to put there Tufo Thundero 40C (already present at the front). Dec 20, 2023 at 12:21
  • This is another example, different from mine. Shimano 105 and Speedero 40C tyres dust-bike.cz/dust-gravel-shimano-105-2x11 This one uses 105 Di2 and promises tyres up to 700cx44C or 650bx47C orrobikes.com/product/terra-ti-105-di2-bike Put it shortly, I think the whole premise of the question is moot. Dec 20, 2023 at 12:26
  • @VladimirFГероямслава There are also many examples of bikes with GRX 2x and clearances greater than 45mm (Specialized Diverge 47mm, Canyon Grizl, 50mm). Hence my "Shimano's reserve".
    – Rеnаud
    Dec 20, 2023 at 12:35
  • I think that once you get to 50mm tires or so, that's when you start to need 1x.
    – Weiwen Ng
    Dec 20, 2023 at 22:59

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