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I know this depends a lot on the model and the shape of the stays and fork, but on the average CX disc frame would it be possible to use 650b wheels with thicker tires, for a more comfortable offroad (gravel) experience?

  • It's generally best to use the size of wheel the bike was designed for and add thicker tyres to those. Play with tyre pressures to find a comfortable ride. – KeithWM Jul 24 '17 at 10:55
  • CX usually have a lot of top clearance for mud. You are only going to pick up like 1 - 2 mm of side clearance. You are going to lose ground clearance and change the geometry. Not something I recommend. – paparazzo Jul 24 '17 at 13:05
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    @KeithWM the trick is that 650b plus a wider tire gives roughly the same outer diameter as 700c with a narrower tire. The geometry is maintained and you get a more comfortable ride. You just need the clearance, and brakes to work. You also need a wide 650b tire. A 47mm 650b tire will be the same outer diameter as a 700x28c combo. – Rider_X Jul 24 '17 at 14:27
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    @KeithWM - This used to be a common conversion 60 years ago. It is gaining popularity again as disc brakes and more frame clearance again make this an easy conversion and there are again wheel and tire options available in this size. It will be (or already is) the next "bit thing" for gravel/adventure riding over the coming years. (And yest a few brave souls have been doing this long before it became mainstream again). – Rider_X Jul 24 '17 at 17:08
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Wheel size swaps in bicycles are generally not possible, but this particular rim and tire size swap is possible with disc brake equipped bikes, if you have the lateral tire clearance.

700c rims are 622mm in diameter at the bead seat. 650b rims are 584mm. The difference in radius - what has to be made up by a bigger tire - is 19mm. Assuming tire width is approximately tire height, going from a 28mm tire to a 42mm tire makes up most of that difference (42mm-28mm=14mm).

As an aside, changing rim diameter without a change in tire size is not recommended. Frames are designed around a certain wheel size and fitting a smaller rim size wheel negatively effects steering geometry and crank-ground clearance.

See this Sheldon Brown article.

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    As long as the outer diameter is comparable (e.g., 650bx47mm will have the same outer diameter as 700cx28mm) your geometry is maintained. The rotational inertia will also be similar, further normalizing the handling. A wide tire on 700c (e.g., 700x42) will actually slow down the handling considerably compared to a narrower tire (pneumatic trail and rotational inertia). The reason these types of conversions are rare is braking and wheel availability. Disc brakes solve this issue and 650b wheels are again becoming plentiful. Be prepared for 650x47 to be the next "thing" in the mags for gravel. – Rider_X Jul 24 '17 at 17:01

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