hi all
I have a dilemma. I have a frame which is capable (apparently) of taking 650b 2.1 and 700x40c tyres.
the 650s are no problem however the 700x40c comes very close to the seat tube less than 5mm clearance.
The 40c was fitted to a 19mm internal width rim. Id like to keep as much volume as possible so I’m reluctant to simply fit a 32c. Id like the best balance between clearance and volume.
My question is if I fit the 40c to a 30mm internal with rim will it reduce the circumference of the tyre sufficiently to increase the clearance between it and the seat tube?
I’m guessing it probably won’t so my second question would be how much narrower in tyre width would I need to go before a wider rim would significantly reduce the circumference? I do appreciate that a reduction in width will have an effect on the circumference anyway.

Increasing the width will make the tyre taller, not shorter, to a certain point anyway, before it starts to go the other way – usually around the width of the tyre.

ok thanks ben

What 700x40c are you trying to fit? Maybe the answer is to fit a different model tyre on the rear, their claimed width doesn’t always match reality. 😉

Isn’t 5mm enough?

I don’t see how a wider rim will reduce tyre outer diameter.

Within any sort of reasonable range of widths, the effect will be tiny.
I would guess that the chainstay width is the limiting factor for 650 but height is for 700.

I fitted a nano and it was less than 5mm clear of the seat tube.
The width with a 650b is fine it’s just the height of the 700c that is the problem.
I don’t expect a huge difference by going for a wider rim but after further investigation it seems a nano would probably be even taller.
I’ve got hold of a 35 c which apparently measures closer to 38 so I’ll give that a go.

What’s wrong with 5 mm clearance? Put a bit of heli tape on the seat tube to protect it and be happy with your 4.5 mm of clearance 🙂