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Orbea Avant disc. This bike is billed as all purpose but I’ve done lots of gravel road races. However, the max tire width is 30. I would like to get to 38 or 40. Should I switch to 650b to do this? Thought of new fork but not sure the rear tire can go an wider either.
 

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Those frames are already made for wider tires.
Those bikes that can use both, have space for something like a 700c x42, and 650b x 47.
The 700c wheel and tire is a little taller than the 650b wheel/ tire. So technically there is space between the chainstays for a larger 700c tire, but clearence at the seat tube becomes a issue. By using a smaller circumference wheel you have space between the chainstays and the seat tube.
 

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The chainstays are almost always the limiting factor, it is easy to measure your frame to get an idea of how wide you can go. The widest point on my 27.5/650B wheelset is about ~12 3/4" to 13" from the center of the hub. Measure the width between your chainstays at the point, and subtract ~8-12mm for mud clearance (~4-6mm on each side) and that is how wide of a tire you could "safely" fit.

My old bike would fit a 700 x 38mm tire with about 8mm of total clearance (4 mm on each side of the tire), I could fit a 650B x 48mm in the frame, but there was barely any clearance, and if the wheel were to go out of true I would be walking home. My new bike is designed for 700x45mm tires, but will also fit a 27.5 x 2.1" tire with plenty of clearance.

In my opinion, going from 700x30 to 650Bx38 or even 40 would hardly be worth it. I don't think you would notice much of a difference, plus your bottom bracket would also be much lower...
 

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The best way to see if you can fit a wider tire in there is to borrow a wheel and try. The difference between 30mm and 38mm tires for me is the difference between being able to ride off-road, and not (at least on the gravel conditions I have to deal with).

Supple wide tires, run at low pressure, make a huge difference. That 8mm isn't just width. It is the air volume difference that is really what is most important.

cf: https://www.renehersecycles.com/wide_tire/
 

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Depending on the frame you have, it might be actually cheaper to buy a new frame with the clearance you want.

But! If that's not feasible, then going 650b is definitely an option. The difference between 30 and 38 is huge!
 

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Definitely frame dependent. I have a 2005 Redline Conquest Disc-R that came spec'd with 700x26 and will take a max 32/33. In my case limiting factors are the CS and ST bridges. The space between the stays allow for 650x47 on a Stans Arch Mk3 and 26x2.3 Compass Rattrapass on Mavic 517 (52mm wide)! I believe this is because the Conquest Disc-R was essentially a modified Conquest cyclocross bike making it a road bike. It'll be my road plus rig as I don't plan on ever running a 700c wheel on this build.

Pics of the RTP clearance...

Metal Iron World Steel Composite material

Shoe White Black Grey Synthetic rubber

Bicycle part Grey Steel Bicycle Synthetic rubber

Blue Brown Yellow Bicycle accessory Photograph
 
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