csacwp
I am ordering a Ti  frame and want to try and ensure my bike is future-proof... How big of tire clearance would I want for gravel and dirt riding with some deeper gravel and mud? I don't do technical MTB trails bit would enjoy riding some simple single track now and then. I ride on road as well as off but don't race (I prefer long rides). Would 45c large enough? 47? 50?
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HollyBoni
650b or 700c? 

If I had the option to choose, realistically with 700c i'd probably say 45 with generous clearance, and space for fenders. Right now generally speaking most 700c gravel tires tend to end at about 45mm, and MTB tyres tend to start at 2.1, but more like 2.2. Before someone starts posting 2 inch tyres, I know there are a few exceptions, but you will find the most options in the 35-45 and 2.1+ range. Also, unless you go with MTB geometry, with a big 700c tyre you're just increasing toe overlap which can be pretty darn annoying and sometimes even a bit dangerous. I'm not sure what kind of frame it is, but based on my limited knowledge I don't think it's too easy to make a frame fit 700x50mm+ tyres with a 68mm BB unless you give the frame really long chainstays. 

With 650b i'd make sure there is enough space for a 50mm with room for fenders, but looking at current frames on the market it doesn't seem that hard to make a frame clear a big 650 while keeping the 68mm BB (Surly Midnight Special, Fairlight Secan etc.)

I think you can do a lot with a 45mm tyre. In my experience after a while it's the rigid fork that will hold you back not the tyre size anyways (and i'm talking about comfort and enjoying the ride, not speed).
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GHC
IMO .... a frame that’ll take up to 45’s with clearance will get you a long long long long way down the trail!  

Can run narrower tires when you dont need them, and 45’s when you do.

tubeless!

(my gravel bike max’s at 38 and the only time i wish it had more is soft sand.......that said, i carry around deep sticky mud)
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epzamora
47 increasingly appears to be a "standard" in 650b, judging by news of new releases. 

eric/fresno, ca.
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NoCoGreg
+1 for what HollyBoni said.  If you're seriously thinking you need more than 700x45 you might want to start looking at something like the Fargo Ti (ie. drop bar mtb).  As the tire gets taller the trail (steering/handling) changes.  Similarly if the bike is optimized around a 700x60 (29er) then the handling might not be what you want if you go to a 45 or 43.  
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imwjl
NoCoGreg wrote:
+1 for what HollyBoni said.  If you're seriously thinking you need more than 700x45 you might want to start looking at something like the Fargo Ti (ie. drop bar mtb).  As the tire gets taller the trail (steering/handling) changes.  Similarly if the bike is optimized around a 700x60 (29er) then the handling might not be what you want if you go to a 45 or 43.  


We have a Fargo with fast 2.2 MTB tires and love it, but our Sutra Ltd. is fine with 50 mm tires it came with. I've tried them both with smaller tires. Maybe it's not possible for the OP here to try before you buy but that would be most important for me. Delaying gratification and trying bikes has made the purchase decision easy and the bikes have been satisfying that way.
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Scree
The Otso Waheela C appears to provide a lot of versatility without mt bike geometry.  It has 50mm+ tire clearance with gravel bike geometry.  I have a cutthroat and love it, but I would like to have something a little better suited for road riding at times.   
I usually run Gravelking 43's and Rambler 45's on my cutthroat for gravel, but sometimes run a 2.2 on the front for some rides.   
The Otso might be in my future?
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