The minimum tyre width will be less than 40mm, but I couldn't say by how much.

Maybe buy a cheap 35mm tyre and see how it goes?

Go to MTBR.com and do a thread search there. you will be surprised how much info you can get there about ... MT's

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Tire pressure will be an essential part of the equation and will also likely create a mis-match between official recommendations and safety guidelines. The tire pressure is actually what is keeping the tire attached to the wheel so at high pressures you can safety get away with narrower tires on a wider rim. However the whole point of getting wide tires is the ability to ride them at low pressures so mountain bike tires and wide road/CX tires are not designed for very high pressures. In any case I would guess their are limited speed gains to be made in terms of downsizing the tire width at the scales you are talking about as long as you inflate them to a very high level. On my CX bike I use tires ranging from 25-40mm depending on conditions. At~80 psi good quality 35-400 mm are not miles all weather commuter 25-28s i.e. hard-shell gatorskins.

Thanks heaps guys. Appreciate

Bunsrltw

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bikemessengersrepresent

Considering a fully rigid 650b+, but also want to attack the city with it.
I can find up to 2.35 slicks, but would like to know how narrow I can safely go. I realise it will change the profile of the tyre and therefore, the ride characteristics.

If you want the bike to do double duty, I'd suggest another set of wheels to make swapping easy. And you can use 700c road wheels and tires in your frame. It gives you loads of tire options.

If you'd rather stick with one set of wheels then use slicks that are at least 40mm wide.

https://www.compasscycle.com/...0bx42-babyshoe-pass/

Great minds think alike! I'm looking at the Pine Mountain as well. I was going to put slicks on the stock wheels and get a nicer set of wheels built up for when I go off road and for racing Xterra next year. For slicks, I've found these: http://www.schwalbetires.com/...d_tires/super-moto-x they're 2.4"/61mm wide and while they aren't true "slicks" they look like a perfect city tire. I'm also looking into getting a cheap 29er 10mm suspension fork as they are mandatory for Xterra.

Edit: as for "how narrow can I go safely" I looked at some ETRO charts and technically 2.4" is just outside of their recommended range for a 40mm rim. I know ETRO is conservative but, that said, I personally wouldn't go narrower than 2.4".

Some Pine Mountain goodness:

I'm with rruff. A second set of 700c wheels with a similar outside diameter would make your life very easy. If you were going to use slicks, I would think you want to stay over 40mm. I've never seen a company recommend tires narrower than internal rim width that I can think of.