Inspired by <link>, I decided a more subtle solution was necessary. So, take regular mountain bike tyre. Give it a ~10mm central section which looks a bit like a road bike tyre. Then, build into the main carcass of the tyre a separate inflatable chamber below this. Then, when changing surfaces you can pump this section up so that it becomes prominent... offering low rolling resistance.
Getting air in and out of the little in-tyre chamber might be difficult. Perhaps one of those football style valves. Or a long valve on the inner surface of the tyre and a special inner tube which allows it to pass through to the rim.-- bs0u0155, Aug 19 2013 Previous HB version Street-to-Mountain_20Tire_20Conversion [bs0u0155, Aug 20 2013] No link, which I'm guessing was supposed to be a hybrid (smooth center, knobby edges) tire, which don't work all that well because they screw up cornering.
With regards to the idea, I think you'd find a problem that your central chamber can't be made tough enough to work for any distance as a road tire while still being flexible enough to sit flush with the tire surface/below the knobs when deflated.
Remember, tires don't typically actually inflate as such, they're supported by the tube which does.-- MechE, Aug 19 2013 Meh, tubeless tyres don't have tubes - and they use the same tyre carcass as well, just a modified (sealed) rim.
I think your problem will be the resultant shape of the tyre when the central bit is inflated. you might get the middle prominent enough to be able to ride in a straight line (maybe) - but cornering would be a real bitch with the rest of the tyre still there on the sides.
Secondly, you'd need some funky rim geometry supporting this central portion, otherwise cornering loads would roll the tyre off the rim.-- Custardguts, Aug 19 2013 I ignored tubeless because they aren't relevant to the point. The tire shape can be deformed slightly, but the general shape is defined at molding, not by inflation.
The best version of this I can think of is essentially a hybrid that has the knobs very high up the wall, and essentially four tubes inside. The innermost tube is always inflated, the outermost tube inflates for road use, and two flanking tubes inflate for dirt use.
That might allow the tire to shift from a very tall narrow profile to a wide shallow tire with knobs engaged. Cornering is going to be extremely poor in dirt mode, and extremely squirmy in road mode-- MechE, Aug 19 2013 ...So we can agree that this wouldn't work, right?
Just carry around a second set of tyres, if you must, and change them out as required.-- Custardguts, Aug 20 2013 I like the idea of dual tires. If a tire could be made that is a mountain bike tire at 60psi, but a road tire once pumped to 120 psi that would be nice. But also there could be two valves on the tire. one valve would be for the sidewalls and another for the circumference. The idea is to form one type of tire or the other by pumping.-- rcarty, Aug 20 2013 "Warning - for drawing-board use only"
(btw - did I miss the promised inspirational <link>, or did [bs0u####] take a spill before getting there?)-- lurch, Aug 20 2013 Right, link up. Phones, no matter how smart, are crap at real internet.
I've ridden tires with a prominent central ridge before, but the low pressure led to most of the advantage being lost in deformation at the bottom.
This idea uses the extra chamber, which is OUTSIDE of the main nylon carcass to a: make the central section a little more prominent, and b: to form a high pressure (relatively) rigid ring around the main tire. This will reduce deformation, which I believe is one of the major reasons for inefficiency in larger, low pressure tires.
The sum total is a smaller contact patch and low deformation. This should reduce rolling resistance.
I'd be quite interested as to what effect it would have on comfort. The two pressures might be complementary in terms of different resonant frequencies and so on. I'm not sure if tires work like that, but hey.-- bs0u0155, Aug 20 2013 The problem is that that high ridge is going to have to inflate and deflate. Which means it is going to have to either be softer rubber with no fabric under- layer, or it will have to have a loose layer of material when un-inflated. In the former case it won't last more than a few feet before flatting, and in the latter it will squirm like crazy and cause slide-outs when cornering in dirt mode.-- MechE, Aug 20 2013 The secondary, peripheral lumen will only be about 4-5mm diameter, when inflated, it will be at very high pressure. It will cause deflection outward and to a lesser extent inward (the relatively inelastic carcass and primary inner tube behind it will ensure this). When deflated, some of the volume will be taken up by the previously deflected primary inner.
I think we'll only be dealing with a couple of mm of (circumfrentially continuous, for extra stability) rubber. Compared to the 5mm cube blocks on my old mountain bike tires, I think this will be relatively stable.-- bs0u0155, Aug 20 2013 random, halfbakery