h a l f b a k e r yWhat was the question again?
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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.
Previous HB version
Street-to-Mountain_20Tire_20Conversion
[bs0u0155, Aug 20 2013]
[link]
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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. |
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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. |
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Remember, tires don't typically actually inflate as
such, they're supported by the tube which does. |
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Meh, tubeless tyres don't have tubes - and they use the same tyre carcass as well, just a modified (sealed) rim. |
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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. |
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Secondly, you'd need some funky rim geometry supporting this central portion, otherwise cornering loads would roll the tyre off the rim. |
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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. |
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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. |
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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 |
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...So we can agree that this wouldn't work, right? |
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Just carry around a second set of tyres, if you must, and change them out as required. |
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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. |
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"Warning - for drawing-board use only" |
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(btw - did I miss the promised inspirational <link>, or did
[bs0u####] take a spill before getting there?) |
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Right, link up. Phones, no matter how smart, are
crap at real internet. |
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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. |
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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. |
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The sum total is a smaller contact patch and low
deformation. This should reduce rolling
resistance. |
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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. |
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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. |
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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. |
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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. |
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