dddd
Joined: 11 Jan 2004
Posts: 3345
Location: NorCal7/16/15 3:44 PM |
Especially when the rim is standard road width, there is a tendency for a smallish tube in a wide tire to pin itself to the inside of the tire casing due to the air pressure, with the rubber pad at the base of the valve stem then not wanting to fall to the inside "floor" of the rim cavity.
When this happens, often the tube will rupture to either side of the reinforced area of the tube near the valve stem, as the stretching tube bulges into the rim cavity.
To some extent, one can prevent this by pulling the stem outward during the initial few strokes of the pump.
But there is still a lot of over-stretching of the tube local to the valve stem, since the reinforced area will not stretch much, possibly forcing the tube in that area to over-stretch to the point of rupture.
Normal tubes have to stretch quite a bit already to fill the inside of the tire, and an undersized one can stretch beyond limits.
An over-stretched tube loses air very fast in response to even a pin-prick, leading me to go the opposite route, fitting somewhat-oversized tubes that more-fully fill the inside of the tire with barely any stretching at all. This allows me to ride all the way home on a slow leak in many instances. These also lose less air day-to-day, and further, will enhance the level of puncture protection afforded by any kind of sealant.
I put just enough air in a tube to give it some shape, without any stretching, then fit it into the tire to see if it fits in without bulging out too far, which could interfere with installaton.
You might find that certain of the 29er tubes will fit into your 42c tires.
Tubes can easily be compared for width by measuring the width across the flat, folded tube inside of the box. The numbers printed on tubes and their boxes can vary wildly versus what the actual tube measures!
Performance sells hybrid-width tubes in two very-different size ranges, but when you take them out of their packaging, they are the same tube! Apparently 28-32c equals 35-42c at Performance, but they are not alone.
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