Matt O'Toole writes:
>> This isn't about MTB's. It is primarily a road bicycle problem.
>> It takes the same pressure to blow a skinny tire off a rim as a fat
>> one, the interface being the inside width of the rim, on which
>> inflation pressure acts (between tire and rim). If your 2.5" MTB
>> tire on a narrow rim can withstand 140psi then you may be able to
>> experience blow-off. On the other hand, starting at 45psi, I doubt
>> that you could get enough pressure even if you have a wider rim.
> So perhaps a 25mm or 28mm tire at 90psi is safer than a 23mm one at
> 120psi? Care to make an educated guess at what the threshold of
> safety is? Wouldn't it be interesting to test this?
Interesting that you mention that, because I am working on a test for
blow-off. I made a valve stem pressure sensor adapter that, when
installed, opens the Presta valve to an aneroid pressure sensor that
sends its readings to a data-logger (about the size of a deck of
cards) that also reads a thermocouple attached to the rim. The data
logger is tied to the inside of the wheel near the hub and has a USB
connector to download test data to a PC where it can be charted.
The plan is to descend Hicks Road near Los Gatos CA using only the
rear brake at about 15mph until the tire blows off. Since this is
essentially a straight run, there is no problem stopping with a flat
rear tire. Each such run will require a new tube and I think the
second run should make clear whether this is reasonably repeatable.
Of course I'll have to cool the rim down with water before running
again and pump the tire to the same pressure.
I will also test a 1950's French touring rim tape that was designed to
prevent tire blow-off but was never recognized as such, and was
therefore, unmarketable from the Cupertino Bike Shop of those days.
After having not ridden down Hicks Rd. since the days of my insulated
tubulars, I was concerned about this descent the last few times I
recently came down that hill and recall reading about a fatality in
2004 on that road... attributed to lack of rider skill.
Not to worry, the results will be announced. There is still a matter
of designing a small circuit board for the thermocouple. I hope to
see a product emerge from this.
Jobst Brandt
[email protected]