Hi Slim,
Thanks for the kind words. While I am not an engineer, my doctorate and profession are in the field of analysis, measurement and research methodology, and I find the topic fascinating...probably to a point far exceeding the general interest of this forum, so I took it off-list some time ago and have been engaged in an extensive correspondence generating lots of links. If you would like, I would be glad to forward my part in this discussion to you. Tire pressure has long been a topic of interest and some dogma for cyclists as well as manufacturers, but evidence is emerging to indicate recent trends ("recent" in terms of the history of cycling) toward ever higher tire pressures may well have topped-out in terms of overall benefit. This is particularly so in terms of bicycles intended for trekking and heavy touring, and especially so in the case of higher-pressure touring tires mounted on narrow, lightweight rims originally intended for mountain bike use instead of touring.
Here is is an excerpt from some correspondence generated by me that relates directly to your concern and speaks to Thorn's warnings of possible rim failure due to excessive pressures...
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...This whole matter of tire pressure is particularly important for trekking and expedition touring bicycles with 26" wheels. When those bikes are carrying a heavy load, it becomes even more important because the pressures must be higher to support the extra weight. How can this be?
Here is the answer: Many of the 26" rims used on touring/trekking bicycles were originally intended for mountain bikes (MTBs or ATBs), cross-country or even racing mountain bicycles, which use wide tires operated at fairly low pressure for maximum traction and comfort. Many of these rims are also very narrow to save weight. Mounting wide MTB tires on narrow rims is generally not a problem because the tires use low pressures. Modern hook-bead rims make this possible by securely retaining the tire bead. "Back in the day" of straight-sided rims, the tire would have simple rolled off when cornering or perhaps lofted off the rim during inflation.
Touring and trekking tires operate at much higher pressures than MTB tires. When a wide _touring_or_trekking_ tire is mounted and operated at high or even excessive pressure, it places a great strain on a narrow rim, and many touring cyclists have literally blown their rims apart. There is ample photographic evidence of this on the Internet. The rims split down the center because wide tire sidewalls create outward leverage on the rim sides under high pressure and push them apart. This can be avoided by using appropriate pressures for the actual load being carried and by using a wider rim. It is the reason why I chose the Rigida Andra rim option to use with the Schwalbe Dureme 2.0 tires on my Sherpa, which will be carrying heavy loads on very poor roads. I could not afford the CSS ceramic coated Rigida Andra, but the regular model is the same design. I wanted the extra margin of safety and toughness and durability, even though it is heavy. It is strong in this application _because_ it is wider and heavy and because by design it has more material to reinforce it against breakage down the middle and on the sidewalls to wear longer under braking. This can easily be seen in Rigida's cross-section profiles comparing their Andra and Grizzly rims.
Splitting rims due to tire pressure almost never happens on 700C tires because the tire and the rim are much closer in size. Most people never use a tire wider than 32mm or 35mm or perhaps 38mm on a 700C touring bicycle. Nearly every 700C touring bike uses touring rims, which are wider than racing rims. For example, my randonneur bike uses 700C Mavic MA2 rims. Those rims will reliably _retain_ tires 42-45mm wide, but the loads placed by such tires on that relatively narrow rim would be excessive if operated at higher pressures, so I have always taken care to run tires no wider than 32mm in cross-section on them at appropriate pressures. It is a matter of appropriate application.
...My own cycling life changed in a positive way by applying principles described in a 1989 _Bicycling_ magazine article on the topic, authored by technical editor Frank Berto. He distilled his interviews with bicycle tire engineers at Michelin (French) and National (Japanese) companies and presented them in both written and graphic chart formats that formed the basis for later work on the subject by Seattle resident Jan Heine. ...Schwalbe, apart from many other suppliers, have been good to recognize no single pressure fits all applications, and publishes charts showing ranges of pressures for each tire size in their line, albeit uncorrected for mass and application and contains no specific recommendations for percentage of rim drop as a consequence of load-pressure-section width relationships.
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Perhaps a separate topic dedicated to the matter of tire pressures, drop, and appropriate rim-tire matching under pneumatic load would make future discussions more easily searched in the archives? I fear deeper discussions of related matters, such as pneumatic trail, could be hard to find under a topic title dedicated to pump selection.
All the best,
Dan.
