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November 12, 2012 5 Comments
[Note: this is #16 of a series of 20]
To build better wheels we constantly must quest for better understanding of the pneumatic systems on which wheels depend. To a casual observer, and that includes most riders, the difference between tubular wheels and clincher wheels is only three-fold:
(1) The tubular is bonded to the rim by adhesive while the clincher grips with continuous hoops that are smaller than the rim's full diameter but can be slipped on before air pressure.
(2) The tubular's inner tube (or equivalent) is sewed or otherwise captured into the tire, so the rider cannot access it while a clincher inner tube is an entirely separate unit that is easily changed.
(3) A tubular rim weighs about 100g less than the equivalent shape and strength clincher rim.
While these are true, they only apply to the rider experience. The underlying physics is vastly different and those designing and building wheels must deal with the physics.
Pneumatic tires were invented and first patented by Scotsman, Robert Thomson in 1846 at the age of 23. Check this guy out. One of the recurring themes in the history of ideas is early discovery of new principles by individuals who were fountains of ideas, but largely unrecognized and uncompensated in their time, who predate the adoption of their ideas by a generation or more. Thomson was such a guy.
Invention of pneumatic tires was undoubtedly the crowning moment in the history of wheels. Suddenly, the very useful shape that underlay all terrestrial transportation was given a supercharge. More than one authority (J.E. Gordon, Structures) declares the pneumatic tire a more important invention than the internal combustion engine. John Dunlop, it turns out, was the man to profit from the idea, applying the pneumatic tire principle to bicycles 46 years after Thomson.
In a tubular tire, air pressure is contained in a closed structure, the 360 degree continuous tube that a sewn up tire represents. As a tubular is inflated, it tries to enlarge, rotate, and assume a smaller diameter shape. We've all seen this effect if we put significant pressure into a tubular that is not on a rim. This constriction in size of the hoop helps the tire assume the diameter of the rim. Small inconsistencies are unimportant. It's as if the tire is molding itself to the rim. Manufacturing tolerances and diameter standards for an industry of tire and rim makers are very relaxed with this system. It's hard to go wrong.
At the same time, recognize that the constricting force of the tire is actually quite low. The tubular tire deforms to match the rim but does not subject it to enough force to cause a significant change in the structure. Example: at 100psi of tire inflation, a tubular wheel loses no spoke tension. The rim supports any constricting force from the tire.
The clincher tire is a different beast from the rim's point of view:
(1) Forcing the rim wider.
The clincher tire pushes out (laterally) to stay aboard. These forces are generated by air pressure. In the case of bicycles, as high as 100psi. In such case, lateral pressure is huge. The outward force of the tire's bead is proportional to the area of the tire casing.
Just as a balloon rises because of upward internal pressure on its shape, bigger is greater. Larger tires are exposed to more pressure (pounds per square inch) and that results in higher lateral forces on the rim. For example, if we wanted to inflate a 2.3 inch mountain bike tire to 100psi, we would have unsustainable forces at the rim. No rim for cycling has been made that could resist that outward force. So it's a great world in which high pressures are preferred by those with small section tires.
At 100psi, clincher rims are spread open with enough force to make their brake tracks non-parallel. Look for yourself with calipers, many rims widen by several degrees. This is elastic deformation so the rim assumes its original shape when the tire is deflated. But when the pressure is present, the brake tracks angled, the belly of the rim is changed in shape. This change can result in a spoke tension reduction of 20% because the rim's cross section is elastically deformed.
(2) Inward pressure by the inflated tube.
A clincher tire puts inner tube pressure in direct contact with the rim. An average road clincher rim is 0.5" wide and presents a 77" length. Surface area equals 38.5 sq in. If air pressure is 100psi, then total force felt by the rim is 38.5 X 100 = 3,850lbs. This 2 ton force is trying to make the rim smaller and is great enough to elastically deform many metal rims more than 0.1" in circumference. This may not sound like much but it has a large and measureable effect on spoke tension. 0.1" circumference = 0.032" diameter, or 0.016" radius. This is nearly one full nipple turn. Imagine the loosening effect.
(3) Tire bead constrictive force.
The recent development of tubeless tires has seen experimentation in tire to rim fit tolerance. In the past rims for 700C, for example, offered a bead seat of 622mm diameter. All tires intended for this fit had diameters of at least 623 so there would be some helpful registration but no interference.
Tubeless specific clincher tires are designed for an interference fit to assure better air sealing. I've measured tubeless tires that, under 400lb bead load, are 621mm in diameter. The interference of such a fit (621 tire to 622 rim) means a disagreement on circumference of 3mm. This may not sound like much but it can amount to thousands of pounds of load especially if the bead material is not elastic. Some tubeless clinchers are using carbon beads rather than Kevlar, so their elasticity is practically zero. I've measured the bead constrictive force of an uninflated tubeless tire that is greater than the forces generated by 100psi inflation. The combination of tight bead and normal inflation load is entirely new in bicycle pneumatics.
(4) The combination of forces.
Some light clincher rims have been observed to lose 40-50% of spoke tension when a tubeless tire is mounted. This worst combination, super tight bead plus air pressure is a burden tubular rims never dream of. A tubular tire, because it contains air pressure within its closed hoop shape, exerts neither of the clincher's forces. The world of a clincher rim is completely different than a tubular rim. And this is before the rider gets on board and starts pedaling.
While the idea of clinchers is appealing, the system is very demanding of the rim. Weight increase of 25-30% is required to support the additional stress. All for the convenience of clincher tire mounting and dismounting? All for the sake of patching flats on the road? Remember, the high pressure of small road tires is the reason for these issues. MTB, BMX, CCX and other tire applications use low pressure and clincher rims are not so burdened.
If you follow my drift, it's no surprise that, across industry, clincher tires are succeeding well for dirt and recreational bikes, wheelbarrows, hand trucks, and wheelchairs. Tubeless is universal for motorcycles, cars, and trucks. Their engines are powerful and their wheels can be overweight to resist tubeless forces.
It sometimes makes me wonder if road bikes aren't better served by tubulars. Or, perhaps, is there a system in our future that can deliver the low rim demands of tubulars but without the adhesives or risk of becoming stranded with a flat? That would be a worthy development.
In any case, now you know more about the hidden forces your rims support besides spoke tension. Designing and building wheels is challenging, but much more interesting if you appreciate the underlying physics.
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