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November 17, 2016 5 Comments
Part 1 showed how clincher rims can alter spoke tension when tires are inflated:
(1) Outward splaying of brake tracks, changing the rim shape, dropping spoke tension.
(2) Inflation pressure down on the rim bed, a constricting force that shrinks the rim causing measurable tension loss.
Tension loss can be confusing to wheel builders following tension recommendations from rim makers. We need to know why this happens and when to worry.
Here in Part 2 we explore how tires, independent of inflation, affect this dynamic.
First let’s review tire and rim sizing for those less acquainted. Why are they made to the sizes we see?
SYSTEMS FOR TIRE AND RIM SIZING
ETRTO has been the world's only clincher tire sizing standard for ages, but oh so vague. It's a theoretical diameter not pertaining to a specific location, but a general zone. Rim makers only know that tires will be occupyimg this area.
Made to this non specific standard, rim-tire combinations can lack adequate safety and often cannot even be mounted. Much trial and error must accompany validation of any rim-tire fit. Pass = seems to fit and no persistent explosive events. Fail = test subjects curse, bleed, are unable to ride their bikes.
UST was developed in 2001 by Mavic, offering tubeless wheels for mountain bikes. For the first time the rim internal profile was specified so tire makers knew what they were supposed to fit. UST is popular but proprietary and French-administered, and sees only modest adoption.There are a number of useful imitations and variants.
(3) Road Tubeless
As UST is not an adequate seal for road, Shimano and Hutchinson offered a new system in 2006. This system is the source of most recent concern over tension drop.
Do these standards make tire mounting predictable? No. Using only rims and tires from industry leading brands, in conformance with these standards, you will regularly experience tires that cannot be mounted and those that blow off without warning.
During mounting, the tire bead drops into the trough creating slack for the unmounted portion. Rims with deep troughs are easy for tires to mount even if they have large total diameter. It is not the rim total diameter that makes a tire difficult to mount. If a tire is easy to mount owing to a deep trough, it is likely to entirely dismount with sudden pressure loss. A dismounted clincher immediately jams in the frame or fork, locking the wheel, leading to an instant crash. For decades, riders have valued a degree of mounting difficulty to minimize such a blow out catastrophe.
As ETRTO does not address this OD-to-trough issue, it does not prevent unmountable and unsafe matches. UST and Road Tubeless were not created to cure ETRTO miss fitting dangers. Instead, they aim to ensure a uniform tire-rim fit for a reliable air seal. Liquid sealants are effective only if the tire-rim fit is precise and snug.
Life with UST and Road Tubeless remains problematic. Fits are often arduously difficult and some are inadequate. Why? The common explanation is manufacturing variations with rims and tires. If the variation range is large then trouble can be expected with extreme combinations: large rim with small tire or large tire with small rim. The other explanation is design deficiency. Which is it?
Confronted with these questions, measurement of tires and rims is a beginning. For proprietary reasons makers do not reveal their tolerances. For rims, total circumference can be measured with a band of spring steel. My band has knobs to hold while measuring.
Once total circumference is known, depth to the bead seat and trough can measured and those circumferences determined. It should come as little surprise that rim technology is at a high standard today. Aluminum extruding and hooping and steel moulds for composite structures are made to high tolerances. If money can be saved in rim making, it will be elsewhere like finish, design, and packaging.
Exacting tolerances are nearly universal. This is confirmed by measurement. While mistakes are possible, you can confidently know your rim diameters are not varying. If a diameter is wrong, then all of that brand and model are wrong. In other words, variations are design driven, not manufacturing outcomes. Rim companies seem able to maintain uniformity on the order of +/- 0.5mm for 700C diameters of about 630mm. Nice.
Here are a few measured built wheels to show rim diameter variety. Bear in mind this table does not display bead seat diameter, trough depth, or ETRTO. While these rim dimensions are quite different, it is the trough depth (not showed) that governs tire mounting effort. Within each brand, uniformity (not showed) is very good.
How about tire sizing? Bead length affects mounting ease, air seal integrity, and blowoff tendency. Measuring bead length requires a dedicated tool. My investigation is a short term study, so the tool is made from laminated hardwood. For long term use the tool needs to be metal. If you want such a device, contact me. I will direct you to a capable fabricator.
This tool consists of two halves that capture a tires beads in grooves. Calibrated (with a wire) to +/-0.1mm length, it uses a 400lbf (1779N) spring, providing force to fully expand the beads but not to elastically stretch them. Distance between halves is added to the known lengths of each side to establish bead length.
Measurements were enlightening:
(1) The tool works very well. Repeatability was outstanding and we were confident of lengths +/- 0.1mm.
(2) Bead length among tires tested is very consistent. Like rims, their manufacture is not subject to much variability. Size within brands and among models is extremely consistent. Examples:
(3) Tire casings stretch with age but beads do not.
(4) Blow outs do not stretch beads. Explosions damage rubber around the bead-rim contact, altering details a particular tire may need to be reliable at pressure. However, bead length is unchanged.
(5) Road tubeless beads are consistently smaller, as much as 6mm shorter, and those with carbon beads fit even tighter to rims. Carbon beads are 2.4X less elastic than Kevlar. All who have mounted tubeless tires can attest to the sheer effort. Once up on a tubeless rim bead shelf, tire fit is very tight.
Some tubeless tires take over 4K newtons to mount. Once mounted, total bead force on the rim (before air pressure) can exceed 600lbf (2670N). With inflation, rim circumference is reduced by 2mm (returning to size when pressure leaves). Double the tension drop can be seen in such circumstances as compared with more conventional, non-tubeless combinations.
(1) Clincher systems are more demanding on rims and tires than tubular.
(2) Tension drop of 10-50% can occur with many combinations. Tire bead length (as well as inflation pressure) can be a serious contributor.
(3) Rim and tire manufacturing deviations are tiny and issues with fit or tension drop is nearly always design, not quality, driven. Of course, trouble can also be caused by inexperience.
(4) Measure everything you can. Learning is a collective effort. Data and experience sharing is the way to move forward. This is the bike industry way and we need it as much as ever.
We need a Part 3 to cover the question of how to compensate for tension drop. Should we be over tensioning in anticipation of drop? There's no one answer but, instead, a lively bunch or relationships at play!
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