Kgf vs Psi, part 3

December 19, 2016 4 Comments

In Part 1 we saw rims deform from tire pressure causing spoke tension drop and discussed the role of tire dimensions, noticing road tubeless clinchers have the most potential to amplify tension drop.

In Part 2 we established that design, not manufacturing inconsistency, makes for odd matches and extreme tension drop situations.

Here in Part 3, let’s ask what builders can do to minimize negative influences of tension drop. After all, builders invest spoked structures with tension and answer for wheel performance.

What are we willing to accept?
First, rim and tire combinations that make for large tension drop are bad news. Culprits: too-deformable rims and too-tight tires. Avoid them, especially in combination. Don’t waste time pondering what to do with inadequate components. Identify troublemakers and insist manufacturers deliver products with more integrity.

Wheel builders should not accept the burden of overcoming obvious component deficiencies. It is job enough to build great wheels from good parts. Designers have ample recourse to prevent assembly-induced knock downs and have no business ignoring tension drop problems that have solutions.

Builders should not need to wonder if exceeding published limits on tension is a good idea when tension drop is severe. That is absurd! If you wonder what vaccinations are needed before visiting a pub, you are going to the wrong establishment!

Meanwhile, constructive experience trumps theory and builders who make compensating tension adjustments (use of washers, etc.) should continue their research. As a rule, however, this is beyond the reasonable duties of wheel building unless you are managing scarce supplies in the developing world, or such.

Never a better time for thoughtful wheel builders to keep good records on build outcomes: tension uniformity, difficulty, tension drop, etc. Map the landscape for yourself, ask questions and share data, and navigate forward with confidence while we wait for component offerings to improve.

What happens with lower tension?
We’ve just left an era for tire pressure, where it’s been mostly “more is better” or “reduce, only as necessary.” Now we have the option to use low pressures in many situations because the total effect is better understood. Thanks Jan Heine, among others.

Similarly, spoke tension has been as high as absolutely possible. Rim makers set maximums that builders match or exceed (even employing supplemental washers to help support higher tension). Is highest tension an unqualified benefit?

Don’t get me wrong, the answer to tension drop is not to simply use lower tension. However, now is a good occasion to consider the role of tension.

(1) Builders agree that tension consistency is more important than magnitude.

(2) Numerous studies confirm tighter wheels are not stiffer unless fractions of a millimeter matter. Compared to contact area scrub and casing deformation, such differences are inconsequential. Like “high pressure = faster ride,” it's more illusion than reality. Best treatment of this topic is Damon Rinard’s. Also check excellent test data by Adrien Gontier.

(3) Spoke fatigue life is better when tension does not regularly reach zero, for example, on every rotation. This rule of material science applies to many material fatigue situations and, with wheels, is easily avoided with relatively low tension.

(4) Very tight nipples better resist loosening because high thread friction counters vibration. Extra tight wheels also seem more stable because components are stretched and settle less. However, there are many engineering solutions to counter vibration and service life yield.

(5) How many wheels taco or crack at nipples? Very high tension can provoke both. Every rim and spoke combination has a buckling limit, a tension at which the rim slumps into an unridable shape. This limit is real, can be calculated, and the closer static tension is to it, the less margin for failure.

Pushing tension without good knowledge of buckling limits is not a job we should undertake. Let’s consider tension as a design option, not an ingredient to always be maximized.

Don’t overlook rims!
Rim design is a major determinant of wheel feel and performance. Consider how tension supports a rim but can also suppress its personality.

When a rim is laced with maximum tension, it is paralyzed, more brittle, and less unique. Such rims all feel similar. Acoustic differences, sure, but hardly any resilience or liveliness. Maximum tension and rim rigidity is optimal for some applications but not always best.

Rinard’s stiffness tests assume 25lbs (11kg) is a typical large wheel side load. The bicycle is a single track vehicle, a category of mobility in which side loads are nearly absent (compared with 3 or more wheels). Even for a typical large side load, nearly all wheels - tight or loose, light or heavy - flex between 1 and 2mm laterally. That's it. Considering the scale of a bicycle and factors like tire scrub and casing flex, it is astounding how little stiffness is at stake with choices of spoke thickness, number, and tension.

In this ferocious sprint, wheel side deflection is not visible, owing to the peculiarities of single track physics.

Even with radically dished rear wheels, the difference in side flex in each direction is only a few thousandths of an inch. Changing from super light spokes to thickest 14G creates an 11% increase in stiffness but that is only a bit more than 0.1mm (0.005”).

For high performance wheels, peak tensions need not always be 150kgf. Sometimes a better riding wheel comes from 100kgf. Lesson: lower spoke tension is a design option, not categorically taboo. Ride quality should be managed, don't reflexively build all wheels to their absolute maximum.

Imagine wheels that improve traction, absorb vibration, dampen harmonics, dissipate point impacts, and not taco. Performance like this can only come from enlightened rim design, a host of spoke elasticity options, and building that selects tension optimal for each combination.

Escape to a brighter future!
We can move beyond tension myopia in which tighter is better. How Tight is Right? Still a good question today. Sophisticated modeling (FEA and structural algorithms) will be soon available to quantify the effects of spoke gauge, material, and tension.

Such tools will help us build the best and most appropriate wheels ever. Vast experience serves the same purpose.

If you already like wheel building and design, the near term is going to be even more fun. For now, be patient and open minded. Spoke tension, rim design, and wheel flexibility needs our fullest understanding and open minds.

Sorry to not offer answers to the specific issue of tension drop but it is a complex subject without simple solutions.

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