Bicycle Mechanics - Max spoke tension vs. gauge

In theory maximum spoke tension should be inversely proportional to spoke gauge, right? All else equal, thicker spokes are more rigid and thus distribute dynamic riding loads over fewer nipples. That’s why, all else equal, double butted spokes build more durable wheels than straight gauge spokes.

Has anyone tried to quantify this relationship and apply it to maximum recommended spoke tension on modern rims? I ask because as a relatively heavy rider (210 lbs) I really appreciate the rigidity of straight-gauge wheel builds but wonder if I need to back down tension to avoid cracking rims (and further, would a lower tensioned straight-gauge build retain a stiffness advantage over a higher-tensioned butted build?). Another way to ask the question would be how do I get maximum rigidity/stiffness out of a given rim?

googlegroups.com > rec.bicycles.tech > Jobst Brandt

Jobst Brandt covers it several times over...

Also being covered on 1-2 threads here as we speak...

=8-)

While you could calculate a max tension based on the spokes. it's moot.

Unless you're building with very light (15/17g or similar) the max tension isn't a spoke issue. The spokes are plenty strong enough to tolerate tensions far beyond what most rims can.

If you still wish to calculate a max tension, use 60,000psi - which is roughly 75% of the yield for the steels used in spokes - and multiply by the cross section area (Diameter/2 x 3.14) and you'll find that a 14g spoke can be tensioned to 300#s tension. (I use 75% to allow for the lowered shear strength at the elbow, plus a safety margin). For butted spoke you need to consider the yield at the elbow and center sections separately, and use the lower number.

You should also note that it's common for rims to fail because of excess spoke tension, which causes spokes to pull through. But spokes rarely (if ever) fail because of excess tension. As a practical matter spokes cannot fail because of excess tension. They would simply elongate slightly which would immediately relive the strain. (this is different than a suspended weight on the end of a wire) Spokes fail through metal fatigue, which results from the flex cycles as the loaded wheel turns.

As to the stiffness issue. That isn't related to tension at all (assuming the wheel is tensioned within a working range). Stiffness (or elasticity) is a function of cross-section and the elastic modulus of the material. It's a linear function so the elongation is inversely proportional to the cross section of the spoke regardless of the initial tension. Simply put, the only way to get stiffer wheels is to use more steel.

The rule of thumb is that you should tension spokes to 1/3 of their yield strength. If that's too much tension for the rim to hold, you should switch to lighter gauge spokes. If lighter spokes makes the wheel too flexible, you should add more spokes or get a stronger rim, or both.
Assuming the spokes have enough pre-load to prevent them from going slack, then tension has no effect on stiffness. Reducing the pre-load tension always reduces the strength of the wheel, all else equal. FWIW I'm around 200 lbs, and I've never felt a wheel was too flexible, although some of my equally large friends tell me that they feel that sometimes.

em, P.E.

The spokes are plenty strong enough to tolerate tensions far beyond what most rims can.


Agreed. But I think you are missing the point. The max tension given by the manufacturer is for an unloaded wheel. What cracks or does not crack the rim are the forces exerted on the spoke holes while the bike is ridden. All else equal, thicker spokes will exert more force on fewer rim holes because they are less elastic, while total force remains the same.

Agreed. But I think you are missing the point. The max tension given by the manufacturer is for an unloaded wheel. What cracks or does not crack the rim are the forces exerted on the spoke holes while the bike is ridden. All else equal, thicker spokes will exert more force on fewer rim holes because they are less elastic, while total force remains the same.

Actually, there's very little increase in tension of individual spokes when a wheel is loaded vs unloaded. The decrease in tension at the bottom is more significant than any individual spoke's increase, with the exact changes dependent on rim rigidity. Butted spokes do protect the rim somewhat by virtue of their ability to deflect more at the same load, which is one reason I almost exclusively use them. But where you see rim cracking it isn't because of stiffer spokes per se, but because builders who use them tend to build to higher tensions.

I build a large percentage of wheels at the opposite end of the spectrum, using light spokes and rims, and consequently rarely crack rims because these simply can't be tensioned as much.

& Unless the spoke tension gage is calibrated to a standard value,
It's relative within just itself.

but it will give you relative tensions on the wheel in front of you..

In theory maximum spoke tension should be inversely proportional to spoke gauge, right?


No because you reach the rim's limits before the spokes'. With lighter/shallower rims you get deformation from stress relieving which suggests you reduce tension because you exceeded the rim's elastic limit, and with heavier/deeper ones you get enough stress in the spoke bed that it eventually cracks from fatigue.



Has anyone tried to quantify this relationship and apply it to maximum recommended spoke tension on modern rims? I ask because as a relatively heavy rider (210 lbs) I really appreciate the rigidity of straight-gauge wheel builds but wonder if I need to back down tension to avoid cracking rims (and further, would a lower tensioned straight-gauge build retain a stiffness advantage over a higher-tensioned butted build?). Another way to ask the question would be how do I get maximum rigidity/stiffness out of a given rim?

Use 32 or 36 spokes and heavier rims where spoke count and rim extrusion have a lot more to do with rigidity than spoke gauge.

In Damon Rinard's deflection tests an otherwise identical wheel (front Mavic X517 with 32 spokes) built with 2.0 straight gauge spokes was 11% stiffer than one built with 2.0/1.5mm (like DT Revolution).

One front front wheel built with a 700C Deep V rim and 28 light spokes (Wheelsmith blades) was 30% stiffer than one built with a Deep V and 16 heavy spokes (Paragon).

A 32 spoke front wheel with a moderate weight rim (Campgnolo Lambda 3-cross with DT 2.0-1.8mm spokes) was 33% stiffer than a 32 spoke wheel with a light rim (Mavic GEL280 3-cross with DT 2.0-1.8mm spokes).

This whole thread is simply another reminder of what's really important when it comes to materials:

It's not so much the material that's important, but how it is applied.

Aluminum AS-IS and how it's applied is a perfect example.

=8-)

To the OP:

Yes, I think your first statement that maximum spoke tension increases with spoke thickness (or inversely to spoke gauge number) is correct. But common sense also tells you, as you hint at in your second paragraph, that there is a limit to that maximum due to rim material, specifically if rims have spoke holes that fail to withstand the stress and crack.

But what I wonder about is what is your goal here? Is it to build the most rigid wheel possible to take your "svelte" 210 lbs and allow you to accelerate ever more so slightly faster? Or to corner ever more, so slightly more precisely?

At 275 lbs, with loaded gear and bike (well over 325lbs), I ride on fairly old box rims, like Mavic MA40's. I tension them just below spec, probably around 200 lbs each on a 36H rear wheel (where most of my load is). I have no issues with wheel flex or spoke pull through. In 25 years, I had spoke pull through only twice, once on a Rigida rim, and another on a Mavic GP4, and those where in the early days of my wheel building when I was experimenting just how a stiff wheel felt.

In the end, I concluded pretty soundly that the flex is negligible compared to the tires/tube and how they are designed to sit on the clincher rim. And I can safely back off the max recommended spoke tension (especially for the back drive side) by changing dish just slightly by putting even a few mm more axle on the non-drive side for any given rear hub as long as I can still shift the chain from smallest cog and I can correct for chainline by spec'ing an ever so wider BB spindle.

Clearly, there's data that supports thicker gauged spokes on the same rim will result in a more rigid rim. But if you're wondering if that impacts how a wheel rides, I'd say it'd be hard to notice because your tire deformation is going to be an order of magnitude greater in effect than the deflection due to wheel rigidity if properly tensioned with pretty much any spoke. And if you're afraid of spoke pull through, well, spec more holes in the rim. Distribute the load over more spokes, and to save weight, spec double butted spokes, lighter tubes, and lighter tires.