# Low Spoke Count Wheels - A Surprise

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**Low Spoke Count Wheels - A Surprise**

I was on bike ride last week with a friend riding a mid-line Trek carbon bike with Bontrager low-spoke count OEM wheels (16 spokes front/20 spoke rear). We noticed his rear wheel was wobbling slightly but not enough to hit the brake pads or make the bike handle oddly. When we examined the wheel at the end of the ride there was a one broken drive side spoke that had snapped at the hub end elbow.

What was most surprising was how little the wheel went out of true and that it could still be ridden. I've always been told that low-spoke count wheels were very vulnerable to spoke breakage and would go wildly out of true if it happened.

Another data point is that Alberto Contador had a similar spoke breakage on a low-spoke count carbon rim racing wheel during a mountain stage of the Giro and manged to finish the stage without serious problems.

I expect the deep section of rims used with these low-spoke wheels are rigid enough to keep the wheels adequately rigid. Maybe the warning has been overdone.

What was most surprising was how little the wheel went out of true and that it could still be ridden. I've always been told that low-spoke count wheels were very vulnerable to spoke breakage and would go wildly out of true if it happened.

Another data point is that Alberto Contador had a similar spoke breakage on a low-spoke count carbon rim racing wheel during a mountain stage of the Giro and manged to finish the stage without serious problems.

I expect the deep section of rims used with these low-spoke wheels are rigid enough to keep the wheels adequately rigid. Maybe the warning has been overdone.

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Low spoke wheels have really freaking stiff rims. That is how they manage the low spoke count. Build one and stress relieve it and you will grasp the fullness.

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Anyway, yes the stiffer rims will help hold up if a spoke breaks. I'm not too convinced that the same will happen for my 16/20 spoke Titans with 25mm rims, though.

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http://www.rouesartisanales.com/article-15087917.html

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I'm feeling snarky, so consider this:

How well a wheel accelerates depends both on the mass and on the moment of inertia of the wheel. Mass is mass, no problem there. But moment of inertia depends on each mass element and how far it is from the axis of rotation. The differential for moment of inertia is in fact dI = r^2dm. The r squared term means that if you move mass in a wheel to double the distance from the axle, its contribution to the moment of inertia is four times as great. For this reason, rims, tires, tubes, nipples, and rim tapes have the biggest gram-for-gram effect on a bike's acceleration of all the bike parts.

People talk about "rotating mass" but often ignore that it's that r squared term that makes it happen. It's not just that the part is rotating but how far from the hub it's rotating. An extra gram of spoke (and especially an extra gram of hub or frame, for that matter) has less of an effect on acceleration than an extra gram of rim.

It would be funny if people started freaking out about the "effective mass" of their bicycle, i.e. force applied to the bike divided by resulting acceleration. It is in fact this "mass" that governs acceleration off the line. A bike's climbing ability is mostly a gram-for-gram deal, though.

Not that I'm really in the business of going fast or having a light bicycle. I'm just feeling snarky.

How well a wheel accelerates depends both on the mass and on the moment of inertia of the wheel. Mass is mass, no problem there. But moment of inertia depends on each mass element and how far it is from the axis of rotation. The differential for moment of inertia is in fact dI = r^2dm. The r squared term means that if you move mass in a wheel to double the distance from the axle, its contribution to the moment of inertia is four times as great. For this reason, rims, tires, tubes, nipples, and rim tapes have the biggest gram-for-gram effect on a bike's acceleration of all the bike parts.

People talk about "rotating mass" but often ignore that it's that r squared term that makes it happen. It's not just that the part is rotating but how far from the hub it's rotating. An extra gram of spoke (and especially an extra gram of hub or frame, for that matter) has less of an effect on acceleration than an extra gram of rim.

It would be funny if people started freaking out about the "effective mass" of their bicycle, i.e. force applied to the bike divided by resulting acceleration. It is in fact this "mass" that governs acceleration off the line. A bike's climbing ability is mostly a gram-for-gram deal, though.

Not that I'm really in the business of going fast or having a light bicycle. I'm just feeling snarky.

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I'm feeling snarky, so consider this:

How well a wheel accelerates depends both on the mass and on the moment of inertia of the wheel. Mass is mass, no problem there. But moment of inertia depends on each mass element and how far it is from the axis of rotation. The differential for moment of inertia is in fact dI = r^2dm. The r squared term means that if you move mass in a wheel to double the distance from the axle, its contribution to the moment of inertia is four times as great. For this reason, rims, tires, tubes, nipples, and rim tapes have the biggest gram-for-gram effect on a bike's acceleration of all the bike parts.

How well a wheel accelerates depends both on the mass and on the moment of inertia of the wheel. Mass is mass, no problem there. But moment of inertia depends on each mass element and how far it is from the axis of rotation. The differential for moment of inertia is in fact dI = r^2dm. The r squared term means that if you move mass in a wheel to double the distance from the axle, its contribution to the moment of inertia is four times as great. For this reason, rims, tires, tubes, nipples, and rim tapes have the biggest gram-for-gram effect on a bike's acceleration of all the bike parts.

Al

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How well a wheel accelerates depends both on the mass and on the moment of inertia of the wheel. Mass is mass, no problem there. But moment of inertia depends on each mass element and how far it is from the axis of rotation. The differential for moment of inertia is in fact dI = r^2dm. The r squared term means that if you move mass in a wheel to double the distance from the axle, its contribution to the moment of inertia is four times as great. For this reason, rims, tires, tubes, nipples, and rim tapes have the biggest gram-for-gram effect on a bike's acceleration of all the bike parts.

People talk about "rotating mass" but often ignore that it's that r squared term that makes it happen. It's not just that the part is rotating but how far from the hub it's rotating. An extra gram of spoke (and especially an extra gram of hub or frame, for that matter) has less of an effect on acceleration than an extra gram of rim.

It would be funny if people started freaking out about the "effective mass" of their bicycle, i.e. force applied to the bike divided by resulting acceleration. It is in fact this "mass" that governs acceleration off the line. A bike's climbing ability is mostly a gram-for-gram deal, though.

People talk about "rotating mass" but often ignore that it's that r squared term that makes it happen. It's not just that the part is rotating but how far from the hub it's rotating. An extra gram of spoke (and especially an extra gram of hub or frame, for that matter) has less of an effect on acceleration than an extra gram of rim.

It would be funny if people started freaking out about the "effective mass" of their bicycle, i.e. force applied to the bike divided by resulting acceleration. It is in fact this "mass" that governs acceleration off the line. A bike's climbing ability is mostly a gram-for-gram deal, though.

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I am really glad to hear this as I ride in continious fear of breaking a spoke on my low count wheels. I have put over 12000 miles spread across WH R-550 Shimanos, and Xero XR1s, weighing 190 lbs with no breaks so far. I am sure it is just a matter of time, since I have never broken a spoke.

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I heard that those reversed-spoke wheels are a devil to true, though.

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I don't think that they're any harder to true but I have to constantly think about which way to turn each nipple in order to tighten the spoke. They come out of the hub in two different orientations so you have to turn the wrench in opposite directions. Also, that itty-bitty wrench isn't really up to the amount of tension those things use. My fingers ached after building my first "Sweet 16" tandem wheel. I've got a much longer handled wrench for those things now.

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How well a wheel accelerates depends both on the mass and on the moment of inertia of the wheel. Mass is mass, no problem there. But moment of inertia depends on each mass element and how far it is from the axis of rotation. The differential for moment of inertia is in fact dI = r^2dm. The r squared term means that if you move mass in a wheel to double the distance from the axle, its contribution to the moment of inertia is four times as great. For this reason, rims, tires, tubes, nipples, and rim tapes have the biggest gram-for-gram effect on a bike's acceleration of all the bike parts.

People talk about "rotating mass" but often ignore that it's that r squared term that makes it happen. It's not just that the part is rotating but how far from the hub it's rotating. An extra gram of spoke (and especially an extra gram of hub or frame, for that matter) has less of an effect on acceleration than an extra gram of rim.

It would be funny if people started freaking out about the "effective mass" of their bicycle, i.e. force applied to the bike divided by resulting acceleration. It is in fact this "mass" that governs acceleration off the line. A bike's climbing ability is mostly a gram-for-gram deal, though.

Not that I'm really in the business of going fast or having a light bicycle. I'm just feeling snarky.

But as compared to the total power available and it's collected sinks, the effect on actual accelleration is terribly minor. If we had the bicycle equivalent of drag races, it would matter on the order of an inch or two by the time one gets to maximum speed. And once you've paid for that inertia, it's yours for free over the next upslope or minor rough patch of road. It's only lost should you hit the brakes. What's more, by helping to even out the effects of naturally imbalanced pedal strokes, it can be beneficial, even when climbing. Depending on all the pesky particulars, a bit of extra weight in the rims (as compared to an equal weight at the hubs) is beneficial for climbs up to a pretty heavy slope.

Alas, 'off the line' acceleration just isn't all that important in our sport. Even our boldest accelerations are mathmatical yawners. Drag racing's nearest analogue - the sprint lead-out - is far more about finding a bread truck to tuck in behind for the first few hundred meters. Mass-inertia variances are swallowed by the much larger physics issues of aerodynamics and simple frictional drag, and simply dwarfed by the combination of luck and strategy.

My own snarkiness, really. For the way I ride, a small bit of comfort and a bit of added reliability will make me much faster than the tiny values corporate marketing arms seem to thrive on.

*Last edited by dangit; 07-04-08 at 11:36 AM.*

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If you're racing, going from 15mph to 30mph 4x per lap x 100 laps, acceleration and polar-moment of inertia of the wheels does become a factor.

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Good point about conservation of power once you've spun-up the wheel.

But with regard to acceleration, are you including track events like the kilo (or at my age the 500m) where you accelerate from a standing start? "Off the line" as you put it. Very important to get to max speed very quickly there, no?

But with regard to acceleration, are you including track events like the kilo (or at my age the 500m) where you accelerate from a standing start? "Off the line" as you put it. Very important to get to max speed very quickly there, no?

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But as compared to the total power available and it's collected sinks, the effect on actual accelleration is terribly minor. If we had the bicycle equivalent of drag races, it would matter on the order of an inch or two by the time one gets to maximum speed. And once you've paid for that inertia, it's yours for free over the next upslope or minor rough patch of road. It's only lost should you hit the brakes. What's more, by helping to even out the effects of naturally imbalanced pedal strokes, it can be beneficial, even when climbing. Depending on all the pesky particulars, a bit of extra weight in the rims (as compared to an equal weight at the hubs) is beneficial for climbs up to a pretty heavy slope.

Alas, 'off the line' acceleration just isn't all that important in our sport. Even our boldest accelerations are mathmatical yawners. Drag racing's nearest analogue - the sprint lead-out - is far more about finding a bread truck to tuck in behind for the first few hundred meters. Mass-inertia variances are swallowed by the much larger physics issues of aerodynamics and simple frictional drag, and simply dwarfed by the combination of luck and strategy.

My own snarkiness, really. For the way I ride, a small bit of comfort and a bit of added reliability will make me much faster than the tiny values corporate marketing arms seem to thrive on.

Alas, 'off the line' acceleration just isn't all that important in our sport. Even our boldest accelerations are mathmatical yawners. Drag racing's nearest analogue - the sprint lead-out - is far more about finding a bread truck to tuck in behind for the first few hundred meters. Mass-inertia variances are swallowed by the much larger physics issues of aerodynamics and simple frictional drag, and simply dwarfed by the combination of luck and strategy.

My own snarkiness, really. For the way I ride, a small bit of comfort and a bit of added reliability will make me much faster than the tiny values corporate marketing arms seem to thrive on.

Tell that to a weight weenie!

It's great how the same physical principle can be used to argue for opposite designs...

I like your style.

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How well a wheel accelerates depends both on the mass and on the moment of inertia of the wheel. Mass is mass, no problem there. But moment of inertia depends on each mass element and how far it is from the axis of rotation. The differential for moment of inertia is in fact dI = r^2dm. The r squared term means that if you move mass in a wheel to double the distance from the axle, its contribution to the moment of inertia is four times as great. For this reason, rims, tires, tubes, nipples, and rim tapes have the biggest gram-for-gram effect on a bike's acceleration of all the bike parts.

People talk about "rotating mass" but often ignore that it's that r squared term that makes it happen. It's not just that the part is rotating but how far from the hub it's rotating. An extra gram of spoke (and especially an extra gram of hub or frame, for that matter) has less of an effect on acceleration than an extra gram of rim.

It would be funny if people started freaking out about the "effective mass" of their bicycle, i.e. force applied to the bike divided by resulting acceleration. It is in fact this "mass" that governs acceleration off the line. A bike's climbing ability is mostly a gram-for-gram deal, though.

Not that I'm really in the business of going fast or having a light bicycle. I'm just feeling snarky.

Here's a thread that evolves into a full-fledged de-bunking.

*Last edited by waterrockets; 07-07-08 at 07:17 PM.*

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I'm sure the car racing folks (especially F1) have the numbers all figured out. They can probably enter the specs of a wheel and calculate exactly how much energy will be saved by reducing weight at a certain distance from the hub. Maybe someone can get the formulas from them.

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