Which hub geometry for the strongest wheel?

Saved by the Aussie, as Colin submitted two good questions. Today we'll address his first:

"Bracing angles.  The Onyx hub has pretty narrow flange spacing.  I'd pretty much assumed that would make a weakish wheel.  But I've heard trustworthy others say that although the spacing is narrow it leads to more equal drive side and non drive side spoke tension and in the real world that gives an absurdly strong wheel.  Flange spacing versus spoke tension variation. Discuss."

First, thank you to Colin for the question. Since it looks like this post a day business will be with us until into the summer, we need the help. Otherwise I'm just going to post lyrics from Paul's Boutique and 3 Feet High and Rising, which would be edifying and wonderful but have nothing to do with bikes.

Back to hubs...

First we need to dissect Colin's question and provide some definitions (which may be mine and not precisely what Colin intended but we'll go with mine)

Flange spacing is the distance between the right and left flanges on a hub. I think everyone knows what flanges are - they are where the spokes connect to the hub. In the chart below, I separate left from right flange spacing. The flange spacing in the absolute, as a sum, is significant, but it's also meaningless without parsing left and right. Right and left flange offsets are from the lateral centerline of the hub to the lateral centerline of the flange. 

Trustworthy others is an internet myth. There are no trustworthy others. Kidding aside, you need to be a critical consumer of information, about hubs, viruses, and everything else. Unlike other so-called "others," we absolutely welcome questioning - you need to be critical of our information just like you need to be with others (just please be courteous, it makes all of this more fun). 

Real world - The place where personal/confirmation/selection biases feel most at home.

Spoke tension variation: every other time I've spoken of spoke tension variation, it has been in context of one spoke's tension relative to its neighbor on that side of the wheel. If you pluck all of the spokes on the right side of the wheel, for example, they should all sound as close to the same note as possible. On a disc wheel, each spoke group (front left, front right, rear left, rear right) will sound different from every other spoke group (the front left and rear right will be fairly similar, as will the front right and rear left). But you want to minimize spoke tension variation within a group. Significant spoke tension variations cause spoke longevity problems (under-tensioned spokes tend to break) and will generally cause an untrue wheel. Better rims allow for more uniform tension, but the wheel builder plays a huge role too.

Tension balance is what Colin meant when he mentioned spoke tension variation. Tension balance is the average tension of spokes on one side versus the average spoke tension on the other side. All else being equal*, the lower the tension imbalance, the better. I worry an awful lot about the lowest tensioned spoke when I build wheels.

Bracing angle is the lateral angle at which the spoke enters the rim (see pic below, the green arrow is right side bracing angle, the red arrow is left side bracing angle). Bracing angle is function of flange spacing and flange diameter, properly referred to as "pitch circle diameter, or PCD" PCD is more accurate than flange spacing since the spoke hole circle's diameter doesn't have a prescribed relationship to flange diameter. Bracing angle is a function of the flange spacing (the wider the flange spacing, the higher the bracing angle) and PCD (the bigger the PCD, the higher the bracing angle).

Bracing angle helps prevents the wheel from flexing laterally (laterally stiff!). Imagine a tall flagpole that you want to keep from swaying too much. If you braced it with something parallel to the direction of the pole (vertically), that something would need to be pretty darn strong to have any effect. But if you tied a piece of dental floss to the top of the pole and anchored that piece of floss so that it was perpendicular to the pole, it would have a huge effect. 

Now I know you're saying "but the floss would only work in one direction, it wouldn't do anything if you pushed against the floss." True, and this is why we don't build wheels using dental floss for spokes. If you used a metal spoke in place of the dental floss, it provide a butt load of support against tension (the pole being pulled away from the spoke) and compression (the pole being pushed toward the spoke). For the purposes of this discussion, we're going to isolate to those two dimensions. 

Ok, we got all that?

Now, I need to address a fallacy here. Many people think that increasing spoke tension increases wheel stiffness. Absolutely false. In fact we tweeted about this like a week ago, totally out of thin air. Once a spoke has any tension at all, it has achieved maximum stiffness. Start mumbling about cross sectional area and Young's modulus and you're all set. But let me repeat this - once a spoke has any tension at all, more tension does nothing to improve stiffness. So what that means is that Colin's so-called "trustworthy others" are not so much.

What a balanced tension ratio does is prevent the "loose side" spokes (non-drive rear and drive-side front disc wheel spokes) from going slack. And to illustrate the benefit there, I will resort to an old illustration. Take a paper clip and straighten it out, then try to break it by stretching it. Not happening. Then bend it back and forth in the middle, and it will break after not that long. That is called cycling fatigue, and when your spokes are too loose they experience this. This is why non-drive side spokes fail so much more often than drive side spokes - you almost never see a broken drive side spoke except when the chain jumps the cassette and mangles the spokes. Spokes don't break from too much tension, they break from not enough tension. Spokes that are too loose go from tension to compression and back again, cycling, as the wheel turns. Balanced tension ratios mitigate this.

Bracing angles, on the other hand, provide lateral stiffness to a wheel. They are the dental floss part of the equation. The higher the bracing angle, the higher the stiffness. But then you have to watch out for tension balance - you can't help one without hurting the other. All hub designs are a compromise. 

I will leave you with the following chart of hubs that we use. We'll refer back to this chart in a future episode, but if I've done a good job today (and you're still awake), this chart will make a whole lot more sense to you now than it would have before you read this post. 

Be well, be distant, and do try to stay sane. 

*all else being equal is an important concept, and is implied throughout

 


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  • Dave on

    Appendage – See today’s post

    Keith – Good question. These spokes you keep breaking are on the non-drive side I guess? Are we talking about disc or rim brake? Do you break front spokes? I think the Williams 31 were based on Kinlin XR31T rims, which should be up to the task, so I question the spoke count, hub, and build. Does HD mean they had a higher spoke count? Aero and fast are more or less the same thing (the wheel doesn’t contribute much other than appropriate width to rolling resistance gains) and you don’t need that much depth to get some aerodynamic speed. Your problem is easily solvable with a $1000 bill and people who spend $2k to $3k on wheels may find full value in them, I don’t. You’re spending much more than necessary at that level.

  • Keith Baumgardner on

    As a 200+ lb rider I struggle to find wheels that will hold up to the abuse given. I am looking for a wheel that will be aero, fast and strong. Do I have to give up one of these variables to get a good set of wheels? I own a set of Williams 31 HD and a set of major brand (insert any major brand that starts with a B) that are aluminum with carbon faring which are pretty good but I keep breaking spokes. I am a teacher by trade so money is not flowing abundantly enough for me to drop 2 to 3 thousand to a set of wheels. And understanding the law of diminishing returns, I understand that at 47 I am going to struggle to keep up with my 17 year in old son (he races and is strong😬), I will continue to look for a sub 1,000 dollar wheel that will aid in aero ability and be strong enough to hold up to the pressure. Thoughts? Btw I am working diligently to get to sub 200 but it’s going to be a few more months, ( or years😆)

    Thanks
    Keith Baumgardner

  • Appendage on

    The numbers in the chart comparing the various hubs look pretty close…or are they?

  • dave on

    We haven’t done a true geeked out tech one in a while. I’ll try to finish the topic tomorrow.

    Definitely use tension gauge, the good Sapim one. My ears are very well tuned to relative pitch (I have good pitch I guess) but it’s useless for absolute values. So I equalize spokes within a group with my ears, but the gauge tells me the absolute values.

  • Geoff on

    The mechanical engineer in me loves it when you talk like that.

    So, do you use a tension gauge when you build wheels, or are your hands (ears?) well-calibrated?


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