Spoke Selection in Bicycle Wheels

 

Spokes can tend to be a bit of a black hole of knowledge to a lot of people. Most everyone realizes that you need to have them as a part of the wheel but not a lot of them actually know why. Even if you simply take the fact that they are needed and accept it on faith you may find yourself wondering then why different spokes exist and why some are selected over others or why a spoke can break in a wheel.

The basics are pretty straight forward. Most all common spokes are made of some variant of stainless steel and feature one side that has a head which nestles into the hub and threads on the other end that are threaded into a nipple (yup) at the rim. The most common head type on a spoke is what we call a “J-bend” as seen in the picture above. J-Bend spokes have an elbow bend in them just before the head itself. Another head type that is becoming more common is the straight pull style of head. These require a hub designed to use them but they are spokes without an elbow bend in them.

Spokes come in different thicknesses. The thicker the spoke – the more it weighs (con) but the higher the fatigue life (pro). The thinner the spoke – the less it weighs (pro) but the lower its fatigue life (con). This is really the 2 main factors we play off of each other when making spoke selections. Over time the industry has played with these thicknesses and run a lot of lab testing as well as real world testing. The engineer in me likes the lab testing for the purposes of heated arguments during coffee breaks at PSIMET central but I insist on making decisions regarding wheel building based on experience in the real world.

Time and time again I have seen “new” companies come out with “revolutionary” new concepts. The ideas they have are usually a modification or straight up duplication of something that has already been tried before yet is always passed off as though this company is somehow magical in innovating this great new way of doing it. Most of the time they just aren’t aware that it’s been done before. After all we are horrible historians in this industry. Other times though you have to wonder if the move is just straight up hubris.

These new and revolutionary ideas seem on the surface like something that will change the whole industry once people try them and yet they tend to fade away in a couple of years. Ever ask yourself why that is? It’s because it just doesn’t work. By that I mean it either doesn’t do what it’s supposed to do, have the benefits it’s is supposed to have, or those benefits just don’t outweigh what this “new” thing costs in comparison to proven bits and pieces. An empty promise laced across the glossy pages of a cycling publication using cool jargon the marketing guy came up with. These are the things that make you either chuckle to yourself when you see them come into your shop a quarter century later to be fixed or that you can’t help eye-rolling at when looking back through old copies of stuff. “OMG They thought that was really going to work?! I remember how much I loved that idea! I feel like such an idiot now.”

Where was I? SPOKES! Sorry. Like all things companies have played with spokes over the years. “Let’s use smaller ones to save weight! I have run the numbers and these are good enough!” SNAP! “Let’s make them smaller but let’s use more of them!” “What happens if we keep them big at the ends so that they don’t break but make them small in the middle?” These all led to innovations. Changes that were tried. Some worked. Some “worked”. Others looked good on paper and in a glossy spread. Over time in this industry we settled on a couple of sizes of spokes. 14 gauge and 15 gauge. These are more commonly referred to as 2.0mm and 1.8mm in diameter.

When mountain bikes came on to the scene hard in the mid to late 80’s into the 90’s a lot of things changed. First off I saw all of the shops I loved changed all of their floor space over to mtb instead of road and there wasn’t a single person working at them anymore who even knew that there were bicycle races being held in Europe. I think I still hold a grudge against mtb because of this reason but I have been changing. ANYWAY – the wheels started to really need to resist a lot of braking power. You needed to resist this with torsional stiffness. You also needed a lot of support at the rim to keep it from folding under impact or rapid load changes or larger load ranges. In short you needed a lot of spokes and those spokes needed to be at an angle.

In the constant pursuit of the old Stronger, Lighter, Faster builders started really looking at the spokes and started to see that while they needed a ton of spokes they didn’t necessarily need them to be as beefy. The 15g spoke really became popular with these guys. These spokes had been used in special applications for climbing or smaller riders in the road side of things. Remember though – smaller = lower weight but lower fatigue life as well. Spokes break when they fatigue out or reach then end of their fatigue life. When holding a lot of the other variables constant – the higher the stress the lower the fatigue life. Stress is load over area. Smaller spoke = higher stress for the same load. That equates to a lower fatigue life. Breaking spokes isn’t what anyone wants to see.

When spokes break due to fatigue (the only real way that they fail through normal use) they will break in 1 of 2 places. 1: At the elbow near the head of the spoke. 2: At the threads (nipple area). Those two features are the only real features on a spoke that introduce a stress riser – a point where stresses are heightened. They don’t break in the middle. In fact due to the elimination of stress risers in the middle of the spoke you don’t even need to use the same thickness of spoke in the middle to have the same overall fatigue life. Enter the double butted spoke.

Double Butted....check it and see! I got a fever of a hundred and three. Come on baby do you do more than dance! Double butted! Double Butted! - with apologies to Foreigner

Double Butted….check it and see! I got a fever of a hundred and three. Come on baby do you do more than dance! Double butted! Double Butted! – with apologies to Foreigner

A double butted spoke is thicker at the ends with a smaller section in the middle. This allows the spoke to be stronger at the mechanical connections or stress risers thus improving fatigue life while still allowing it to be smaller in the middle – shedding weight – without sacrificing performance. Wheel builders have slowly moved to flat our specifying that 14g (2.0mm) is the size they want to use for the ends of the spoke. This then allows us to use 15g in the center (1.8mm) to save weight. Some crazy guys also said, “If 1.8mm is fine then what about 1.5mm? It HAS to be better, right?”

Well….define “better”. Lighter? Yes. Will it have a vastly shorter fatigue life? No. So what’s wrong with it? Fair question. Some call them a pain to build with. That is true. They aren’t the easiest to build with because they do tend to store up a lot of twist or wind-up but that’s not really an issue in my book. To me they just stretch way too much for the tension that is placed on them. It seems like the stress they see (load/cross-sectional area) is too high in relation to the material’s yield strength that the spokes just don’t function the way I like them to. They don’t “stand up”. This “standing” is what really helps hand built wheels shine and in particular it’s what makes our wheels and build quality really a level above what many if not most in the marketplace offer. Don’t ever seem to have to true a particular wheel? That’s because it stands. Everything seems solid as a rock? Wheel is standing.

At some point someone got a bright idea and decided to start shaping spokes – changing their cross sectional shape. Spokes are generally drawn wire so their shape nets to a round shape. If you form it after the fact though you can put all sorts of new cross sections in place. So let’s try airfoil shapes, right?

Cross-sectional shapes of spokes. Most common modern iterations.

Cross-sectional shapes of spokes. Most common modern iterations.

Right now the most common modified shape on the market is oval. Argue why as much as you want but really let’s not mix messages here. Zipp created the premium wheel aftermarket and they chose the Sapim CX Ray as their main go-to spoke and put a lot of time and effort into touting it as the correct choice because it was more aero than a round or a bladed spoke. Here’s the real thing that keeps builders coming back to it: the low weight and the amazingly crazy high fatigue life.

The low weight comes from the fact that Sapim started with a laser model spoke. This is a spoke that is a round 2.0 spoke with a butted 1.5mm center section. These are the spokes that just won’t “stand”. They then form that spoke into the oval cross sectional shape. This moves the metal and cold works the stainless steel. Cold working it pushes the yield stress and ultimate strength of the material higher. This gives the spoke it’s higher fatigue life. Tradeoff seems that they are a bit more brittle in application. This is something that never comes out on paper. This is what you learn from spending thousands of hours at races seeing these things do the work they are meant to do. Spend enough time at a race and you will see a wreck. If it’s a crit then odds are that is someone has any wheel damage then they most likely have broken spokes. If they have broken spokes – odds are they have CX Rays.

Doesn’t mean CX Rays are bad spokes – quite the contrary. They are simply the spoke that is ubiquitous at that level of product. The failures are in the middle of the spokes. This is always because something struck the spoke during the wreck. A skewer from an opposing bike – a crank arm – a water bottle – an opponent’s shoe. It’s a foreign object that will cause the failure.

So the CX Ray has just ended up being that little sweet spot of all of the properties you like to see in a spoke. It still have the 14g mechanical connections for strength and higher fatigue life. It has the low weight wanted in heavily butted spokes like a laser but the work hardening has eliminated the drawbacks that those smaller/lighter spokes have in their round form. So why not just chuck all the rest and build everything with CX Rays?

Some do. Some performance wheel lines use nothing else other than CX Rays. The tradeoff though is $. Costs for a CX Ray are roughly 7 times more expensive than a standard double butted spoke (2.0-1.8 Race model spoke). The expense is a consideration. It can add $80-$100 to your wheelset price through us. Sometimes it’s worth it (carbon aero wheels). Sometimes it’s not (cyclocross wheels that tend to have foreign objects contact the wheel during normal use). Luckily we help you balance out these issues and decisions when we take you through our quoting process. On the wheels we have in our handbuilt catalog that you can simply buy right from the site have been reviewed for application and the spoke choices have been made based on the needs of the application.

Hopefully this sheds some light on the topic of spoke selection in wheels and how spokes tend to break. I might cover some more on the topic of breaking of spokes or standing of wheels. I know I miss writing a lot in here and have found that it’s really an important part of who we are and where we came from.

Rubber side down and be sure to pick the right nipples in life.

Twisting them is what this whole business is about.

Twisting them is what this whole business is about.

PSIMET
11 Jackson St E Dundee, IL - 60118
(847) 428-2600