Saturday, September 5, 2009

Does Heavier Equal Stronger?

It's often the case that adding material, and therefore weight, to a component will make it stronger and more durable. With rims, for example, this is generally true. So does a heavier wheel always make for a stronger, more durable and more resilient wheel? No. Anytime we have an opportunity to make a wheel lighter while maintaining the same or greater durability, we try to take it. Our alloy cross wheels, the REV-25x, for example weigh 1460g. Not spectacularly light but given what they are built for, they are significantly lighter than most other alloy wheels being sold for cyclocross use. So how do we make a stronger, lighter wheel? We've carefully sourced all our components but much of the credit goes to the often overlooked spoke.

Invisible components

Next time you're looking over some mid to high range road or mountain bikes, check the specs to see what headset, bottom bracket and cassette are used. Are they on par with the other components on the bike? Probably not. These are very important components but most manufacturers will source them as cheaply as possible since most people won't bother to look at these parts in much detail. Spokes too, are largely invisible to the consumer. Most people can differentiate between round or bladed and silver or black, but that's about it. The subtle variations in thickness, forging techniques, nipples, etc. and the associated benefits and costs are easy to overlook. It's no surprise then that most manufacturers will use cheap spokes to cut costs.

We use Sapim CX-Ray spokes for all our wheels. These are the gold standard for spokes and are very expensive. What makes the CX-Ray so great is that it is both the lightest available steel spoke and also the strongest and most aerodynamic. Most wheelsets in the same price range of ours use much cheaper spokes. A straight 2.0mm (14 gauge) spoke weighs about 7 grams. A CX-Ray spoke weighs about 4.5 grams. On a 24/28 spoke wheelset, this is a weight difference of more than a quarter pound. Aerodynamic advantage is more obvious. Round objects don't go through air very well. Oval objects do. A round spoke with a 2.0mm cross section will be significantly less efficient than and oval spoke with a 0.9mm cross section.



But wouldn't a thicker, heavier spoke be stronger?


When discussing shear strength (the force needed to cut through it) yes, the 14 gauge spoke is stronger. Generally speaking though, when we throw around terms like strength and durability with regard to spokes, we are talking about fatigue resistance. And no spokes has been tested to have better fatigue resistance than the CX-Ray. As the wheel rotates, the spokes at the bottom of the rotation lose tension to some degree. Each spoke on the wheel is cycling from a lower to higher tension with every rotation of the wheel. These constant stresses on the spoke will eventually lead it to fail, typically at the spoke head or where the spoke leaves the nipple. Straight gauge spokes have the least fatigue resistance. Their uniform thickness does not allow them to efficiently absorb the stresses that a wheel experiences as it rotates and the spoke will break much sooner than a more flexible spoke. The CX-Ray is 14 gauge (2.0mm) at the spoke head and nipple threads but swaged (or butted if you will) to 17 gauge (1.5mm) at the center. That butted section is then forged to an oval shape that is 2.3mm x 0.9mm. This makes for a very flexible spoke that is much better at absorbing the stress that the wheels experiences as they rotate under load. There is a reason that top mountain and BMX racers insist that their wheels are built with these spokes and it doesn't have much to do with their aerodynamic properties.


The nipples are also important. Unlike just about every other nipple on the market that leaves the rim at a fixed angle, the Sapim Poly-ax nipple allows for a subtle angling of the nipple as it leaves the rim. This allows for a straighter path for the spoke from the hub to the rim, resulting in less stress to the spoke.



So why isn't it more widely used?

As I mentioned before, it's very expensive. The CX-Ray costs more than 8 times as much as a straight gauge spoke by Sapim and more than 10 times as much as straight 14g spokes from Asian suppliers. These costs can be hard to justify when the buying public has very little knowledge about what makes one spoke better than another and when the stronger spoke is paradoxically much lighter than the weaker one. Being a smaller manufacturer (I can only build so many of these things), we can ignore the mass market and give our customers a bit more credit when it comes to evaluating our products based on the important details that are often lost in the marketing of most products in this industry. This topic probably deserves its own post (and every time I try to write it, it comes out way too arrogant) but I genuinely believe that there are a lot of bad products out there simply because they are designed around what consumers think they want instead of what experts know is best (cheap ceramic bearings is a good example of this).

How can we use such expensive componentry on such reasonably priced wheels?

I've answered this here to some extent but it basically comes down to selling direct to the consumer and disciplined cost management. We're a very good size for what we do. We can purchase our materials in large enough quantities to maximize economies of scale, minimizing variable costs. But since we're so small, fixed costs can be kept to a minimum. We're happy to grow our business organically so we have no debt to service or outside owners to please and we don't have to sell too many wheels to sustain our operations.

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