We’re riding in the world of the future, and not just in a Bluetooth-enabled-seatpost sense. So many of the otherwise timeless components we rely on are often souped-up with subtly fancier shapes and materials. Our shoes feature proprietary brand-name versions of what once was “rubber.” Our lubricants have proprietary brand-name versions of what once was “oil.” But there’s one component that, try as they may, the bike industry just hasn’t managed to reinvent: Spoke nipples.
Sure, there are some with nifty sockets at the top to make wheels easier to build, or locking agents baked onto the threads right out of the box, but the alloys that actually make up those nipples are still either aluminum or brass.
Brass. The stuff of doorknobs, trombones, and nautical sextants, has been the dominant material in spoke nipples for over fifty years. Not stainless steel, like the spokes themselves. But why? No other load-bearing fastener on our bikes is brass. And isn’t it best to match like materials to avoid electrochemical corrosion? This isn’t the sort of thing we’ve ever really needed to know, so I reached out for some answers from Scott Boyd, the product manager at Sun Ringlé, corporate sibling of Wheelsmith Spokes.
“I had to go back and dig a little too,” admits Boyd. “Nipples were made of steel years and years ago. On the Schwinn Continentals and Suburbans and such. Brass came into play well before both our times.” First, brass nipples aren’t 100% brass, which is itself technically an alloy of copper and nickel. Brass nipples are usually coated with black oxide or silver, but grind down the coating, and you’ll notice the bright, warm material beneath it. Brass alloys often include aluminum, arsenic and lead. Interesting sidenote; Sun Ringlé switched to low-lead nipples on many of its wheels, specifically those to be used on kids’ bikes, to avoid needing to put the Prop. 65 warning about those nipples being “known to the state of California to cause cancer, birth defects or other reproductive harm.”
Brass is naturally slightly softer than stainless steel. It allows for some more stretching as load is put on it. “When you think of a spoke, it’s always in varying amounts of tension,” Boyd explains. “That’s whether you’re riding, or when the wheel is being built.” Nuts and bolts—which is essentially what nipples and spokes are—hold together because, when tight, there is some very slight deformation in the threads. The material pushing back against that deformation is why a bolt will tend to stay tight, and why split lock washers are sometimes needed to help. Especially when spokes are under such unpredictable levels of stress, that extra deflection offered by brass keeps the friction slightly more constant.
“Also, it’s a natural lubricant,” adds Boyd. If both spoke and nipple were stainless steel, a problem called “galling” would be likely to occur. Galling means that some amount of one material is, essentially, being scraped off one material and deposited on another, leaving one surface with tiny craters and the other with tiny bumps. It’s a similar effect to friction welding, where extreme force combined with a sliding or spinning motion between two surfaces, results in them being bonded.
Speaking of bonding, brass and steel are dissimilar materials, which we know ought to be a no-no if you want to avoid corrosion. But not all materials will behave the same. The likelihood of “galvanic corrosion,” which is what we mean when we talk about dissimilar metals corroding when put together, depends on each material’s “anodic index.” That measurement is based on how reactive a material is with gold. The more similar the anodic index of two metals, the safer it is to put them together. Given the other benefits of brass nipples, the difference in the anodic index of brass and steel is plenty low. Interestingly, though different types of aluminum react differently, any type of aluminum nipple is more likely than brass to suffer electrochemical corrosion than brass.
Titanium is also unlikely to react with steel, but the weight savings would be negligible. “There have been titanium nipples. You can find them on places like Alibaba,” says Boyd. “But manufacturing and price are prohibitive. And the weight advantage is much less than aluminum.”
It’s actually pretty refreshing to see a little BCE technology on our bikes. It’s a reminder that the laws of physics apply to everything, even the future-bikes we’re riding today. So, until some new wonder material is discovered, or until somebody actually makes a full-carbon wheel that isn’t too expensive, XC-only, or just a novelty, we’ll be sticking with the classics.
Photos: Ryan Palmer