The Secret the Bike Industry Don't Want You to Know
I feel dirty. That click-bait title! [throws up a little in his mouth] Still, I literally have no idea for how to caption this article.
Bicycle wheels are one of humanity's greatest inventions! A bunch of spokes, crossing each other a couple of times, a hub, a rim a tire. Motorcycle wheels evolve from this, too. A well made , steel spoked wheel, with a quality hub and rim, can be lighter and stronger than carbon fibre. Well, cheap carbon. (Stop buying your carbon parts from Ali Express! You don't what they're really rated at. FFS!) And then I was looking at a buddy's Pinarello he's selling on orders from his missus. Heartbreaking but, happy wife, happy life.
This bike is a nice piece of European carbon and design, and I've seen it many times, but at my buddy's new years party day-after, I was looking at the "Pin" and suddenly realised it had a 21 spoke rear wheel! Aside from the low spoke count for a back wheel (I'll come back to that), ODD NUMBERED SPOKING?! WTAF!!!
It's triplet lacing! There are twice as many spokes on the drive side as on the non drive side. Do the maths, and that's 14 on the drive side and 7 on the non-drive side. It's one of the last bikes ever built before the UCI's 7kg rule, that a bike must never be less than 7kg. I think I have the figure right. I'm a weird-beard cycle tourist, a slow rider. Apart from Le Tour, I don't follow racing, or much care for it, really. Forgive me. But I do love when a sport pushes engineering's limits. A 21 spoke rear instinctively feels like the limits are being pushed. So, why?
Bicycle wheels are strong because they keep all of the forces balanced through the directions best suited to each force applied. The spokes are in tension in such a way that the rim is held in balanced compression against itself. You and your bike _hang_ from the top of the wheels, you're not resting on the contact patch... as such, anyway. The weight of the ride is applied to the hubs, the hub hangs from the top of the rim via the spokes, the spokes' tension is transferred to the rims and these forces are applied in compression around the rim, around both sides, to the ground. It enables a massively strong wheel at very light weight, by applying the forces to the materials in the way that maximises those material properties. Road bike, mountain bike, cargo bike - the weakest part of any is the frame, yet the lightest parts are the wheels.
The biggest problem with this design, though, is the back wheel is built lop-sided, to accomodate the derailleur. Modern dearailleurs being up to 12 cogs wide, this dishing is quite large, and it puts a HUGE amount of extra tension on the drive-side spokes, compared to the non-drive side. Too much tension. You'll often hear riders complain about breaking drive-side spokes more than left-side spokes, and drive side spokes are harder (therefore more expensive for most people) to replace. Thus we come to my mate's 21 spoke rear.
Leaving aside the "crazy" of so few spokes in the weel that does the "hard work" for a second, it actually looks like "sex." It's radially spoked, too, no crossing points. Say what?! Here's the sweet thing, the drive side spokes, when plucked like a guitar string, ring at roughly the same pitch as the non drive side. The tension is near equal! This was a bike built for racing, so radiallly spoked rear? The drive losses that are said to exist in back wheels with radial spokes are obviously not that great as this model won so many races before the minimum weight rule, it's been said to be the bike that caused it.
So, the reason this wheel works is, the tension is balanced on both sides of the wheel. 2 spokes in each triplet sharing the tension on the dished side, 1 spoke in the triplet on the other side, also having about the same tension, all of them forming a perfect, three-sided pyramid of forces in perfect balance. All of this done 7 times around the rim. Not counting the weight of the drivetrain, a wheel that's actually stronger than a 20 spoke front, and as strong as a conventional 24, or even a 28, spoke rear, in fact. With 21 spokes!
I want this! I want this on "Buster," my touring/gravel bike, so much!
But parts... bugger. Can it be done with standard parts? Actually, yes, just. It's a bit mullet, but it is doable, with 32-spoke hub and rim and 24 spokes. It can also do crossings for added circumferential force tolerance under power and load. See the schematic picture up there. ^
Starting with the drive side, we have 16 spokes, in a double crossing pattern, alternating inside and outside on the flange, crossing the next adjacent spoke, then the first spoke from the previous pair, ending at the rim. They're the dark grey spokes. The 8 non-drive side (red) spokes only use every alternate flange hole, starting between each drive side pair at the flange, crossing once between the drive side crosses, meeting the rim between the drive side "parallel" spokes. This leaves every second hole on the non-drive flange unfilled and every fourth hole on the rim unfilled. There are little rubber plugs available in hardware stores that could plug the unused rim holes, to keep out road "gunge." Elegant, no?
I want to build this! Spokes are cheap! I need to manually calculate the spoke lengths because this is not a pattern you can do in any of the online calculators I can find. I did say it was mullet, it's mullet as fuck, in fact, but it should result in a 24 spoke wheel stronger than a 28 spoke wheel and maybe as strong as a 32 spoke wheel, because the internal forces are not fighting against each other. The best part is that it uses off-the-shelf parts! No drilling your own rim, no searching out Ali Express hubs (21 spoke hubs exist, but not 21 spoke rims... and Ali Express... please) and stronger for commuters, gravel bikes, tourers, MTB and cargo bikes. This is also the pattern I will be building into the back wheel of my recumbent lean steerer, as well as for my gravel/tourer workhorse.