Low Spoke Numbers, pt 2

January 31, 2023

Part 1 explored the roots of low spoke number wheels. Historically, they were a weight saving strategy that, by the 1980’s, were boosted by aerodynamic testing. Here, in Part 2, we get practical. What are the concerns and opportunities of low spoke wheels?

Rims are key
Low spoke wheels are rarely trouble because the best employ a stout rim. Rim strength counts more than spoke number, size, or tension; more than hub type or spacing. Increasing rim strength, especially radial (vertical), spreads loads and can more than compensate for missing spokes.

One way to make a rim stronger is to increase mass (weight). More efficient is to increase section (width or depth). As brake calipers have limited capacity for width increase, depth is an obvious way to grow. While more difficult to form into hoops, deep rims deliver huge increases in stiffness, which reduces individual spoke loads.

Deep rims also look more aero due to their knife-like profile. Their side view is also a big change from shallow rims. The riding public thinks they’re aero because of their slicing shape and can spot them from a distance. That’s a  trend that self propels.

Easy to Build
Wheels with few spokes and very strong rims are easy to build. Today’s deep section rims practically build themselves and a reduced number of spokes makes building a faster puzzle to solve.

Don’t get me wrong, low spoke number wheels have sacrificed strength, but they are usually easier and faster to true. Take their building seriously but enjoy a the simpler task. Reduced spoke number wheels are neither high tech nor involve special skills. That impression, advanced by 30 years of marketing, is not going away. Don’t be surprised when riders act like building aero wheels is rocket science.

Can low spoke wheels be made stronger or stiffer with higher spoke tension? This point comes up too often and the simple answer is “no.” Rigidity is a product of many elements. Quick release integrity, axle flexibility, and tire pressure are all variables. From a rider POV, it also includes the frame and fork. More spoke tension is not the key to wheel stiffness any more than more tire pressure is the key to low rolling resistance. An element, yes, but super high tension is too often recommended.

The Dave Scott Solution
Here’s a great wheel saga that illustrates how simple low spoke wheels can be and how you can apply ingenuity to a wheel trend. In the 1980’s we were lucky to build some wheels for Ironman legend Dave Scott. He was sponsored by Shimano and they supplied 36h hubs. But the racing scene was keen on low spoke wheels since the 1984 Olympics. Dave was the King of the Ironman, for those youngsters among us. He won 6 Ironman titles and placed second 3 times between 1980 and 1994. Araya rim company in Japan, like Shimano, was less conservative and made 24h aero rims. We just had to figure out how to use the sponsored hubs with those rims. Turns out it’s easy. We spoked the front wheel radial—use 2 hub holes, skip one, use 2 more, skip one, etc. until the 24h rim is filled. Six empty holes remained on each front hub flange.

For the rear, we spoked the left (non-drive) side radial using the front wheel strategy. On the drive side, we used pairs of hub holes, with an empty hole between each. The illustration below shows this strategy. The silver rim has 12 red spokes attached because it is a 24 drilling and we’re looking at the drive side. The black hub is a 36 so 18 white spoke holes are on the DS flange. Red spokes are in 2/3 of the holes. The question—how to calculate spoke length? We find that a normal spoke length calculator can be used but fractional cross numbers are needed. Spocalc by Damon Rinard (on the Sheldon Brown site) does this. The number, if I remember correctly, is 2.17. Not cross 2 or 3 but in-between. The spokes are nearly tangential as they leave the hub. With a basic 24h wheel, tangential spoking is X2.

The aero wheel scene grew throughout the 1990’s with most made by custom builders. Bladed steel spokes became available and builders got proficient preparing hub holes to take them. See here. We chose the Velocity Deep V, settling on 36h Ultegra hubs (economical, durable, and plentiful). We used the X2.45 from Dave Scott days with Wheelsmith ACE blades. The result was a fast pair of wheels that found their way onto high end road bikes in the Bay Area. Dozens were built with great results.

The biggest success was Mavic’s Ksyrium that, beginning in 1999, dominated the category for a decade before serious competition. It made a fortune, combining low spoke numbers, good looks, perfect timing, and clever graphics. 

Spoke Placement
Despite Ksyrium's huge commercial success, the design that arguably pushed the hardest and earliest at low spoke numbers was Rolf. Rolf Dietrich was early to discover, and then harness, an aero rim's unusual strengths.

Early aero rims could be 18mm wide by 30mm deep. They were new to cycling as hooping the shape was previously unfeasible—too much deformation to control.

They are massively more stiff in the vertical (y) axis than side to side (x). Wheel designers gradually picked up the opportunity of such shapes.

Rims are always vulnerable to lateral wobbles with low spoke numbers—these aero shapes were no different. However, deep section means they can support vertical loads over huge arcs. Rolf’s experiments showed that paired spoking leveraged this characteristic. At first glance, the Rolf wheel idea seemed counter intuitive because we were collectively unfamiliar with deep rims.

This paired wheel hugely loads the rim radially where two spokes attach. Meanwhile, between pairs the rim is without bracing. But supporting such loads is the aero shape’s forte. Whether you use or admire such spoking, the lessons apply to all. Thanks, Rolf!

Takeaways
Don’t be intimidated by low spoke wheels, just make sure there’s a robust rim.
Tailor made, matching components are not required.
Many such wheels are largely visual gimmicks—harmless, distinctive looks and tiny aero benefits.
Always be careful and don’t take any wheel for granted.
Test ideas thoroughly—don’t experiment on customers!

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