It’s so easy, these days, to establish the exact spoke length for any wheel. Just visit one of the Web’s many free calculation sites (DT’s, for example). But it wasn’t always so easy; in fact, the correct lengths were hard to discover, closely guarded secrets. For more than a century, wheel builders relied on charts and notebooks filled with observations. That’s the wheel world I entered in the early 1970’s, and here’s a glimpse.
For those of you who don’t build wheels, let me mention that the spoke length needs to be within a millimeter, less than 1/3 of one percent accurate. Otherwise you run the risk of too long (pops your tire) or too short (unsightly and weak) spokes. Compounding the task, manufacturing tolerances of rims, spokes, and hubs vary and they stretch and deform during building. With such a moving target, a tiny error in length calculation can become a time consuming failure. Here is the basic mathematical formula for spoke length determination:
* a = distance from the central point to the flange, for example 30 mm,
* r1 = spoke hole circle radius of the hub, for example 35 mm,
* r2 = nipple seat radius, equal to half the ERD of the rim, for example 301 mm,
* m = number of spokes to be used for one side of the wheel, for example 36/2=18,
* k = number of crossings per spoke, for example 3 and
* α = 360° k/m
For most bicycle wheel builders, this is an intimidating formula.
Up until the early 1970’s, builders relied on hand kept notes and published charts. Here’s a chart by Sturmey Archer, the famous English hub maker:
Schwinn, Raleigh, and other manufacturers created pages and posters with “our spoke lengths.” Unfortunately, they were often inaccurate. Craftsman builders kept their own notes. Wheelsmith’s book was kept by brother Jon and myself. It consisted of hand laid-out pages, holes reinforced with masking tape (how quaint), and entries according to observed results. Sections by rim maker, our log eventually contained 38 pages with about 35 lengths per page; more than 1,000 recorded lengths.
Here, for the very first time, is a page from the Wheelsmith log of lengths.
This is page three from the Super Champion tubular rim section. You can’t imagine the care we took with this book. It virtually defined our capacity to build custom wheels efficiently and it recorded, for our own satisfaction, how much territory we had experienced. These pages look like a travel-worn Passport book: dirty, filled with stamps from exotic places, almost a diary, rich with authenticity. Honestly, it was fun.
However, the future beckoned and everyone wanted change. Real change, right now. Howard Sutherland and his gang of East Bay bike nerds threw their efforts behind a comprehensive book of cycling standards. Howard gave us a spoke length determining system. You looked up your hub, spoke number, and cross pattern; then consulted a “rim correction factor,” and PRESTO - you had the length. This system worked great and we rarely needed our dog-eared spoke length notebook again. Here’s a page from an early Sutherland Manual. These are rim correction factors with our own notes.
By the late ‘70’s, the energy in the wheel scene was fierce. We had Spence Wolf at Cupertino Bike Shop importing every obscure length of Robergel spokes from France (the World’s best at the time) and also custom drilled Super Champion rims (48 spoke, 650B, etc.), Phil Wood in Campbell was making superb sealed bearing hubs in every drilling and configuration. Howard did his manuals and Jobst Brandt published The Bicycle Wheel, the seminal work on tensioned wire wheel physics. He also developed a wonderful spoke tension gauge.
Jon and I were motivated to make our own contribution. Besides collaborating with Specialized to design the first commercialized mountain bike wheels, we created the Wheelsmith tensiometer (many thanks to the late, great Norm Ogle). While admiring Jobst’s design, ours was cheaper to make and sell. It was also the dawn of handheld calculators and we figured one could be harnessed and dedicated to spoke length calculation. The Spoke Length System, with its rim measuring rods, was born.
Now, for the first time, non-mathematical builders could establish the correct length for wheels EVEN WHEN the hubs and rims had never been seen or used before. The consequence was huge. A host of innovators threw themselves at the opportunity. Phil introduced his spoke cutting machine, WTB began making extraordinary hubs with inspired dimensions and features, Keith Bontrager cut down Mavic rims for MTB use, Specialized came out with proprietary hubs and rims, and pretty soon we had a host of players from Mistral rims (later to become Bontrager), to Ringle hubs, to Ross Schafer’s Salsa quick releases, to Tom Ritchey’s hubs and rims, Don Millberger’s Nipple Driver, and many more.
The floodgates were opened between 1974 and 1984. A decade to remember, and a truly fun one for me.