Wheelbuilding Tip #17 - Understanding Tire Sizes

[Note: this is #17 of a series of 20]

To truly become a wheel guru, you need to understand more than just how to build. Crucial to wheel performance is your choice of tire and there's more to tire selection than many so called experts know.

The ISO system is an excellent beginning to the question of sizing. Tires are given an ETRTO number that designates the rim diameter for which they're intended. Our 700C standard is ETRTO 622. So a 622 tire will fit a 622 rim. Likewise, for MTB, the standard is ETRTO 559. Match these numbers and you're on pretty safe ground.

This completely overlooks the assembled size. Clincher tires become larger or smaller depending upon the rim on which they are mounted. You can appreciate the absurdity of buying a particular tire size and assuming this defines its inflated dimension.

For example, a tire's inflated size is a combination of casing and rim, not just the tire's labelled width. Technically, the tire's cross sectional bead to bead length must be added to the rim's inside width. This total, divided by π, is the actual tire width.

In practice, this affects the labelled width of tires by more than 10mm, hardly the sort of accuracy that informs wise component choice. So, appreciating the assembled size of a tire and rim is our goal. 

What good is ignoring that a 2.3" MTB tire mounted to a narrow rim is actually 2.1? Or a 23mm road tire on a wide rim inflates to 25? Such oversights don't help us master and enjoy our sport. Size matters just as pressure matters. Any tire designer can tell you that.

The great news is a recent device that enables anyone to anticipate the inflated size of a tire-rim assembly. It's the result of the directDimension work group. And proactive tire and rim makers could add the directDimension number to their products so any user could easily determine the outcome of a particular fit.

dD Tool

First, measure the inside width of the rim using the short straight scale. Save the number.

This rim is 7.

Second, measure the inside length of the tire casing. MTB tires fit the larger tool end. Road tires fit the smaller. This number, length from bead to bead is effectively casing width because the scale is millimeters divided by π.

This tire is 50.

These two numbers, the dD values, are a composite calculation of circumference, bead diameter, and casing thickness for both tire and rim. Add them together and you have the inflated size of this combination, good to within 1mm. In this case, that number is 57mm.

If you are a pro-active rim or tire maker, you could put this number on your product to begin. Then a consumer could anticipate the exact resulting size without trial mounting and inflating.

Perhaps you're a skeptic but are you sure you're not a Luddite? Without this tool (and system) we are as blind as if we had no tire gauges. How much pressure would you like? "Just make it medium hard, please." No, we're quite past that phase.

Let's bring the same insight and order to tire and rim fit. And let wheel builders, since we guide the masses in these matters, lead the way.

Visit the directDimension website to download an explanatory white paper. You can order a dDStandard tool as seen above from the Wheel Fanatyk store. 

Feel free to print out a copy of the tool above, size it so the rim scale (1-15) is exactly 44mm long. Glue it to some stiff paper and start measuring. dDStandard is a free and open initiative. Just label appropriately so the standard does not become misused or mistrusted.

The point of this post is less to sell you the tool than to sell you on the reality that rim size affects tires. Too often overlooked despite great advances in rim and tire design. This knowledge makes you a more enlightened builder regardless of how you apply it.

What the Future Holds

2013 looks to be an amazing year for wheels. How could that be and why are we talking about it?

Before diving into business, check our brand-new Facebook page!

As you know, Wheel Fanatyk gives me the rich privilege of close contact with wheel builders around the world: Africa, Asia, East and West Europe, Canada, and across the US. We provide digital tensiometers (over 100 in its first year, yeah), spoke cutters (a big investment and indicator of trends), wood rims (identifies artists, restorers, and fashionistas), P&K Lie stands (non plus ultra of building tools), hand wrought wood dish tools, and more. Every transaction is a conversation. The narrowness of our offering is also the uniqueness of this database.

This journey has brought me closer to the cutting edge of the wheel world. Many of the bike industry's most capable and passionate designers are in the wheel category. Steve Hed, Josh Portman, Jason Schiers, Bill Shook, Paul Lew, Steve Lee, David Nayer, Stan Koziatek, really too many to list. Similar to the stunning impact suspension designers had on MTB, these folks are bringing serious innovation and change to wheels.

Consequently, be ready for opportunity. The role of the individual builder is too often overlooked. Are they favored or out-gunned? I can say from first-hand experience that opportunity for custom builders has not been greater since the defining years of the 1970's that gave rise to Wheelsmith, Chris King, WTB, Phil Wood, Weyless, American Classic, Specialized, Bontrager, Rolf, Zipp, HED, and the dramatic initiatives of Mavic and Shimano. Of course, Campagnolo and Fulcrum were close behind. 

Today, the majors offer a level of technology for VERY SPECIFIC APPLICATIONS that cannot be approached by custom builders. But the essence of cycling is diversity. So, while some needs (Kona Ironman, Pro peloton) are nailed, almost out of reach, many others go unserved. To be a custom builder is to follow fast-emerging trends and niches too small for the majors. There are many:

(1) Rider weight and strength

As body weight increases among the cycling public (while it decreases for elite pro's), there arises a discrepancy in design. Custom builders can address this need. Majors cannot.

(2) Fat Bikes 

Alaska => Pugsley => LA, are finding many fans. Which of you would love a fat bike for fooling around? Getting these double wide rims on clever hubs (generator, 11-spd internal, disk brake, etc.) is the work of custom builders.

(3) 650B

From genteel urban navigating to the most tortuous Paris-Brest-Paris exploits, this size may offer a perfect compromise of performance and rider sympathy. Credit Kirk Pacenti, Jan Heine, and others for this sensible trend. And so many endurance riders require generator hubs, a perfect storm for an individual builder.

(4) 36" wheels 

Don't look now, but this unlikely wheel diameter is finding fans. Leave it to Colorado eccentrics like Black Sheep ZAMer are creating a new aesthetic. And guess what you can't find from Mavic? 36" wheels!

(5) Road disk wheels

No need to tell you that every major will have serious models soon. But for a year or two, these are up for grabs. 

(6) Fixie fashion wheels are too wacked and regional for majors to embrace. Hence, loads of great offerings. You can find every color and depth of rim, tall retro hubs, wood and bamboo, high and low budget. Urban and campus cycling will continue to push aesthetic boundaries.

(7) E-bikes 

They are flooding onto the streets in selective cities. Soon, they'll be all over your neighborhood too. Driving this trend: green fashion, those tolerant of 2-wheel exposure but not 2-wheel perspiration, aging or disabled riders without other options, huge advances in battery/controller/motor technology, and the emergence of pedal-assist. Pedal assist blends athletic riding with electric assistance. If you haven't tried, you are missing a big surprise. Not to mention pushing electric trailers that attach to any bike (including Shiv). These bikes take special rim diameters, odd spoke gauges and lengths, and a huge variety of hubs. Perfect for informed individual builders.

Can you see why the future is so promising for custom builders? It's not a trend away from the majors. Those companies (Mad Fiber, Enve, Easton, Mavic, etc.) will be working tech miracles for the foreseeable future. The point is, their success supports opportunity for common sense, rider functional solutions that only custom builders can serve. 

Look for more industry insight in posts to come. This is a world we all share and it's the best news in the bike industry (oops, my bias is showing). To close, please enjoy this postcard from Jude Kirstein of Sugar Wheelworks in Portland. 

© 2012 Sugar Wheel Works

More on the Russian Truing Stand

Happy to report that my post about Bill Woodul's Russian team truing stand stimulated a bit of conversation among team mechanics. Jim Ingram (perennial motor driving wrench and former Campagnolo Support Program director) recalled that the Russian Federation shared a building with their Space Program. That must have generated a few interesting projects.

Rumor resurfaced and was confirmed that titanium versions of this tool were made. Ninja racing mechanic Brad Purfeest shared one he bought from a Russian Federation Director who was visiting the US for Masters Racing. Wow:

Functional jewelry.

You can see the obvious similarities and small improvements on the stand Bill gave me. What is it about tools that transcend their purpose with beauty and zealous over-design? Like Hector Castillo said, "it's bike porn." In the healthy sense.

Similar beauty can be seen anywhere mechanics are expressing their passion for tools and compelling mechanisms. Racing motorcycles, South Bend or Graziano lathes, Snap On hand tools, we've all got our lists.

And some of the best the little gems are ones we find at flea markets and thrift shops.