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.