There is a trend in today's road and triathlon wheels – they're getting fat. What's the reason behind this seemingly unstoppable expansion of girth? http://www.yoeleowheels.com/23mm-rims-700c-88mm-carbon-tubular.htmlDepending on who you ask, there are many benefits from using wider rims. First and foremost, you can use a wider tire at little-to-no aerodynamic penalty. When the rim's width more closely matches your tire's width, the airflow around this system tends to be smoother than the lightbulb shape of a wide tire and narrow rim. http://www.yoeleowheels.com/23mm-rim-width-700c-carbon-tubular-60mm.html
But why would you want to use a wider tire? For starters, they reduce your risk of pinch flatting at a given tire pressure. As well, you can use a lower air pressure which absorbs more road shock, and tends to have a reduced Coefficient of Rolling Resistance (Crr). http://www.yoeleowheels.com/23mm-wide-carbon-rims-tubular-38mm.htmlFor those not in-the-know, Crr is like a golf score – lower is better (and faster). Lastly, wider tires tend to have better grip on wet or irregular road surfaces. Gone are the days of road racers using 21mm tires at 150 psi (10 bar) for everyday usage. http://www.yoeleowheels.com/23mm-wide-bicycle-rims-clincher-50mm.htmlBy 2012, we've seen nearly a universal adoption of 23 and 25mm tires by the Pro Peloton, at pressures ranging from roughly 90 to 115 pounds. For the cobbled Spring Classics races, they often use up-to-28mm tires and even lower pressure.
On to the more subjective benefits of wide rims, some say they improve handling or cornering, due to the resulting casing shape of the tire. There is also a loosely thrown-around idea that wider rims unilaterally reduce rolling resistance – in and of themselves. At this time, I say the jury is out; nobody seems to be able to back it up with real numbers. http://www.yoeleowheels.com/clincher-rims-700c-carbon-88mm.html For example, suppose you have a 23mm tire. You mount this tire on both a 19mm wide and 23mm wide rim. Your tire's contact patch on the 19mm rim is long and narrow, and with the 23mm rim is short and wide (relatively). The wider rim increases air volume slightly, and makes the tire's sidewalls more vertical. In theory, the tire casing has to deflect less to make the same size contact patch – easy enough. http://www.yoeleowheels.com/carbon-tubular-rims-700c-88mm.html
Does that automatically mean that the Crr is lower? Some manufacturers suggest that this is the case, but nobody is producing hard numbers to prove it – or at least showing them to any of us. Zipp Technical Director, Josh Poertner, http://www.yoeleowheels.com/carbon-road-rims-700c-clincher-60mm.html challenges the idea. He suggests that there may be a very, very small improvement – but neither their testing, nor any independent testing they've seen can prove it outside of the margin of error of the test (on the order of .0000125 Crr improvement). My suggestion is that saying that the wide rim itselfimproves Crr, is like claiming that the chicken came first, when it's really the egg. http://www.yoeleowheels.com/carbon-rims-bicycle-tubular-60mm.htmlYes, a wide rim allows you to run a wider tire and lower pressure – which lead to lower Crr. But it isn't the rim itself that lends to this improvement; or at least not enough that we can actually measure it. I'll go ahead and insert the voice of reason for you: Go worry about something else! Sleep more! Eat less Twinkies!
Now that we understand the theory behind it, how do we go about measuring our rims? In tubular rims, you can only measure outside-to-outside at the braking surface. With clinchers, however, you have the ETRTO (European Tire and Rim Technical Organization) internal width, as well as the outside-to-outside measure.
The part that really matters is the ETRTO – this is what determines how wide the tire casing spreads. That measurement generally correlates to an outside-to-outside rim measurement, but it can vary. http://www.yoeleowheels.com/carbon-clincher-rims-38mm.html
For example, a narrow 13c clincher rim measures 13mm on the inside walls. This generally correlates to an outside width of 18.5-19.5mm, but it varies depending on manufacturer and rim wall thickness. An “average” road rim used to be a 13c or 15c, and many of the new wide rims are 17c. A 17c rim generally lends to an outside measurement of about 23mm. http://www.yoeleowheels.com/carbon-bike-rims-tubular-38mm.html
Keep in mind that – at least with clincher rims – there is a specific range of tire sizes that you can “get away with” for a given rim size. Go with a tire that is too wide for your rim, and it can feel excessively squirmy while cornering, especially at lower air pressure. http://www.yoeleowheels.com/carbon-bicycle-rims-clincher-20mm.htmlGo with a rim that is too wide for your tire, and you run a real safety hazard – at some point those tire beds won't stay seated (for example, if you try to use a 20mm tire on a 30mm rim).