There are lots of sources of information regarding what the real benefits of running wider rims on a road bike are, but by and large it seems there is not much data supporting the hype. It is my intention in writing these articles to disseminate information from my own research and testing to show the true differences and characteristics of each type of rim on tire performance. This particular article is only the beginning of a series of experiments which will hopefully shed some light on the hypotheses behind the wide rim craze, and to help you, the consumer, understand if this technology could benefit you.
Today we look at two different rim profiles and their effect on tire shape and volume. The starting point for this experiment is simply to look at the difference in height and width of the tire from the outermost part of the rim. The vertical distance to the outermost part of the tire to the rim is the compression zone. This is the space between the ground and the rim, when there is zero load on the tire, that is used to prevent pinch flats and keep the rider suspended on air and not on the hard sidewalls of the rim itself. The tire's width can vary with different inner rim widths, and so inner rim width and tire width are measured to help gain an understanding of air volume and how much support is gained from having a wider rim.
My hypothesis is that the inital contact patch is similar with the wider rim, but the wider tire profile and increased radius means that when load is placed on the wheel, the increased radius will make earlier contact with the ground, thus reducing the depth of compression in the tire under load, which will reduce forward rolling resistance by reducing internal casing friction within the tire itself through reduced compression.
Our data collection begins with our two most popular road rims, the classic Mavic Open Pro, aka "The Reference Road Rim" according to Mavic, and the Velocity A23, aka "The New Standard" from my perspective. The data I have collected are quite simple, but the techniques I have used are designed to guarantee their accuracy. For my measuring tools, I have used a Fowler Vernier caliper, an SPI machinist's square, and a Mitutoyo 0-1" and 1-2" micrometer. Both micrometers have torque limiters in the thimble so as to prevent distorting the work that is being measured and creating a false reading. The vernier caliper was aligned against the machinist square that was resting against the sidewall of the rim to ensure perpendicularity to the rim for the most accurate and consistent readings. Each reading was taken a minimum of 5 times, recorded, and I have averaged each value and will now present you those numbers.
GP4000 Average Sidewall Thickness: .612 mm,
GP4000 Average Tread Thickness: 2.897 mm
Inner Rim Width, A23: 17.738 mm
Inner Rim Width, Open Pro: 14.562 mm
Mounted Tire Width, A23: 25.024 mm
Mounted Tire Width, Open Pro: 23.350 mm
Open Pro Rim Height: 18.47 mm
Open Pro Rim Height with Tire: 40.564 mm
Open Pro Tire Profile Height: 22.094
A23 Rim Height: 19.375 mm
A23 Rim Height with Tire: 42.005 mm
A23 Tire Profile Height: 22.630 mm
These data help us create a profile for the air inside the tire in a cutaway view that allows us to calculate the volume of the tire overall. Why does the volume matter? Having a larger air profile means having a larger, stronger support, which could allow the tire to roll easier. Additional tire height could be helpful in reducing pinch flats from a pothole or road plate. If the tread radius is increased, as you push into the casing of the tire with the blunt corner of an obstacle, the tire has more force to push back. As well, in order to receive a pinch flat proper, the tire will have to be depressed further. Adding air volume could also mean slower deflation between air fill-ups as a nice side effect in, for all you lazy no-fillers out there. I always tell my customers to fill up every day, but I myself generally fill up every 3 days. Do as I say... Anyways, presented here below are the profile area values and their corresponding volumes.
Open Pro Air Profile Area: 217.7 mm^2
A23 Air Profile Area: 247.3 mm^2
Open Pro Air Volume: 436.246cc
A23 Air Volume: 497.330cc
Our basic findings are this: The average difference in sidewall width from A23 to Open Pro is 3.176mm, and this creates a 1.674 mm difference in tire casing width, effectively increasing the tire's radius. This increase has raised the tire profile height by 0.536 mm on the A23 rim. All of this helps add 14% to our GP4000 tire's air volume which hopefully will reduce the rolling resistance and increase pinch protection.
Right now there are many more options on the market to fulfill the wide rim market demand. Pacenti Cycle Design has launched their SL23 road rim, as well as their SL25 disc road and cyclocross rim. Velocity has now just released another rim called the Quill. It is 24mm wide and even lighter than the A23 by 30g. The A23 will soon play second fiddle to the Quill due to its slightly heavier weight, but it should remain a strong contender for heavier riders. The SL23 will work well for heavier riders at 450g with a little extra support for the big boys. Kinlin has released a 24mm rim known as the XR-31T. The Kinlin is tubeless, as are the Pacenti and Velocity options.