Tire Dimensions for Road Bikes
Have you ever wondered if the dimensions of your tire can make you faster? Can they improve your aerodynamics or lower your rolling resistance?
Perhaps speed isn’t your primary concern. You might be trying to figure out if a particular tire will fit on your bike without rubbing the frame.
As the owners of a wheel company, we’re interested in all of the above.
Making You Faster
We’ve been studying how tires can make you faster for the past few years. At the end of 2015, we conducted what is rumored to be the most extensive tire study to ever pass through the A2 Wind Tunnel in North Carolina.
During that study, we discovered how tire pressure can make you faster by improving your aerodynamics.
We also studied the aerodynamics of a list of the most popular road tires. We combined the aero data we collected with the rolling resistance of each tire (thanks Tom Anhalt) to find the fastest tires you could put on your FLO wheels.
The speed of your tire comes down to the two things I mentioned above, aerodynamics and rolling resistance.
Your tire has to interface well with your wheel for great aerodynamics. If you combine the right tire with an optimized rim shape, you create an excellent aerodynamic system.
At the end of our study in 2015, Jon and I felt we had a strong understanding of the aerodynamic side of the equation. By optimizing our wheel and tire interface, we were able to reduce the drag of our original FLO 60 by 22.2%. We also created the FLO 45—a 45mm wheel—that was faster than our first generation 90mm wheel.
That said, we realized we knew little about how a wheel could help lower rolling resistance. We hypothesized that optimizing both aerodynamics and rolling resistance would be the next breakthrough in developing faster wheels. So naturally, we go to work.
When looking at Tom’s rolling resistance data, we noticed that wider versions of a tire rolled faster. You can see this when comparing the 25c Continental GP 4000S II to the 23c Continental GP 4000S II and the 25c Zipp Tangente Speed to the 23c Zipp Tangente Speed.
Tom’s data suggested wider tires were faster and we wanted to see if this theory was true.
We started by taking thousands of tire measurements to see how tire size was affected by different variables. Three variables change the height and width of your tire. They are:
- Tires size
- Rim size (internal rim width)
- Tire pressure
I’ll be honest. It doesn’t take a genius to figure out these three variables. Our main goal was to see how much each variable—mainly rim size and tire pressure—changed the dimensions of a given tire.
Let’s look at each variable.
On the sidewall of your tire, you’ll see a listed size. It typically looks something like this:
Let’s break these numbers down.
“700” is the installed outside diameter of your tire in millimeters.
“C” is the bead seat diameter (BSD). BSD is the outside diameter of the rim bed where the clincher bead of the tire connects to the rim. BSD is more of historical value that is rather useless for the end user.
A 700c wheel has a BSD of 622mm. The “b” in a 650b gravel wheel represents a BSD of 584mm. Like I said, rather useless data.
“23” is where things get a little misleading. The 23 is supposed to reference the width of the tire in millimeters. However, the installed width of a 23c GP 4000S II varies drastically.
By changing tire pressure and internal rim width, we noticed the installed width of a 23c Continental GP 4000S II measures anywhere between 22mm and 28mm. A far cry from 23mm.
Why Should You Care About Tire Size?
Knowing the installed height and width of your tires will help you avoid frame interference (frame rub). If your bike will accept 26mm tires, buying a 23c tire doesn’t guarantee the tire will fit on your bike. If your pressure and wheel combo result in a measured tire width of 28mm, you’re out of luck.
Additionally, some wheel companies state optimal aerodynamics are achieved when the widest part of the rim is at least 105% of the inflated tire width. If you own a set of those wheels, knowing the dimensions of your tire is useful. At FLO we believe tire aerodynamics is governed more by tread pattern than inflated width, so don’t worry about the 105% rule if you own our wheels.
Rim Size (Internal Rim Width)
The internal rim width of your wheel is the measured distance between your clincher hooks. As internal rim width gets wider, the width and height of your tire changes.
What Does All Of This Mean?
As many of you know, we are developing a gravel wheel. One of our design goals is to create a wheel that optimizes rolling resistance. While we do not have the final equation yet, we have collected a ton of useful data that we wanted to share with our readers.
This document below allows you to estimate the measured height and width of common road tires on your wheel. We weren’t able to test every tire known to man, but we picked some common tires our customers like to use.
For FLO customers, we’ve included the internal rim widths of our wheels to remove a step.
|Wheel||Internal Rim Width (mm)|
|FLO 45 CC||17.0|
|FLO 60 CC||18.0|
|FLO 90 CC||17.0|
|FLO DISC CC||18.0|
|FLO 60 AC||19.4|
|FLO 90 AC||19.4|
|FLO DISC AC||19.4|
We hope you find this document useful.
The document is embedded below in this article. If you want to share the article, please feel free to use this link FLO Cycling – Tire Dimensions – Road.