What is the largest size tire I can put on my Cervélo?
Recent advances in wide rim technology and the rising popularity of options beyond the ubiquitous 23c tire have prompted Cervélo to update the definition of tire clearance on our latest models. Not only have we designed our new bikes to fit larger tires, we have developed a new way of specifying maximum allowable tire size for each frame.
In the past, road bikes came with ETRTO defined aluminum alloy rims and 23c tires, which measured somewhere around 23mm wide. With the advent of wider rims, wider tires and disc brakes, bicycle frame design for tire clearance has changed drastically in only a few short years. Further compounding this was the 2016 release of new ISO standards for bicycle safety, which specify legal minimum clearances between the tire and frame.
For our latest models, we have expanded on the concept of starting with a design-basis tire dimension by specifying a maximum tire-plus-clearance model when we initiate design. In the past, we attempted to find the largest tire possible in a given size, install it on the largest rim, and establish that as our design goal. However, we found that there are simply too many variables to continue with this method. Moving forward, instead of stating that “all such-and-such-size tires” will fit, we will provide the actual design-basis tire dimensions chosen for the bike model (in the form of wheel diameter and installed tire width), and require that these allowable dimensions are not exceeded.
NOTE: When considering a prospective tire’s compatibility with the frame, it’s important to respect the maximum measured size of tires, rather than relying on the manufacturer's nominal values, which are dependant on real-world tire pressure, loading, rim width, rim ERD, and tire-mold accuracy.
Failing to adhere to these design-basis tire dimensions can result in damage to the frame or fork. Circumstances that promote wheel flex under hard efforts, mud or ice buildup during spring riding, or picking up a rock from a freshly paved road, may cause an oversized tire to impact the frame. These are all realistic risks and should be taken seriously.
Figure 1: Tire to Frame Clearance
The image above depicts our tire-clearance model for a typical new frame design. The frame, in grey, is manufactured using a hard-tooled mold, which means the surfaces will not vary significantly from the initial design. The tire is blue with the design basis tire dimension given in green. The actual frame design clearance model is shown in red. The difference between these two dimensions is 4mm (per side), which is the ISO required minimum allowable clearance for safety.
To determine what tire clearance models to use, we started differently. Rather than trying to measure everything, we chose specific wheels and tires for different frame categories, including the latest designs and the most popular ones. We worked with our retailers, consumers and sales team to look at consumer trends, as well as talking with wheel and tire suppliers. Furthermore, new research has confirmed the old knowledge that larger tires inflated at lower pressures provide a more comfortable ride. But in addition, it has shown that by using wider wheels and modern tires, the rolling resistance of this type of setup is not necessarily any worse than a thin tire at high pressure. This has lead to many consumers pushing to go to wider tires on all kinds of bikes that they would not have considered previously. Based on all this, we measured a range of wheels and tires as detailed below.
How (Actual) Tire Width is Measured
- The tire is inflated to maximum recommended inflation pressure (as listed on the tire sidewall).
- Using a calibrated vernier caliper, the width of the tire at the widest point is measured at four points (12, 3, 6, and 9 o’clock). The resulting widths are averaged to give the final result.
How (Actual) Tire Diameter is Measured
We believe the industry best practice to measure diameter is to perform a tire rollout test.
- The tires are inflated to maximum recommended inflation pressure (as listed on the tire sidewall).
- To get an accurate result it is critical to keep the bike straight during the rollout. We built a metal track that ensures the bike stays straight, and is long enough to accommodate five wheel revolutions.
- The starting point is marked on the tire and the bike is rolled for five complete wheel revolutions, ideally. You should rollout for as many revolutions as you can fit and your tape measure will work for. This is usually around 3.
- We repeat this rollout five times to ensure accuracy. When doing at home you can just do a single measurement.
- The resulting distances for the five rollouts are averaged, and then are divided by five (or however many revolutions were done) to give one wheel circumference. Circumference is translated into diameter by dividing by pi (π) or 3.1415927.
Design Basis Tire Dimensions
Using all of this information we then had to actually come up with what dimensions to choose. Rather than trying to pick one single tire clearance model, we decided to have different ones for our different series of bikes. It is also important to note that tire clearance is not ‘free’. There is a tradeoff between stiffness, weight and tire clearance so having space for 35mm tires on a TT bike will come at a price. We used our experience to balance tire sizes and stiffness values for each series, based on how they are used.
What does all this mean? The tires equipped on your Cervélo complete bike were chosen to ensure the legally required minimum clearances to the frame and fork are met. If you want to change the tires (or wheels) on your Cervélo to something different than what it came with, you need to ensure that the tire width and resultant diameter are less than or equal to the maximum allowable values given for each frame. Use the above described methods for measuring the maximum width and diameter of the specific tire size and wheel combination you would like to use - DO NOT rely on the size of the tire listed on the sidewall. Remember that a tire’s width and diameter can change drastically depending on the exact wheel it is mounted on.