By Andrew Reimann

Mechanical Monday: Wider Isn’t Necessarily Better When It Comes to Tires

Over fifteen years ago, bicycle enthusiasts continued extensive rolling resistance testing for different tire compounds, but also decided to include the wider 25mm tires. The results they discovered astonished them. The assumption was that a wider tire would simply mimic the contact patch on the road, except on a slightly larger magnitude, meaning that rolling resistance would only get worse with a larger size tire.

Instead, when testers pumped tires up with the same pressure from a 23mm tire and a 25mm tire, they discovered that while the contact patch was wider, it wasn't nearly as long. The total surface area was smaller with the larger tire. Over the next six years, triathlon and time trail forums jumped on the data and confirmed the results with tests of their own.

The 25mm tire slowly became the new standard in solo endurance events. In the last few years, not only has the tire size been hailed as the better performing standard without much in terms of qualifying remarks, but already performance enthusiasts are looking toward the 28mm as the ideal size. I have even seen articles claiming that we will eventually use 41mm tires once we can overcome the aerodynamic drag with a wider wheel size.

Not only are these trends becoming silly, but they are missing a large factor in the performance a rider gets from their tires.

Riding the Prototype

When combating physics, physics always wins. This showed up for us recently. Over the last three weeks, our elite sponsored athlete, Kathryn Cumming, has been testing our new women’s prototype as we perfect the geometry for all kinds of road conditions.

Recently, we tested the clearance and capabilities of the frame with tires that measured out to 28mm. She reported that the ride quality was extremely smooth, especially when pairing the tires with a steel frame. She included that the tires felt like they increased her confidence and speed in the middle of long descents. Sounded like an absolute winner so far, but she instantly noticed a big drawback.

When starting up from a stoplight, starting to climb a hill, or coming out of a corner and getting back to speed, she noticed a sizable lag when she used the larger tires. When she went back to the 23mm tires, she could feel the potholes and bumps in the road more clearly, but she was able to accelerate out of corners with a lot more speed, and she was not feeling like something was dragging against her when she got out of her saddle on a climb or she shot off from a stoplight.

Rotational Mass

Pretend that you have two wheels of the exact overall weight. One has a heavy hub and a light rim; the other has a light hub and a heavy rim. If you put both on a hillside and let them go at the same time, the wheel with the heaver hub would accelerate down the hill faster than the wheel with the heavy rim. The latter has more rotational mass that it needs to overcome than the former.

What makes the 28mm vs. 23mm difference even worse in terms of acceleration is twofold: first, a 28mm tire doesn’t just redistribute weight, it adds it to the part of the bicycle where added weight costs you the most. It may sound like the difference in tire weight is almost negligible, but in reality, the average additional weight is between 110g and 160g per set (depending on type of tire). That is around the equivalent of adding between a 90 to a 120mm length stem worth of weight in tire material.

Second, not only is this added mass to the outside of the wheel, but a 28mm tire is also far bigger in diameter than a 23mm tire. And no, we're not talking a paltry 5mm difference here. Cateye offers some sizes that we have seen our consistent with our own findings. They size up a wheel with a 23mm tire at 2096mm in diameter, while a 28mm tire measures out to 2136mm. That is a solid four centimeter difference; in seat tube measurements, that's the difference in two frame sizes!

In other words, not only are you just adding 110g-160g where it hurts the most for acceleration purposes, but you are also adding that rotating mass beyond the diameter of a wheel with a 23mm tire. That's a compounding amount of work, and if you are riding in a group with a sizable amount of stop and go traffic, you are putting in significantly more effort in order to accelerate alongside the rider with the 23mm tires. Your brakes are also required to do a lot more work trying to slow you down as well.

Remembering the Value of the Tests

We are not trying to harp on larger tires, just the way they are beginning to be marketed and discussed. 

There is a very good reason we built frames to allow for a tire with a 28mm width. There are plenty of applications where a larger tire size reigns supreme. Over cobblestones and rough roads, riders with thinner tires waste loads of energy as their bike bounces up and down, devastating their forward momentum. On long, flat rides with no traffic signs, the superior rolling resistance of the larger tires is obvious. In a solo time trail, especially one with a flying start, you’d have to be a fool not to use the widest tire in your disposal as long as it doesn’t hurt your aerodynamics. Why? In this situation, a wider tire’s weakness in acceleration is its strength in momentum. When you actually get that rotational mass up to speed, it wants to hold you there. In a flying start at a flat race course, the rider with the wider (heavier) tires is literally given the advantage of a superior flywheel over their opponents.

Over the last three years, there have been a large number of articles published touting the benefits of wider tires, usually describing how many seconds you can shave off a solo event. While this is technically true, it hardly tells the full story. To make matters worse, there has been a run of marketing claims, such as one ad describing that a company’s 25c tire is 10 watts “better” than the 23c version.

We have to remember how all of these tests showing the superior rolling resistance are being run. Typically the different tires are being tested at a predetermined speed. These same tests don’t measure the amount of watts it takes to accelerate each wheel up to that speed.

This is a long way of saying that if you are looking at a tire for an event where low rolling resistance is king, or are looking to smooth out a ride, you’ll likely love that wider tire. Just don’t expect to throw those same tires on a stop and go group ride, a criterium, or a miss-and-out track event at the velodrome and expect the same benefits to carry over. Like most things in cycling, there is a trade off to be made.

Just make sure you're making that trade for the right reasons.

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*Note: this increase in diameter applies to wider tires, not wider rims. Going wider with tubeless and clincher rims counter-intuitively decreases the overall diameter of the full wheel.