How does rim width affect gravel tyre performance?

Si Richardson looks at Shimano’s GRX range of wheels to find out how their varying rim widths affect tyre performance

Clock18:37, Friday 1st March 2024

It’s not that long ago that rim widths were an overlooked part of a wheel’s design, resigned to a figure that frequented the specifications sheet, but was overlooked by most cyclists.

That’s all starting to change. More is now understood about the importance of rim widths and manufacturers are increasingly focussing on the measurement when designing their rims. This is for good reason, as rim width has a big impact on performance, altering comfort, aerodynamics and rolling resistance.

All of these things become especially important for gravel riding, where a much wider range of tyres are used.

To find out more about the subject, GCN’s Si Richardson put Shimano’s GRX and GRX Carbon wheels to the test to see how they compare. The GRX is narrower, with a 21mm internal width, while the carbon is 25mm wide, but how would this affect his ride?

Check out the video at the top of the page to find out, or read on for a guide to the importance of rim widths.

What is rim width?

The vast majority of road and gravel wheels have 622mm diameters; these are commonly referred to as 700c wheels. Some bikes have slightly smaller 650b wheels, which have 584mm diameters, but few stray from these two measurements.

Wheels start to diverge, however, through their rim widths. As the name suggests, this is the width of the wheel rim, except it is measured in two very specific places.

The first is the external rim width, which is the measurement between the rim’s outer edges. This has a huge influence on aerodynamics as a wheel’s profile can be optimised to work in tandem with a tyre to smooth airflow. Optimal, in this instance, is when the rim is five percent wider than the tyre.

Then there is the internal rim width which has a bigger bearing for most riders who aren’t as bothered about aerodynamic performance, but do care about ride feel. It is the distance between the innermost edges of the rim wall.

Relationship between rim width and tyre performance

Wheel rims obviously play an important role, helping to secure the tyre in place, but the rim width also alters the performance characteristics of the tyre, governing its width, among other things. This can have a big impact on tyre performance.

Tyre width is one of the most important factors affecting how your bike rides. Wider tyres provide more comfort and are faster on anything bumpier than tarmac, effectively providing a bigger spring. They can be run at lower pressures which means more squish, so they can absorb more bumps and vibrations from the road or trail, which allows you to roll with much less effort. You’re less likely to get impact punctures too.

Wider is only better up to a point. What point that is depends on you, your riding style and where you ride. Generally, when you go up to 40mm, 45mm and beyond while using the same rim, the tyre changes shape, which in turn alters its dynamics.

Pro and cons of narrower rims

A tyre on a narrower rim will bulge out, taking on the shape of a balloon. There are some positives to this, namely added comfort, as the tyre will overhang over the side of the rim. It can then compress more easily at a given pressure.

The wider the tyre, the more overhang there will be, which in turn leads to more comfort, although this does come at a sacrifice. Too much overhang can make your tyres feel 'squirmy' when cornering.

To combat this, the walls of the tyres need more support. You can build that support into the tyre itself, something a lot of mountain bike tyres benefit from because they are even wider, but this is likely to be detrimental to the feel of the tyre. The other solution is to use wider rims.

Pro and cons of wider rims

So, a wider rim allows you to run a wider tyre without having to worry about excess squirm when cornering. That’s only the tip of the iceberg, though, as wider rims come with other performance benefits too.

As previously mentioned, wide rims can be aerodynamically optimised for specific tyre widths. The idea is that the rim and tyre work together to provide an optimised shape for airflow.

Then there’s the matter of rolling resistance, the lesser-mentioned hidden force working against cyclists. Aerodynamics take all of the headlines, but rolling resistance is up there in the top-three forces that act on a cyclist.

According to a study on Bicycle Rolling Resistance, rim width affects rolling resistance, albeit only factionally. In its tests, a 4mm increase in rim width decreased the average rolling resistance across a range of pressures by 0.3 watts per wheel. A fairly miniscule amount, although the lower the pressure you run, the greater the difference. At really low pressures, it might be as much as 0.5 watts per wheel.

Rim width can have a big impact on ride feel and performance, then, but how noticeable are different rim widths when riding? Watch the full video at the top of this page to find out.

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