Cycling wheel rim width vs tyre width - is there a simple relationship with aeroydnamics?

PH587141 • 4 August 2025
It has often been said that to have good cycling wheel performance, there is a "rule" that the external rim width of a wheel should be at least 105% of the width of the tyre, or that there is a simple relationship/formula that will predict the aerodynamic performance of a wheel based on rim width vs tyre width.

Whilst this may have rung true with older models of wheels which shared a V shaped or flat sidewall shape, modern rims come in a range of shapes which vary in terms of their aero performance due to a number of factors, not just external rim width. Rim depth, the vertical location of maximum rim width, nose profile (at the spoke entry point), dropoff angle to nose profile and "brake track" angle are all critical components that add up to allow for a fast rim shape.

The data below is a subset, taken from a wind tunnel test of deeper time trial/triathlon wheels, both with normal spokes and composite multispokes (3 and 4 spokes), exploring if there is a simple relationship between the external width of a wheel and its performance, all with the identical same tyre.

The goal of this test was to establish if there was a simple relationship between the external rim width of a wheel and its aerodynamic performance, in particular looking to see if a wheel with >105% external width in relation to the tyre width was more or less aero than one with <105% dimension.

The tyre used for this test was a Vittoria Corsa Pro Speed in 28mm width.

WIND TUNNEL

When interpreting wind tunnel results it is important to understand yaw angle weighting.


Real world cycling is affected by changes in wind speed and direction.

The angle of attack of the wind on the cyclist is referred to as “yaw angle”, and this is affected by four components - rider speed/direction as well as the wind speed/direction.


While cycling on the road, you will experience a range of yaw angles. Very high yaw angles are much less common than lower yaw angles, and this can be expressed as a weighting function - for example at 45kph you spend ~80% of your time at 7.5deg yaw or less. At <40kph this extends out to 10deg, and at >50kph you will typically see 5deg or less.


We tested the aerodynamics of these wheels at our typical test speed of 45kph. This allows us to get good clean data (compared with testing at lower speeds, where the resolution isn't as good) which can then be applied to lower speed calculations later. As an example, a 10w aerodynamic drag saving at 45kph is equivalent to 4.7w at 35kph.

WIND TUNNEL

The results from the wind tunnel testing are shown in a chart below. 


In this data set there is no relationship between the external width of a tyre and rim and the overall aerodynamic performance of the wheel. For example the wheel with the highest CdA had a tyre/rim width relationship of 112%, whereas the wheel with the lowest CdA had a relationship of 113%. In the 9 wheels shown in the chart, the wheel in 5th place had a relationship of 93%.

From this data set we can say conclusively that there is no simple relationship between tyre width and external rim width when looking to establish aerodynamic performance, across a range of wheels from normal spoked wheels to multispoke composite wheels.


Instead, each wheel must be analysed individually for aerodynamics. Wheel aerodynamics is complex, and so extensive testing has to be completed to provide recommendations on the best tyres to use for that particular wheel in terms of aerodynamics, but also rolling resistance.

Track cycling inner tube Crr testing: TPU vs latex

by PH587141 • 30 July 2025
Reducing rolling resistance is imperative to improved cycling performance, particularly during high speed track cycling. A tyre with a high rolling resistance will require more power to travel at the same speed, whilst a low rolling resistance tyre requires less power. We have previous shown that top end clincher tyres, such as the FMB/AeroCoach Pista Ultra 3000 have extremely low rolling resistance. We have also found that for road tyres at road pressures, latex inner tubes are the best option for reducing Crr compared with TPU tubes. In this test we explore the difference between TPU and latex tubes but at higher, track relevant pressures and with a track specific tyre rather than a road tyre.

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Dr Xavier Disley of AeroCoach & Chris Hall are performing a road bike fit at AeroCoach HQ

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by PH587141 • 23 October 2023
Chris Hall came to visit us at AeroCoach HQ in order to improve his bike fit on his Cervélo road bike, specifically focusing on aerodynamics for road bike time trials. Holding different positions such as in the drops, the breakaway aero position and on the hoods we explored the impact on Chris' body position and how that would relate to his aerodynamic drag. Watch the video to improve your road bike fit for aerodynamics: Xavier and Chris discuss narrow handlebars, turned in levers, torso angle, aero helmets and more!

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Using clip on aerobars on a road bike is a great way to improve your aeroydnamics. Your body accounts for around 80% of the aerodynamic drag of the bike and rider system. In this test we explore the impact of holding different positions on a standard drop handlebar, alongside an aerobar position using a set of Ascalon extensions. A lower aerodynamic drag will result in less power required to travel at the same speed, or for the same power will cause an increase in riding speed, when not travelling up steep gradients.
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