Today, we hear a lot about “modern rim shape”. What’s behind this is simply wider rims. Some have succeeded in convincing cyclists that a wide rim makes a better wheel and nowadays, each brand launching a new wheel has to make sure its rims are at least 1mm wider than their competitors.
But what they have forgotten in their equation is the tyre. Nobody talks about the shape that the tyre takes when the rim width increases. Nobody talks about what is the right size tyre to put on those wide rims and everybody forgets about safety standards and norms.
So, talking about rim width alone is only doing half the job and it becomes pointless. It’s only when you look at both variables of the equation (tyre + rim) that it makes sense. Performance and safety are also at stake.
2. HOW TO MEASURE WIDTH
2.1 Rim Width
Rim width is measured at 2 points :
- Outer width (A): most often measured at the brake track
- Inner width (B): measured inbetween the rim hooks
The outer width (A) will mostly have an influence on the look of the rim, but also on the aerodynamics of the wheel-tyre system. But it has zero influence on the comfort, rolling efficiency or tyre shape.
The inner width (B) is the one that defines the tyre shape, by pinching its sidewalls. It influences the air volume and tyre width.
So, this is really the one measurement we have to look at when discussing rim width to tyre width in the respect of rolling efficiency, comfort and air volume.
2.2 Tyre Width
As we just mentioned, the width (section) of a tyre is directly linked to the rim on which it is mounted. It is also linked to the amount of air pressure.
To neutralise those variables, a norm defines how a tyre section should be measured. This norm called ETRTO (European Tyre and Rim Technical Organisation) has defined all the safety and performance parameters that a tyre and rim should respect to be compatible with one another. Although the name says “European”, this organisation’s work is the reference for all international and national norms in the bike industry (ISO, CEN and their national declination).
To measure a tyre section, ETRTO has defined:
- The rim width on which each tyre section must be measured
- The pressure at which it must be measured, taking into account that the casing of a brand new tyre will stretch within the first 24 hours of inflation, so the measurement should be done after those 24 hours
For example, a tyre said to be a section of 23mm should be measured on a rim of 15mm internal width at 6 bars (87 PSI) after 24 hours.
A tyre said to be a section of 28mm should be measured on a rim of 17mm internal width at 6 bars (87 PSI) after 24 hours.
Only the casing of the tyre should be measured. If the tyre has side knobs or an extra rubber layer down on its shoulder, this should not be taken into account.
But at the end of the day, it’s the manufacturer’s responsibility to indicate the right dimension of the tyre and one should always be able to rely on those indications so the tyre choice can be made safely.
3.1 How the tyre section influences the rolling resistance and the tyre pressure
To increase your speed for the same effort, the Rolling Resistance Coefficient (Crr) must be as low as possible.
Our measurement shows that the Crr decreases when the tyre pressure increases:
However, the Crr decreases when the tyre section increases. The graph bellow shows that at equal tyre pressure and rim width, the Crr gap between a 23mm and a 25mm tyre (for a given tyre) is about 0.3/1000:
So, when reading the 2nd graph in light of the 1st one, you can see that this 0.3/1000 gap is equivalent to a pressure difference of 1.5 bars / 22PSI.
In short, the Crr of a 23mm inflated at 100PSI, is the same as that of a 25mm tyre inflated at 80 PSI.
While increasing the tyre section, the rolling resistance is maintained, while hugely improving your comfort.
And if your goal is to lower your rolling resistance, you can just go for a wider tyre and keep the same riding pressure.
The same would go with even bigger tyres (28mm) but then weight and inertia will come into account and there will be a trade off between rolling resistance, low inertia and weight.
3.2 Better ride quality with both wider rims and wider tires
The “ride quality” is influenced by many factors and cannot be reduced to just one single number. It’s more a question of “feeling” and, as such, it is a personal matter and doesn’t have just one universal definition.
However, in most instances, cyclists refer to ride quality in terms of comfort and high “speed to effort” ratio.
The air volume is a good way to look at it.
More air volume means a stronger protection against snakebite, by increasing the distance of the gap between the ground and the rim edge.
As a result tyre pressure can be lowered to improve comfort and traction without increasing the chances of pinch flats.
So, how to increase the air volume? Here are few measurements:
|Width||Width change||Air volume difference (mL)||Difference|
|17mm rim||Tyre 23–>25||144,25||15,0%|
|25mm tyre||Rim 17 –> 19||24,04||2,2%|
A 2mm increase of the tyre width alone has a much higher impact (+15%) than a 2mm increase of the rim width alone (+2.2%).
And of course, the combination leads to both air volume increases, which is even better.
3.3 Better aerodynamics with wider rims
For a long time the general thinking was that to be faster a part (let’s say a wheel) had to be narrow to reduce its frontal surface. It’s true, but this doesn’t take into account real world conditions where the wind doesn’t always blow right in front of the cyclist.
With high section rims becoming more and more popular it also became obvious that the ease of handling of such wheels had to be improved.
Based on those 2 factors research began to improve the wheel’s aerodynamic performance. Two facts became obvious:
- The tyre had to be part of the equation: you can spend hours developing the right wheels but as soon as you change the tyre the behaviour of the system in windy conditions changes dramatically.
- The best way to achieve amazing aerodynamic performance is to define a rim-tyre shape that is as close as possible to a NACA airfoil profile.
The widest point of such a profile is quite far from its leading edge. On a bicycle wheel, that means that the widest point of the rim-tyre combination must be below the tyre itself. In short, the rim has to be wider than the tyre.
Doing that, the trailing edge radius gets rounder to the benefit of the wheel’s stability in a crosswind.
The frontal drag (at 0° of yaw angle) becomes higher but at every other yaw angle this design lowers the drag and improves the stability of the wheel.
The graph bellow compares two 40mm wheels fitted with the same 23mm tyre. The blue curve is a rim with 19mm outside width, whereas the pink one is has 25mm outside width.
The gap is 2,5 Watts in favour of the wide rim, based on our yaw angle weighted law (http://www.engineerstalk.mavic.com/why-weightings-should-be-applied-to-wheel-drag-data-to-measure-aerodynamic-performance/)
But, if you use a 25mm tyre on the 25mm wide rim you’ll be back to the blue curve and even higher since the frontal area has become bigger!
So, widening the rim for aero benefits is really efficient as long as the tyre remains narrower than the rim’s widest point.
If you think “I have a wide rim, so I can ride a wide tyre”, that’s true but means that you’ll annihilate the aero benefits of the wide rim. But maybe you’re pursuing another goal, like more comfort or lower rolling resistance…
Tyre width and rim width have to be made for each other.
If the tyre is too wide relative to the rim width, it will not be well supported by the rim and the ride may feel flimsy. There is also a higher chance of pinch flats.
Equally, if the tyre is too narrow relative to the rim width, it will get a flat shape that is bad for rolling efficiency and ride feel. For a MTB tyre this can result in the side knobs constantly coming in contact with the ground even in a straight line. The tyre will not deform as it is supposed to and the grip and rolling efficiency is no longer guaranteed.
But most importantly, there is a safety issue.
Some tests performed by several tyre manufacturers, including Mavic, have shown that a narrow tyre on a wide rim has a higher chance of severely coming off the rim.
Norms are setting barriers and rules to ensure rider safety.
ISO4210 et ETRTO regulations have recently redefined the tyre and rim combinations usable in the best security conditions. An essential parameter has been added to these combinations: the tyre inflation pressure.
The result of this work is summarized in the table below and applies to use of tubeless tyres.
For the use of an inner tube, you should add about 15% to the pressures indicated in the table.
TC: Tubeless-compatible rim
TSS: hook-less tubeless rim
1 bar = 14.50 psi
The gray cells indicate that no inflation pressure has been determined such that the combination of that tyre width with that (internal) rim width can be used completely safely.
If the tyre or the rim indicate a pressure different from that indicated in the table, it is the lower pressure that should be respected.
- A wider tyre and rim allow for better rolling efficiency while improving comfort
- A wider rim improves aerodynamic performance (low drag and high stability) as long as the tyre width doesn’t increase at the same time
- Safety is a key item. Norms have defined the working parameters of tyre to rim widths and should always be respected
For safety matters always make sure your wheel and your tyre bear the ETRTO mention and that their sizes match according to the chart in this article.
To find out the best combination for rim and tyre, taking into account who you are and what kind of ride and road you’re riding, download the MyMavic app for iOS and Android.