One of the most significant trends in road bike, mountain bike and gravel bike design has been the move to ever-wider tyres. That’s resulted in an increase in wheel rim widths too, because there’s a need to support the wider tyres properly to ensure they perform optimally and safely.
The improved accessibility of carbon rims, in place of alloy, has helped enable the increase in rim widths by reducing the weight and increasing the strength of the material required to make a rim wider.
Here’s everything you need to know about rim width.
What is rim width?
The width of a bicycle wheel rim is the distance across the rim from edge to edge. That’s as opposed to the depth, which is the distance measured from the outer circumference to the inner circumference of the rim.
Rim width is usually quoted as two figures:
- Internal width – the distance between the innermost edges of the rim wall
- External width – the distance between the rim wall’s outer edges
It’s worth noting that some aero rims (mainly seen in road bike wheels) bulge out further lower down the rim profile. In these wheels, the overall external rim width will be greater than the external rim width at the tyre bead.
The internal rim width (in the position where the tyre beads sit) is where official measurements for tyre compatibility and safety are taken, and are the vital measurements to pay attention to.
Why does rim width matter?
There are a number of reasons why rim width is important.
Firstly, it determines which tyre sizes will fit safely.
As a general rule, a very wide tyre on a narrow rim will be unstable thanks to inadequate support for the sidewalls and large air volume.
Similarly, mounting a tyre that is too narrow for a wide rim risks insecure fitting and damage to the rim if you strike an object as you ride.
Rim and tyre widths are determined by the ETRTO (European Tyre and Rim Technical Organisation), with a range of sizes that are deemed safely compatible. You should never ride a wheel and tyre combination outside of the safe range.
Although most wheel makers publish both internal and external rim width measurements, it’s the internal width that is most important when it comes to safe compatibility with tyres.
In general, a tyre will be wider than the visible external rim, but there are limits to how much wider it can be before it falls outside the safe range.
That said, there are examples of external rim widths that measure wider than the widths of the tyre, while remaining safe. This is usually in pursuit of maximising aerodynamic efficiency.
Rims have become a lot wider in recent years, with some road bike rims having an internal width of around 25mm or more, whereas 17mm or even narrower was a more usual width a few years ago.
External rim widths have also grown in tandem. External widths of 28mm or more are now common, and aero rims designed for maximum efficiency may be significantly wider still.
Hunt’s 60 Limitless Aero Disc wheelset has rims with an external width of 34mm, for example.
There’s been a similar expansion in the width of mountain bike wheel rims, with some as wide as 40mm across the rim bed.
Rim width, tyre volume and tyre pressure
As rim widths grow, the real width of a mounted tyre may increase too.
Most road bike tyres and gravel tyres are now designed around an ETRTO standard rim with a 19mm internal width, meaning they should measure true to their stated size on such rims.
Using rims with an internal width greater than 19mm will therefore see the inflated width of those tyres increase in tandem.
A nominally 700 x 28c road bike tyre may actually measure around 30mm wide on rims with a 21mm internal width, for example.
This results in the volume of air contained in the tyre being significantly larger. For example, a 700 x 28c tyre will contain around 15 per cent more air when stretched to a 30mm width than at 28mm on a narrower rim.
You can also get the extra volume without the additional weight a nominally wider tyre would bring.
In turn, that means that tyre pressure can be lowered because the weight of the bike and rider is supported over a larger surface area.
Road riders in particular typically run much lower tyre pressures than they did a few years ago, because doing so can bring benefits in comfort and rolling resistance.
The effect is less pronounced with mountain bike tyres, which have much larger air volumes already. Incremental increases in rim and tyre width lead to proportionately less increase in the air volume in the tyre, although there are other effects of increased rim width, as described below.
How to measure bike rim width
There are two widths to measure: the internal width and the external width. The internal width is more important for the safety of the wheel-tyre interface, while the external width is significant for frame clearance, aerodynamics and brake compatibility (if you’re running rim brakes).
Typically, wheel makers will quote rim width on spec sheets for their wheels.
If not, to get an accurate measurement of rim width, you need a set of Vernier calipers. These can measure the width of your rim precisely, either digitally or with a scale embossed on the caliper.
There are two sets of jaws to Vernier callipers and you use the main outside-facing ones to measure the rim’s outer width and the smaller inward-facing ones to measure the internal width.
You can also get a rough estimate, down to around a millimetre, using a ruler to eyeball the rim width.
How to match rim width and tyre width
There are various online resources you can refer to, including Schwalbe’s guide to recommended tyre widths for different rim widths. Other tyre makers, including WTB, have similar charts and most wheel makers will also provide a recommended tyre width range for their wheels.
Mountain bike tyres can usually be a little wider relative to the rim width than road bike tyres, because they have more inherent structural rigidity.
These recommendations will be based on the ETRTO’s recommendations, and should always be adhered to.
What’s the best rim width?
There’s no one answer to this question, but it will depend largely on the kind of riding you want to do, and the experience you want to have from your wheel and tyre combination.
As we’ve mentioned above, rims have become progressively wider over the last few years.
In general, modern road bike rims are between 19mm and 25mm wide, whereas mountain bike rims are between 25mm and 35mm wide. Gravel bike wheel rims are towards the top end of the range for road bikes.
Wider rims can offer a more rounded, less light-bulb-esque tyre shape that can improve handling. They can also offer aerodynamic advantages, because a wider rim is typically better able to recapture the airflow from the tyre and help reduce turbulence.
The main drawback of wider rims is that, all else being equal, they tend to be heavier than narrower ones. Though the performance benefits generally outweigh the negatives created by that increased weight, it may be something to consider.
If your tyres are towards the upper end of the spectrum – 28mm or above for road tyres, 42mm and above for gravel tyres and 2.6in and above for mountain bike tyres – your rim width should be wider for optimum tyre support.
You also need to consider your frame’s clearance. A wide tyre on a wide rim may push how much space there is between the frame and the perimeter of the tyre.
Road bike rim width
Opinions on rim width have changed significantly over the last few years. So whereas 17mm or narrower was a typical internal rim width eight or 10 years ago, 19mm is now a more usual starting point, and many road bike wheels now have much larger internal widths.
HED was the first brand to promote the benefits of wider rims and we remarked on the 23mm external width of its HED Ardennes wheels and the ancillary benefit of better aerodynamics and lower tyre pressures as long ago as 2008.
It took more than 10 years for the mainstream to catch on, but wide rims are now a feature of wheels from almost every wheel maker. For example Zipp’s wheels typically have 23mm internal width rims and ENVE’s go as wide as 25mm internal.
Mountain bike rim width
As with road bikes, there’s been a trend towards wider rims on mountain bike wheels.
With hard turns under compression a feature of mountain biking, the ability of wider rims to provide a less light-bulb-shaped tyre profile and reduce the tendency of the tyre to deform when turning is the key reason, just as with road bikes.
Older mountain bike wheelsets tended to come with 25mm internal width rims, but this has increased to 30 or 35mm internal in many modern mountain bike wheelsets.
Note that whereas mountain bike tyre widths are usually expressed in inches, rim widths are given in millimetres.
There are a couple of issues with going ever-wider though.
First, wider rims may lose rigidity over narrower ones because there’s more material between the point of attachment of the spokes and the rim’s edge. That potentially makes the rim more prone to damage.
Weight also increases. That’s less of an issue with carbon rims, which can be built stronger for a given width, while still being lighter than alloy, which is a driver of the trend for carbon wheels in mountain biking in particular.
Secondly, as rim width increases, the tyre profile becomes more flat, so there’s more of the tyre’s tread in contact with the ground. That’s great for straight-line grip, but means the knobs on the edge of the tyre designed to give you cornering grip aren’t there when you do make a turn, so your ability to turn at may speed actually decrease.
Zipp makes a benefit of the decreased rigidity of wider rims with its 30mm internal-width 3Zero Moto wheelset’s rims. These have a single-wall carbon rim (meaning it doesn’t have a hollow mid-section, as many other rims do) and are designed to flex under cornering load. This results, Zipp says, in less chance of rim and tyre damage, greater traction and a smoother ride, among other benefits.
As with mountain bikes, gravel bike wheels have to cope with high loads on uneven surfaces. Gravel bike tyres are wider than road bike tyres, so considerations of tyre-wall support lead to wider rims than on road-going wheelsets.
Although many gravel bikes come fitted with road-going wheelsets, it’s increasingly likely they will have gravel-specific wheels with wider rims.
Mavic’s Allroad gravel wheels, for example, have rim widths between 22mm and 25mm depending on the model, with many of the latest gravel wheels from most brands sitting around the 25mm mark.
650b gravel bike wheel rims tend to go even wider, to better support the wider tyres typical with this wheel size. For example, ENVE’s G-series wheels are 23mm internal width in 700c, but 27mm internally in 650b size.
Hookless rims and tyre width
Carbon bicycle rims are increasingly going hookless, with a straight profile to the edges of the rim rather than having a rounded hooked section at the top edge to help retain the tyre.
In a hookless rim, a close fit between the rim and the tyre holds the two together and prevents the tyre from popping off the rim.
This relies on precise tolerances in making the rim and the tyre, which haven’t always been the case in the past.
Although hookless rims are only compatible with tubeless tyres, some wheel brands are known to specify which tyres will work with their hookless rims and don’t recommend using tyres other than these. Some tyre makers will also stipulate that their tyres should only be used on hooked rims.
The main advantages of hookless rims are that they can be made lighter, and there’s less manufacturing complexity, which can make them cheaper.
The tyre-to-rim interface may also be smoother, which some manufacturers cite as an aerodynamic advantage to hookless rims.
Hookless rims are easier to use in mountain bike wheels, where tyre pressure is a lot lower than with road wheels. At higher road wheel pressures, there is a greater risk of the tyre separating from the rim, particularly if hitting an object at speed, which could result in explosive loss of pressure, damage to the rim and a crash.
Brands such as Zipp and ENVE, who have introduced hookless beads on many of their road bike wheelsets, recommend a maximum tyre pressure (typically 72.5psi / 5 BAR or less), which is much lower than has been typically used by road cyclists in the past.
This gives headroom for sudden increases in tyre pressure seen when the tyre is compressed by hitting objects, as described above, and helps prevent blow-offs.
The trend to wider rims for road bike wheels helps here, because you don’t need to run such high pressures for the same support.
For optimum performance and safety, then, it’s important to pay close attention to wheel manufacturers’ recommendations for compatible tyre sizes and pressures.