One of the most significant trends in road bike, mountain bike and gravel bike design has been the move to ever-wider tyres.Thats resulted in an increase in wheel rim widths too, because theres 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.Heres everything you need to know about rim width.What is rim width? Some aero wheels reach their maximum width mid-rim. Hunt wheels The width of a bicycle wheel rim is the distance across the rim from edge to edge. Thats 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 walls outer edgesIts 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? Rim and tyre width both have a significant effect on tyre profile. Katherine Moore / Immediate Media 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, its 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. Wider rims are a trend in both road and mountain bikes, assisted by the use of carbon fibre. Alex Evans / Our Media Rims have become a lot wider in recent years, with many modern road bike rims having an internal width of 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.For example, the Hunt 5AM Limitless Ti_UD Carbon Spoke aero front wheel has a rim with an external width of 34.5mm and the rear is 30mm wide, both with 23mm internal width.Theres 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 A greater internal rim width can plump 28mm tyres up to over 30mm. Ashley Quinlan / Our Media 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 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. Actual tyre width varies significantly with rim width. Enve 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 Vernier calipers enable you to measure the internal and external width of your rims accurately. Russell Eich / Immediate Media 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 youre 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 calipers and you use the main outside-facing ones to measure the rims 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 Many brands have guides to help you match rims and tyres. Continental There are various online resources you can refer to, including Schwalbes guide to recommended tyre widths for different rim widths. Other tyre makers 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 ETRTOs recommendations, and should always be adhered to.Whats the best rim width?Theres no one answer to this question, and it will depend largely on the kind of bike you're riding and the experience you want to have from your wheel and tyre combination.You also need to consider your frames 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 HED was an early advocate of wider rims for road bike wheels. Steve Sayers / Our Media Opinions on rim width have changed significantly over the last few years. So whereas 17mm or narrower was a typical internal rim width a decade ago, 19mm is now usually the absolute minimum 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, Zipps aero road wheels have 23mm or 25mm internal width rims. ENVEs go as wide as 25mm internally, although its newer SES 4.5 Pro wheels are 23.5mm and its SES 6.7 Pro wheels are 22.5mm wide internally.Mountain bike rim width A wide rim may mean the side knobs are not positioned to provide optimal cornering grip. Jack Tennyson / Our Media As with road bikes, theres 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 theres more material between the point of attachment of the spokes and the rims edge. That potentially makes the rim more prone to damage.Weight also increases. Thats less of an issue with carbon rims, which can be built stronger for a given width, while still being lighter than alloy, which is one driver of the trend for carbon wheels in mountain biking in particular. A tyre will deform less under cornering load if it's supported by a wider rim. Elite Wheels Secondly, as rim width increases, the tyre profile becomes more flat, so theres more of the tyres tread in contact with the ground. Thats great for straight-line grip, but means the knobs on the edge of the tyre designed to give you cornering grip arent there when you do make a turn, so your ability to turn at speed may decrease. The Zipp 3Zero Moto wheel's rim is designed to flex under asymmetric load. Zipp Zipp makes a benefit of the decreased rigidity of wider rims with its 30mm internal-width 3Zero Moto wheelsets rims. These have a single-wall carbon rim (meaning it doesnt 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.Gravel bikes As gravel bike tyre width has increased, so has the width of gravel bike wheels. Scott Windsor / ourmedia 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.45mm is now a typical width for tyres fitted to newer gravel bikes and rim widths have expanded to keep pace. The Newmen Streem G.62, for example, has a 27mm internal width, although its external width balloons to 45mm. The Zipp 303 XPLR is 32mm wide internally, while the ENVE G SES 6.7 Pro betters even that at 35mm.Hookless rims and tyre width Hookless rims can be lighter and cheaper, but they require precise tolerances. Warren Rossiter / Our Media Hookless rims have been the norm for performance MTB wheels for some time. These have 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. Hookless tech has spread to gravel bike wheels too and, to some extent, to road wheels.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 havent always been the case in the past.Hookless rims are only compatible with tubeless tyres. But, particularly for road wheels, some wheel brands publish a list of which specific tyres and widths will work with their hookless rims and dont recommend using tyres other than these. Some tyre makers will also stipulate their tyres should only be used on hooked rims. A hookless rim has a straight-edged profile to the rim. Hunt Bike Wheels The main potential advantages of hookless rims are that they can be made lighter, and theres 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.Pros have called for hookless rims to be banned for road racing and the UCI has toyed with a ban, which is one reason many newer performance road bike wheel rims are designed with mini-hooks. ENVE's newer SES Pro wheelsets have mini-hooks rather than being hookless. Ashley Quinlan / Our Media Brands such as Zipp and ENVE, who have introduced hookless beads on many of their road bike wheelsets, recommend a maximum tyre pressure of 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 dont need to run such high pressures for the same support.For optimum performance and safety, then, its important to pay close attention to wheel manufacturers' recommendations for compatible tyre sizes and pressures.For more information and advice, on topics such as silencing noisy disc brakes and repairing punctured tubeless tyres, check out our workshop hub page.