What you need to know about tyre pressure, rim width and the limits of safety

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Take a look at the sidewall of any bicycle tyre and you’ll find a collection of numbers. One set pertains to the dimensions of the tyre, while the other is concerned with inflation pressure. Both are fashioned after the markings that appear on automobile tyres and both provide crucial information for the safe use of the tyre. Australian tech editor Matt Wikstrom dives into the detail.

It has been well over 100 years since the pneumatic tyre was invented and for most cyclists, it is easy to take the safety and reliability of the system for granted. But for tyre manufacturers, safety remains a primary consideration for every new tyre that is created even as they work to improve weight, rolling resistance, grip, and puncture resistance.

Wrapping a balloon around a rim is not an easy thing to do. The major challenge for early inventors was keeping the tyre on the rim. Adhesive was an early solution embraced by Dunlop for its first tyres, while Edouard Michelin developed a detachable tyre that was clamped to the sides of the wheel rim. The latter employed an inner tube and was easier to repair, but Michelin’s system was very heavy, so racers gravitated towards lighter tubular tyres.

As work on the modern clincher progressed, it soon became apparent that the design of the rim had a critical role to play, which eventually led to the creation of the hooked rim profile that now defines clincher rims. While those hooks look like they are designed to keep the tyre on the rim, they are more important for guiding it into place as it is inflated. What actually keeps the tyre on the rim is a near perfect match between the diameter of the bead of the tyre and the rim where the tyre will sit.

With this in mind, tyre and rim manufacturers were quick to recognise that standardised rim and tyres sizes were needed to ensure the safe fit of tyres, but up until the 1960s, standards were only a national phenomenon. This led to some pretty profound incompatibilities when tyres were traded between nations because the nomenclature that had been developed did little to distinguish between incompatible rim/tyre combinations.

Common versus standardised tyre size nomenclature
The ETRTO’s tyre nomenclature removes a lot of the confusion surrounding tyre and wheel sizes but most consumers are still unfamiliar with it.

Having wrestled with the same issue for automotive tyres, the newly-formed European Tyre and Rim Technical Organisation (ETRTO) stepped in to standardise bicycles tyre and rim sizes in the late ‘60s. Where once tyres were defined in terms of outer diameter, the ETRTO concentrated on the diameter of the tyre bead and converted all measures from inches to millimetres.

The new system reconciled all of the various national standards, and by the early ‘80s, it would go on to become international standard ISO 5775. Importantly, the system is only concerned with clincher-type tyres and rims (including tubeless variants) and does not extend to tubular tyres, which continue to be defined by traditional standards.

Sidebar: an introduction to the ETRTO

Founded in 1964, the ETRTO is a non-profit organisation that is devoted to the “alignment of national standards to achieve interchangeability of pneumatic tyres, rims and valves in Europe as far as fitting and use are concerned.” It is only concerned with the technical aspects of these products as they apply to any wheeled vehicle, be it a racing car, family vehicle, tractor, or a bicycle.

Headquartered in Belgium, the ETRTO is comprised entirely of industry representatives, primarily from Europe, however manufacturers from other nations (e.g. Canada, USA and the UK) are able to participate in the organisation’s activities. Members must simply manufacture a tyre, rim, or a valve, and have no corporate affiliation with an automobile manufacturer.

While the organisation has undertaken to standardise rim and tyre dimensions, including tolerances, it is not a regulatory body that is able to enforce its recommendations. In fact, it is not necessary to be a member of the ETRTO — or even to adhere to its standards — in order to manufacture and sell rims, valves, and tyres. The only manufacturers that have any obligation to adhering to ETRTO standards are its members, which include Mavic, Campagnolo, Schwalbe, Continental, Michelin, Goodyear and Pirelli.

Tyre and rim sizing according to the ETRTO

Once the ETRTO/ISO adopted bead-seat diameter, it was able to simplify the nomenclature for tyre and rim sizes so that compatible combinations could be easily identified by the user. If a tyre and rim share the same bead-seat diameter, then they promise to be a safe match.

Unfortunately, while bead-seat diameter has been in widespread use for at least a few decades, and is displayed on both tyres and rims, it remains an unfamiliar measure. Most shoppers think of road wheels and tyres in terms of the traditional moniker (700C) rather than the 622mm that describes the bead-seat diameter for each. This is something that the industry recognises, which is why many tyres are labelled with both the ETRTO designation and the traditional size.

Deciphering the ETRTO’s nomenclature on tyres is not difficult, since there are just two numbers: the first refers to the width of the tyre casing (excluding the tread, measured in millimetres) while the second represents the bead-set diameter of the tyre (also measured in millimetres). Some examples of the ETRTO nomenclature for common tyres sizes are shown in the table below:

table of ETRTO nomenclature for some common bicycle tyre sizes
The ETRTO’s standardised nomenclature comprises just two figures, one for tyre width (left) conjoined with the bead-seat diameter (right). All measurements are in millimetres.

While the ETRTO designation is typically printed on or moulded into the sidewalls of most tyres, it is less common to find it on wheels and rims. As a result, a side-by-side comparison of any given tyre with a wheel/rim may not always be possible, which does a lot to undermine what the ETRTO nomenclature has to offer.

Also missing from the majority of wheels and rims is any mention of the internal width of the rim. This is important, because in an age where rim widths have been growing rapidly, the ETRTO has found that a tyre can now be too narrow for a rim. For example, the organisation cautions against fitting a 23C tyre to a rim with an internal width of 17mm or more because there is an increased risk of the tyre coming off the rim, especially when high inflation pressures are used.

That’s because a wider rim allows the beads of a clincher tyre to sit further apart, and as the tyre loses its bulbous shape to assume a rounded profile, there is a dramatic increase in the amount of tension on the sidewalls. It’s enough to challenge the strength of the tyre bead, and tests by ETRTO members have shown that there is an increased risk of a blowout.

This added risk accounts for the organisation’s conservative recommendations around specific rim/tyre width combinations, which are set out in the table below:

Table of ETRTO recommendations for tyre/rim width combinations
The ETRTO recommends that a tyre must be considerably larger than the width of the rim bed.

Like bead-seat diameters, these recommendations are not widely known, and there is no indication on the sidewalls that any specific tyre width must be matched to a specific range of rim widths. So for the user, it is difficult to knowingly abide by these suggestions, especially when these recommendations are at odds with those of some wheel manufacturers (For example, Zipp states that its latest carbon clincher rims that have an internal width of 19mm have been optimised for a 25mm tyre, while its 21mm rim beds are best suited to 28mm tyres).

It’s not the only instance where non-compliance has occurred within the industry. During the early years of tubeless tyre development, some rim manufacturers actually increased the bead-seat diameter of their rims to provide a tighter fit for the tyre. It was a step that was taken in the name of innovation that arguably improved the safety of the system, yet it occurred in direct contravention of existing standards.

Adding further to confusion in this realm is the growing disconnect between nominal tyre width and measured tyre width, especially for road tyres. The nominal tyre width that is shown on a tyre is always measured for a standardised rim, however the actual rim width used to make that measurement is not openly declared, and tyre manufacturers rarely provide any information on how it might change for other rim widths. As a result, designated tyre width has lost much of its original meaning.

On top of all of this confusion and ambiguity, there is the influence of quality control. Mass manufacturers like Mavic, Campagnolo/Fulcrum, and Shimano are known for very strict quality control, but it’s an expensive undertaking that smaller companies may not be not equipped (or inclined) to match. It makes for another layer of product variation that goes a long way towards accounting for those instances where there is a poor, even unsafe, fit of a tyre on a rim despite matching bead-seat diameters.

While ETRTO nomenclature is a common sight on tyres, it is much harder to find this information on rims, so ensuring a suitable match can be difficult for consumers.

The year ahead for the ETRTO: defining a safer tyre fit

Speaking with industry representatives, there is a feeling that the ETRTO has had trouble keeping up with the rapid pace of change in rim and tyre design in recent years, and at least some of the the organisation’s recommendations have become outdated. Be that as it may, the ETRTO is working on updating its standards and recommendations for 2019, with a key priority being a much clearer definition for the tyre/rim interface for a safer tyre fit.

At this stage, it is too early to say what this will look like, but Mavic’s specifications and tolerances for its new road tubeless system might provide a clue on what to expect. With the goal of providing a tubeless system that is 100% safe, the company has tightened tolerances for the bead seat diameter of its rims and tyres, started measuring how much its tyre beads stretch under load, and is insisting on lower tyre pressures.

While changes to the ETRTO’s standards are likely to be incorporated into the international standard (ISO 5775) at some point, any changes in manufacturing practises are likely to be slow in coming. As mentioned above (see sidebar), only some manufacturers will have any obligation to abide by a change to the ETRTO’s standards, while compliance with the international standard is, at least in part, voluntary.

Tyre pressure recommendations

While the ETRTO and ISO 5775 spell out minimum tyre pressures on the basis of size and intended use, the final recommendations that are printed or moulded onto the sidewalls of a tyre are determined by the manufacturer. Some recommendations take the form of a range, while others simply define a maximum .

The parameters that are used to define these recommendations appear to vary from one manufacturer to the next, however there is an underlying theme of safety. In this regard, the maximum pressure is the most important number to abide by, though some brands, such as Maxxis and Continental, admit that they build their tyres to withstand much higher pressures.

The consequences of exceeding any recommended maximum should be obvious, though the risk of a blowout is not solely associated with the tyre. The rim also has an important role in deciding the fit with any given tyre. A relatively small reduction of just a few millimetres in the bead seat diameter of the rim can have a profound effect when high inflation pressures are used.

Most tyres recommend a range of pressures though it’s only the maximum that needs to be adhered to. The risk associated with over-inflating a tyre should be obvious: BOOM.

Some rims, especially carbon rims, stipulate a maximum allowable tyre pressure that can be be significantly lower than the tyre. Unfortunately, this is not often shown on the rim, so there is a real risk that it will be overlooked by owners that have a preference for high tyre pressures. This becomes especially important when the wheels are used in mountainous terrain, because the heat from extended periods of rim-braking will increase air pressure in the tyre.

Road cyclists have traditionally favoured high tyre pressures with the hope that it will reduce rolling resistance, and it’s a practise that continues today despite the fact we now have a much better understanding of the performance benefits that lower pressures and wider tyres have to offer. Tyre manufacturers have a good appreciation for both and some have reduced their maximum recommendations to stop consumers from over-inflating their tyres.

For those tyres bearing a range of tyre pressures, the lower recommendation can be derived a variety of ways, too. Some brands directly test the risk of a pinch-flat at various pressures to come up with a reliable number, while others concentrate on the weight of the rider (or the total load). The final recommendation will vary with the size and construction of the tyre, where larger and more robust tyres can be used at lower pressures.

Nevertheless, the indicated minimum remains a recommendation rather than a strict limit. Indeed, at least some manufacturers recognise that experienced riders are able to use lower tyre pressures without the same consequences that might afflict a novice. There is also an understanding that no matter how extensive any testing effort is, it is still not possible to reproduce every set of conditions that a rider may experience.

A pinch-flat or a cut tyre remains the biggest risk when using low pressures, but there are other hazards, such as damage to the casing and/or an increase in wear. The former manifests as long cracks in the sidewalls (which are quite distinct from the short cracks caused by perishing), while excessive wear on the tread outside the centreline of the tyre is consistent with the latter. Users of tubeless tyres also face the added risk of burping (where air escapes from the tyre).

Tyre pressure recommendations typically decrease as the width of the tyre increases but users are still free to use pressures that are less than the recommended minimum.

Putting tyre pressure recommendations into perspective

Most cyclists intuitively understand that the amount of air in a tyre can have a profound effect on its performance. Comfort, grip, and speed can all vary with tyre pressure, however the recommendations that appear on the sidewalls do not address any of these aspects. As discussed above, they are concerned with safety, and while adhering to them won’t prevent a blowout or pinch-flat from occurring, they will serve as a good starting point for any rider using a tyre for the first time.

Minimising the rolling resistance of a tyre, maximising aerodynamics, and/or improving comfort or grip is another matter altogether. These are all things that tyre manufacturers understand well for their tyres, including the fact that they are subject to a number of variables — such as the rider’s weight and local riding conditions — that cannot be anticipated. It’s a multifactorial challenge, and ultimately, something the individual must determine for him- or herself by experimenting with different pressures.

Any pressure that is derived in this manner is going to be more meaningful than a manufacturer’s recommendations. For racers, there is the promise of strategic advantages, while enthusiasts and/or commuters can maximise their comfort and grip for a more enjoyable time on the bike. All that is required is a decent track pump with a reliable gauge and the motivation to check and adjust tyre pressure on a regular basis.

Acknowledgments: This article was made possible by input from and helpful discussions with representatives from Campagnolo/Fulcrum, Continental, Kenda, Maxxis, Schwalbe, and Trek.

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