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How do tire manufacturers determine the min/max tire pressure for their products, specifically tubeless tires? Seems somewhat arbitrary to me and likely rim choice plays a role. From a practical point, how much risk is there to ride an under or over-inflated tire as I suspect we all do at one time or another? What are your thoughts?
This is a great question. And, yes, rim choice does play a role. Not only is the method of attainment of those numbers not widely understood, but so is the meaning of them. While more riders appreciate the value of lower tire pressures than they did 40 years ago, there are still many riders who actually pump to the max tire pressure molded into the tire sidewall.
Tire pressure certainly affects the rolling resistance. In our test of 15 Paris-Roubaix tires, we tested each at a range of air pressures to find the sweet spot for each tire. We tested the Specialized Turbo Cotton, the Continental Grand Prix 5000 TL, the Vittoria Corsa Control 2.0 Graphene TLR, and many more.
Tire companies come up with these numbers in different ways, so I’m attaching answers to your question from a number of tire manufacturers.
As for risks, going below the minimum pressure increases the likelihood of damaging the rims. There is also some danger of dislodging the tire from the rim under hard cornering. Going above the maximum pressure is more dangerous, IMHO. This can result in blowing the tire off of the rim, which by definition would happen very rapidly and hence may not give the rider sufficient time to prepare for it and avoid a crash.
Here are the answers from four tire manufacturers.
For maximum riding pressure, a tire is tested with the biggest inner width allowed in the ETRTO rim/tire combination table and must surpass a time at 110 percent of that pressure.
As for a recommendation of the lowest pressure, it is more critical, and more variables come into play; that’s why from Conti side we do not state a lower value. For your own way to find the lowest possible pressure, you should make sure that, even at the highest possible impact in a pothole situation, your tire doesn’t get compressed until the rim touches the stone. Mostly depending on the desired grip and comfort, you can adjust your pressure to this point.
– Jan-Niklas Voß, product management BC – Two-Wheel Tires
“Our internal rule for tire pressure indication, in general, is quite simple.
Tire pressure indications are two on our labels: Rideable pressures are expressed as both minimum and maximum in bar and psi, and within these are the pressures that the tire is most effective: the range you should be in. This is considering an average rider with a certain weight range and skills. There is a slight tolerance here, and that depends on rider skills, and this generally applies to lower pressure. Pros can ride at a slightly lower pressure than indicated but we are talking of top, experienced riders.
For the max pressure: this is indicated at the end of the label and expressed in kPa. This is the max operating pressure the tire can be used.
Above that pressure, there is no meaning nor benefit to exceed this indication, really zero gain if not the contrary. We also state that this is the safe limit of the tire.
On the hookless side, we follow the same rule, but we have a limitation given by ETRTO that limits max pressure to 5bar (72.5psi) for all tires, so we still have the min/max rideable pressure, but max pressure is limited to ETRTO regulations.
– Alex Brauns, president Challenge Tech, Ltd.
“Indeed, this is a good question that shows how dark this topic is for the regular cyclist. What is the optimal/best-performing tire pressure in conjunction with the rim width and type (hookless or crochet, tubeless or tubed type) and the tire width?
What factors influence the initial tire pressure while riding (heat from the climate, friction from the road, heat from the action of the brake pads on the rim – if any…)?
The precision of the pressure gauge can vary up to 1 bar (15psi) from one gauge to another.
With this in mind, each manufacturer (rim and tire) defines the maximum pressure that its rim or tire must support and designs it accordingly, according to its know-how, experience, proprietary technologies. Usually, the max pressure that is written on the product (or user guide) by the manufacturer includes a safety margin to take into account points 2 and 3 above.
At Mavic, this margin is 140 percent; if we write 10bars on the rim, it means we have tested it at 14bars.
Testing is pretty simple: once you’re satisfied with your engineering work and you have prototypes, you inflate to the max pressure + safety margin and wait for the tire to blow out (or rim to fail), or not.
In the norms (ISO and ETRTO), the only max pressure that is stated is the one for hookless rim: 5 bar (72.5psi), regardless of rim and tire width. Any other type of design and associated max pressure is up to the manufacturer. But whatever the max pressure text molded on the side of the tire, it must comply with the standard ISO4210 part 2, “edition 2015” – the tire must not blow out during a 5-minute test when inflated at 110 percent of the max pressure (written on the tire).
Note that I’m only mentioning max pressure here. The min pressure is not related to safety, but to performance and reliability.
And most of the time, riders tend to overinflate, more than to underinflate.
— Maxime Brunand, Mavic road category manager
“Maximum and minimum pressures indicated on the tire sidewalls, as well as on any official communication piece, are set in order to be safe and provide reliable performance for whatever rider weight on whatever (approved and ETRTO compliant) rim. This means the worst combinations are set as boundaries of testing, as we cannot control those parameters (weight and rim and riding conditions).
1. Do not blow out of the rim. It is set with a safety margin aimed to consider possible variables which may be difficult to control (rider+bike total weight, pump precision, delta altitudes, sun, impacts, rim’s tolerance, etc.). It is crucial to always check the max pressure of both tires and rims and be in the safest situation (especially with hookless rims).
2. Provide sufficient grip even for very light users.
Watch out! Not always, but very often, this value exceeds the one of the rim itself, meaning that the rim shoulders are not necessarily strong enough to sustain such loads of pressure pushing inside out. This is a point of attention we always highlight in our communication to consumers.
1. Support the rider’s weight and dynamic load (bumps, leaning, etc.) at the “heavy” side of the spectrum.
2. Provide sufficient performance (handling and grip) even for heavy riders, again with a safety margin.
These conditions are tested through a series of tests, both standardized by ISO and internal aimed to validate the points above. Max pressure: outdoor, high pressures indoor tests, (ex. Burst pressure), etc.
Min: outdoor, indoor test at low pressure (deflection, bead seating, etc.). On different rims, of course.
It is very difficult to evaluate the impact of going under or over-inflated, it depends on many factors: for example, a very light rider, using a very wide rim at low speeds, might not face risk by riding slightly below the minimum pressure. On the other hand, this situation for sure is way more critical for a heavy rider on a narrower rim. Going above the max pressure depends a lot on the rims and on the riding conditions (bicycle left in the sun, delta altitude) and on the weight of the rider (high weight means more stress in use on the whole part of the tire), but this is much more critical in terms of safety between the two to respect.
In terms of the minimum, if the user has total control of those variables, it might be reasonable to go below, but a tire manufacturer can never state that this is okay, because it cannot guarantee those variables are always met (at the beginning of the ride, or throughout it…).
I hope this gives the whole perspective of why we always and only recommend to not exceed the tire + rim + tape indicated pressure limits.
–Samuele Bressan, Head of Global Marketing – cycling division, Pirelli Tyre SpA
As you just saw, those numbers imprinted on the sidewalls are not obtained the same way by all manufacturers. I hope you see the point of neither exceeding the max pressure nor inflating below the min pressure.
Lennard Zinn, our longtime technical writer, joined VeloNews in 1987. He is also a custom frame builder (www.zinncycles.com) and purveyor of non-custom huge bikes (bikeclydesdale.com), a former U.S. national team rider, co-author of “The Haywire Heart,” and author of many bicycle books including “Zinn and the Art of Road Bike Maintenance,” “DVD, as well as “Zinn and the Art of Triathlon Bikes” and “Zinn’s Cycling Primer: Maintenance Tips and Skill Building for Cyclists.” He holds a bachelor’s in physics from Colorado College.
Follow @lennardzinn on Twitter.