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Why are tire makers such lying liars? Or more specifically, how can I know what size my tire is really going to be when I mount and inflate it? Or even more specifically, why does my Bontrager xr2 2.6 measure NARROWER than my old Nobby Nic 2.35? – Ibis s35 rims 35mm inner, 41mm outer dimension according to Ibis. For the record, I want the biggest tire I can squeeze in my frame (Ibis Ripley v.3), cause in my books, phat is where it’s at.
MSC Tires: For an industry that is obsessed with standards this is a very valid question and one that I’m sure is pondered by many when looking at tyres on the hook in dealerships or online. What size will it be in real life? There is actually a standardised system for obtaining the measurement you seek, but it is not without its faults for the consumer. A uniformed set of rules for establishing the dimensions of tyres and rims has been sought since way back in 1952 when a unified system was first muted for the motor vehicle industry.
It took a further decade of negotiations and adoption by the trade to establish the European Tyre and Rim Technical Organisation, also known as ETRTO. It wasn’t until the late 1960s when it began to broaden its reach and assess the multitude of different bicycle tyre dimension labelling systems world wide. Yes, it was once worse than today! It then created a singular way to present the sizes. This new agreement was later set in stone by way of the ISO5775 regulations, an international standard for labelling the size of bicycle tyres and rims.
We are familiar with observing sizes like 29” x 2.40” on our mountain bike tyres, packaging and advertising. This is the common language that riders use talk to each other about tyre widths, but this is only provided as a consumer friendly number to guide us. This is not actually the ETRTO standardized size. If you look closely at your tyres, you will see a small set of numbers embossed down at the bead in a format similar to 60-622 marked on the tyre below.
This is millimetre-based sizing for the width of the carcass and the diameter of the tyre’s inner bead. This is the international standard that unifies manufacturers. Road is a little less complicated because that sector has retained the French mm markings and there is very little tread to cause discrepancies.
Now this is the catch: See how I said “carcass”, not tyre width. The big fault with ETRTO and the ISO5775 is that it does not include the tread. The profile of the tyre’s tread can change the real life width significantly. If you have a very rounded tread profile where the tread blocks come down lower on the sides, then that will increase the width of the tyre beyond the carcass width. If you have a very square profile tread, with fairly upright and straight-edged side tread, then the real life width may not be much greater than the ETRTO calculation.
Tyre profile can also be affected by another factor; the width of the rim it is paired with. If you run a 2.5” tyre on a 25mm internal diameter rim (the inside width between the two rim sides) this means the bead is contained within a 25mm space at the bottom. This naturally pulls the side tread down a little and would make a higher volume tyre rounder. If you ran the same 2.5” tyre on a 35mm internal diameter rim, the beads would be further apart and therefore the shoulders or side tread would be better supported by the tyre sidewalls and would therefore be in a higher position. Tyre and rim brands should provide some recommendations for rim widths for their tyres to guide you.
However, these are not the only places where discrepancies can arise. During the manufacturing process, tyre companies are constantly developing new materials and carcass layups and technologies. These all have a different effect on the expansion of the tyre under inflation. Whilst computer simulation and calculations are very sophisticated, there are too many variables to accurately calculate the true expansion of the tyre when inflated at “x” pressure on “x” width of rim. For absolute accuracy they have to wait until the mould has been manufactured and it has been measured in real life conditions, by which time, measurements will usually have been applied to packaging and other media ready for retailing.
Whilst tyre brands could technically be unified by adopting ETRTO guidelines, this only applies to the carcass. That leaves them with the problem of providing the consumer with a measurement for the whole width of the tyre that they can relate to. As in your situation, it is very important to prevent frame rub, damage or having an undersized tyre causing you to be unhappy with your purchase. The truth is that brands need to take their customer’s problems and turn them into solutions and provide greater information on the real life sizing of their products. But as easy as it is to say, that is difficult in real life. Every season dimensions change, XC wheels previously running at 19mm internal width a few years ago, grew to 23mm, then 25mm became standard. Next season there will be an array of XC wheel sets available at 27mm. The market constantly changes so it makes it very difficult for manufacturers to constantly update the information. If you think that was complex, wait till you realise we have been decimalising the inch now for years. We are so used to saying 2.40” inches, but what is that?
So in answer to your question, it is not so much of a lie that manufacturers tell you, it is more a case of which truth.
Vittoria: Tire width is a moving target. For every tire width, there is a specific rim width that it was designed for. When riders use these tires on rims of other widths, it changes the actual tire width. Often, it takes 1-2 years for a tire to come to market, and in that time, rim widths may change slightly, which can be a challenge for product designers.
I can’t speak for Bontrager or Schwalbe, but it would make sense that the tires you mention were designed on different rim widths, given the widths, per ETRTO standards.
The general rule of thumb is for every 2mm the rim width grows, the tire width grows about 1mm. However, this effect is exaggerated as the rim width approaches the tire width (as a proportion). Given that, let’s assume the 2.35 tire you mention was designed on, for argument’s sake, a 23mm internal rim, which was a popular ETRTO internal rim width for 2.35 tires. You then mount these on a 35mm (inner) Ibis rim, and surprise, the tire is a lot wider than 2.35… Quick, post your disdain everywhere!
Let’s do some math though… Your rims grew by 12mm internal, so using the general rule from above, that means the tire should grow 6mm (maybe even more, due to the proportionality). That is roughly ¼”, so 2.35 + .25 = 2.6. Imagine that.
This is further exaggerated by some tire brands under-sizing their tires to begin with. I won’t mention any specific brands, but it does help to maximize their claims.
What makes an MTB tyre quiet rolling?
MSC Tires: Great question, and i’m slightly curious as to the motive. Is your bike all black, you wear black kit and like to stealth up on your mates on the trail? We all know that guy that likes to look “stealth” but if you’re considering how quiet your tyres roll I think this might be a level up again, “matte black” stealth. Do you own a Mondraker Foxy because you thought its sharp angles would show up less on radar?
Any way, regardless of your motive, what makes for a noisy tyre? Whilst there are many factors there are three main ones, split around 35/35/30. The tyre, the terrain and you!
The first two are what I call noise causing factors, the later is what I call an amplification factor! The fact is that the variables of tyre type and ground conditions are amplified by the weight or pressure that is pushing the tyre into the terrain, and the biggest weight of any moving bike, even e-bikes is that of the person riding it. So, if you want to go all Hunt for Red October and enter “Silent Running” when chasing down your mates on the trail then you’d better skip the pre-ride pastries!
The next biggest factor that makes an impact is the terrain. Soft fresh loam can be almost silent, although your squeals when you slide out on that “fresh” all natural secret trail may still give away your location. Gravel trails make for a frenzied and superbly unnerving, “riding on marbles” sort of sound, even when running something pretty quiet. General trails and fire road offer the best baseline. Here, other factors can become more evident.
If you’re riding to the trails of an evening, when it’s generally less busy and you don’t have as much background noise, you will probably not notice the regular car road noise that much. It has become a pre-adapted noise you are used to. But, if a Land Rover Defender comes down the road at any speed, you will hear that regular “hum” of the tyres on the tarmac from a greater distance. This is because the tread on the Land Rover tyre is more widely spaced compared to normal road vehicle tyres. There is great correlation between what provides better rolling resistance and what makes for a quiet tyre, so a regular central tread pattern would be beneficial.
Now, why do I say central tread? Because, if things are set up pretty well, you should spend more time riding on the central tread blocks than the outer ones, hence they are traditionally made with a harder compound to balance the wear with the softer side treads. So that means that if you run a tyre with chunky side treads, all is not lost if your centre treads are not too far apart. For this to work, you also have to factor in tyre pressure. Whilst most tyre profiles appear slightly rounded, as in the cross section of the tyre, when you add your 60-120 kg of body and equipment weight, the tyre will flatten out to a certain extent. That’s why a lot of Enduro riders will pump their tyres up for the first big transition climb of the day then let some air out at the top. The extra air supports the tyre and means it doesn’t flatten out under their weight so much.
If it doesn’t flatten out so much then when riding along on a fireroad surface, then only the central tread blocks will be in contact with the terrain. Reducing drag and, yes, you got it, noise.
We have ticked off tread pattern and pressure, but what about compounds and the outer diameter? Compounds that are too soft will cause drag and thus, noise. Also, if you run too hard a compound, it will also make noise when its tread hits the surface. So, a medium twin compound should be best. We know from diagrams that the 29” tyre has a lesser angle of contact with the ground. This means that the tread blocks engage with the terrain at a slighter angle and this is what reduces the rolling resistance, but it could also be a factor in reducing the noise created when the tread blocks meet the ground.
Now, to be all fair and equal, there is a flip side to the 29” size over the, say, 26”. That bigger wheel also has a larger contact patch so there are more tread blocks engaging with the ground at any one time. It’s one of the benefits of 29” when climbing to have increased grip, however this could have a negative effect on noise.
Lastly, you have tyre volume. Higher volume tyres run at lower pressures will help absorb some of the variation in the terrain and help to deaden sound, but there is a flipping point where the increased amount of tread on these wider carcasses actually causes increased noise, like with Fat Bikes for example, which are the equivalent of trying to catch your mates on a Panzer Tank.
So, this is a difficult one and an element of guess work is required. But, without any scientific testing or sound analysis, I would recommend a mid volume 29” x 2.30” tyre, in a twin medium compound with regular central tread blocks as the perfect tyre setup to stealthily catch your mates on the trail! With blacked out decals, obviously.
However, these days, if you’re just riding along minding your own business, don’t have a drone following you, you aren’t recording your latest Vlog on your GoPro, or screaming Yea Boi! to your latest Bromance riding buddy, then you’ll be more likely to be able to sneak up on people at your leisure!
Vittoria: Most of the time, tire rolling resistance is measured as the wheel is in a perfectly upright position. This means that the center portion of the tread is the factor that matters most for rolling resistance, and noise.
As the leading-edge of the knob is the portion of the tread that hits the ground first, this is often the cause for resistance and noise. This gets worse as more spacing is added. A lower, tightly packed center tread can help to reduce this. However, as the rear edge of the knob provides the braking traction, a low tread height can sometimes negatively affect performance in other ways.
To get around this, product designers have been forced to get creative. An example of this is the Vittoria Terreno Dry Gravel tire. The fish-scale style tread pattern positions thousands of ramps in an interlocking pattern. This allows the casing to remain flexible, while the leading edges of the scales create VERY little noise and resistance. However, under braking, thousands of little braking edges stand up, providing confident braking traction.
This tread has been very popular in Gravel… and the concept was further proven by winning the European, French, German, Canadian, and Pan-AM XC MTB championships in 2018.
How [dangerous] would a tire be to ride with a loose fitting bead? Is there’s some safe tolerance for loose fit between rim and tire bead, with intended use and inflation pressure? Asking because I saw some bead hook illustrations (e.g. Stan’s) showing a tire floating, not touching the bead seat, and recall the times I saw a tire come off a rim in World Cup MTB DH racing.
MSC Tyres: The illustrations that you refer to may show a marginal gap between the bead and the rim simply to show the individual component parts with some clarity. In these scenarios it is purely for illustrative purposes to show the shape of a rim profile, tyre shape or the difference between two specifications such as hookless and hooked rims etc. It is only an illustration and in reality the bead will always be engaged with the sidewall of the rim when setup correctly.
In general, for a tyre to stay on the rim when riding it requires an internal pressure, whether by an inner tube expanding or the internal air pressure of a tubeless setup. This pressure pushes the bead into the the rim wall causing enough friction between the two to hold it in place. The situations where tyres have come off the rim as you describe are when the force between the tyre and the terrain has exceeded the internal pressure holding the tyre on. Historically, tyre detachment would usually appear quite suddenly after the puncture. Now, with modern day technology such as inserts and improved rim profile design, deflated tyres are often able to stay on the rim for the length of a World Cup Downhill course. This increases safety significantly and allows the rider to run out to the finish to avoid a DNF or in some cases still collect some points if the puncture was not too far up the track.
Tyre inserts provide protection to the rim and additional pressure on the bead, especially if they are a relatively tight fit. They act as a physical pressure pushing the bead into the rim wall, even without any air pressure. This, added to the adhesive function of a good quality sealant in a tubeless setup, can create quite a strong bond. More recently we have seen the development of rims to include a better “bead lock” or bead retention lip which is a small ridge or raised line set back from the internal rim wall by about 3-5mm, depending on the rim width. This lip means that when the tyres are inflated the bead is pushed over it and into a position close to the rim wall. Even when air pressure is reduced, the bead is held in place, not only by the rim wall and insert but this small lip on the interior side. Altogether this causes significantly less risk of the tyre coming off the rim.
In relation to your question as to the danger involved in riding with a loose fitting bead (Insufficient air pressure), it varies from very dangerous on a basic setup to the level that some riders, with quality inserts and a top set up, will be seen to ride enduro stages at up to 70% pace with no air in the tyre.
Obviously these scenarios would be against manufacturers guidelines and due to the variables in rim/tyre compatibility, sealant and insert performance that is unlikely to change! In relation to the safe tolerance, the best option is to follow the manufacturer’s recommended guidelines for air pressures which are usually embossed on the tyre sidewall.
In short, floating beads unlike floating rotors are for illustration purposes only!
Vittoria: There is no benefit to a loose fitting tire. The tire should fit tightly on the wheel, creating a safe and secure interface. This also helps for inflation, especially with tubeless tires.
Despite being designed with ETRTO standards in mind, there are still acceptable manufacturing tolerances, which will (in some cases) make certain combinations of rims and tires fit differently. Sometimes all it takes is an extra layer of rim tape to help get the tire to inflate easier, but this should never be used as a way to make the tire “safe” in terms of fitment. If you ever have doubts, simply err on the side of caution.
At the World Cup level, athletes have the latest technology, and best quality equipment. However, they are also pushing the equipment harder than anyone else, especially in the DH segment. As a former DH racer, I personally cracked a head-tube off a frame while racing, but this would never have occurred in a normal use scenario.
Despite years of design and testing, it’s not inconceivable to see a tire burp and then peel off in competition, however it is fairly rare, especially as these bikes are professionally maintained. Therefore, a loose fitting tire is not something that anyone would intentionally use in a competition.
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