Why tubeless tyres can lead to looser spokes than those with tubes inside
Did you know that a tubeless tyre puts more compression force on a hooked rim than one with a tube? That increased force effectively shrinks the diameter of the rim.
Most wheelbuilders worth their salt know this to be true and can prove it by measuring the drop in spoke tension. It’s not such an issue with the low tyre pressures seen in mountain biking and gravel riding, but it’s an entirely different story on the road where high pressures can commonly result in 10-20% drops in spoke tension.
Roval certainly knows that – the brand’s latest road racing wheels are clincher-only with Roval stating this provides a lighter-weight wheel system versus one reinforced for tubeless use. Meanwhile, the latest gravel wheels from DT Swiss and Roval now dictate different maximum pressures depending on whether you run tubeless or tubes.
But why does a tubeless tyre exert more force on a wheel? Surely pressure is pressure? Wanting to know more, I reached out to a few engineers responsible for making wheels.
The general assumption is that it’s the tighter and stiffer bead of a tubeless tyre that exerts more force onto the rim. While in theory this can be a factor in the equation, it’s not at all the main factor. Rather the ‘why’ is largely due to the exposed rim area for the tyre pressure to act on.
“We know from measurements in our test laboratory that, if we mount the same tubeless tyre and the same system pressure with and without a tube, there is a relevant difference in the spoke tension deviation,” explained a representative from DT Swiss.
“If you pump pressure into your tyre system, whether tubeless or tube-type, the pressure acts on every surface within this system. This means that the air pressure stretches the tyre towards the outside and tries to expand it, but on the other hand also presses on the rim and compresses it.”
Looking at hooked (crochet) style rims, the difference between tubed and tubeless setups comes down to the transition between the tyre and the rim. “In a tube-type setup, the tube seals this transition area,” the DT Swiss rep continued. “Even if the tyre expands due to the pressure and detaches from the rim horizontal bead surface, the [tube-equipped] system [still] seals horizontally directly at the transition between tyre and rim. The pressure area on the rim extends exactly between the tyre beads.
“With a tubeless setup, the pressurized area of the rim bed can increase. When the tyre expands due to the air pressure in the system, the tyre may no longer seal optimally on the horizontal surface but rather on the last vertical surface of the rim. This lifting of the tyre increases this rim surface on which pressure is exerted and thus there is a greater stress on the rim and the entire system wheel.”
Jon Partington of Partington Cycling provided a similar explanation. “If you were to take a wheel without the tyre assembled and look at it, you’ve got the floor of the rim which extends from the centre all the way out to the exterior of the sidewalls,” he said. “In a tubeless wheel the entirety of that area is acted on by the air.
“With an inner tube, you’ve got not only the wall thickness of the tube that narrows that area but then you’ve also got a bridging of the corners and crevice areas. So [with a tube] your effective exposed area to tyre pressure is less and therefore the resulting force is less and the resulting change in [rim] diameter is less.“
That exposed area to the air pressure is key. Simply, more area at the same pressure equates to more force. “The air pressure compresses the rim in any case, but the effect is greater with a tubeless setup,” said the DT Swiss representative.
What about hookless rims?
The explanation above is most applicable to the more intricate shaping of hooked rims, but what about the new wave of straight hookless rims that require the use of tubeless tyres?
According to Partington: “If you were to run a tube in a hookless rim, as you don’t have a hook, you’ve got less opportunity for an inner tube to bridge and you might get a more true projection of the area for the air pressure to act on.”
Or in other words, the simpler shaping means the exposed area will be much the same regardless of running a tube or tubeless, and therefore the resulting force on the rim will be largely unchanged.
This is not to say a hookless rim is better at managing force from air pressure than a hooked rim, but rather there is little differential in force between running a tube or tubeless.
Wheel design considerations?
OK, so tubeless setups have the potential to provide a larger air surface area than a tubed configuration, but why does this matter? Well, building wheels for tubeless use isn’t simply a matter of creating the right-shaped rim bed and making it airtight; it needs to be equipped for the increased forces, too.
“At the first instance we didn’t appreciate how significant the difference would be between tubes and tubeless with changes in spoke tension,” said Partington whose sole current product is a 1,200 g disc-brake tubeless wheelset. “It’s [approximately] a difference of a factor of two.”
According to Partington, this is especially important to know when building a stiff wheel with stiff spokes. “Any change in diameter, [it] has a more significant effect on spoke tension,” he said. “So if you’re chasing a stiff wheel, it’s even more of a challenge.” By contrast, a wheel with more compliant spokes will be less sensitive to a compression of the rim’s diameter.
DT Swiss is obviously well aware that tubeless setups cause more spoke tension drop than running a tube. In addition to designing for the increased rim force, many of the brand’s road and gravel rims now also state different maximum pressures depending on whether you’re #teamtubeinside or not.
So if high-pressure tubeless setups are known to cause greater spoke tension drop then should wheelbuilders counteract this by increasing the tension? Some wheelbuilders say yes, while others say no. And DT Swiss sides with the latter.
“All of our max [spoke in relation to rim] tensions are calculated without a tyre on,” a company rep said. “Our recommended max build tension of 1,200 N would be considered high for a wheel with a tyre on it. As the tension drops when the tyre is inflated this brings the ridden wheel tension down, though the wheel goes through a loading and unloading cycle as the wheel rolls, causing some spokes to have increased tension during the cycle while others decrease (every single revolution).
“This allows the increased tension during the cycle to still be within range for the max spoke tension. If the wheel was re-tensioned after the tyre is inflated, then during the loading/unloading cycle, spoke tension would regularly surpass the recommended max tension causing premature spoke failure.”
So tensioning a wheel beyond its maximum recommended limits isn’t recommended, but a number of experienced wheelbuilders will tell you that an inflated tubeless tyre can throw off the trueness and/or dish (how the rim sits centered over the hub axle) of a wheel.
Perth-based wheelbuilder Adrian Emilsen of Melody Wheels knows this all too well. “If I know a customer is planning to run road tubeless then I will ask if we can set them up in the workshop before sending them out,” he said. “This gives me a chance to check over the wheel as it will be used.
“Frustratingly, the tension drop, while relatively uniform, can throw the wheel slightly out of true. Thankfully the majority of road tubeless wheels now are disc. This means that the tension ratios are more even and the lower-tensioned spokes will generally be at a functional range where they are unlikely to have issues with spokes working/vibration loosening.
“Rim-brake tubeless setups with traditional hub geometries don’t excite me at all. The non-driveside rear spokes already struggle to get adequate spoke tension, and the road tubeless only adds to these woes.”
Playing it safe
The reality is that our tyres are increasingly growing in width with lower recommended tyre pressures. And thankfully these factors largely negate many of the worries associated with tubeless tyres making our rims smaller and our spokes looser.
Still, it’s worth remembering that tubeless and tubed setups are not the same thing. Judging by the warning stickers found on an increasing number of rims, your pressures should certainly be lower for tubeless than what you may be used to with tubes. Both Enve and Silca offer good resources for recommended tyre pressures for modern road wheel systems.