Our brand new range of Carbon Disc wheels was recently launched and has generated a huge amount of interest. They embrace the latest rim technologies to enable the wheelsets to deliver some of the lightest weights available on the market. Industry magazines and websites often ask questions about the hookless carbon rims used and the benefits they provide. Hunt owner Tom Marchment recently answered a few and we thought the answers might be useful if you wanted to be completely up-to-date on the latest tech:
Can you clarify how and why you've been able to include hookless rims and the benefits they provide?
We have already had good experience with hookless rims on other wheels we have developed. The key benefits of hookless rim design (as is attested to by their use on all modern car, motorbike and many mountain bike rims) is that the rim wall can be stronger and yet require less material (saving weight) and complication (reducing potential material failures) when compared to a traditional hooked rim.
Hooked rims have been used as the easiest option for containing the bead during the high pressures that can be reached in a rim-braked road bike wheel and tyre, there are ways to achieve this with hookless as well, but that’s mostly irrelevant to this discussion about disc brake wheels. The physics of braking means that pretty much all your kinetic energy is transferred to heat during braking. In rim brake wheels there is a significant amount of heat energy being transferred to the rim, tyre and air contained within the tyre, all of which can significantly raise tyre pressures during a long descent well above the pressure set by the rider before embarking on their ride.
There have been several factors in rim design over the past few years that have reduced this issue including wider tyres and rims which have more material and air to disseminate the heat reducing the peak pressure during a long heavy braking descent. However, the change to disc brakes has completely removed this issue as of course all braking heat is dissipated at the center of the wheel, not the rim. So disc braking has immediately opened bicycle rim design to the benefits of hookless rims which is why companies such as ourselves Stans and Enve are utilising hookless in our latest designs.
Our 30Carbon Aero Disc wheels are a great example of the benefits as the rims compare extremely well on weight (at just 380g) against hooked models especially when their broad 27mm width is taken into account. Of course reducing weight at the outer edge of the wheel delivers a strong improvement for climbing and an exponential improvement in acceleration.
What is the benefit of the 30Carbon Aero Disc's rim width?
When discussing the benefits of wider rims it is useful to consider the internal and external widths as they are often not as strongly related as it can first appear. Aluminium wheels tend to have very thin side walls as the material is denser than carbon so increasing the thickness of the wall adds considerable weight. The inner width of an aluminium rim would normally only be a maximum of approx. 5mm difference to the external width, say 17internal and 22external. There is not such a strong relationship with carbon fiber rims between inner and outer widths as carbon fiber has a lower density so it is possible, and sometimes required for strength reasons such as rim braking, to have a thicker sidewall and thus a greater difference between the inner and outer rim width. So many ‘wide’ rim brake carbon fiber wheels may be 27mm externally and only 16mm internally = 9mm difference.
What are the benefits from externally wider rims:
1) Aerodynamics; it is now widely recognised that smoothing the air-flow across the tyre and rim is beneficial for reducing lower air pressure zones which increase drag. There are still many rims that are only 20mm wide externally and if used with 25mm tyres there is obviously a considerable difference and this disrupts the airflow. Hence the theory is that reducing this difference should reduce drag so an externally wider rim should be faster. There is obviously a lot more to discuss in this section regarding scientifically recognised aerofoil profiles and patents on rim shapes but that is a whole different thesis!
2) By having an externally wider rim you of course make room for an internally wider rim, but as discussed above this relationship is not necessarily fixed.
What are the benefits from internally wider rims:
1) Increased air volume; Increasing the internal width of a clincher/tubeless-ready rim allows the beads of the tyre to be spaced more widely and thus increase the measured width and cross sectional area of the air chamber and thus the overall air volume held within the tyre and rim. This can have the effect of making say a 25mm tyre measure as wide as a 28mm tyre on a conventional narrower rim (and so delivers the widely recognised comfort and lower rolling resistance benefits, as well as the ability to run lower pressures) without adding any extra weight to the tyre.
2) Changing the shape of the tyre; having the tyre beads space more widely not only increases the overall volume but it also changes the shape of the tyre from that similar to a balloon, with it’s pinched tear drop shape to a more U-shaped profile. This wider support for the tyre reduces sideways flex when cornering and thus improves handling. It is also thought that this can have an effect of reducing the chances of pinching an inner tube (pinch or snake bite flat) on edges such as pot-holes as there is less curve to the sidewall for the tube to be trapped in during an impact.
Why have you gone for the rim shape you have?
There are some theories that aid the initial design such as feasibly eliminating low pressure areas as the air flows over the tyre to the rim (having a wider rim as close to the tyre width as possible aids this) and having a blunt profile to improve cross wind handling preventing airflow from suddenly creating low pressure areas off a sharp rim nose both inform our initial design. My father (John) is a chartered engineer and my brother (Peter) is a Cambridge University materials science graduate so having them both working here at Hunt | TheRiderFirm means we can work with our rim designers at the factory to utilise computational fluid dynamics software to model how the rims perform aerodynamically. All this means we are very confident that our rim profiles offer an excellent balance of handling and aerodynamic benefits for the real-world rider.
What has not spending thousands on wind-tunnel testing allowed you to do regarding the quality of the components that comprise the wheel?
Our ethos has always been to design purpose focused wheels, so we always choose the highest specification wheel components that are most suitable for that type of riding. A great example is the Pillar spokes that are made with high-grade Swedish Sandvik stainless steel and are triple butted and bladed. Both the butting and blading process are expensive and complicated but they add exceptional strength by work hardening the spokes. They also feature Pillar’s Spoke Re-enforcement (PSR) where the head of the spoke butts out again to 2.2mm to combine very low weight with extra strength, this is a patented technology. These are the same specification as the spokes used in some pro-tour wheels which sell for more than twice the price of this wheelset.
Our other issue is that we feel real-world wind conditions and wind obstacles for road riding are so erratic that it can be somewhat misleading to simply carry out a nice controlled wind tunnel test with the aim of producing some great numbers to talk about. However we are already looking at real-world aero testing in the future as we have a few irons in the fire with some leading experts.