And so, once again, some of the world’s top athletes are about to set forth on the highlight of the professional road racing calendar, Le Tour de France. Before a pedal has even turned, top riders have come into form, fallen out of form and retired from the event due to injury.
As with the riders, the bikes need to be in best shape possible to perform well across the three-week event. And of great concern to both the elite and serious recreational cyclist are wheels – and the engineering that goes into them.
A bike wheel is composed of three main components:
1) the rim (peripheral part of the wheel) has the specific inner width and diameter of the bead seat (a hoop that forms one edge of a tire) so that it can only be fitted with tyres of matching dimensions. Rims are made for both high-pressure beaded tyres or tubular glue-on tyres.
2) the hub (central part of the wheel) consists of an axle, bearings and a hub shell.
3) the spokes, which lace the rim and hub together under tension
The rules of the Union Cycliste Internationale (UCI) indicate that any wheel can be used during cycling competitions, provided:
- the wheel’s diameter is between 70cm and 55cm, including the tyre
- there are at least 12 spokes with different shapes allowed (i.e. round, flattened or oval), provided no dimension of their various sections exceeds 10mm
- the wheel comes with certification that its rupture characteristics are compatible with those resulting from an impact sustained during a “normal use of the wheel” test (to be obtained from a laboratory approved by the UCI)
Within these broad parameters, wheel manufacturers have come up with a large number of designs that have been approved by the UCI, all of which have been guided by the following considerations:
The power required to accelerate a bike is affected by the weight of the bike plus the rider system. The weight of the system influences its inertia – the term given to the resistance the system offers to changes in its motion.
By using lighter wheels, cyclists can accelerate with less energy, but they can also enjoy a better handling of their bike. There are different ways to reduce the weight of the bike wheel, such as reducing the number of spokes or using carbon fiber.
Studies have shown that, for road grades steeper than 6%, novice and recreational cyclists can maintain a higher bike speed using a light and non-aerodynamic wheel compared to a 500g-heavier aerodynamic wheel.
The amount of power required to accelerate a bike or maintain a given speed is also affected by the aerodynamics of the wheels. Wheels contribute 10% to 15% of the total aerodynamic resistances/drag force cyclists have to overcome.
Because it’s located just behind the seat tube, the resistance caused by the rear wheel is about 25% lower than front wheel. The aerodynamic characteristics of bike wheels are generally tested in wind tunnels, at relative wind velocities (velocity of air relative to the bike) and yaw angles that are typically observed on the road (between 30 and 60km/h).
A reduction of 2-3% of the total aerodynamic resistance can be obtained through optimisation of the bike wheel aerodynamics. Over a 40km time trial, aerodynamic wheels can provide performance improvements ranging from 60 seconds in elite cyclists to 67 seconds in recreational cyclists, compared to conventional wheels.
Different methods are employed to improve the aerodynamics of bike wheels, such as designing deeper rims with toroidal (doughnut-shaped) cross sections to replace thin rims with flat profiles. The different shapes can have an effect on how well the wheel works at yaw such as a crosswind.
Rim depths of between 45-60mm seem to offer a good aerodynamic compromise (low aerodynamic resistances and good handling). Thanks to the ease of molding carbon, subtle improvements in rim design can be readily incorporated to improve wheels aerodynamics.
Reducing the number of spokes and also using shaped spokes can also reduce the drag of the wheel. Solid disc wheels generally have low aerodynamic resistances. This is why pursuit track cyclists are using solid disc wheels on their front and rear wheels. The video below – of the Australian Olympic men’s pursuit squad – is illustrative.
However, road cyclists should not run a disk on their front wheel if there is any chance of wind, as the side forces that apply to the front wheel will make the bike difficult to handle.
Because of their structure (rim and hub laced together with spokes under tension) bike wheels are designed to minimise power losses rather than to reduce the vibrations generated across road irregularities, although shallow rims give a smoother ride than deep rims.
The stiffness of the wheel will help determine how much power can be transferred from the hub to the rim. This is particularly important for the rear wheel, as cyclists expect the power they generate at the pedals to be translated into sharp accelerations of the bike.
The power transfer is also influenced by the lacing pattern of the spokes. These include no crossings (radial spoking), paired spokes, multiple crossings in both dimensions (lateral and longitudinal).
The rigidness of the hub is also important for maintaining the wheel’s bearing alignment and minimising power losses.
Generally speaking, stiffness is improved by using a stiff and deep rim, large spokes, a high bracing angle, and high-tensioned spokes.
4) Material resistance
Heat dissipation from braking is an important design consideration in wheels, given carbon composite is a poor conductor of heat compared with aluminium. Some carbon rims have failed after heavy braking, such as that employed while descending mountains, due to the resin becoming soft due to overheating.
If you are riding on cobblestones – such as during Paris-Roubaix or Strade Bianche races (see video above and top) – the wheel’s impact resistance becomes a very important factor. In such conditions, wheel design is adjusted through an increase in the number of spokes in order to ensure durability.
Because light carbon composite materials can have less impact resistance than other available materials, their use for the rims has to be carefully considered if good impact resistance is expected.
Will any of these factors make enough of a difference to influence the Tour results come July 22 in Paris? Will they make enough a difference on your local crit race or sprint effort at the end of a weekend group ride? They just might.
This article was co-authored by Raoul Luescher, Director of Luescher Teknik.