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:
1) Weight
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.
Both the total weight of the wheel and the weight distribution across the different wheel components have an effect on the longitudinal and rotational components of the wheel’s inertia.
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.
Low density carbon rims (top-end wheels) allow a reduction of wheel weight compared to alloy rimmed wheels or high-density carbon rims (basic wheels).
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.
2) Aerodynamics
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.
3) Stiffness
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.
Further reading:
Seamus Gardiner
Citizen
There's a debate in the MTB world about wheel size at the moment. I assumed that the diameter of the road wheel was fixed by the UCI.. Clearly not, as your article points out.
I'm surprised that the 650c wheel size has not transmitted from the triathlon world to cycling. I'm told they are lighter and stronger and more aerodynamic (although rolling resistance is greater, apparently) Is this true and if so why wouldn't professional teams use them?
Joel Mayes
Bicycle Mechanic
Probably because a 700C wheel is already more then strong enough, and the other factor balance out.
Could also be an accident of history, The industry has chosen two sizes and economies of scale limit experimentation outside the norm. even for pro teams as the money isn't in supplying the pro racers but selling to the people who think they need the same gear.
So if you have/want a bike which uses unusual sized wheels (i.e neither 559 or 622) you are very limited in the make of tyres and rims available to you.
terry lockwood
maths teacher
Fender don't make money out of selling stratcastor guitars to professionals but by selling them to wannabes. Buying the dream rather than living it.
Seamus Gardiner
Citizen
That sounds like someone who's never played a fender or ridden a race bike.
Peter Ormonde
Peter Ormonde is a Friend of The Conversation.
Farmer
Amazing science ... and at some point it might have some spin-offs that have some significance beyond watching these lycra clad greyhound types sweating all over the Pyrenees... but strewth I wish someone would explain the mystical tactics employed by racing teams... seems like alchemy to me. I reckon the commentators are just making it all up.
Nonetheless I shall be watching the race again this year - and taking far more notice of the rims and spokes thanks to this piece.
Seamus Gardiner
Citizen
mmm... spinoffs. Don't know about that but a good wheel, in itself, is a marvel to behold.
terry lockwood
maths teacher
Wow! Obviously the tiniest differences make all the difference in these bike races. I had wondered about why the solid wheels were never used on the open road. I am still at a loss as to why recreational riders go for highly efficient bikes when exercise is the aim of the game. I thought extra resistance would be the go. A jump suit with batwings to try and catch the wind. The old 1970's marlin bars as per the chopper. A couple of miles and you would really know you have been riding!
Gary Murphy
Independent Thinker
If I see anyone posting comments to this article in the hours the TDF is on - there will be trouble!