What rim height?
Wheels are one of the most aerodynamically effective parts of the bike. With high, favourably shaped rims, the speed can be increased by up to 1 km/h without additional power. The contour of the rim determines at which angles of attack the profile works particularly well. The higher the rim, the faster it is. However, the effect is small, with a good rim shape the difference between 30 and 80 millimetres in height is only a few watts. High rims also have disadvantages: not only are they somewhat heavier, they also act like sails.
Strong crosswinds generate forces on the rim that affect the steering. High rims are not easy to steer, especially in gusty winds. The combination of low air resistance with the lowest possible crosswind sensitivity is ideal. Rims around 50 millimetres high are a proven compromise; even higher rims only make sense for time trials, triathlons and in light winds. A not insignificant side effect: Taller wheels tend to be stiffer and more durable because the rims are more stable and the spokes are shorter. Heavy riders can therefore choose higher rims, while small and light riders should favour slightly flatter ones, at least on the front wheel, because they feel crosswinds more strongly.
Wheels: carbon or aluminium rims?
It's a question of the requirements and the price. If you are looking for a material that is suitable for competition and good aerodynamics, there is no way around carbon - a good, lightweight aero profile can only be moulded from carbon. Carbon also has an advantage in terms of weight. However, the difference is only significant when compared to inexpensive aluminium models - high-quality aluminium wheels can even be lighter and cheaper than the cheapest carbon models. Prices for decent material with carbon rims start at around 1000 euros per set; the top models from major brands cost 2000 euros and more.
Aluminium wheels start at a few hundred euros for the simplest technology, which weighs around two kilos per set. The more expensive the lighter: you can get aluminium wheelsets weighing around 1500 grams for less than 1000 euros. Important for racing bikes with rim brakes: Braking on carbon is not optimal. Depending on the product, you have to accept limitations such as low braking performance in the rain, high wear of expensive special pads or even rims collapsing under braking heat - in the worst case, all effects together. From the experience of our tests, only the well-known manufacturers have this under control. For extreme requirements (heavy riders, lots of luggage, steep hills), aluminium rims are the safer choice.
System impeller or customised?
Most wheels sold today or installed in road bikes are system wheels. This means that all components come from the same manufacturer and are matched to each other. Their greatest strength is the usually better stiffness-to-weight ratio compared to individually assembled wheels. The suppliers can also pull out all the stops when it comes to aerodynamics.
DisadvantageIf something breaks, you have to fall back on these special components.
It is not always certain that these are available; for older models in particular, special spokes can be difficult to obtain. However, customised wheels - whether built by yourself or by a professional wheel builder - can also have advantages: Depending on your own requirements or preferences, hubs, spokes and rims from specialised manufacturers can be freely combined. For example, particularly durable specimens or special visual requirements can be realised. The availability of spare parts, especially spokes and nipples, is also usually better.
Wheels - Which inner width?
Rims for road bike wheels can be between 15 and more than 30 millimetres wide. The rim width depends largely on the tyre width and therefore the question: What do I want to ride? One thing is clear: the wider the tyre, the wider the rim should be. Although there is a wide tolerance, wide tyres can also be ridden on comparatively narrow rims without any safety concerns. The recommendations of the ETRTO (European Tire and Rim Technical Organisation) provide guidance, although these are considered outdated in the bicycle industry.
For optimum performance, tyre and rim manufacturers tend to recommend wider rims, including combinations that would not even be permitted under the ETRTO. Background: On the one hand, aerodynamics are improved because the rim and tyre form a more favourable profile. On the other hand, the riding characteristics benefit: On wide rims, tyres bulge wider and with more volume, which means that the carcass is under more tension at the same pressure. This makes the tyre firmer. However, this has a greater effect on steering behaviour than on suspension.
The wide rims make the tyre slightly less bouncy, but the ride is much more agile and laterally stiff. This creates the potential to lower the pressure - by up to two bar - and combine more comfort with agile steering behaviour. For 25-millimetre racing tyres, an inner rim width of 17 to 19 millimetres is considered optimal, for 28 millimetres around 20. Gravel bikes with tyres around 40 millimetres are best ridden with 24 millimetres or more. The matrix is based on current recommendations from the tyre manufacturer Schwalbe.
Clincher, tubeless or hookless?
The terms refer to the rim bed and indicate the type of tyre for which the wheels are suitable. Clinchers are the classic: folding tyres with an inserted inner tube have been standard for decades. Tubeless or tubeless-ready rims can also be ridden without a tube; a latex-based liquid then seals the tyre and rim. The rim well is either completely closed or sealed to the spoke holes with a special rim tape.
There are also grooves at the edges into which the tyre bead literally hooks. Today, most current road bike wheels are prepared for use with tubeless tyres. Classic clincher tyres with inner tubes can also be ridden on tubeless rims; the reverse is not possible. Hookless rims are still relatively new; they do without the small shoulder on the rim flange, which makes them lighter and more stable. Only a few manufacturers offer rims of this type, prominent examples are Cadex and Zipp.
Special care must be taken when choosing tyres: They may only be ridden with tubeless tyres, and not all tyre models are approved for these rims. In addition, the maximum pressure is significantly lower, sometimes only five bar. However, because the sidewalls of the tyres can support themselves better - the effect is similar to that of the wider rims - this is sufficient.
Wheels - How many spokes?
The number of spokes and their arrangement largely determine how stiff and durable the wheel is. Fewer spokes make the wheel lighter and aerodynamically better. Today, 20 to 24 steel spokes are typical for road bike wheels. However, it is difficult to make a generalised statement about what makes sense, as other factors also play a role. For example, a wheel can be laterally stiff even with few spokes if they are tensioned more tightly.
The prerequisite is that the hub, spokes and rim can cope with this in the long term. The spoke design also plays a role: spokes with a round cross-section are more durable and stiffer than thinly rolled bladed spokes. If you want to be on the safe side, 24-spoke wheels are a good choice. If you have particularly high demands, for example a high system weight, it makes sense to go to a wheel builder: wheels with 32 or even 36 spokes can withstand extreme loads if they are well made.
Which storage technology?
A rough distinction is made between conical bearings and industrial or deep groove ball bearings. Today, the former are only found on very inexpensive models; until a few years ago, only Shimano continued to use this type of bearing even on high-quality wheels. Sealed industrial bearings require less maintenance and are easier to change. High-end models are occasionally fitted with ceramic bearings whose balls and/or raceways are not made of steel but of a high-strength ceramic material. They last longer and run more smoothly; measured against the much higher price, however, the advantages are hardly relevant in practice.
Where are there restrictions on compatibility?
When buying a bike, you have to make sure in several places that the wheels fit the bike.
Axles: For disc brake bikes, 12-millimetre thru-axles are now standard, and the installation widths (front 100, rear 142 millimetres) are almost the same everywhere. For rim brake bikes with quick-releases, only one standardised dimension (100/135 millimetres) has been common since the 1990s. Be careful with wheels from the transition period (2015-2018): There were disc wheels with quick-release axles, which is sometimes still the case with very cheap wheels today. Occasionally, there were also 15 or 10 millimetre thru axles. Many wheelsets can be converted to the different dimensions with adapters, but these are not available for all models. The best chances are with the big manufacturers such as DT Swiss or Mavic.
Freewheel: The freehub body on the rear wheel must match the gearing, but not all wheels are available with all freehub types. Shimano and SRAM freewheels are widespread and are available for most models. There are far fewer wheels for Campagnolo drivetrains. Shimano has been using the same standard for decades (exception: the current twelve-speed Dura-Ace), which also fits SRAM drivetrains with up to eleven sprockets. SRAM introduced the special XDR freehub with the twelve-speed eTap AXS groupsets. Campagnolo differentiates between road bike components (nine- to twelve-speed) and the N3W freehub especially for the gravel components of the Ekar groupset.
Brake discs: A multi-tooth mount known as a centre-lock, on which the disc is secured with a central nut, is common. Less common are 6-hole mounts, which require different brake discs, but can also be installed in any wheel.
Jens Klötzer
Editor
