One of the most important traits of a wheelset is its role in transferring your efforts on the bike to an outcome on the road, so matching the right wheels for your riding style is important, as is having a reliable set that still offers sound levels of performance.
Anatomy of a wheel
A bicycle wheel consists of four main components, all of which have an influence on weight, performance, and durability. Upgrading (or downgrading) these components can affect the ride quality, your effort output (speed) and braking performance so it's worth knowing a little about each, and how that relates to improved performance on the road.
Rim: The wheel's rim sits on the outside of the wheel and has two main functions; to hold the tyre and provide a braking surface (for rim-brake bikes, not disc-equipped bikes). The rim width will affect the tyre width which could have a significant impact on rider comfort, and the material of braking surface will impact braking performance.
Hub: The hub sits at the centre and provides the axis of rotation. Within each hub, front and back, is the axle which attaches the wheel to the bike. On the rear wheel the hub features splines which a cassette attaches to. The bike's chain wraps around the sprockets of the cassette, and in association with the crankset and shifters, forms the bikes drivetrain which propels it forward. As well as propelling the bike forward, all road bike wheels (excluding fixed-wheel bikes) will use a rear hub with a 'freehub' mechanism which allows the bike to coast.
Spokes: Material that connects the hub and rim. The number of spokes will vary between front and rear wheels, the rear often featuring more spokes to improve strength and stiffness. More spokes typically means a stronger wheel but that comes with a weight penalty. Spokes are made from varying materials and come in different shapes to either optimise strength or improve aerodynamics, sometimes both. Most commonly spokes are made with steel wire, but can vary greatly in shape and diameter.
Nipples: Spokes attach to the wheel via a special nut called a 'nipple'. Nipples are important for adjusting spoke tension which will 'true' a wheel, making it straight when spun.
What makes a good wheel?
Choosing a good wheel will depend largely on its intended purpose, however whilst difficult to nail all aspects, ideally a good set of wheels will be durable, have dependable hubs, provide confidence-inspiring braking, be stiff for power transfer, yet also be lightweight.
Lightweight wheels keep rotating weight down by having a shallow rim profile and low spoke count. As well as reducing overall weight, a fringe benefit of this is comfort. Deeper wheels are fast but the ride quality is often described as 'harsh', conversely, lightweight wheels often provide good levels of compliance. Quality lightweight wheelsets will typically be below 1,400 grams, some extremely lightweight wheelsets coming in under 1,000grams for the pair!
Aerodynamic wheels aim to be as fast as possible by reducing drag. Aerodynamic wheels are typically greater than 40mm deep at the rim and are becoming wider as well. This speed does come at a cost with deep-section wheels more susceptible to crosswinds which can make them difficult to handle, and the extra material does add weight.
Wheelsets not so focused on performance commonly have features that make them more appropriate for everyday use or general training. The braking surface is aluminum providing better performance in all weather conditions when compared to carbon fiber, higher spoke counts are used to aid strength, and rim width is wider to cater to larger tires. As a result of these features, weight increases with quality wheelsets typically ranging from 1,500 - 1,800 grams.
Wheelsets designed for loaded touring or to withstand regular use under heavier riders (120kg +) are typically hand-built with higher spoke counts of 32 or even 36 spokes. With the extra strength, wheels in this form typically weigh in excess of 1,900 grams.