Upgrading your wheels is one of the first and probably best ways of significantly improving the performance of your bike. As wheels and every one of their components is rotating mass, even the slightest shaving of weight saved here will pay dividends in your bike's acceleration and climbing capabilities.
There are also obvious aerodynamic advantages of a slim race wheel with a low profile tyre over say a broader, more bulky rim with a fatter, lower pressure, slower rolling tyre on a touring wheel.
But ask any seasoned roadie which part of the bike is the most difficult to work on - especially now exotic materials are taking over from simple shop-sourced stuff - and the reply is nearly always wheels. Replacement parts like rims and spokes are not as commonly available as they once were either, unless you go the more traditionalist route and opt for handbuilt wheels, which tend to be better value for money anyway.
What to choose...
Most cyclists go for clincher rims as the range of tyres is far greater and they are easier to fit than the sprint type that requires the user to glue the tyre to the rim. That said 'tubs' or wheels with 'sprint' rims are widely preferred by elite time triallists as the tubular tyres can withstand pressures of up to 180psi that some believe to provide lower rolling resistance. Cyclists in the pro ranks still prefer 'sprints' too, as a punctured tubular can be ridden on until the service car catches up with a replacement.
Some time ago the UCI brought in a set of safety standards that have to be met in order for a wheel to be allowed for use in UCI sanctioned events. Go to the UCI website.
As a general rule the stiffest wheels have thick, oversized rims, spokes and hubs with a shorter spoke path while softer wheels tend to have a longer spoke path and lighter rims.
The perfect wheel
Most people want a wheel that is light enough yet sufficiently strong to hold their weight without shaking loose or causing chain rub on the climbs. So this is where picking the right wheel to suit you comes in.
Due to the need for space to accommodate the cassette, the left hand spokes of a rear wheel are unavoidably looser than the right hand spokes which can lead to the rims rubbing on the brake blocks. Some manufacturers get around this by using an asymmetric rim for the rear wheel with the spoke drillings offset to one side, while others use hubs with a short distance between the flanges to equalize spoke tension.
Low profile rims are without exception lighter, easier to source and more economical to buy. They also remain unaffected by strong cross winds. Deep section rims offer a small aerodynamic advantage and are well suited to time trials - they're increasingly seen in road races too. Midi-section machine-built wheels like Mavic's Ksyrium have taken cycling into the new century with their good looks but can be pricey, with spares less easy to source than for traditional hand-built wheels.
Stiffness and 'brake rub'
We've found the difference between the stiffest wheel and the least stiff was very little. But even that tiny difference meant a few flexed enough to brush the brake blocks when rocking the bike out of the saddle on steep climbs.
Interestingly when paired with a similar frame (light but not super-stiff ) there was less brake rub, highlighting that not only should you choose the right wheel for you and your riding but also one suitable for your bike..