Rim Profiles and the Effect on Aerodynamics

Aerodynamics is key in addressing the way a wheel should be built.  I personally love a deep section rim with fewer spokes because it changes the entire ride characteristic of your bike and requires less effort to maintain speed.  I use a 66 mm. deep carbon wheel for all around riding and they really shine on race day.  I use shallower aluminum rims for training where the increased weight and wind resistance aids in training.  Switching over to the deeper rims for racing just makes the bike feel so much faster.  Deeper section rims provide the best aerodynamics to stiffness ratio.  The only downfall of an aerodynamic rim is that more material is needed to make a deeper section rim and they tend to be heavier. Deep section rims range from around 30 mm. to 90 mm. deep. Carbon fiber rims provide the best benefit of a deeper aero profile without the weight penalty of an aluminum rim.  Carbon rims can be built much deeper than aluminum rims but with much less weight than a similar profle aluminum rim.  Case in point: a Mavic Open Pro rim with an 18 mm depth weighs about 430 grams.  A 46 mm deep carbon rim weighs 450 grams.  Because carbon is very strong it can be built with less spokes, thereby reducing overall wheel weight compared to the Open Pro built with 32 spokes, and it will still be stiffer.  Carbon rims are commonly found in incremental depths averaging 35 mm., 45 mm., 65 mm., and 80 mm.  In this regard they are ideal, but quite expensive.  A good compromise is to use a 27- 38 mm. deep aluminum rim to keep the cost and weight down when looking for an affordable aero wheel.  The deeper the rim the more aerodynamic and stiff it will be.

This is not to say that lower profile rims cannot be built to be stiff.  A traditional box section rim can be built using more spokes to get be quite stiff.  In effect, this will increase the weight and lower the drag co-efficient, making them a less aerodynamic option.  Most aero aluminum and carbon rims are built with fewer spokes, typically between 16-24 on the front wheel and 20-28 spokes on the rear.  Using fewer spokes reduces the overall weight of the wheel and provides better aerodynamics by decreasing turbulent airflow or increasing the drag co-efficient.  Traditional aluminum box section rims vary in depth  from 18-27 mm. Rim depth greatly affects the ride quality of a wheel.  Lower profile or shallow depth rims provide a lively ride quality that has better rebound and can dampen the ride.  This, in part, is due to the fact that there is less material to provide rigidity as well as the use of longer spokes required to build the wheel, which provide more “spring”.

If you watch the spring classics such as Fleche Wallone, Flanders or Paris-Roubaix you’ll see a large number of riders on traditional box section aluminum rims built with 32 or 36 spokes per wheel.  This is because the tenacious nature of these races drives riders over the roughest roads and teams use them because they provide the greatest amount of strength and comfort in the most adverse conditions.   Increased spoke count on a wheel improves durability and makes them easier to repair by more evenly distributing the tension of the wheel to all of the spokes.

Reduce spoke count wheels have greater tension which puts a significantly higher amount of strain on the rim.  The high tension makes for one responsive wheel that can transform a bike from feeling soggy to crisp.  But because of this higher tension, when a spoke breaks on an aluminum wheel, the rim will often permanently distort making it irreparable even when a spoke is replaced and the wheel is brought back up to the correct tension. Catastrophic failures of this nature make exotic aluminum wheels a gamble when riding over cobblestones.  Carbon Fiber rims do not suffer from this malady as the material has what is referred to as infinite memory.  Meaning, it is highly elastic and will continue to bend and return to its original shape.  Every material has its limitations in terms of memory but carbon fiber has very high strength to weight ratio and requires incredible force to break when strained laterally.  Carbon has low impact resistance but is incredibly strong when stressed compared to aluminum.  These properties in addition to its compliant ride quality and dampening characteristics make it an ideal material for rims used when riding on rough surfaces and many professional riders use carbon wheels in the spring classics.  Of course, cost is a major factor when considering using carbon rims and teams have a quiver of wheels provided by sponsors.  The traditional box section rims are still used by traditionalists who rely on their tried and true durability.

In addition, the box section rims used by professional racers are tubular rims. Tubular rims are built without the taller bead of a clincher rim and require that the tire is glued to the rim.  I will cover tubular vs. clincher tires in another article.  The tubular rim can be constructed using less material and is inherently lighter than their clincher counterparts. Tubular rims are used for this purpose and provide the benefit of running larger volume tires at lower pressure, contributing to a smoother ride quality from the added cushion, making them faster on rough roads or for cyclocross.

When considering the type of material to use when building a rim it is important to take into consideration the type of conditions that they will be subject to.  After all a wheel set is an investment that you want to get the most out of without having to save them for special events or ideal riding conditions.  Professional riders use carbon rims for all types of racing and training but they require more maintenance than an aluminum rim because the braking surface is more prone to wear when introducing dirt, sand and rain between the pad and rim.  Big teams have big budgets and wheel sponsors provide them with multiple sets of wheels to use day in and day out.  For the everyday cyclist who is considering investing in a set of carbon hoops regular maintenance will prevent wear but be advised, if you ride all winter and never clean them after riding in the rain and slush the braking surface will wear significantly faster than an aluminum rim.  Many people use them for everyday riding because the performance of the wheels is addictive, but if you want your investment to last then it is best to save them for race day or use them in dry weather.




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