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 36h 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 as they 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 profile 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 aerowheel. 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. Select aero bladed spokes with a flat cross section to slice through the atmosphere and you will really reduce drag. 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”.
Reduced spoke count wheels (ie. 24h, 20h) have
greater tension which puts a significantly higher amount of strain on the
rim, pulling it in opposite directions. 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 are commonly built with higher tensioned spokes but don't suffer from radical distortion if a spoke breaks. Carbon has excellent elasticity and will
continue to bend and return to its original shape. This means that replacing a broken spoke will bring it back to the original shape without distortion. The reinforcement of shape memory polymers makes carbon a malleable material to create distinct aero shapes with. When it cools it yields a contrasting material that yields under incredible force yet can be engineered to provide great stability and stiffness (or flexibility) in specific directions. This is important when creating a wheel that rides smoothly but has high lateral rigidity. 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 against it's layup. It has good impact resistance to a degree. A force that would dent an aluminum rim would crack a carbon rim, relegating it to the scrap heap while an aluminum rim would still be safe, but lumpy.. There is a breaking point, but it is quite high and difficult to convey the exact amount of force required for failure. A carbon tube used for a bicycle's top tube, say on a Tr#k requires approximately 3,600 lb's of force to break. When it comes to wheels there are a number of factors initiating dynamic force on it's makeup from the rider, road, and the inertia of two said masses converging at high rate of speed.
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. If you have any question about carbon rims being strong enough watch this... 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 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, or lighter weight materials using a higher thread count (Think of how nice higher thread count sheets feel...) 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. A traditional 32 spoke tubular rim excels in this type of terrain where aerodynamics is not as influential. See Sheldon Brown's article on aero wheels that disseminates the stark differences between 36h traditionally laced wheels and 18 spoke 58mm deep carbon aero wheels - yet, somehow magically aligns them in sharing similar lateral and radial stiffness as only Sheldon Brown can do.
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 and improve braking 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 and they are, in fact,
quite durable. The investment made for a good custom wheel set that is designed
for the rider will pay dividends down the road and will be a pleasure to ride
day in and day out. Increased aerodynamics benefits the road rider of any size defeat their greatest enemy and get to the point - which is to go faster.
-Jens Voigt
-gene
gene_ruiter@cycleloft.com
