It’s no big secret that
aerodynamic wheels provide a significant benefit when it comes time to hammer on
the bike. But what’s up with all the options? There seem to be as many
variations of bike wheels in the world as there are bike companies. While
freedom of choice is great in theory, it doesn’t help you when you can only
afford one set. Only a handful of athletes can stock their garage with a quiver
of wheels for every type of training and racing. So how do you combine hard
science with practical sense when determining the correct wheel set to
supplement your collection? Finding the answer, as you see below, is a
multi-step process.
Step 1: Know Your
Options
We can squeeze most of
the world’s wheel options into three categories.
Least-aero: Standard flat or box
rims with various patterns and numbers of round metal spokes that are usually
very durable and relatively inexpensive.
More-aero: Bladed spoke wheels with
aerodynamic rims.The best of these
will decrease a person’s 40-kilometer time-trial time on a flat course by
roughly 1.5 percent (or 50 to 70 seconds) versus the standard rims above. In the
“More-aero” category, there is a range of options.Aerodynamic rims come with different rim
depths, and the spokes will range in number and vary from round to bladed.In a strict drag-related sense, the
deeper the rims and the more bladed the spoke, the faster the wheel.
Most-aero: The most efficient
set-up is a rear disk wheel with a composite-spoke front wheel. This set-up can
knock another .5% (10-20 seconds) off a 40k time when matched up against the
best aero rimmed and bladed spoke wheels. These are heavier wheels and are the
least durable of the three options.
Step 2: Weighty
Matters
It all sounds pretty
simple so far: Slap on more aerodynamic wheels and you’ll reduce your time. On
level roads, even the added weight of a disc wheel is negligible; since reducing
the drag force (that is the force it takes to move the wheel through air) by 10
grams can negate an increase in actual weight of 1,000 grams. Problem is, once
the road tilts up, weight must be considered, and the arguments over using a
disc versus an aerodynamic spoked wheel get heated.
Once speeds drop under
eight mph, as they would on a sustained and steep climb, aerodynamic drag
contributes minimally to the total force you must overcome through pedaling, but
at 25 mph this number jumps to 90 percent of the resisting force. The general
speed where aerodynamic drag trumps weight and calls for aerodynamic wheels is
12 mph. And even in courses with sustained climbs, the weight of a good aero
wheel set (1500+ grams) is not much heavier than a light-weight wheel set (1100+
grams), and the aerodynamic speed advantages can be significant.
This fact was
demonstrated to us at this summer’s U.S. National Cycling Festival in
Seven-Springs, Pennsylvania. The time-trial course included
significant and steep rolling hills – enough to lead many racers to opt for
lighter aero-rimmed, spoked wheel sets instead of heavier disk rear/composite
front combinations. Out on the course though, we saw that those weight concerns
were unfounded. Sure the climbs brought speeds below the standard 12 mph
barrier, but since the high speed descents allowed momentum to be carried into
and up the next uphill, the most aerodynamic set-up (disk/composite) resulted in
faster times.
Step 3: Speed vs.
Durability
But besides weight,
there are a few other downsides to the “most-aero” option in racing wheels. One
is durability: these wheels are made for racing, not for enduring thousands of
miles of training. Additionally, these wheels have an “apparent mass” which
makes them harder to get moving. Basically, the more weight found farther from
the axle, the more force it takes to make that wheel turn.
Step 4: Race
Directors
Another factor to
consider is the variety of events you want to compete in. If you’re racing
criteriums, time trials, road races, and possibly triathlons, a disc wheel may
only be useful – or legal – in a few events each year. On the other hand, a
relatively light weight, “more aero” spoked wheel may provide very good
performance in the highest percentage of your competitions.
Coach’s
Opinion
If you’re training and
racing on just one set of wheels, a durable aero rim with bladed spokes is your
best option. It gives you the aerodynamic advantage of reduced drag and the
flexibility to reap the benefits of that advantage in almost any conditions.
Kirk Nordgren is a Coach for
Colorado Springs, Colorado-based Carmichael Training Systems. For more
information on the latest in training, fitness, and nutrition go to www.trainright.com.
