In the triathlon world, an analogous confusion is widespread. Look through the bike ads in any triathlon or cycling magazine and youll notice two performance characteristics touted above all others: lightness and aerodynamics. Which is more important? Lets do some comparisons.
Im not going to present any equations in this article. Instead, Im going to present the results of some hypothetical calculations so that we can set out upgrade priorities more intelligently. Before I do, though, let me tell you where I got the numbers, in case you want to do your own research.
All the calculations presented here came from Tom Comptons superb web site called Analytic Cycling (www.analyticracing.com). Analytic Cycling performs the computations and also presents the equations.
It is accepted lore that lighter bikes are better bikes, and that reducing weight on wheels in particular has the best effect of all, because wheels rotate. Ive heard otherwise level-headed folks claim that weight on a wheels rim counts for six times the weight elsewhere on the bike. Not so.
The rotating weight penalty only affects acceleration, and we really dont have to worry about acceleration very much. The effect of a bikes weight during acceleration is small, and the effect of a wheels weight is very small.
Prove this to yourself: Run up a flight of stairs. Then run up a flight of stairs carrying a wheel. Finally, run up a flight of stairs carrying a spinning wheel. You get the idea: The weight of the wheel, spinning or not, has very little effect compared to what it takes to accelerate your body.
The other and more important reason why the effects of rotation matter little is because we dont accelerate often. If you are doing kilos on a velodrome, then worry about it. Maybe. But in the typical 40K bike leg, we accelerate exactly once, with an additional partial acceleration at the turnaround. You cant even measure the effects.
If you are going up a hill, weight counts. But at a constant speed (uphill or not), weight on a wheel counts exactly like weight on the bike, or weight on your body.
Moreover, if that extra weight on the rim serves to improve aerodynamic efficiency, then it is worth it. Lets do a comparison.
Mr. Lightwheels weighs 165 pounds, and rides a bike weighing 17.6 pounds. His twin brother, Aero, is exactly the same. Both can maintain 250 watts of power output through a 40K bike leg, and both face identical wind drag, except in their wheels.
Mr. Lightwheels has conventional wheels lightweight box-section rims with 32 round spokes. Aero has something like Tri-Spokes. (Pick your choice of aerodynamically optimized wheel; Im using typical numbers.) Standard wheels arent much lighter, if at all, than aero wheels, but just for fun lets say the conventional wheels are ultralights weighing 200 grams less per wheel.
In a flat 40K time trial, who will win, the rider who is lighter, or the rider who is more aero? The answer is that the rider with aero wheels will finish more than 28 seconds ahead of his lighter brother. This includes the effects of the startup acceleration. Even if the bike leg goes steadily up a three-percent grade, the rider with the more aero wheels will win.
Only when the grade exceeds 3.7 percent does the bike with lighter wheels have the advantage. And thats 3.7 percent over the whole race, not just the uphill half of a rolling course.
Other analyses have shown that aerodynamically efficient wheels are always better, even in bike racing events like criteriums, with the exception of hill-climb events. Even when they weigh more, they are better.
In a flat 40K time trial, the aero wheels would have to weigh many pounds more before their weight soaked up their aero advantage.
That takes care of the wheels, but what about weight elsewhere? Lets do another comparison.
We have two riders competing against each other in our hypothetical time trial. The first is Mr. Heavybike, whose bike weighs 22 pounds. Then we have Mr. Lightbike, with a 17.6-pound bike.
The riders are identical, each weighing 165 pounds and able to produce 250 watts of power in our time trial. The bikes are also identical, except in weight. The riders have equal aerodynamics. These are typical values for riders who can do a one-hour 40K.
If you look at the physics, the weight has no input into the calculation of drag on a flat course, except in one place: rolling resistance. This is the drag created by the tires against the pavement, which is actually heat generated by the rubber as it deforms across the contact patch.
When we do the math, 250 watts will propel Mr. Lightbike at 24.99 mph, and Mr. Heavybike at 24.96 mph. That gives Mr. Lightbike about a three-second advantage over 40K.
When was the last time you lost a race that long by three seconds? How much is that three-second gap worth to you? Thats the consideration when you pay a bunch extra for lighter parts, and only you can assign that value. But remember, we are comparing bikes that are 4.4 pounds different in weight, not 4.4 ounces.
On a six-percent hill climb, it matters more. Mr. Lightbike will go 10.13 mph, and Mr. Heavybike will manage only 9.93 mph. Thats eight or 10 seconds difference every mile or so. Of course, no 40K time trial is at a six-percent grade over the whole length (thats an 8,000-foot climb!), and Mr. Heavybike will gain some of that advantage back on the descents.
What if the reason why Mr. Heavybikes bike is weightier is because it is more aero?
Lets suppose that Mr. Heavybike is only one percent more aero on his bike. To achieve that, he would only have to reduce his frontal surface area by 1 percent, from 0.5 square meters to 0.495 square meters. This is pretty easy unless Mr. Heavybikes position is already very carefully refined.
A one-percent reduction in aerodynamic drag will increase his flatland speed to 25.03 mph. Thats faster than Mr. Lightbikes 24.99 mph.
But what if Mr. Heavybike borrowed his machine from Mr. Reallyheavybike? How much would his bike have to weigh to soak up a one-percent aero advantage? The answer is about 11 pounds more than Mr. Lightbikes machine.
So, Mr. Lightbike is stylin at the start line with a 17.6-pound bike, but Mr. Heavybike will match his performance on a 29-pound bike if hes only one percent more aero.
Even on a hill climb, being more aero helps, but not nearly as much. On our six-percent hill, Mr. Heavybike wont be able to hang with Mr. Lightbike even if he is over 20 percent more aero.
On a three-percent grade, though, Mr. Heavybike will match Mr. Lightbikes speed if he improves his aerodynamic efficiency by 10 percent. Ten percent is not so hard to achieve by improving body position (depending on your starting point), but its impossible to achieve by having a more slippery bike.
Lets sum up. If you are riding in a hill climb time trial, then lighter is better. But for flat or rolling bike courses, aerodynamic efficiency has a much greater effect than weight. Go forth and spend wisely.