Is a Lighter Bike a Better Bike?
Triathlete Magazine, October 1999
by Rick Denney
In the car-racing world, there’s a saying: Horsepower sells engines; torque wins races. No, I’m not going into a discussion of horsepower and torque. My point in bringing it up is that even highly experienced practitioners don’t always understand the importance of different characteristics. In our world, we face the same confusion. Look through the ads in Triathlete, and you’ll see two recurring themes. Whatever is being advertised is either lighter or more aerodynamic. Which is more important? Let’s do some comparisons.
I’m not going to present any equations in this article. Instead, I’m going to present the results of some hypothetical calculations so that we can more intelligently set priorities. 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 Compton’s superb web site called Analytic Cycling. 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 has the best effect of all, because wheels rotate. I’ve heard otherwise level-headed folks claim that weight on a wheel’s rim counts six times weight elsewhere on the bike. Let’s send that familiar bovine to its long rest before doing anything else.
Weight on a wheel affects the wheel’s rotational moment of inertia. The moment of inertia is a measure of how the weight is distributed on the wheel. The common wisdom is that rotational weight counts twice what non-rotational weight counts. That’s because to accelerate a bike, you have to pick up translational momentum, which is what you gain moving down the road, and you also have to pick up angular momentum, which is what you gain spinning a wheel.
The rotating-weight penalty only affects acceleration, and we really don’t have to worry about acceleration very much. The effect of a bike’s weight during acceleration is small, and the effect of a wheel’s 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 that the effects of rotation don’t matter much is that we don’t accelerate much. If you are doing kilos on a velodrome, then worry about it. Maybe. But in the typical 40-km bike leg, we accelerate exactly once, with an additional partial acceleration at the turnaround. You can’t even measure the effects.
If you are going up a hill, then 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.
If that extra weight on the rim serves to improve our aerodynamic efficiency, then it is worth it. Let’s do a comparison. Mr. Lightwheels weighs 165 pounds, and rides a bike that weighs 17.6 pounds. His twin brother, Aero, is exactly the same. Both can maintain 250 Watts of power output in a 40-km bike leg, and both face identical wind drag, except for 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; I’m using typical numbers.) Standard wheels aren’t much lighter, if at all, than aero wheels, but just for fun let’s say that the conventional wheels are ultralights that weigh 200 grams less per wheel.
In a flat 40-km 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 over 28 seconds ahead of his lighter brother. This includes the effects of the startup acceleration. Even if the bike leg goes steadily up a 3% grade, the rider with more aero wheels will win. Only when the grade exceeds 3.7%, does the bike with lighter wheels have the advantage. And that’s 3.7% 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, except for hill-climb events. Even when they weigh more, they are better. In a flat 40-km 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? Let’s 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. Both riders are identical, each weighing 165 pounds and able to produce 250 Watts of power in our time trial. Both bikes are also identical except for weight. Both 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, and it is really 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 3-second advantage in 40 km. When was the last time you lost a race that long by three seconds? How much is that three-second gap worth to you? That’s the consideration when you pay a bunch extra for lighter parts, and only you can assign that value. But remember that we are comparing bikes that are 4.4 pounds different, not 4.4 ounces.
On a 6% hill climb, it matters more. Mr. Lightbike will go 10.13 mph, and Mr. Heavybike will only manage 9.93. That’s 8 or 10 seconds difference every mile or so. Of course, no 40-km time trial is 6% over the whole length (that’s an 8000-foot climb!), and Mr. Heavybike will gain some of that advantage back on the downhills.
What if the reason Mr. Heavybike’s bike is heavier is because it is more aero? Let’s suppose that Mr. Heavybike is only 1% more aero on his bike. To achieve that, he would only have to reduce his frontal surface area by 1%, from 0.5 square meters to 0.495 square meters. This is pretty easy unless Mr. Heavybike’s position is already very carefully refined. A 1% reduction in aerodynamic drag will increase his flatland speed to 25.03 mph. That’s faster than Mr. Lightbike’s 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 1% aero advantage? The answer is about 11 pounds more than Mr. Lightbike’s 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 he’s only 1% more aero.
Even on a hill climb, being more aero helps, but not nearly as much. On our 6% hill, Mr. Heavybike won’t be able to hang with Mr. Lightbike even if he is over 20% more aero. On a 3% grade, though, Mr. Heavybike will match Mr. Lightbike’s speed if he improves his aerodynamic efficiency by 10%. 10% is not so hard to achieve by improving body position (depending on your starting point), but it’s impossible to achieve by having a more slippery bike.
Let’s 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.