Original episode & show notes | Raw transcript
A common belief in the cycling community is that “rotating weight” on a bicycle—primarily the weight of the wheels, tires, and rims—is significantly more impactful on performance than “static” or “non-rotating” weight (like the frame or the rider). The often-cited rule of thumb is that saving one gram of rotating weight is equivalent to saving two grams of static weight.
This idea stems from a valid physical principle: a rotating object possesses rotational kinetic energy in addition to the translational (or linear) kinetic energy it has from moving forward. Therefore, accelerating a rotating object requires energy to both increase its forward speed and increase its rotational speed.
However, as argued by the podcast host, a physicist, the practical, real-world impact of this additional energy requirement is often vastly overstated, especially for the amateur or “weekend warrior” cyclist. This document will delve into the physics and the specific calculations presented to illuminate why this is the case.
To understand the discussion, we must first define the two types of kinetic energy involved.
This is the energy an object possesses due to its motion in a straight line. It is the energy required to move the entire mass of the bike and rider from one point to another.
The formula for translational kinetic energy is: \(KE_t = \frac{1}{2}mv^2\) Where:
m is the total mass of the object (e.g., rider + bike).
v is the linear velocity (speed) of the object.
Every part of the bicycle and rider has translational kinetic energy.
This is the energy an object possesses due to its rotation around an axis. For a bicycle, this applies primarily to the wheels.
The formula for rotational kinetic energy is: \(KE_r = \frac{1}{2}I\omega^2\) Where:
I is the moment of inertia of the object. This is the rotational equivalent of mass; it measures an object’s resistance to being spun up or slowed down. It depends not just on the mass of the object but, crucially, on how that mass is distributed relative to the axis of rotation. Mass further from the center of rotation contributes more to the moment of inertia. This is why rim weight is more significant than hub weight.
ω (omega) is the angular velocity of the object (how fast it is spinning), typically measured in radians per second.
For a simple hoop or a bicycle rim where most of the mass is at the outer edge, the moment of inertia (I) can be approximated as: \(I \approx mr^2\) Where:
m is the mass of the rim.
r is the radius of the rim.
The total kinetic energy of a rolling wheel is the sum of its translational and rotational kinetic energy.
The host uses a practical, high-intensity cycling scenario to quantify the actual energy and power differences between heavy and lightweight wheels: accelerating out of a corner in a criterium race.
Scenario: A cyclist accelerates from 35 km/h to 40 km/h.
Time for Acceleration: 1.5 seconds.
Total Mass (Rider + Bike): 80 kg.
Wheel Comparison:
Lightweight Rims: 1000g per pair (a simplified, extreme example).
Heavyweight Rims: 2000g per pair (a full 1 kg heavier).
Note: The host simplifies the calculation by assuming the entire mass of the wheelset is located at the rim to represent the most extreme case for rotational weight savings.
Power is the rate at which energy is used. To find the power required to accelerate the wheels, we calculate the change in total kinetic energy (translational + rotational) and divide it by the time taken.
Power = tΔKEtotal=t(KEfinal−KEinitial)
Translational Power: The energy needed to accelerate the mass of the wheels from 35 km/h to 40 km/h.
Rotational Power: The energy needed to increase the spin of the wheels from the rotational speed corresponding to 35 km/h to that of 40 km/h.
The Results:
Power for Lightweight Rims (1000g): ~11.6 Watts
Power for Heavyweight Rims (2000g): ~23 Watts
The difference in power required just to accelerate the wheels is approximately 11.4 Watts.
While a saving of ~12 watts sounds substantial, it must be viewed within the context of the rider’s total power output required for the acceleration. The rider isn’t just accelerating the wheels; they are accelerating their entire 80 kg body and bike mass.
Calculation for the Total System:
Putting It Together:
Total Power with Heavy Wheels: 771 W (for rider+bike) + 23 W (for wheels) = ~794 W
Total Power with Light Wheels: 771 W (for rider+bike) + 11.6 W (for wheels) = ~782.6 W
The ~12-watt saving represents only a 1.4% to 1.5% reduction in the total instantaneous power required for that specific acceleration.
Important Caveat: This calculation excludes air resistance. At these speeds, overcoming aerodynamic drag requires a significant amount of power (the host estimates 200-300 watts just to maintain speed). When you factor in the power needed to overcome drag, the percentage saving from the lighter wheels becomes even smaller.
The analysis leads to a clear conclusion: while the physics of rotational kinetic energy is real, its quantifiable impact on performance for most cyclists is marginal.
Extreme Example, Small Savings: The scenario used a very large weight difference (1000g) between wheelsets. A more typical high-end wheel upgrade might save 200-300g. The power savings in such a case would be a fraction of what was calculated, likely only 2-4 watts, which corresponds to a sub-1% difference in power output during a hard acceleration.
Context is Everything: The moments where rotational weight matters most are during accelerations. In steady-state cycling (like a time trial or a steady climb), the effect of rotational inertia is negligible. You only have to spin the wheels up to speed once.
Cost vs. Benefit: Very lightweight wheels are extremely expensive. The host argues that for a non-professional cyclist, the thousands of dollars spent to save a few hundred grams of rotating weight could be far more effectively invested elsewhere.
The podcast suggests several areas that offer a much greater return on investment for improving cycling performance than chasing marginal gains in rotating weight:
Aerodynamics:
Aero Helmet: Can save significantly more watts at speed.
Improved Position: A professional bike fit can yield a more aerodynamic and powerful position, offering sustained benefits.
Wind Tunnel Time: For serious competitors, this provides the ultimate aerodynamic optimization.
Training and Support:
Coaching: A structured training plan will yield far greater performance improvements than any equipment upgrade.
Training Aids: Investing in things that make training easier and more consistent (e.g., a good indoor trainer, quality apparel).
The idea that saving rotating weight is twice as beneficial as saving static weight is a dramatic oversimplification that misleads many cyclists. While the principle is physically sound, the magnitude of its effect is very small in the context of the total forces a cyclist must overcome (inertia of the total system, aerodynamic drag, gravity, and rolling resistance).
For elite athletes where a fraction of a percent can be the difference between winning and losing, optimizing every possible variable, including rotating weight, is a logical pursuit. However, for the vast majority of riders, the significant financial cost of ultra-lightweight wheels does not justify the minuscule performance benefit. The money and focus are better directed towards improvements in aerodynamics and, most importantly, structured training.