Check out or latest addition to the product line-up: CYBREI Cranks and components!
Check out or latest addition to the product line-up: CYBREI Cranks and components!
Both bike brands and riders like to throw various saving figures around without specifying any context, which makes any such claim completely meaningless and empty.
Here is a quick guide on how to interpret and understand power savings in all three domains related cycling performance:
Claims about “watts saved” in relation to aerodynamic drag are often misrepresented due to the lack of specificity and the failure to describe the underlying measuring conditions.
As a rule of thumb, any such claim that does not state at least the test speed can automatically be disregarded. The power required to overcome drag increases with the cube of speed. For this reason, valid comparisons can only be made when comparing measurements performed at the same test speed.
The test methodology used for cycling aerodynamics also needs to be considered. The metric we are looking for aero testing is the value called “CdA” (which you can read more about here). When we compare the aerodynamic performance of two different parts, we compare the final CdA they produce in testing.
The difference in this CdA value is then used to calculate the difference between the power levels required to achieve a set speed. The resulting value represents “how many watts do you save” with that particular setup change.
The same upgrade – same change in CdA means radically different power savings at different speeds. For example, a 10% CdA reduction (0.16m2) saves 6W at 30km/h, 14W at 40km/h and 27W at 50km/h due to the non-linear nature of aerodynamic drag.
Based on this comparison, it might seem like the savings at low speed will not be as impactful due to the small absolute numbers, but the reality is completely different when you compare them proportionally – A 10% reduction in CdA will yield a ~ 10% reduction in aerodynamic drag across a wide range of speeds. For this reason, stating a percentual CdA difference is a much better comparison that “watts saved”.
In simpler terms:
While “6W saved at 30km/h” might not sound like much, it actually means a 10% reduction in aerodynamic drag. At the same time “6W saved at 50km/h” is equates to a marginal difference of just 2%.
(All examples were calculated using the Gribble interactive model.)
Power savings from rolling resistance are relatively simple to work out, as their relationship with speed is linear.
The rolling resistance itself (as a force) depends on the coefficient of rolling resistance (often labeled as Crr) and the vertical load on the tire.
This is the part that often leads to confusion: testing companies (e.g. bicyclerollingresistance.com) seem to be really accurate when describing the testing conditions:
“All numbers are for a single tire at a speed of 29 km/h / 18 mph and a load of 42.5 kg / 94 lbs.
Use the formula: RR (Watts) = CRR * speed (m/s) * load (N) to calculate rolling resistance at a given speed and load.”
However, it must be noted that just because the testers emphasize the “single tire”, nature of the test, it does not mean that your saving for two tires will be double that amount!
The rolling resistance is directly proportional to the total vertical load, AND THE VERTICAL LOAD ONLY! It does not matter how many tires the load is distributed to! (As long as they are of the same construction = same Crr of course)
To calculate the exact savings you will experience with your own setup, you need to input YOUR values to following the equation:
RR (Watts saved) = ΔCrr (the CRR difference of the 2 tires you are comparing) * speed [you are interested in comparing(m/s)] * load [your total system weight * 9.81 (N)]
Despite the fact that this term is by far the simplest of the three, I still see a lot of confusion in “watts saved” in the drivetrain department.
The amount of power lost in your bicycle drivetrain is directly proportional to the power output: a system that is 98% efficient will lose 2W at 100W, 4W at 200W and 8W at 400W.
In any claimed saving, the author MUST specify what kind of power output is the drivetrain loaded with during the measurement.
Unfortunately this is often not the case, with many claims venturing even beyond what is mathematically possible, e.g. “product XYZ saves 15W at 200W“.
With most road bicycle drivetrains performing in the 96-98.5% range, such an improvement would yield a drivetrain with an efficiency of over 100% – which frankly is nonsense.
Approaching this boundary leads to diminishing returns. Applying multiple performance upgrades, even if effective, will not add up in a linear fashion.
