Aero Insight - Aero vs. Weight

JB
posted by Jean-Paul Ballard
July 13, 2020

What makes you faster… aerodynamic savings or weight savings, and in particular, how about when it comes to rotating weight? To help answer this question for Global Cycling Network (GCN), we at Swiss Side put our various advanced tools and know-how to work, with wind tunnel testing, course simulations and a simple set of scales. Is Aero always KING? …   

Watch the GCN video "Why Rotating Weight Doesn't Matter On Your Road Bike / GCN Tech Debunk A Common Cycling Myth" and read the Rotating Weight Simulation Summary below:


Simulation Setup Overview:

  • Wind tunnel test data and course simulations were made to compare the time saved or lost using aero wheels (HADRON Ultimate 625) with lightweight climbing wheels (Lightweight Meilenstein).
  • For the simulations we make the assumption that the identical tyre is used and thereby the rolling resistance (Crr) was kept the same for both wheels.
    • Weight of Swiss Side HADRON Ultimate 625 set: 1600g.
    • Weight of Lightweight Meileinstein set: 1165g.
  • 4 configurations were run (A, B, C, D) changing from the aero wheel to the lighter weight climbing wheel, independently accounting for differences in aero, weight and rotational weight, step by step.
  • Each config was run over 3 different course types (climb, TT and criterium) to calculate the time required to cover each course with each config.
  • The data was supplied by GCN. The course was generated from the supplied data and the simulation tool was used to replay each course using the actual power and rider / system weights.
  • For each config and each course, the simulation was run to consider various wind conditions and the aero effects: no wind, wind at 5kph (averaged over all directions), wind at 10kph (averaged over all directions).

 

 

 

In the results tables presented below, the time deltas in each row, are the deltas give relative to the previous configuration.     

  • So configuration A is the baseline reference (with the aero wheels).
  • Configuration B shows the time delta due to the isolated weight effect due to climbing and in-line accelerations (but without rotation weight effects).
  • Configuration C shows the additional time delta due to the isolated rotational weight effect (relative to configuration B).
  • Configuration D then shows the additional time delta due to the isolated aero effect (relative to configuration C).
  • The final Total D-A shows time delta of the total effect of changing the wheels from the HADRON aero wheels to the Lightweight wheels, (taking into account all factors together, as would be the case in the real world).

           

Results:

Climb:

  • “Morning_Ride (Sa Calobra)”
  • Length: 9492m
  • Av Speed: 18.2km/h
  • Av Slope: 7.1%
  • Av Power: 322W
  • Total Time: 31min 19sec
  • Power sensitivity: -5.0sec/W (time saving per additional Watt power output).

 

 

Conclusions:

  • The isolated weight saving with the Lightweight wheel set alone gives a -6.7sec time improvement on the Sa Calobra climb.
  • The isolated rotational weight reduction with the Lightweight wheel set brings a negligible (<0.05s) time difference.
  • The isolated aero effect with the Lightweight wheel brings a time penalty of +2.3sec in wind still conditions and up to +14.8sec with a 10km/h wind.
  • The total effect on the Sa Calobra climb is that the Lightweight wheel set brings a -4.4sec time saving in wind still conditions but this becomes a time penalty of up to +8.0sec with 10km/h wind due to the lost sailing effect. The rotating weight reduction from the lighter wheel weights has a negligible effect.

 

TT:                 

  • “25mile_TT_pb_of_49_03_project49_completeditmate”
  • Length: 40’259m
  • Av Speed: 49.2km/h
  • Av Slope: -0.3%
  • Av Power: 282W
  • Total Time: 49min 04sec
  • Power sensitivity: -3.9sec/W (time saving per additional Watt power output).

 

 

 

Conclusions:

  • The isolated weight saving with the Lightweight wheel set alone gives a -0.3sec time improvement on the TT course.
  • The isolated rotational weight reduction with the Lightweight wheel set brings again a negligible (<0.05s) time difference.
  • The isolated aero effect with the Lightweight wheel brings a time penalty of +29.5sec in wind still conditions and up to +105.7sec with a 10km/h wind.
  • The total effect on the TT course is that the Lightweight wheel set brings a time penalty of +29.3sec in wind still conditions but this increases up to +105.3sec with 10km/h wind due to the lost sailing effect. The rotating weight reduction from the lighter wheel weights has a negligible effect.

 

Criterium:     

  • “Help_For_Heroes_Criterium_1st”
  • Length: 42’817m
  • Av Speed: 40.2km/h
  • Av Slope: 0%
  • Av Power: 235W
  • Total Time: 63min 52sec
  • Power sensitivity: -5.1sec/W (time saving per additional Watt power output).

 

 

 

Conclusions:

  • The isolated weight saving with the Lightweight wheel set alone gives a -2.4sec time improvement on the criterium course.
  • The isolated rotational weight reduction with the lightweight wheel set brings a very small -0.7sec time saving.
  • The isolated aero effect with the Lightweight wheel brings a time penalty of +20.5sec in wind still conditions and up to +81.2sec with a 10km/h wind.
  • The total effect on the criterium course is that the Lightweight wheel set brings a time penalty of +17.4sec in wind still conditions but this increases up to +78.2sec with 10km/h wind due to the lost sailing effect. The rotating weight reduction from the lighter weight, has a tiny (negligible) effect on delivering time savings.

 

Discussion:

Very interesting is that the rotating weight (inertial) effect is tiny or negligible. This was expected for the TT and hill climb courses where there is no acceleration, but initially surprising for the criterium event, which has many acceleration and braking phases. For the criterium, the isolated rotating weight reduction due to the ~400g lighter weight wheel set, only accounts for a -2.4sec time saving over a 63min 52sec race time. This is only a 0.06% time saving. But this does not consider the aero benefit on top of this which we see is far more potent.

However upon further more detailed analysis, the reasons become clear. Energy is not lost with a heavier wheel upon acceleration. This energy is simply stored in the wheel like a flywheel. This energy is only lost in braking phases. Analyzing the braking phases in the criterium event, these account for a total of less than 2% of the riding time. Furthermore, since the relative weight difference between the wheels is still relatively small, and this small reduction in rotating weight can only yield any benefit for less than 2% of the riding time, it is not surprising in this context that the sensitivity to the wheel inertia is so small and the time savings predicted by the simulation are tiny. The rotating weight effects are an order of magnitude smaller than the overall (static) weight effect due to climbing and in-line accelerations.

The potency of aerodynamics for time saving is highlighted by these simulations to be by far the dominant factor. Even on the climb, the aero effects, in particular the sailing effects with the presence of wind, show to be the dominant performance parameter.

In terms of importance there is a clear ranking:

  1. Aerodynamics. (First order effect).
  2. Weight. (Second order effect).
  3. Rotating Weight. (Third order).

Conclusion: AERO is KING!