WIND TUNNEL TESTING METHODS & RESULTS.

SAME DRAG with LESS SIDE FORCE than the best 80+mm deep ‘big brand’ wheels! …HADRON IS HERE!

Following months of aerodynamic optimisation work in CFD and the extremely positive results achieved (see ‘Update 7’ http://archive.swissside.com/735), it was time to put the final Hadron designs to the test in the wind tunnel. The entire development methodology where a weighting system driving both the drag as well as side-force performance measures, across the entire cross-wind angle range, was put to the ultimate test.

…And the Hadron performed amazingly well, proving to outperform what is regarded to be the world’s leading aero wheel set!

Swiss Side assembled what would undoubtedly be considered the most experienced aerodynamics team to ever take part in a wind tunnel test in the bicycle industry! The team of aerodynamicists had a combined experience of over 50 years in Formula 1 aerodynamics and wind tunnel testing.

The purpose of the test was not only to test the Hadron back-to-back against the world’s best aero wheels, but also to evaluate the different methods of testing, with the goal of defining which is the most representative and efficient way for testing bicycle wheels in the wind tunnel. In addition, all of Swiss Side’s other wheel models including the Heidi, Franc, Gotthard andMatterhornwere tested against their respective leading brand competitor models, in order to determine where they sit on the aerodynamic leader board.

1. TESTING METHODS

There are varying opinions on the most representative wind tunnel testing methods for bicycle wheel development. At Swiss Side, we are careful to remain objective and scientific about our engineering approach and we let the data do the talking. Therefore, using the vast experience in aerodynamics and wind tunnel testing within our team, we decided to evaluate multiple methods for the wheel testing. Below is a table describing the various relevant testing methods together with the pros and cons of each. Highlighted in grey are the methods chosen to evaluate in the wind tunnel.

The Basics:
The wheel or complete bike with wheels are setup in the test section, held in place by stays which are linked to extremely accurate balances below the floor, capable of measuring all the forces and moments which act on the bike mounted above. For the purposes of our bike wheel testing, we are mainly interested in drag and side force. The wind tunnel facility Swiss Side chooses to use has an ‘open jet’ test section. The wheels rest on rollers which are driven by electric motors to rotate the wheels at the correct speed relative to the airspeed in the test section. The entire installation can be rotated on a turntable in the test section to simulate a wide range of yaw (cross-wind) angles. The wind tunnel can be run at various airspeeds, our primary test speed being 45km/h. When a test is conducted, the data from the balances are logged continuously together with the airspeed, air temperature, pressure and density. The balance ‘zeros’ are also recorded before and after each run.
 
Repeatability:
One of the most important factors for wind tunnel testing is the repeatability. During the testing session, repeated tests of a given configurations are performed. The results of these repeats should be as identical as possible. Without ensuring adequate repeatability of identical configurations, then the comparison of results between different configurations cannot be reliably made. This is a fundamental principle of wind tunnel testing whether it be in Formula 1 or for testing bicycle wheels.

 
How real is real enough?:
As a part of our CFD investigations and the model setup, we had already evaluated numerous configurations including stand-alone wheel, wheel with partial bike frame, complete bike and complete bike with rider. From this it was possible already to analyse and understand the flow fields to determine which elements influence the wheel performance, and which are required for ensuring a realistic simulation. It was noted, for example, that that upper torso isn’t important for measuring relative wheel performance.

In addition, members of our team had extensive experience with wind tunnel testing using static human dummies and the extreme challenges that this method poses for repeatability. Even the smallest movements of the static dummy between runs (for example when changing wheels), can lead to significant changes to the flow field and the measurement results. The rider accounts for 75% – 80% of the total drag on a road bike. So even the smallest movement of a dummy in the wind tunnel causes large changes in the drag force measured which can completely swamp the relatively small force differences measured between the various test wheels.

Testing with static dummies was therefore ruled out. Nonetheless, we were still interested in verifying if dummy legs had a significant impact on the flow field in the wind tunnel and, in turn, on the relative test results and trends between different wheels. To do this in a repeatable way, the only method was to use dynamic (moving) legs which would pedal. The sampling time for each yaw (cross-wind) angle was increased so that the measured data could be averaged over numerous pedal rotations.

In addition, two less complicated configurations were also evaluated- front wheel alone and wheels on complete bike frames. Note that both road bike frame as well as time trial frame geometries were tested.

 
What was tested?:
Hadron prototype wheel sets were constructed using rapid prototype (3D-printed) covers. These were tested back-to-back against various ‘big brand’ aero wheels of various profile depths, including what is regarded as the world’s leading deep profile aero wheel.

In addition, all of the top end Swiss Side wheel models (Heidi, Franc, Gotthard & Matterhorn) were tested against their ‘big brand’ counterparts to establish their relative aerodynamic performance.

All wheels were tested with the same tyres and tyre pressure. In addition, various other tyres were tested to establish the tyre shape sensitivity on the aerodynamic performance of the wheels.

Smoke visualisation was also conducted and high speed photographs made in order to correlate the flow fields with the CFD flow visualisations. The smoke visualisations also make for great marketing material!

One of the Hadron prototype wheel sets, built using structural rims with non-structural rapid prototype covers.

 

2. RESULTS
 
Different testing methods – Conclusions:

 

All three testing methods  (1. Front wheel only;   2. Wheels on complete bike frame;   3. Complete bike with dynamic leg dummy) offered good consistent and repeatable results. Moreover, the relative performance offsets measured between the different wheels was the same independent of the testing method.

Nonetheless, some interesting effects were observed with the complete bike and dynamic leg configurations, which offered good directions for future development.

The ‘front wheel only’ tests offer the best resolution for demonstrating the relative wheel performance. The following graphs show the results. Again, the relative wheel performance offsets and characteristics are the same as with the complete bike simulation methods.

 

Drag Results-  Hadron:

***World’s best deep profile aero wheel-  reference wheel for Hadron comparison.

 

The benefits of the Swiss Side Hadron (62.5mm deep profile) seen in this graph are:

–         Stall point delayed from 11° to 14° yaw compared to reference wheel (82mm profile).

–         Almost identical drag magnitude and characteristics as reference wheel but with lower overall drag at high yaw angles.

–         Negative drag (thrust) consistently achieved from 13° to 15°.

–         Huge performance advantage over equivalent ‘Big Brand’ 60mm aero wheel model.

–         Very poor aero performance of ‘Big Brand’ externally machined wheel shown for reference.

 

Side Force Results-  Hadron:

***World’s best deep profile aero wheel-  reference wheel for Hadron comparison.

 

The benefits of the Swiss Side Hadron seen in this graph are:

–         Significant reduction in side force compared to reference wheel, with increasing reduction with increasing yaw angle:  Greater than -20% at high yaw angles.

–         Note that reduced side force shown by the inferior drag performance wheels is normal. The lower drag effect of good aero wheels comes from the flow remaining attached (not stalling). The attached flow produces lift as on an aircraft wing. This lift is the side force. It is not possible to have low drag in cross-wind without generating side force. The Hadron however successfully achieves an improved balance of low drag together with lower side force compared to the reference wheel.

Drag Results-  All wheels:

***World’s best deep profile aero wheel-  reference wheel for Hadron comparison.

 

All of Swiss Side’s wheel models outperformed their equivalent ‘Big Brand’ counterparts across the range. Some observations from the test results are:

–         Swiss Side Matterhorn (30mm carbon climbing wheel) offers extraordinary aero performance for this profile depth, showing typical aero wheel characteristics with delayed stall until 7.5°. It massively outperforms its ‘Big Brand’ equivalent wheel model in aerodynamic performance.

–         Swiss Side Franc, with its wide bladed Sapim ‘Aero’ spokes offers a slight improvement in aero performance over its lighter weight brother, the Swiss Side Gotthard.

–         Swiss Side Heidi & Heidi Shadow offer excellent aerodynamic performance for a 23mm profile wheel, almost on par with the Gotthard.

–         The competitor wheels with externally machined rims for reinforced spoke attachment, showed extremely poor aerodynamic characteristics. These are the only wheels which increase drag with cross-wind.

Side Force Results-  All wheels:

***World’s best deep profile aero wheel-  reference wheel for Hadron comparison.

 

The side force characteristics of Swiss Side’s other wheel models was as expected. Some observations from the results are:

–         Swiss Side Matterhorn showed an elevated side force response due to the low drag, lifting effect in the pre-stall region up to 7.5° yaw. After the stall point, the side force drops to similar levels of the other less aerodynamic wheels.

–         Swiss Side Heidi & Heidi Shadow offered the lowest side force which is in line with its drag response.

–         Again the competitor wheels with externally machined rims for reinforced spoke attachment, showed the highest side force levels of the lower profile wheel models.

3. ANALYSIS – What these results mean in the real world?
 
Final Performance Results:

 

Referring back to our previously published ‘Update 4’ (http://archive.swissside.com/735) and ‘Update 7’ (http://archive.swissside.com/596), it is again important to highlight that the entire Hadron development process was designed to produce a wheel which gives the best all-round real world aerodynamic efficiency and consequently the best overall real world performance. For this reason, the performance weighting system was implemented, taking into account both drag as well as side force.

The final performance results relative to what is regarded as the world’s best deep profile aero wheel, against which the Hadron was tested back-to-back, are:

So for a slight 1.1% increase in drag, the Hadron offers 5.9% reduction in side force, combined with a delayed stall characteristic and improved high cross-wind angle performance. Note that these characteristics in particular also offer benefits at low riding speed because for a given wind speed, the effective cross-wind angle is higher. Based on an aerodynamic efficiency performance function which gives a 70% weighting to drag and a 30% weighting to side force:

 

The Swiss Side Hadron offers a 1% performance
improvement over the world’s best aero wheel.

Note that this has been achieved with a lower 62.5mm profile compared with larger 80+mm profiles of the Hadron’s direct competitors. Also importantly, this wheel will be an aluminium-carbon hybrid clincher, with a 23mm wide braking track (therefore offering ideal interchangeability with other wheels) and will be competitive in weight to its full carbon competitors. Finally, not to be forgotten:

 

The Swiss Side Hadron will be less than 50% the cost of the
world’s best aero wheel and other ‘Big Brand’ equivalents.

What does this mean in more simple terms?:
As aerodynamic efficiency is hard to gauge, we have made an equivalent calculation of what this means in terms of weight on a bike.

 

A cyclist climbing a 10% gradient at 10km/h, with a total rider + bike weight of 80kg, requires a power output of approximately 235W. For every additional kilogram weight, the power output increases by approximately 3W.

 

The difference in terms of aerodynamic performance between a typical low profile wheel such as the Swiss Side Heidi and the highly aerodynamically developed Swiss Side Hadron is 10W of power*.

*(Based on aerodynamic drag measured at 45km/h). 

 

So based on these calculations, aerodynamic efficiency can be easily equated to weight:

 

The Swiss Side Hadron can offer a 10W power
saving over a typical low profile wheel.
 
10W is the equivalent of 3kg weight on a bike.

In the context of power outputs, significant drag savings were measured with the time trial bike over the standard road bike. The relative wheel performance offsets and characteristics however remained unchanged. Using the weighted average drag data, the power saving of the time trial bike frame tested, directly compared with the road bike frame was in the order of 37W!

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