posted by George Cant
May 06, 2014


SWISS SIDE HADRON secures its place at the top of the leader board, OUT-PERFORMING the best ‘big brand’ aero wheels! …

Swiss Side has been back to the wind tunnel with the production specification Hadron wheels. The purpose of the test was three fold:
1. To confirm the aerodynamic performance of the production Hadron wheels.
2. To conduct further tyre aerodynamic testing following the interesting trends seen in the previous wind tunnel test session (
3. To put the Hadron up against further top-level ‘big brand’ competitor wheel sets in the search of a wheel that can match the Swiss Side Hadron’s aerodynamic performance.

In conclusion, the final production specification Hadron wheels were taken to the mountain passes of the Swiss Alps for testing and rider evaluation on the road.



1.1 Performance Confirmation Test Method:

Before launching into the production phase of Swiss Side’s new Hadron wheel set, it was important to take the final production specification wheels to the wind tunnel to confirm their aerodynamic performance. In addition, some further fine details were evaluated. The run list for this part of the test looked as follows:

A) Hadron prototype vs production spec wheel.
B) Spoke holes open / closed. (There are small clearance holes around the spokes passing through the non-structural cover. The effect of this was measured).
C) Soft / stiff cover laminate. (Various carbon cover laminates were considered ranging from very stiff to very soft. The difference was the weight. Therefore any aerodynamic differences between soft and stiff covers were quantified).
D) High-speed test on the production laminate to check for any irregularities in aerodynamics performance.

The confirmation testing was done using a front wheel only. As detailed in the Hadron wind tunnel testing methods report (, the ‘front wheel standalone’ configuration offers the best resolution for evaluating and comparing the individual wheel characteristics. The results are consistent with those measured on complete bike frames, with or without dynamic dummy legs. This choice of testing method was important for this round of testing as the differences in the aerodynamic forces between the configurations tested was expected to be quite small. Therefore maximum resolution was required. The testing speed was the same as in all our wind tunnel tests, namely 45km/h.

1.2 Performance Confirmation Test Results:

A) The results showed that the performance of the production specification wheels exceeded that of the prototype wind tunnel wheels built with rapid-prototype (3D printed) covers.

B) Taping closed the spoke holes offers a very small benefit. Therefore for final production, the spoke clearance holes were reduced to the minimum.

C) The laminate stiffness had a slight influence only at the high yaw (cross-wind) angles where the side force is at it’s highest. The ultra-light laminate showed to be at a slight detriment, therefore the intermediate laminate weight was chosen for production (+15g per wheel).

The graphs below show the results. All wheels were tested with the identical reference tyre, which was also used in the previous wind tunnel test- Continental GP 4000s. The production spec wheel is the blue curve, although it should be noted that in final production the spoke clearance hole size has been reduced which will close the gap to the taped hole run shown in green.

d) The final part of the performance confirmation testing was the high-speed evaluation. The maximum speed was limited by the maximum possible wheel roller speed. This was 75km/h. The yaw angle was also limited to 16 degrees as higher angles are unrealistic at this speed. The Hadron production wheels were then tested under these conditions to ensure that there were no unexpected aerodynamic characteristics. There were not. Indeed, the performance characteristic was identical as at the lower speeds, except with the separation point delayed beyond 16 degrees. This is consistent with what is expected aerodynamically at higher speed.

2.1 Tyre Aerodynamics Test Method:

From the first wind tunnel test, interesting trends were observed between smooth wall and rough wall / textured tyres. The best tyre from the first test was the Continental GP4000S. Furthermore, the effect of tyre shape, in particular wider tyres and used tyres was evaluated.

The Hadron has been extensively developed in CFD to have a reduced sensitivity to tyre shapes, as well as to minimize any impact of the steps between tyre & rim and between braking surface & rim aero cover. (See CFD report- This robustness was further evaluated as a part of the tyre test, by testing different widths of an identical tyre model.

The clincher tyres tested were:
1. Continental GP4000S (reference tyre).
2. Continental Grand Prix-TT (GP-TT)
3. Continental Grand Prix-23mm (GP-23)
4. Continental Grand Prix-25mm (GP-25)
5. Continental GP4000S used (≈10’000km)

Tyres 1 – 3 above were tested to evaluate the effect of sidewall roughness / texture.

Tyres 3 & 4 were tested to evaluate the effect of tyre width using an identical tyre model (construction & tread).

Tyre 5 (used) vs. Tyre 1 (new) was chosen to evaluate the effect of local changes to the tyre shape. Note that the used tyre has a slightly squared off top due to the wear.

Two of the ‘big brand’ competitor wheel sets tested also had their own custom tyres installed (tubular tyres in this case). In these cases, direct ‘same tyre’ comparisons with these wheels were not possible.

2.2 Tyre Aerodynamics Test Results:

The results confirmed the trends and observations from the first wind tunnel test. Most importantly:

Tyre sidewall roughness / texture plays a very important part in wheel aerodynamics.

The reason for this is due to ‘boundary layer’ effects. The boundary layer is the layer of air near the surface of an object. Directly on the surface, the flow velocity is zero and the thickness of the boundary layer is given by when the flow velocity reaches 99% of the freestream.

There are two types of boundary layer, laminar & turbulent. The laminar boundary layer remains thin and in discrete layers. Eventually this transitions to a turbulent boundary layer in which the flow is mixing through the thickness. The laminar boundary layer remains as a thin sub layer below the turbulent layer. The turbulent boundary layer is more tolerant to separation because it is energized by high-energy freestream flow, which is drawn in and mixed into the boundary layer.

The tyre plays a critical part in the wheel aerodynamic system, because it forms the leading edge at the front of the wheel. The tyre surface has a tight radius of curvature on shoulder. The airflow here is stressed very highly and can easily separate from the surface (like stall on an aircraft wing). Therefore it is important that the boundary layer is turbulent in this region to make it more robust to separation. It is possible to control the transition point from the laminar to turbulent boundary layer. This is known as boundary layer tripping. On the tyre, this is done with sidewall roughness / texture.

Boundary layer tripping is very important in wheel aerodynamics.

The tyre shoulder shape is also important, because reducing its radius of curvature also reduces the stress on the airflow. However it is of secondary importance to the boundary layer tripping.

Note that this is no different in the world of Formula 1 aerodynamics and is a known challenge within these circles. Swiss Side’s hugely experienced aerodynamics team is well used to dealing with this issue, because F1 tyres, in particular F1 wind tunnel model tyres play a primary role in aerodynamic development.

One of the key tasks of this wind tunnel test session was to evaluate the level of sidewall texture required to trip the boundary layer. It was already established that the aerodynamic performance of completely smooth sidewall tyres is very poor regardless of shape, due to the prolonged laminar boundary layer and in turn, early separation of the flow. However in contrast, the GP4000S with its level of sidewall texture successfully trips the boundary layer.

The GP-TT tyre was selected for evaluation as it had only a mild roughness on the tyre sidewall. However as can be seen in the graphs below (light blue line), this was not adequate for tripping the boundary layer and resulted in early separation characteristics. Note that this was also the case on all wheels tested.

The Continental Grand Prix tyres in both widths- GP-23 (Dir-2) & GP-25 in the graph, offered an improvement over the GP4000S (23mm wide), bringing the Hadron to a new even higher level of performance again! The aerodynamic performance of the GP Tyre in both the 23mm and 25mm widths was almost identical, with only the expected deviation in characteristic occurring around the separation point. Furthermore, the GP4000S also shows almost identical performance to the GP tyres, again with the only changes occurring at the critical separation point. Therefore…

This confirms the Hadron’s tolerance to different tyres shapes.

However an interesting discovery was made with the GP tyres. These tyres have a directional tread pattern with some directional channeling. Therefore it was decided to test this tyre in both rotational directions (Dir-1 & Dir-2 in the graphs) to see any influence on the results, in particular in the separation point. Interestingly the reverse direction (Dir-1) offered the best aerodynamic performance. This is clearly linked with the boundary layer tripping, indicating that the reverse direction tread better energizes the turbulent boundary layer. This indicates that there is even more potential in further delaying the separation point with tyre sidewall texturing and will be further investigated in future wind tunnel tests with more tyre models.

3.1 Competitor Comparison Test Method:

As reported in the previous wind tunnel test report (, the Hadron achieved better performance than the industry benchmark aero wheel. So the search was extended to find a wheel which could outperform the Hadron. Absolute top specification (and very expensive) ‘big brand’ wheel sets were selected for the challenge.

Where possible, all competitor wheels are tested with the identical GP4000S reference tyre used by Swiss Side for all comparisons. However in this case, two of the competitor wheel models tested were tubular wheels and came pre-installed with their own custom aero tyres. Note that these tyres also had sidewall texturing, clearly designed for boundary layer tripping. (See tyre photos above).

The top spec ‘big brand’ competitor wheel specifications tested were:
• Competitor 1 – 60mm deep x 27mm wide carbon tubular wheel. -Custom tyre with blending strip from tyre to rim.
• Competitor 2 – 58mm deep x 25.3mm wide carbon clincher wheel. -GP4000S reference tyre.
• Competitor 3 – 50mm deep x 24mm wide carbon clincher wheel. -GP4000S reference tyre.
• Competitor 4 – 75mm deep x 24mm wide carbon tubular wheel. -Custom tyre.

3.2 Competitor Comparison Test Results:

So how did the Hadron stand up to this latest battle?

With reference to the following performance graphs, only one wheel was found which could put up a proper fight. That was the ‘Competitor 1’, a full carbon tubular wheel with custom aero tyre and a (heavy) aerodynamic blending strip to smooth the gap between the braking surface and the tyre. Admittedly, its performance was superior in the low yaw (cross-wind) angle range, however the Hadron was significantly stronger in the mid to high yaw ranges with it’s best tyre, the GP-23. Considering side force, although the Hadron and ‘Competitor 1’ showed very similar performance in the low yaw angles, ‘Competitor 1’ however showed improved (lower) side force characteristics at the higher yaw angles.

Very close results then but which wheel won the fight?…

Overall, the Hadron won the drag battle, however ‘Competitor 1’ won the side force battle. So overall the performance can be said to be very similar. However, when considering the cost price…

The cost price of the Hadron is less than HALF it’s nearest competitor.

The Hadron maintains its position at the top of the leaderboard.

The Swiss Side team understands that these are bold claims that the Hadron has outperformed all the competitor aero wheels tested to date. Therefore consistent with the Swiss Side brand approach of offering complete transparency to their fans and cyclists alike, along with their total confidence in the Hadron wheel performance, Swiss Side will host a wind tunnel test during the 2014 Eurobike week and will be inviting media editors to bring any wheel set along to put up against the Hadron.

This will be a David vs Goliath battle of the market leading aero wheels!

The test will be run in a fully independent and transparent manner. All wheels will be tested in the same conditions, with the same tyres (where possible) and the data will do the talking and be made public by the press to all.

The final part of evaluation for the production specification Hadron wheels was out on the road. Swiss Side has the luxury of the ideal testing ground at the front door… the Swiss Alps. As detailed in the previous structural design update (, the original Hadron prototype wheels have been out on the road accumulating mileage since mid 2012 without any issues. So from a structural integrity evaluation standpoint, this milestone was already complete.

The main purpose of road testing was to evaluate the rider feel to the wheels, particularly in windy conditions. Furthermore, checks for rattles, whistling, or other unexpected noises were done. Some of the roughest and toughest mountain passes were chosen for testing, ensuring that all types of road surfaces were tested. At the same time the Hadrons were tested at both very low speed on long climbs, as well as at absolute top speed on steep descents.

All the riders who have ridden the Hadrons to date cannot fault them. Particularly in gusty wind conditions the wheels perform phenomenally without any overpowering or off-putting side force or steering inputs to unsettle the rider.

With absolute industry leading aerodynamic performance, aluminium braking surfaces, light weight and clincher construction, the Hadron is an unbeatable wheel set!

Members of the Swiss Side Aerodynamics Team testing the Hadron production wheels back-to-back with Gotthard wheels.