CeramicSpeed has collaborated with British aerodynamicist Simon Smart of Drag2Zero to develop a new derailleur cage that claims to save 2.5 seconds over a 25km time trial ridden at 50kph or 7.2 seconds over 40km.
According to the Danish company, the CeramicSpeed OSPW Aero reduces drag by 40% on average and at some points up to 60% compared to a stock derailleur.
CeramicSpeed says the OSPW Aero has been a “continuous dialogue with Simon Smart spanning over two years, where extensive wind tunnel testing has influenced multiple design iterations.” The end result is a new carbon-reinforced PA cage wrapped around its existing “mechanically optimized” oversized aluminium pulley wheel (OSPW) design.
According to Smart, given that the rear derailleur cage only represents about 1% of the total system drag, he and CeramicSpeed had a challenge on their hands particularly as the cage sits on the most complicated area of flow on the bike, and also given the position and angle changes that come with gear changes.
Smart claims that ultimately they found the sweetspot between aero and mechanical whilst still being able to utilize the OSPW layout - the 13-tooth upper and 19-tooth lower pulley wheels (or 15-tooth upper for SRAM configurations). The OSPW Aero “refines aerodynamics by focusing on frontal area impact, as well as smoothing and controlling the trailing wind.
It had taken, says Smart, “meticulous effort to keep the weight of the aerodynamically optimized cage down [though at the moment there's no weight quoted] while retaining critical system stiffness for precise and crisp shifting.”
It may sound like a highly niche marginal gain for time trial or triathlon bikes (opens in new tab) only, but CeramicSpeed explains in the white paper (opens in new tab) published with the press release that it conducted tests with a TT bike (Canyon and Scott) and aero road bikes (opens in new tab) (Factor, Specialized and Scott) and says the relative differences between pulley cages were the same. Therefore: “We can assume that within the realms of repeatability of the testing that the data is valid for any bike type.”
Additionally, “Initial tests with a disc wheel and 60mm deep rear wheels proved to have minimal difference on the aerodynamics of the pulley cage.” The white paper explains that at very high yaw angles, there are slightly different sensitivities. The effect of wheel choice is lessened from the non-drive side, and higher from the drive side when using a disc wheel compared to a medium depth aero rim. The conclusion regarding wheel type: “Overall, the disc wheel v 60mm deep wheels makes little difference to the weighted average.”
CeramicSpeed notes in a section titled ‘design constraints' that the dynamic nature of a derailleur and pulley cage results in changing aerodynamic positioning throughout use: the drag on the pulley cage will be reduced when the cage is in the most swept back position (small chainring and smallest cog on cassette). And the drag is higher when the cage is swept forwards (large chainring and largest cog cassette). Therefore it carried out all testing in the vertical rotational position (the large chain ring and smallest cassette cog).
The testing itself took place outside using an anemometer to measure crosswinds as well as in the Silverstone Sports Engineering Hub (opens in new tab) wind tunnel. It included testing pulley cages in isolation, complete bike only, as well as bike with rider and mannequin scenarios.
CeramicSpeed admits in the white paper that when a very high accuracy is required to map the performance of small bike components, the best compromise is to test on a complete bike which simulates the blockage around the pulley cage. “In an ideal world,” it continues, “all testing would be performed with a moving rider in order to simulate the fluctuating pedal wake. However, the measurement fidelity with moving legs is not sufficient to map small design changes through all yaw angles. It is therefore common practice to develop many components with a bike only.”
In other words, the gains from small changes to the design could be so small as to be unnoticeable in real-world situations.
Is it UCI legal since fairings are banned? One of the aims of the project was that in order to adhere to the UCI fairing regulations, the cage would have to be designed “in a structural manner and not only a fairing.”
Although it's hard to see how it isn't a fairing, since a derailleur cage doesn't require a cover for its structural integrity, the system has already been used in the Giro d'Italia by Israel-Premier Tech and Intermarche-Wanty-Gobert.
This isn't the first time CeramicSpeed has got involved in aerodynamics as well as shifting: in 2019 it proved that its radical prototype 'Driven' driveshaft system was also more aerodynamic that a standard drivetrain (opens in new tab).
The OSPW Aero comes in three versions: for Shimano 12-speed 9250 and 8150; for Shimano 11-speed 9100 and 811 and for SRAM Red/Force AXS. All three cost the same €739 and are available from CeramicSpeed's website (opens in new tab).
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Simon Smythe is Cycling Weekly's senior tech writer and has been in various roles at CW since 2003. His first job was as a sub editor following an MA in online journalism. In his cycling career Simon has mostly focused on time trialling with a national medal, a few open wins and his club's 30-mile record in his palmares. These days he spends most of his time testing road bikes, or on a tandem doing the school run with his younger son.
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