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Posted by Cy Scott

Engineering The EPIC 3.4 Wheelset System

Tkwl3 43t 0 paraview super Tkwl3 43t 0 paraview grande

Each of our new wheels was designed with computational fluid dynamics (CFD) for optimal performance in the aforementioned use cases. While most companies will design and test a single rim profile (or cross-sectional area) and extend this single profile to different rim depths, each of our wheels, whether it be the 3.4 front wheel or the 5.6 front wheel, has a completely unique rim and drag profile. Our CFD was done on the SimScale platform (simscale.com), a brand new company that offers all types of engineering analysis tools in a cloud-computing environment, which allowed us to rapidly test more variations than would otherwise be physically possible. You can read more about our CFD processes here if you’re interested:

https://www.simscale.com/customers/tokyowheel/

http://www.deskeng.com/de/?s=tokyowheel 

Our CFD simulations have rendered aerodynamic drag reductions of 10 to 20 percent over our old models (across different yaw angles). The front wheel rim profile of each wheelset was iterated on to reduce drag in both the direction of travel and the direction perpendicular to travel to give these wheel amazing handling even in the harshest of crosswinds. The rear wheel rim profile was then iterated with a seat tube and optimized to reduce drag only the direction of travel at lower yaw angles (0 – 10 degrees). Consequently, the front wheels have a more blunt shape and are thicker than their rear wheel counterparts.

Designing Tokyowheel's 3.4 series wheelset posed an interesting engineering problem: we wanted a light-weight wheelset that could deliver the best handling to performance ratio possible for our climbing and sprinting rider. To accomplish this, we created individual design studies for each wheel (front and rear). We hypothesised that by designing a front wheel for crosswind stability, and a back wheel for pure aerodynamic advantage, we'd end up with a higher-performing wheelset overall. This post is just a snip-it of the engineering and design processes used.

The 3.4 series covers our climbing and sprinting use case. As such, this use case requires the lightest-weight construction possible and agile handling/steering in all weather conditions. To meet these requirements, I first set out to design the lightest wheel in the entire line-up, and the front wheel of the 3.4 series. This front wheel would begin with a 33 mm depth rim, which is the shallowest rim we could design due to structural loads and material selection. We collaborated with our manufacturer during much of this early stage of development to determine what the most efficient carbon lay-up would be for such a wheel.

The actual profile of the wheel is designed with and around a tubeless 25 mm tire, for reduced rolling resistance and maximum traction. This tire geometry consequently set certain design constraints on the bead hook and braking surface width of our rim. Our clincher design, as a 1st level requirement, must be able to accommodate any brand of tire our customer might want, and do so without effecting the brake calipers or fork of their bike. As such, we used our own heritage angled braking surface. The angled braking surface improves the overall aerodynamics of the wheel by smoothing the interface between the tire and the rim and reducing net drag on the rim. Ultimately, the more elliptical and symmetrical shape provided by this 3 degree angling of the braking surface results in a more functional aerodynamic wheel in all conditions. With these parameters set, I began the process of CAD modeling and CFD analysis to arrive at the most optimal profile for crosswind stability.

The design phase required the use of many modern engineering tools, as well as research into the latest concepts in aerodynamics. Design started with the CFD testing of our entire previous Epic series, the 2.5, for a control group and standard. I decided to simulate the wheels at the 3 most performance indicative yaw angles: 0, 15, and 45. The 45 degree yaw angle was chosen in order to best measure the lateral (or side) forces on our wheels that contribute to steering instabilities.

I then started the long and iterative process of designing a new rim profile in CAD and running CFD analysis to optimize the shape. Ultimately, the winning rim profile was chosen based upon the raw data received from post-processing and numerical methods. This front rim provided the best balance of reduced drag in both the direction of motion and the lateral axis. I believe it is important for our customers to understand the engineering involved in this type of decision. Below is an illustration from Falcon Pursuit of the various forces (drag) acting on a bicycle wheel during travel. Please note that this image is the property of Falcon Pursuit. They are awesome! (www.dontbeadrag.com)

As you can see, the yaw angle of a wheel has a great effect on the effective drag felt by the rider. Any angle past 0 degrees produces force vectors that cancel each other out in the direction of travel. The effective drag, or resultant drag force, is simply the subtraction of the force component of the lateral direction (Z-axis) from the force component of the forward direction (X-axis). This resultant drag force is what most wheel companies today report to their customers and is the very reason why you might find negative drag forces reported on some wheels.

The force component in the lateral direction is a vector that is actually acting in the same direction as the wheel: forward. These negative drag forces will only occur at the higher yaw angles because, as you can see, the higher the yaw angle the greater the magnitude of this lateral force component. I believe thoroughly, as many other engineers do, that these reported negative drag numbers are less representative of real world conditions and only look good paper; a rider will never actually experience a forward “push” from their wheels, they will only feel the relative ease at which said wheel moves in a crosswind.

So in an effort to decrease this lateral drag force in our front wheel, we chose to optimize it at higher yaw angles and probe the CFD simulations for the lateral axis. This analysis resulted in the more rounded profile you see before you now. It is the perfect compromise between crosswind stability and reduced drag in the direction of travel. In addition, our analysis shows that the new 33 mm front wheel reduces the drag on the wheel at 0 degrees of yaw by 20%! With drag savings in the lateral axis of up to 10%, compared to our previous model.

The 3.4 rear wheel followed the same line of design and subsequent analysis as above, but was instead optimized for reduced drag solely in the direction of travel, and at the shallower yaw angles of 0, 5, and 10 degrees.

We hope you like them!

-Cy


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