January 16, 2014
STRUCTURAL DESIGN & ANALYSIS.
One of the core parts of the Hadron wheel development process is the structural design and optimisation of the wheel assembly. As always, our goal at Swiss Side for the Hadron is to engineer an absolute top-level wheel set in its category and to bring it to the market at a price 40% below that of the leading brands. With the Swiss Side team’s wealth of experience from Formula 1 and the sports product design sectors, together with clever engineering choices, this has, once again, been made possible. In this update, we detail the methods used and the engineered choices made in the structural design process for the Hadron wheel set.
The key criteria which drive any wheel design are weight, stiffness, strength and durability. The rim design, hub design, spoke count and spoke type all play a role in these criteria. Also the materials and construction strongly influence the design process. Carbon fibre, aluminium and steel all have quite different properties and characteristics which influence the design. Together with the material choice, the construction of rim (e.g. clincher or tubular) impose different design constraints. Nonetheless, the step-by-step method we use is the same regardless and we apply the same approach across our entire pallet of wheel models… St. Bernard, Heidi, Heidi Shadow, Franc, Gotthard, Hadron and Matterhorn.
SWISS SIDE’S STRUCTURAL DESIGN & ANALYSIS METHOD
1. DESIGN TARGETS:
Our structural design targets for the Hadron were to make the wheel set as light weight as possible but strong enough to cater also for heavier riders, as well as, all types of roads, smooth or rough. From a usability and end price point of view, we wanted to keep an aluminium clincher rim structure and braking surface. We also focus heavily on our stiffness to weight criteria. How stiff is stiff enough? Our target here was to offer good lateral stiffness performance without making the ride totally uncomfortable in terms of vertical stiffness. Finally, nothing short of excellent was the target for durability. In summary, for the particular wheel construction and aerodynamic profile depth we are designing, our target weight is in the mid-1600g range.
2. FIRST CONCEPT:
With the targets above in mind, our initial specification was as follows:
For the rim construction we have been testing prototype hybrid aluminium-carbon clincher wheels for two years to assess their performance and durability. (See photo further below). This construction consists of a minimum profile structural aluminium rim with a non-structural bonded carbon fibre fairing. The huge benefit of this construction is that any shape and depth of aerodynamic rim profile is possible without impact on the structure. Also aluminium braking surfaces are kept for optimum braking performance, as well offering no hassle to swap between aero wheels and standard training aluminium wheels. This is a very versatile solution and the weight penalty compared to a full carbon clincher equivalent is absolutely minimal, but for a fraction of the cost to produce. We have been extremely impressed with the prototypes and therefore chose this construction for further optimisation for the Hadron wheel project.
Spokes & Spoke Count
To ensure excellent strength, durability and optimal aerodynamics for the Hadron, a spoke count of 18-front and 24-rear was chosen. For the front, the 18 ‘straight pull’ spokes are arranged in a purely radial configuration. For the rear, our much developed ‘tension optimized’ rear hub with ‘2-to-1’ spoke lacing configuration was the natural choice. This features all ‘straight pull spokes’ with 8 radial spokes on the non-drive side, and 16 cross-laced spokes on the drive side. This offers lower overall and more balanced rear spoke tension for the same resultant stiffness, coupled with improved deflection response characteristics with wheel bump.
For optimal durability and aerodynamics, clear choice was for the market leading Sapim CX Ray spokes. These offer the absolute minimal aerodynamic section, with the lightest possible weight but with the highest fatigue (load cycle) life of any spoke on the market.
For the Hadron we are using Swiss Side’s absolute top end ultra-light straight pull hubs. These are our tried and proven designs used on all our top end 2013 wheel models.
The front hub consists of a minimal design for the lowest possible aerodynamic drag and weight. The rear hub features spoke tension optimized flange designs with easily interchangeable and simple to maintain cassette body.
We use only the best quality sealed cartridge bearings from Enduro for minimal maintenance and uncompromised performance. A bearing choice of top end stainless steel or ceramic-hybrid is available.
3. FEA- FINITE ELEMENT ANALYSIS:
The Finite Element Method of structural modeling is vital to the design process. Using the computer models of our wheel designs, we can apply virtual loads to the assembly to allow us to look at the internal stresses in the wheel structures, in turn allowing us to optimise the design of the individual components.
It is very important to correctly simulate the real life loading conditions. This is not a trivial task as many forces are in play. These include the spoke tension, tyre pressure, plus the vertical and side forces transmitted via the tyre contact patch, through the wheels into the bike frame.
In our FEA process, we model the true wheel assembly geometries and importantly the individual materials and associated properties of each component in the wheel. We also need to be careful to correctly model the interface between the components so that the wheel structure reacts as it would in reality. Like this we can identify areas of critical stress where reinforcement is necessary but also areas of low stress where material can be removed to reduce weight. We can also estimate the expected wheel stiffness and deflection under load.
An image of the Hadron front wheel structure with amplified
deflection in a fully loaded configuration.
Another very important consideration in analyzing the FEA results is fatigue. Fatigue is the cyclic loading of a structure. In a bicycle wheel, with every revolution the spoke tension changes and the rim bends. This results in variable loads in all the components of the wheel. As time goes on, the number of load cycles also accumulates. Time microscopic cracks unavoidably occur at areas of stress concentration. For example, on a bike wheel, this typically occurs around spoke holes. The local stress at the tip of the cracks is further increased causing it to then propagate through the structure, eventually causing it to fail.
There are many additional considerations which affect the fatigue life. Most importantly, the type of material used. Different materials have different fatigue characteristics. Steels typically have level of stress where the fatigue life is infinite (known as the endurance limit) whereas this is not the case with aluminium. The metallurgical microstructure also plays a major role. Cast metallic components have a much more brittle structure than forged or extruded components for example, causing cracks to propagate faster and at lower stress levels. Surface quality, finish and any surface treatments also play a role.
Therefore, the true allowable level of stress in the components needs to be carefully considered hand in hand with the fatigue life so as to ensure the durability of the product.
However FEA is just a simulation of an ideal world and is not the be all and end all. There are many other factors which influence the real structure and loading such as variability in the production and manufacturing tolerances. The way people use and even abuse their wheels plays a role, but also environmental considerations such as temperature, salt, grit, and the associated abrasion and damage. All this needs to be considered carefully and a certain margin of safety in the design needs to be applied. Ultimately, real life testing and experience is invaluable and plays an equally important role in the design process.
4. DESIGN OPTIMISATION:
Once we have a complete overview of the FEA results, we can build a better picture of what challenges or compromises each design may present. We then go through a process of comparing the weights, stiffness, production challenges etc, between the various designs to hone in on a final direction. The input from our customers, followers and team riders is also important here as the end goal is to produce the most desirable wheel design for our customers. All of these factors are considered for defining the ultimate design specification for a final detailed loop of design and analysis.
5. PROTOTYPE TESTING:
After a few loops of refining the design, prototypes are produced for testing and evaluation. From our design and experience at Swiss Side, but also together with that of our manufacturing partners, we begin the testing process. Remember that Swiss Side uses the same manufacturers and quality of production as the “Big Brands”.
Our testing protocol looks as follows:
- Stiffness testing.
– The static (not rotating) lateral and vertical deflections of our wheels are tested at various loads and at various points on the wheel to ensure that stiffness is at the levels expected.
- Endurance bench testing.
– We use a test bench which can apply combinations of vertical and lateral loads to the wheel whilst rotating at a set speed. This rig runs for a set period of time under higher than normal loads to get a first look at the durability of the design. If it passes this test, we proceed to the next level.
- Destructive bench testing.
– Using the same test bench, the wheel is run again at a fixed combined vertical and lateral load which is significantly higher than what the wheel would see on the road. The wheel is tested at set speed and must withstand the loads for a certain period of time before failing.
- Road testing, structural performance evaluation.
– Once the wheel has passed the bench testing steps, it’s out onto the road for the structural performance evaluation. Here we test the wheel out under some extreme riding conditions to ensure that it performs as expected. This includes sprinting acceleration tests, high speed braking tests, coasting, and riding some pretty nasty routes with plenty of potholes!
- Road testing endurance evaluation.
– In this final step we tap into our team riders to muster kilometers on the prototype wheels. The wheels are swapped around between the riders to maximize the mileage. Once a set distance has been covered on the prototypes without issue, the stamp of approval is given for production.
On the left: The latest Hadron prototype structural test wheels (without carbon covers).
On the right: The first evolution Hadron test wheel set, endurance testing since mid 2012.
6. FINAL DESIGN CONFIRMATION AND RELEASE:
Once the new wheel design has passed through the rigorous loops of design, optimisation and testing, it’s all go for production. In parallel, numerous other parts of the project plan such as the graphic design, packaging, accessories all come together at this point. A new wheel is born!
Stay tuned… HADRON is coming!