From a Lightweight Bike to the Lightest Aero Bike
When starting to work on the SL1 frame, it was clear it should be a bike that is as balanced as possible, taking weight, stiffness, comfort and aerodynamics equally into consideration. Also, we wanted to keep it simple and easy to use. We decided to start from a structurally optimized bike, and then alter the shapes to be more aerodynamic without losing the structural efficiency.
So our first step was to find an optimized structure. Each tube was analysed on what kind of loads it experiences, and then we looked for the best geometry to withstand those loads, making it stiff where we want stiffness, and allowing it to flex where we want comfort. For all of our frames, we create a base model, assuming aluminium in 1 mm wall thickness, to compare stiffness and weight to our benchmark models. A train energy density plot shows us which areas are important for stiffness, and where we can save weight.
A great example for how this optimization process is the top tube. Our analysis showed quite significant torsional load close to the head tube which tapered off rapidly to an almost pure lateral bending. So the top tube starts with a roundish cross section close to the head tube, which is a very efficient shape to resist torsional loads. But as the torsional load diminishes towards the seat tube, the cross section morphs into a wide, but low profile, which is best to provide lateral stiffness while allowing for some vertical deflection.
Cross-section of the top tube near head tube (left) and near seat tube (right). While bending properties are quite similar, the cross-section near the head tube provides three times as much torsional stiffness as the one near the seat tube. The reduced cross section towards the seat tube allows for significant weight savings without losing much stiffness.
Another example for such optimization is the seat tube. The analysis showed that it carries very little load in most load cases, so we kept it as small as possible. The cutout for the rear wheel does not only create enough space for some big tires, but also increases vertical compliance.
The top tube to seat stays connection shows to be very important for torsional stiffness, which is why we went for a strong, triangular design and decided against dropped seat stays.
Last step in this process was a comparison to our benchmark model, which was modelled using the same wall thickness and material properties. The results showed that we can achieve similar performance at an 8.7 % lower weight
Now that we had a structurally efficient structure, we had to make it aerodynamic as well. We focussed on the frame elements exposed to the wind, namely the fork, head tube, down tube, seat tube, and seat stays. The challenge was to lower the drag without compromising the great stiffness and weight that we had achieved at this stage. We set the goal to optimize the aerodynamics as long as the impact on stiffness to weight is not bigger than 5%.
A good example of what kind of aerodynamic gains could be achieved without losing much in structural efficiency is the down tube. Typically, the down tube of a lightweight frame as a quite wide and flat surface towards the front. We changed that to a profile that has round leading edge transferring into a truncated NACA profile. With those changes, torsional stiffness, and the circumference (and therefore weight) stayed almost the same, while we realized a massive 76.5% drop (or 3 Watts at 45 km/h) in drag.
Typical lightweight climbing frame vs. Road SL1 down tube.
Sometimes, aerodynamics and structural efficiency even go together. A typical, almost round head tube provides good torsional stiffness if you look at the tube alone. But if you look at the complete frame, a deep, aerodynamic head tube shortens the top and down tube, creating an even stiffer connection. Therefore, our head tube is as deep as the UCI allows.
Aerodynamic testing has shown that everything that is in front of the rider has the biggest impact on drag. For this reason, we developed our integrated cockpit that hides all cables from the wind, saving up to 6 Watts over a traditional cockpit.
Sometimes, small details make big differences. Placing the bottles as low as possible in the frame results in a drag reduction of around 1.5 W. Leaving the space above the rear brake open allows the air to pass through, further minimizing drag.
A frame that weighs below 800 g (raw) that plays in league of aerodynamically optimize frames. Doing this second development stage of aerodynamic optimization is something not often done when developing lightweight frames, but it clearly paid off. With a minimal weight increase of 30 g, we reduced drag by more than 100g. This is beneficial in almost any riding situation, even when climbing.
But even more important than all the performance parameters was the goal to develop a bike that you actually want to ride. That is why our integrated cockpit is compatible to standard handlebar, the cable routing is simple, the BB threaded and the seatpost round. It is easy to work, it will accept the parts you want to ride, and you can adjust it to match your fit.