E-mobility is one of the major trends of the car and automotive industry. So in this week's "How it's Made" we break down the manufacturing process chosen by the designers of one of the most limited vehicles on the e-market, the Cake KALK electric motorbike.
We will take a closer look at the design challenges the engineers of this electric off-road motorbike had to overcome and explain the materials and technologies used during the manufacturing process.
Let's get started...
The frame: off-road strength vs. efficiency
Off-road performance and electric drive technology are two concepts that are not really compatible with each other. When it comes to the frame the requirements are contradictory: a strong and stiff frame, designed to withstand the forces during landings from high jumps and downhill rides, but also lightweight for high energy efficiency.
To solve this puzzle, the engineering team selected aluminum 6061 as the material of the frame, because of its strength-to-weight ratio. This material is an excellent choice for applications in the automotive industry or bicycle frames because it has ⅓ the density of steel but about half its strength - and also cost half as much.
The frame composes of rods manufactured with material extrusion. One advantage of this manufacturing process is the ability to transform aluminum alloy logs into objects with hard to machine and complex cross-sectional profiles. These profiles maximize stiffness while minimizing weight (and are therefore ideal for the bike).
Check out this video to see material extrusion in action:
The suspension system: how to stop the bounce
For off-road bikes, shock absorption is a crucial requirement. Important here: back wheel and swing arm, the first touch points after long jumps.
The swing arm of the bike holds the back tire in place and is a made from 6061 aluminum extruded tubes similar to those used for the frame.
The linkage between the swing arm and the frame is shown and marked in the image above. This little component carries the heaviest loads on the bike.
To reduce the risk of early stress failure, this linkage is CNC machined, using a higher grade aluminum. Al 7075, an aircraft grade material, can be heat treated to reach a strength comparable to steel, while still being about ⅓ the weight. The CNC machining process gives the part excellent and fully-isotropic physical properties.
Another important factor of the shock absorption are the wheels. As a rule of thumb, the more spokes a wheel has, the higher its resistance to impact and its shock absorption. While jump and x-trial bikes may have up to 48 spokes, the engineers stuck to the industry standard of 32. Why? Most likely a design decision and sufficient for the average user of the bike.
The heart of the wheel is the hub. Its job is to spread the impact forces equally on the rim. Since this part needs to have high strength to absorb all impacts and has a rather simple shape, it was CNC machined out of Al 7075. This guarantees a perfect powerflow within the part and eliminates the risk of stress failure.
To increase the strength of the rims, some unique adjustments to the inner profile were made. Three chambers were added within the rim to support it against possible deformation. This follows again the concept of adding multiple walls within the structure, similar to the extrusion process of the frame, to increase strength while keeping additional weight minimal.
See how rims are normally made in the video below:
The tires: Grip with a small footprint
For off-road tires, it's all about the grip. Mud, wet conditions, leaves or stones should not affect the riding performance. This is usually achieved with soft, thick tires and deep tire profiles. The ideal tire for an efficient and energy-saving ride, needed for racing bikes, are as thin as possible, hard and have no profile.
To find the right balance between off-road performance and reducing rolling resistance, the tires have the off-road profile but are relatively narrow. With only 24 inches of width, the nylon tires are only have as wide as standard dirt bike tires (45-50 inches).
The Power transmission: How to make it fast
Like most electric vehicles, the KALK does not need a gear system or a clutch: power is transmitted directly from the electric motor to the rear wheel! This reduces the weight and keeps the overall weight of the bike at around 70 kg. This allows it to have a range of up to 80 km, with a top speed of 80 km/h.
If the sound of an electric vehicle is as acoustically pleasing as a motorized one is up for discussion. To offer options to both sides, two different ways of connecting the motor and rear sprocket are offered. A chain connection, for riders that prefer the "traditional" bike look and sound, and a belt for those who prefer to keep the bike completely silent.
The rear sprocket itself is constantly moving part and the only motor shaft of the bike. It needs to be of exceptional quality. CNC machined 7075 aluminum does that job. In contrast to other CNC machined aluminum 7075 parts of the bike, it is anodized in a post process. This means that a ceramic layer of up to 125 μm is applied on the part. A smooth and aesthetically pleasing surface, but also higher corrosion and wear resistance are the results.
The brakes: Sometimes you need to stop
Occasionally it might is useful to reduce the speed of the bike, to avoid major injuries or accidents. To guarantee a safe breaking process the rotor discs need to be hard, heat resistant and reliable. A material that perfectly fits these requirements is stainless steel.
The rotor discs are laser cut instead of CNC machined. The reason for that is the thickness of the rotor discs. With only 3.2 mm they basically have a 2D shape. These shapes are hard to CNC machine because parts tend to start vibrating during the machining process. The results are less accurate parts outside of their tolerances. It is also faster and cheaper than a CNC machining process because of the almost nonexistent set-up process.
Never seen a laser cutting machine at work? Check this video:
The right balance between strength & lightweight
The majority of KALK bike is made out of aluminum 6061, a general-use aluminum alloy with an excellent weight-to-strength ratio. Smaller parts that are exposed to higher forces, like the linkage between swing arm and frame, or the stem and handlebar, are made of a higher grade and stronger aluminum alloy, aircraft grade aluminum 7075.
The material selection and construction (e.g. aluminum extruded frame) clearly show that weight was an important factor in the developing process of the bike while keeping the material properties strong enough for off-road performance. The overall result, a well engineered and light off-road bike that might be a fun and enjoyable gadget.
Should you buy it? Luckily this question has already been answered for you - the 50 units of the bike are already sold out.