Everyone dreams of having that crazy light bike, but what are really the thinking and compromises that has to be done when designing an E bike, in order to make it durable and lightweight?
First of all, the frame, the main component of every bike. There are a few different materials to make it from:
-Steel (cheap but heavy)
-Aluminium alloys (cheap, relatively light)
-Titanium alloys (light but expensive)
-Carbon fibers (very light, expensive)
One of the main advantages of carbon fibers is that they're easily formable in complex shapes and that allows to conform around the battery easier than with steel for example. Not only the form can be more complex, but the overall weight is reduced, the only drawback being the price, which can be explained by the manufacturing process of a frame: molding and layering the fibers by hand (most of the time). That's why carbon E bikes cost from 3000€. Titanium and steel are less used in E bikes, the first being expensive and complicated to weld, the latter being heavy (added to the electric components, you can easily end up with a pretty hefty bike). We're left with aluminium alloys, which are largely used on the middle of the range (for all types of bikes). Their weight and their price make the ideal compromise. You can expect your E bike frame to weigh between 1,8 and 2,5 kg.
In order to design a frame, you have to determine which electric system you're gonna use. There are various systems out there (we were talking about it all here), and the frame should fit the motor and the battery, depending on their respective location. Among motor and battery manufacturers, there are Bosch, Bafang, Shimano, Fazua or Mahle. The main difference being the position of the motor on the bike (they're all in central position except for Mahle, which is in the rear hub). The overall weight of the system plays a major role on the weight of the bike. To compare, Bosch and Bafang motor systems weigh around 4-5kg while Mahle only weighs 3,6! So if you want a lightweight bike, pick a lightweight motor system.
Then, the components. To insure comfort while riding, one may want to install a suspended fork or a shock, but that's gonna add some weight. Thus, in order to not add unnecessary weight to the bike while keeping it comfortable to ride, you can choose to run bigger tyres or more supple tubes for the frame. It's all a question of compromises and intended use of the bike.
The drivetrain components have more flexibility on their weight depending on the chosen system: a belt drivetrain is in general lighter than a chain one thanks to the lack of a derailleur and cassette. However it's possible to put on a geared hub but this solution is rather heavy. The brakes have a minor incidence on overall weight but it's always good to consider them if you want a truly light bike. Contacts points (saddle, pedals..) also have their part to play on the weight, but the question remains the same as with the frame regarding materials to choose from.
Finally, "options" can be considered to render the bike more secure and more practical: mudguards, racks, dynamo hub and lights, a kickstand... But all of those add up!
To conclude, the main component of a lightweight design is compromise. A choice has to be made on materials, the most adapted to the intended use and price point, without sacrificing weight.
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