Every Single Gram or Ounce a Light Ebike Spares is Pricey

“Light” has lately become one of the most prized attributes for ebikes. How light should your ebike be?

Advantages of lighter eBikes most manufacturers hustle to offer

The majority of new models lately launched by quality electric bike brands are light e-bikes, not only e-mountain bikes but also gravel, trekking and road electric bikes. These models are sleeker, they almost conceal their electrical components: batteries, motors and sometimes even displays and control units are very discrete or even barely visible. You can find road e-bikes weighing less than 12 kg, or 26 pounds, and e-mountain or trekking e-bikes weighing less than 16 kg, or 34 pounds. These e-bikes are more elegant, easier to handle, for instance when loading them on a car, swifter and more agile on paths and tracks, easier to park and to carry upstairs, etc. They consume less energy, allowing lighter and less expensive batteries or longer range. The questions at stake are: how much more should you spend for each gram or ounce spared? The lighter the better ? Well, it depends…

Lighter e-bikes are pricier than normal eBikes

That’s because raw materials and technologies deployed for their production are more expensive. Carbon is way more expensive than steel, and even a steel seat post, for instance, requires a pricey high quality steel in order to be thinner and therefore lighter. Since torque is the force delivered by the electric motor on the wheel, what determinates acceleration, carrying and climbing capacities, one could evaluate the price worthiness of an ebike by its price per Newton meter of torque. That would be a gross approximation, similar to evaluate a car based on its price per horsepower. So for an 85 Nm of torque KTM Macina Sport 620 2021 costing 2,699 € VAT included, one should reckon 31 € per Nm of torque. By comparison, with a Mahle X20 motor, delivering only 23 Nm of torque on ebikes averaging 8,000 €, the price per Newton meter is around 347 €. The above portrayed Flyer Upstreet SL 3.12 ebike, 18 kg with a 55 Nm torque, has no shock absorbers, no lights, but reduced pedalling resistance and silent belt drive and hub gears. At 3,999 €, it costs 72 € per Nm. The new lighter eBikes have made this comparison less meaningful. Previously, more torque implying more robust frames and components, it was the hallmark of better performance and quality e-bikes. Now, the lighter the ebike, the less torque its motor can afford to deliver, since less weight increases the importance of leg power. Nevertheless, should you have a slightly weak leg power or need to climb daily, more torque would easily compensate five or six additional kilogramas, enabling you to ride faster.

The hidden costs of lightness

Lightness, however, comes at a price which many people do not fathom when choosing less heavy components: more stiffness. To lighten a bicycle we usually switch from aluminum frames and components to their carbon fiber counterparts. The weight saving is obvious but the gap in stiffness is very high. Stiffness of a material means the ability of that material to resist elastic deformation, that is, a force acting directly on it.

Usually stiffness is denoted by Young’s modulus (or elasticity), which is nothing more than the ratio between the applied force and the resulting deformation. The greater the modulus, the stiffer the material and therefore the greater the force it will be able to resist. A carbon frame or component will deform much less than its aluminum counterpart, so it will better transmit the force impressed on the pedals but conversely will make the cyclist feel more the roughness of the terrain, since vibration absorption will be less. Also, the stiffer the material, the more brittle it is, that is, it will break with less yielding, thus giving fewer signs of failure. Consequently, carbon frames are more fragile than aluminium and steel ones.

Cost of installing lighter components

Let’s assume we must replace an aluminum handlebar, weighing 269 grams for a price of 105€, and want to mount a carbon fiber one, costing 305€. The weight saving comes to 79 grams, for a price difference of 200€. We now divide the price difference by grams saved obtaining:

200 € / 79 gr = 2.5 € / gr

In the end, every gram saved would cost us 2.5 euro. Now the question is: is it worth spending 2.5 euro for each gram spared on one’s bicycle? As always, the answer is only one: it depends. In this case it depends on whether or not you have another way to lighten your bike.

Let’s make a very trivial comparison:
A 700×23 racing tire of a well-known German brand costs 67.90€ and weighs 175gr. A less valuable model costs 57.90€ and weighs 205gr. Wanting to switch from the less valuable tire to the lighter one we would have a saving of 60gr at a price difference of 20€ (considering we need to change both). Let’s do a bit of serendipitous math:

20€ / 60g = 0.33 € / gr

Basically for each gram of weight spared we would pay a price six-time cheaper then if we changed the handlebar. Moreover, the total weight savings are almost equal (79 gr vs. 60 gr) but at an expense 10 times smaller!

So before you rush out to buy a stem, handlebar, seatpost or wheel with composite spokes, stop for a moment and think about where it really pays to spend the money, but more importantly, where to do it for the best possible results.

Your muscle power and riding habits should determinate your choice

The lighter the ebike, the more your muscle power weighs. Moreover, if you also demand agility from your e-bike, for instance to challenge narrow curves and tough trails, a light weight is precious. Consequently, it should be worth spending more money for a lighter electric bike. On the other hand, if you daily commute to work on hilly roads, and prefere to spare your energies, the bigger 85 Nm torque of the aforementioned KTM can make its heavier weight almost irrelevant, but would be valuable for a relaxed, pleasant everyday commute.

Reckon your torque per weight ratio to choose the best ebike for you

Consider that the average torque of a given individual’s leg power is about 100 Newton meters, going from 50 to 180 Newton meters, depending on age and athletic prowess. Let’s imagine the case of David who is a bit weak, his legs delivering a torque of 75 Newton meters, and weighs 85 kg. With the above mentioned KTM, his total weight riding would be 110 kg for 160 Nm of torque, i.e 1.45 Nm of torque per kg. With the Flyer, his total weight riding would be 103 kg for 130 Nm of torque, i.e 1.26 Nm of torque per kilogram. Therefore, David would be faster, and also better master climbs with the KTM.

Now let’s take the example of Jeff, athletic young man weighing 70 kg, his legs delivering a torque of 140 Newton meters. With the KTM, his total weight riding would be 95 kg for 225 Nm of torque, i.e 2.36 Nm of torque per kg. With the Flyer, his total weight riding would be 88 kg for 195 Nm of torque, i.e 2.21 Nm of torque per kilogram. Therefore, Jeff would still perform better with the KTM than with the Flyer, but could prefer the latter because of its agility, lightness and so on. In the end, with the KTM, David would gain 0.19 Newton meters of torque per kilo, Jeff only 0.15. Not a big gap anyway.

Lightweight e-bikes make a difference especially on trails and handling

Delivering less torque, lightweight e-bikes perform worse than heavier high torque e-bikes on asphalt roads. Where their agility and lightness matters most, is both on trails and when handling is needed, be it climbing the stairs or loading them onto a car, a metro or a train. If one enjoys riding with very little or sometimes even no assistance, they are definitively the best choice.