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Bench Talk for Design Engineers

Bench Talk


Bench Talk for Design Engineers | The Official Blog of Mouser Electronics

Technology Whose Name is Mud or a Look at eMTBs David Fambrough

(Source: Flystock/Shutterstock.com (left) www.ebike-mtb.com (right))

May is National Bike Month, which promotes the many benefits of bicycling, and showcases the evolution of bike tech and encourages more folks to give biking a try.

While many prefer to stick to the paved surfaces when cycling, others see their bike as an escape vehicle and a way to connect with the wilds of nature. Here is where dirt and advanced technology have no choice but to become one. And one of its testing platforms and latest area of advancement is with Electric Mountain Bikes (eMTBs).

What is it about bikes and our desire to constantly want to reinvent the wheel, so to speak? Let's take a look at the technology behind creating a stronger bond with the great outdoors. As you'll see, engineering is at the hub of new development and cycling innovation—changing the way people are pedaling over the mountains and through the woods.

Quest for smaller, lighter, cheaper and more powerful batteries

Tesla's Battery Day marks one of the most highly anticipated moments on the tech calendar each year. At this event, huge advances in battery technology are unveiled in a pursuit to manufacture electric vehicles that are more powerful, longer lasting with greater range and less expensive. It all comes down to being able to fulfill Elon Musk's vision for Tesla of selling a fully electric car for $25,000 USD within three years.

But how does this technology trickle down to eMTBs, and how will it impact their development going forward? Let's start off by looking at the batteries eMTBs use now.

Currently, most eMTB batteries are 18650 Lithium-ion (Li-ion). Li-ion batteries were first developed in 1985 and they have been the main driving force behind electric vehicle development as they are rechargeable and you can get a lot of power from them without them taking up too much space. Measuring 18mm x 65mm, about the size of your finger, the batteries are welded together in packs, connected in parallel. Remove the plastic shell, basically it looks a bit like a bunch of AA batteries all joined together.

The all-important number with batteries is watt-hours. This, like you might think, measures how many watts can be delivered in an hour. For example, a 250 watt-hour (Wh) battery could drive a 250W motor on full power for one hour while a 500Wh battery could drive it for two. Most e-bike batteries sit between 300 and 550 watt-hours. The end goal with batteries is getting as many watt-hours as possible. But if you just keep adding cells, you'll start adding weight and volume.

Looking at the positives, manufacturers aren't content to stand still. There is still plenty of innovation still transpiring in this size of cell. In fact, Bosch introduced its 625Wh Powertube battery last year, offering a huge range despite the smaller cell size. The internals of these cells are still being perfected too with engineers experimenting with different cathode and anode materials to boost capacity.

That's not all. One of the other big problems of increasing the volume of a cell is that it makes it harder for heat to escape. If a battery gets too hot, it has to operate at reduced power or risk damaging itself permanently. eMTBs are often out in the sun all day, so better heat management of an 18650 cell could lead to better performance over a longer period of time.

Tesla-inspired battery technology joins the ride

For those of you out there that follow Tesla, it should come as no surprise that Tesla doesn't currently use 18650 cells. Rather, they currently use the 21700 standard with cells measuring 21mm x 70mm. Tesla developed this cell with Panasonic in 2017, and its larger volume means it can be packed with more anodes and cathodes to hold more energy. Tesla also claims it has a longer lifespan and requires less charging.

Recently, riders are starting to see these cells come into eMTBs with the Specialized Turbo Levo. This bike has a massive 700Wh battery providing a generous amount of ride time, making it one of the biggest you can get on an eMTB today. Additionally, its motor delivers up to 565 watts of power and 90Nm of peak torque. Overall, the Specialized Turbo Levo stands at the pinnacle in the industry for its performance and technology.

However, Tesla being Tesla, they haven't stopped with the 21700. Their recent announcement at Battery Day shared the next evolution of its cell technology. The Tesla battery has gone up in size again, this time far more significantly to 4680 or 46mm x 80mm. According to Drew Baglino, Sr. VP of powertrain and engineering at Tesla, this new innovation boosts the energy by five times, ups the power by six times, and increases the range of a car using these batteries by 16 percent in relation to the 21700.

The advantages for eMTBs of a more powerful battery are clear. You can either deliver the same power in a smaller, lighter package, which makes the bike handle and look more like a traditional mountain bike, or you can keep the battery the same size and boost the range of the bike. But before you get too excited, remember that it took roughly two years for the 21700 tech to trickle down from Tesla Model 3 cars to the Specialized Turbo Levo. And like all new technology innovations, expect the costs to be steep (new technology is rarely cheap) just like the mountains its designed to help climb.

Other key engineering factors at play

A common complaint engineers often hear is weight, eMTBs are too heavy. However, eMTBs have to withstand up to 50% higher loads on the trail than regular mountain bikes. As a result, engineers have to factor in braking loads, especially on the fork. Brakes also need to incorporate bigger rotors and feature four-piston brake calipers to provide enough stopping power. Suspension kinematics, front and rear, along with the bearings and linkages have to be rethought and designed for strength and durability. Individual components like seat post and handlebars are often pushed to their literal breaking point. All of this quickly adds up.

Using current industry data, the maximum permissible weight with an active riding style reaches its limit at 150kg. To go beyond that, engineers would have to develop frames and components that are sturdier and more robust, designing them differently than standard mountain bikes. This would make the bikes much heavier and riders would have to be content with eMTBs that weigh 27–29kg. The question, or the proverbial elephant left standing in the room, is… who really wants to ride such a heavy bike?

The frame material of choice for over a decade has been carbon fiber. It's lightweight and can be sculpted and manipulated to increase the frame's stiffness and flexibility or compliance for a better ride quality. The biggest downside to carbon has been its reputation for being brittle, and how an unblemished but weakened frame can suddenly fail catastrophically at unexpected moments.

Continual engineering and experience working with carbon have lessen this to some degree. However, several new approaches using hybrid composites show promise of increasing safety with minimal or no decrements in the features that make carbon fiber so highly valued in the bicycle industry. Hybrid composites blend carbon fiber bonding strands (called “plies”) with another material such as plastic or steel in the same ply. Innegra Technologies, Dyneema and REIN4CED are a few companies blazing the trail for others to follow.

Ready for some dirt?

The automobile industry has Ehra-Leissen, Nürburgring or Millbrook Proving Ground to put their new vehicle technologies to the ultimate test. Recently, I came across a 2021 eMTB test featured on YouTube. In the rugged, rock-strewn desert of St. George, Utah, twelve different manufacturer's eMTBs are tested and tortured by a panel of professional riders. Check out how far eMTBs have come and where they still need to go in the future. But one thing is for certain: this growing genre of mountain bikes continues to evolve and get even better each year.

Although not everyone that will read this blog is a cyclist, thanks to National Bike Month, we can all marvel at the bicycle industry's advances in technology, especially when engineers decide to venture down the path less traveled. And that has me and Mother Nature smiling. Now go out and fling some mud in the name of technology.

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David Fambrough is a technical writer for Mouser Electronics. He’s adamant that Lost in Space, Star Trek and James Bond have had a strong role in inspiring innovation and new design.

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