How Electric Mountain Bikes Work: An Interactive Guide


Electric mountain bikes are one of the fastest-growing trends in the cycling industry today! However, many people don’t actually understand how they work.

While researching the topic myself, I found that I had to dig around in quite a few places just to get a simple and thorough answer! So, I decided to create my own guide explaining how electric mountain bikes work. My hope is that this will help to demystify the topic and remove some of the controversies around them.

The infographic below walks you through the most simplified version of how they work. However, I also cover all of the parts, their feature and functions below in far more detail.

Because there are a lot of details around the features and functions of each component, I created a table contents so that you can jump directly to a section of interest.

However, I still highly recommend reading the entire way through so that you understand the system end to end.

Table of Contents:

Common questions about electric mountain bikes

Before jumping into the meat of explaining how an electric mountain bike works, I’ll first answer a couple of common questions. This way we’re all on the same page before jumping into the meat of it.

What is an electric mountain bike?

Sometimes also referred to as an eMTB, an electric mountain bike looks just like a regular mountain bike. The difference is that it has a small integrated electric motor to assist with propulsion.

While there are slight differences in their classification depending on how and when they deliver power, I’ll be focusing on the pedal-assist version as it most closely aligns to their use in electric mountain bikes.

Bikes that engage power via a throttle instead of pedal-assist are more commonly used for commute purposes instead of the trail. So I won’t be covering that variety in this article.

Are electric mountain bikes legal?

Yes, electric mountain bikes are legal. However, the country and state that you’re riding in may have specific laws and regulations around their use. The main limitations are related to the bike’s power and the maximum speed that it can propel you on its own. Exceeding these limits will simply cause an EMTB to be treated as a motor vehicle instead of a bicycle (for which there are different laws).

Around most of the United States, this limit is 20 miles per hour and an electric motor of 750 W (1 hp). Each state will have its own rules and regulations, so make sure to verify the actual requirements in your state of residence. They may also have additional requirements such as the need to wear a helmet, minimum age to operate, trails that you’re allowed to use them on, and having a driver’s license.

You can also refer to the International Mountain Biking Association (IMBA) website for more details about electric mountain bikes.

Will I still get a good workout on an electric mountain bike?

Yes! Just because you have small motor assisting you, doesn’t mean that you still won’t be able to get in a good workout. Many riders find that they end up spending more time on the trail and covering far more mileage thanks to the extra assistance.

Instead of spending most of your time grinding up long climbs, you’re able to enjoy more downhill sections and get the benefit of a more balanced full-body workout. The extra help may also allow you to reach new areas that were previously out of your reach due to fitness or time reasons.

Older riders, or those recovering from injuries, find that electric mountain bikes are a great way to ease back into the sport or to help them keep up with more fit riders.

How electric mountain bikes work

Now that I’ve covered some of the most common initial questions, let’s jump in to understand what’s actually powering an electric mountain bike.

As crazy as it might sound, there are really only two new components on an electric mountain bike compared to a non-powered one. Those two components are an electric motor and battery to power the motor.

EMTB Motor

Let’s start by first covering the motor options available and how they work. As I previously alluded to, there are different types of electric bike motors. These motor types are hub-drive and mid-drive. The key difference between the two is how they power the bike.

Hub-drive versus Mid-drive motors

Hub-drive motors work by attaching directly to the front or rear wheel hub. The motor directly applies its power to the wheel for assisted riding. By comparison, mid-drive motors are situated in the middle of the bike and instead apply power to the chain. Mid-drive motor systems are almost exclusively used in electric mountain bikes for reasons that I’ll cover now.

While these may seem like minor design differences, the mid-drive motor system has some massive advantages over a hub-drive when it comes to mountain biking. First and foremost, a mid-drive allows you to take advantage of your gears. This is incredibly important for mountain biking where you will frequently have to take on steep climbs. Hub-drives are stuck using a fixed gear and are less adept at applying power during ascents.

Secondarily, the central placement of the motor in a mid-drive allows the bike to remain more balanced and nimble. By comparison, hub drive bikes situate the weight on either the front or rear wheel. This placement makes them less than ideal for remaining agile on the trail.

While mid-drive motors tend to smaller and lighter, the lower weight for weight’s sake alone isn’t necessarily an advantage. The fact that you do NOT have that weight awkwardly placed on the wheel makes it much easier to perform tire-related maintenance. Trying to wrangle wheel on or off that bike that has several extra pounds of motor strapped to it isn’t an easy task!

To be fair, I should point out that there are some disadvantages to mid-drive motors.

First, because they connect to the drivetrain it can create more wear and tear. Hub drives operate completely independently, so it’s not a concern for them. Secondly, the larger number of moving parts comes with an increased chance of part failure.

However, even with considering these disadvantages, a mid-drive motor is the only real option for electric mountain bikes in my opinion.

EMTB Battery

Battery location

As can be expected, modern electric mountain bikes are paired with Lithium-ion batteries for power. While they can be more expensive than other types, Lithium-ion batteries are typically lighter and offer more energy storage. More energy storage = longer riding time 🙂

When it comes to mounting on the bike, there are two options – internal and external. As the name implies, internal batteries reside inside of the down tube. This allows them to be hidden from sight and better protected! Just glancing at an eMTB with an internal battery, you may not even realize that it’s there. The tube itself will be larger than normal, but otherwise, it’s a clean design that can be very aesthetically pleasing.

Some internal batteries can be tricky to remove for charging, whereas others are very straightforward. This will vary model by model, so it’s just something to keep in mind if you do choose to purchase an eMTB with an internal integrated battery.

External batteries are mounted to the outside of the down tube where a water bottle may have otherwise resided. Typically a mounting base plate attaches to the tube itself, and the battery then slots into the plate.

Bolts, zip ties, and other attachments may also be used to help secure the battery in place if it’s been installed as an after-market addition. Unless you plan on modifying the bike’s frame, external batteries are almost always used for an electric mountain bike conversion.

Mounting location for internal versus external batteries.

Battery size & specs

One of the most significant aspects of the battery is its storage capacity. Not surprisingly, batteries that have more storage capacity are both larger and more expensive. Their energy storage is measured in Watt-hours. When looking at battery specs, you’ll usually see this abbreviated as “Wh”. Common ranges fall between 300 – 700 Wh, with larger ones hitting the 750 and even 1,000 Wh mark.

Because larger batteries are also physically larger, you’ll want to be careful to watch their physical dimensions. For after-market conversions, you’ll need to ensure that the battery physically has room to fit inside your bike’s frame.

As batteries get larger, they also get heavier. Depending on their storage capacity, most batteries weigh in somewhere between 4 – 7 pounds. While that may sound heavy, it’s usually not noticeable to the rider because the battery is mounted to the bike. The extra weight can actually be helpful for maintaining traction in turns. However, the extra weight is a completely different story if the battery isn’t mounted to the frame and you’re instead carrying it in a backpack!

Most electric mountain bike batteries take around four hours to reach a complete charge. Two hours is a good estimate for sneaking in a half charge as needed. You can expect the battery to last around 1,000 full recharges during its lifetime. Exactly how much mileage you get out of each individual charge will be highly dependent on the battery size, terrain, and how much you rely on it for assisted power.

How the motor powers the drivetrain

Ok – now that you understand the two primary components let’s jump into how they actually help move the bike! I’ll once again be focusing on mid-drive motors because they are the most applicable to electric mountain bikes.

A mid-drive motor mounts directly into the bike’s bottom bracket. This is exactly where a regular crank would be in place. However, we now instead have a small motor that’s attached to the front chainring and cranks for assistance.

The primary mechanics of bike motion still function the exact same way. You push the pedals, the chainring turns and rotates the chain, the chain spins the back wheel. The only difference is that when the motor senses the rotation of the pedals it kicks in to help. This is why it’s referred to as a “pedal assist” electric bike. The motor only helps when you’re actually turning the pedals.

The electric motor is of course powered by the battery that’s also mounted on the bike. How much power is delivered is typically controlled by the “mode” that the motor is set to. Every manufacturer has its own array of modes, but you can always find a low “eco” mode and higher “turbo” mode. Eco modes typically deliver 50% assist, while the turbo modes can range up to 300% assistance!

While these high-powered modes are great for helping you up steep hills, they also drain the battery the fastest. So try to use them sparingly in order to maximize your battery life during the ride. Most eMTBs come with small displays so that you can see statistics like the current speed and mode. They also often display the remaining battery life. This is key to avoid running out of battery far from home 😉

To reserve the battery, you can shut off the motor during long downhill sections where you won’t be needing its assistance. Because of the regulations around maximum speeds, you’ll find that if the motor is left on won’t provide any additional power once you reach a certain speed. This is monitored and controlled via sensors.

Torque versus Cadence pedal assist systems

You’ll notice that the key way in which the engine is engaged boils down to the rotation of the pedals. However, just how much power the engine delivers is controlled by two possible mechanisms in the pedal assist system. Power is regulated by torque, cadence, or a combination of both.

Cadence pedal assist systems work by measuring the rate at which the crank is turning. This would be the revolutions per minute (RPM) of your pedals. Using a series of magnets, the system is able to calculate how fast or slow the pedals are turning and modulate the power accordingly. For example, if you were spinning at a comfortable 70 RPMs but slowed down to 30 RPMs while climbing a hill, the motor will sense the change and increase its power output.

While the system is simple, it does have its downsides. Since it relies on the placement of sensors to calculate rotational speed, there can be delays in power delivery that create a less than natural riding sensation.

Torque-based pedal assist systems improve upon the concept by using a sensor connected at the bottom bracket that measures torque (rotational force). While there are also variations that measure chain tension, these are less sensitive and less reliable than bottom bracket versions.

As you push harder on the pedal, this translates into increasing amounts of torque at the bottom bracket. The torque sensor reads the increasing or decreasing amount of torque. It then sends a signal to the motor to increase or decrease power. This tight coupling between effort and power assist leads to a very intuitive and natural-feeling ride.

Some modern mountain bikes combine cadence and torque-based sensors to provide the ultimate control for power delivery.

Power modes

As briefly previously mentioned, power modes are the final factor to look at. Think of the power mode as simply a setting for the engine. As the torque or cadence sensor sends its signal to the motor, this value can be modulated by the controller. This is essentially a software layer that uses logic to determine how to modify the signal from the sensor before it reaches the engine.

After receiving the signal from the sensor, a higher mode will tell the engine to supply more power. Conversely, low modes will tell the engine to provide less assistance.

The number of modes will vary by manufacturer, but most electric mountain bikes have three. While the names vary, you can think of them as essentially a high, medium, and low setting. The lowest version is often referred to as “eco” because it uses the least amount of battery power. The highest modes are usually called something like “turbo” or “boost”. These should be saved for the steepest of hills because they drain the battery the fastest.

Let the countdown begin as to when a manufacturer will take a page out of Tesla’s book and come out with an “insane” or “ludicrous” mode 😉

Best electric mountain bikes 2019

While electric mountain bikes are still in their relative infancy, many top manufacturers have gotten behind the movement and are producing awesome bikes! If the last several year’s progress is a sign for what’s to come, then this is going to be a really exciting space in the coming years.

With your new understanding of electric mountain bikes, it should make it easier to understand price differences. Aside from the bike’s themselves, the primary differences will be in the motor and battery size as well as the complexity of the controller and displays.

Unfortunately, you’ll notice that the list price for all of these bikes is over $5,000. While this is a bit pricier than most of us probably want to spend on a specialty bike, they are still some fantastic bikes. The fully integrated nature provides a smooth experience that lets you take it straight from the shop to the trail for enjoyment.

If this cost sounds a bit too steep for you, then I’d highly recommend taking a look at doing an electric mountain bike conversion. I’ll cover this in more detail below.

Best electric mountain bike conversion kit

While the price of a new top eMTB may be prohibitive, conversion kits have gotten REALLY good in the last couple of years. Most of us who have been in mountain biking for a few years have an older backup bike that gets little to no use. This is the perfect situation to convert it to an electric mountain bike!

bafang makes some of the best electric mountain bike conversion kits. They offer kits in a variety of battery, chainring, and display options to help meet your budget. More importantly, these kits come with mid-drive motors that are perfect for mountain biking. Their motors are some of the quietest, and the installation is straightforward.

Rather than simply take my word for it, take a look at how highly reviewed their products are on Amazon (click here for Bafang conversion kit at Amazon.com). Make sure to pick a chainring size matching that of your current bike so you can use the same chain. Then, choose how large of a battery and type of display you’d like.

Depending on which option you choose, the kit price can range widely. However, it’s still far cheaper than buying an off-the-shelf electric mountain bike. Plus, you’ve got that old mountain bike sitting around collecting dust that could be put to good use again! 🙂

Conclusion on electric mountain bikes

While electric mountain bikes will likely continue to be a hotly debated subject, there’s no doubt in my mind that they’re here to stay. The simple fact that several large bike manufacturers are creating their own electric mountain bikes at this point is enough market validation for me. If there was no money in it for them, they simply wouldn’t bother.

Putting the economics aside, it’s hard to argue with the fact that electric mountain bikes open up the hobby to more people. The positive outcomes that can result from more people getting into mountain biking are hard to argue against.

Whether you’re past your prime or recovering from injury, electric mountain bikes help open up new avenues. They let you ride further and harder than you might otherwise be able to, while still providing a great workout.

Zach Reed

Hi, I'm Zach Reed and I'm a Colorado-based mountain biker! I spend my weekends cruising around on a Wasatch Peak Comp 29er and love every minute of it :) For more information, take a look at my dedicated about me page.

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