Q&A: How We Test Our Bikes

The GSD and HSD wouldn't be nearly so useful without their impressive hauling abilities. Whether you're taking the kids to school, doing the weekly grocery run, or transporting materials for a home improvement project, the ability to carry heavy loads is essential.

But as you can imagine, carrying heavy loads at high speeds—and stopping all that weight once it's in motion—puts enormous strain on a bike's frame and fork. And while making cool bikes is important to us, making safe bikes is an even higher priority. That's why we put the GSD and HSD through rigorous testing to prove that they're safe and reliable up to the claimed load limit. 

In this article, we'll tell you everything you ever wanted to know about how we test our bikes. By the time you're done reading, we hope you'll understand what goes into our claim that the GSD and HSD can safely carry your most precious cargo.

What are the load limits for the GSD and HSD?

We've designed and tested the GSD and HSD to the following load limits:

Bike = weight of the bike plus batteries; Rider(s) = weight of the cyclist and any passengers

The MGVW includes the weight of the bike and the rider, plus any accessories, cargo, and/or passengers.

How about the load limits for the rear rack?

The rear racks have been tested, too. Here are their capacities:

How do we know these loads are safe?

In short, because we designed these load limits into the bikes from the very beginning, and then ruthlessly tested the bikes for strength and durability at every step along the way.

Tell me more. How does Tern's testing process work?

We start by defining the limits under which the bike will operate, including:

With these limits in mind, we use specialized software to simulate the stresses we expect the bike's frame and fork to face. The software helps get us in the right ballpark before we start making real prototypes.

Next, we make and ride a series of prototype bikes. We typically put a few thousand miles on these, making improvements with each new prototype design. 

To make sure the product development team gets well-rounded feedback from all kinds of riders, we encourage team members organization-wide to ride these bikes. Our Bike to Work program provides incentives and support for team members who commute to work using our bikes, including models still in development. Feedback from Tern team riders across a wide variety of roles helps the product development team make improvements to the initial design.

After this lengthy iterative design process, we begin lab testing the frame and fork to ensure that they can withstand the forces associated with the claimed weight capacity. We perform multiple rounds of internal testing on-site at our factory using the relevant testing standard (more on that later). 

During internal testing, we generally try to "test to destruction" to identify the weakest parts of the frame or fork. However, a few of our recent frames have just been too strong for that—and at that point, our lab turns the machines off.

Next, we move on to even more demanding testing at an external laboratory. For the GSD and HSD, we worked with EFBE Pruftechnik, a German company that is an international leader in bike durability testing.

How does EFBE testing work?

EFBE testing simulates hundreds of thousands of cycles of multi-directional stress to the bike's frame and fork. For example, one testing station simulates pedaling forces, while another simulates braking forces. The EFBE testing protocol truly takes the bike's frame and fork to its limits using a relevant testing standard.

What standard was used to test the GSD and HSD?

Funny you should ask! Identifying and applying a suitable testing standard was one of the big challenges we had to overcome in creating these bikes.

The most widely used bicycle testing standards come from Europe. For translation reasons, these standards are called EN Standards.

Because bicycle design is moving forward so quickly, the EN standards for bikes still lag behind a bit. Up until recently, a standard appropriate for cargo bikes did not exist. However, Germany has now developed a comprehensive and rigorous draft test standard for cargo bikes that could serve as the basis for a future EN standard.

This German draft standard takes into account the bike's claimed load as well as the weight and geometry of the bike. That means the testing for a cargo bike that claims a capacity of 120 kg vs. 140 kg vs. 200 kg is drastically different, which makes sense since the forces applied to the fork and frame increase dramatically with increased gross vehicle weight.

The founder of EFBE was one of the principal authors behind this new draft standard. Using findings from the standard drafting process, EFBE has developed its own custom testing protocol for cargo bikes called the Tri-Test, which is even more demanding than the draft EN cargo bike standards.

The GSD and HSD have both passed the Tri-Test up to the claimed load limits, which means they exceed the requirements of the German draft cargo bike standard.

What makes the Tri-Test so demanding?

The easiest way to explain the Tri-Test is to compare it to the EN City/Trekking Bike standard, which was previously the only standard available for testing urban bikes.

The EN City/Trekking Bike standard allows the use of a separate frame for each test—meaning the frame and fork is "fresh" at the start of each test. Each test typically applies 100,000 cycles of force with a maximum load of 85 kg.

By comparison, EFBE's Tri-Test includes 10 different stress tests. EFBE requires that one single frame must pass all 10 tests—so any damage to the frame or fork caused by one test will carry over to the remaining tests. And each test is performed with massively higher loads—proportionately higher based on the claimed weight capacity of the bike. 

Knowing just how intense the Tri-Test process is gives us peace of mind that the GSD and HSD are fit for use. For instance, the amount of stress applied in one of our GSD fork tests was over 300% greater than the amount applied to a fork in the EN City/Trekking Bike test. 

Why does that matter? Imagine the amount of braking force applied by a strong disc brake when trying to slow a 200 kg vehicle—the fork must be properly designed to be safe in those conditions (and as you can see from the GSD, you end up with a pretty burly-looking fork!).

How were the rear racks tested?

There is an EN test standard for racks to test their strength, but we find that this standard isn't comprehensive enough. 

Just because a rack is strong enough to support a certain weight doesn't mean that it's stiff enough to prevent uncomfortable and unnerving sway while you ride. You don't want to feel sway when you're threading between cars in traffic, and your child on the back rack moves slightly. 

So, in addition to testing the rack at the claimed weight capacity (and then some) using the EN standard, we do hundreds (or thousands) of kilometers of ride testing to ensure that the bike is comfortable and safe when the rack is fully loaded.

Do all cargo bike companies test their bikes this way?

Some do, but many don't.

In the bicycle industry, companies are only required to self-certify that their bikes meet certain safety standards. But the lack of a standard for cargo bikes has made this hard for cargo bike companies. In fact, most standard test machines can't even accommodate a long cargo bike. 

The cargo bike companies that are committed to testing devise their own custom tests that require them to make custom jigs with companies like EFBE. It's a costly and time-intensive process, but we think ensuring the safety of riders and passengers is more than worth it.

So does this mean my GSD or HSD is indestructible?

Sorry, but not quite.

Even though these impressive bike frames are very strong and stiff, you should still be sure to operate the bike within safe limits. That means paying careful attention to MGVW and following safe passenger-carrying guidelines.

We encourage you to have fun and challenge yourself to do as much as you can with your GSD or HSD. But please don't feel the need to find out if you can break it—we've been there and done that, so just enjoy the ride!