Closeup image of the FLIT M2 rear swing arm

Engineering Takeover: Rear Swing Arm

Matt explains the role of the rear swing arm and the challenges it has presented in the M2’s design.

When it comes to folding ebikes, it’s all about the details. One of the critical components ensuring the safety, performance, and overall experience of a tri-folding ebike, such as the FLIT M2, is the rear swing arm. In this blog post, we look at how the rear swing arm works and the challenges FLIT faced in perfecting it.

The Rear Swing Arm’s Role 

Each part of the M2’s design is a combination of form and function that must hold up individually and also complement the rest of the design. This is especially true for the rear swing arm. Because the M2 is designed to let the rider sit upright, the rear swing arm must be strong enough to support the majority of the rider’s weight over a lifetime of use. It must also provide features for tensioning the chain, mounting accessories, and retaining the bike when folded. Further constraints are imposed by cable routing, desired fold size and weight, styling, and manufacturing processes.

Folding ebike regulations, like EN15194 in Europe, are designed to ensure the safety, durability, and structural integrity of ebikes. The rear swing arm must meet the rigorous mechanical tests outlined in these regulations, as well as satisfying all of the constraints listed above. On top of this, at FLIT we have our own more stringent internal mechanical testing procedures that each part of the bike must pass. 

Matt gives us a tour of the M2’s rear swing arm and explains its role and design challenges

Design to Prototype One: A Journey of Improvement

The FLIT M2's rear swing arm being testing on a rig at FLIT's Cambridge workshop and HQ

Above: A prototype M2 rear swing arm being tested on a rig at FLIT’s Cambridge workshop

The initial prototype, while meeting regulatory requirements, posed a challenge in terms of ride comfort due to the rider’s positioning. The FLIT engineers recognised the need for a better balance, which led to the decision to shorten the frame while extending the rear swing arm’s length.

This adjustment, however, meant that we had to review the materials and manufacturing processes used. Through careful redesign, FLIT achieved the desired length without adding any extra weight to the final folding ebike and preserving its compact fold.

Prototype Two: Pushing Beyond the Standards

Once we had tested to the formal industry standards and passed, we moved on to real-world testing to ensure the rear swing arm could really handle the rigours of daily use. This testing involved scenarios such as carrying loads far beyond the recommended weight limit, hitting potholes, riding off large kerbs, conquering steep hills, and riding in wet and gritty conditions.

While the rest of the M2’s frame performed well, the rear swing arm exhibited early signs of deformation under more extreme conditions, such as hitting a large pothole at speed under heavy load. In response we took the following actions:

  • Temporarily halted production.
  • Launched a thorough investigation to find the cause of the distortion.
  • Undertook redesigns and material changes to double the strength of the rear swing arm without significant weight gain.
  • Sent new designs to manufacturers.
  • Explored re-tooling but ultimately found this unnecessary.

Next, our manufacturing partners delivered an updated prototype rear swing arm for testing. The maximum force on the updated arm exceeded FLIT’s test requirement, giving us the confidence to push on to building the first full prototype built to the final design and on manufacturing tooling.

A Journey Through Detail and Dedication

Having spent time with hundreds of riders, and as regular cycle commuters ourselves, we know that ebikes need to be able to withstand real-world conditions outside of a regulator’s handbook. As we push to make the lightest and most compact folding ebikes on the market, we also know that we need to consider lots of trade-offs, for example between weight vs strength or compact fold vs rider comfort.

Our years-long process of designing, testing, and adjusting, all with the help of our customers and partners, helps us to hit the right balance when it comes to giving FLIT riders the best experience, whether that’s riding through busy city streets or folding their bike to take on a packed commuter train. This attention to detail is also what helps FLIT ebikes stand out from the crowd.

The design and manufacturing process is often not linear, and the M2 has been no different.  Although it’s a part that riders don’t usually pay much attention to and has proved to be a technical challenge, the rear swing arm has become symbolic for us of the dedication needed to create an excellent product.

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