BMW's carbon-fibre swingarm replacement concept

Flexi carbon-fibre could replace a conventional swingarm
Flexi carbon-fibre could replace a conventional swingarm

Amongst the landmarks in motorcycle design, the swingarm stands out as one of the greatest. But now BMW have come up with a revised version of the idea that takes advantage of their extensive experience with carbon fibre.

With the launch of the HP4 Race in 2017, BMW snuck in before Ducati’s 1299 Superleggera to offer a full carbon-framed superbike.

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While the £68,000, track-only BMW wasn’t exactly a machine for the masses, it demonstrated that they were serious about transferring their knowledge of carbon fibre to motorcycles. A next-gen version, based around the ShiftCam engine from the latest S1000RR, is surely on the way, and a new patent suggests the firm might have another ace up their sleeve when it comes to weight reduction.

The HP4 Race had a conventional aluminium swingarm, despite using carbon for its frame, bodywork and even wheels, but the new design shows a chassis with moulded-in rear suspension.

The term ‘swingarm’ doesn’t really apply, since it doesn’t have a pivot to swing on. Instead the design relies on the fantastic properties of carbon-fibre, which can be made incredibly flexible – think carbon fishing rods, pole vault poles or archery bows – or immensely rigid, like the HP4 Race’s frame, simply by changing the layout of the fibres within.

The two properties can even be combined, orienting the fibres in such a way that the component can bend in one direction and remain rigid in others, and that’s what BMW are doing here.

In the centre of the ‘swingarm’ there’s a hollow carbon box section but BMW have added four layers of carbon above it and another four below, all with the fibres aligned so they run from front to rear. These allow the arm to flex up and down, providing rear suspension movement.

What’s more, the layered pattern of the fibres mean they act like leaf springs – eliminating the need for a conventional coil spring. Composite or carbon leaf springs are already a known technology, pioneered by the Chevrolet Corvette in the early 1980s. They’re far lighter than steel springs and can be designed to have linear or rising spring rates.

Of all the companies working on carbon fibre bikes, BMW are best placed to succeed. The firm already have mass-production facilities for composite parts including entire car monocoques, and the firm’s relatively affordable, i3 electric car shows they can make carbon parts quickly to a price.

The BMW HP4 Race gets a carbon fibre frame

Carbon revolution explored

  • Engine clues: Similar engine mountings to the HP4 Sport suggest work has been underway for a while.
  • One-piece: Carbon frame is moulded in one piece with the swingarm, reducing the number of components to cut weight and cost.
  • No spring: While the rear spring isn’t needed, the chassis shown here still has an upper shock mount – a damper will still be required.
  • Pre-load kit: A coil-over shock using a lightweight helper spring allows preload tweaks while the carbon leaves do the heavy lifting.
  • No pivot required: Layers of carbon are laid longitudinally above and below the rear arm act as leaf springs.
Ben Purvis

By Ben Purvis