Advanced materials cycle to advantage

Tom Shelley reports on some of the advantages to be obtained and problems to be overcome in making even a relatively mundane product out of advanced materials



An electrically assisted bike, made out of carbon fibre composite, and its light alloy based derivative, show that working with advanced materials gives new freedoms in design with one hand and pose new problems with the other.

Because of the immense strength and rigidity of the base material, it is possible to produce something much more stylish than a conventional bike, with a back wheel which is only mounted from one side, a totally enclosed mechanical drive, and a configuration that folds into a neat package.

On the other hand, fastening and joining methods used in conventional construction are often not suitable and alternatives have to be found.

The 'evolution' has been developed by Richard Thorpe, managing director of Karbon Kinetics. A road test of the development prototype behind London's County Hall showed it to be a delight to ride, with and without the electric assist. At the end of the trial, it was quickly folded and fitted into an oblong black bag, 250mm x 480mm x 600mm, neat enough to be trollied into the bar of the nearby Mariott Hotel without causing comment.

The use of high strength material allows the 250W AC sensorless induction motor and inverter to provide a lively acceleration as well as a top speed of 15mph and range of 35 minutes without pedalling on level ground. Hand sized, the motor slips on the front hub as in the Ultra motor equipped bike described in Eureka April 2003, but is even smaller, although not nearly so quiet.

Thorpe commented that "The non-traditional frame represents new challenges for the designer - namely the long, cantilevered seat post. The use of a single composite tube instead of an aluminium tube offers substantial weight savings. On the other hand, the clamped adjustment point can pose problems. Thin walled composite tubes are extremely sensitive to variations in clamping forces. If clamped too tightly, local delamination can occur that greatly compromises the structural integrity of the tube." Thorpe described several solutions: a bottom stop for the seat post to lessen the required clamping force, with a possible twist-to-lock device to limit force, a metal insert bonded inside the base of the carbon tube, or increasing the wall thickness of the tube in this area. The ideal solution, he insisted, should only be settled on after full finite element analysis and fatigue and proof testing."

A small team of UK motorsport companies has offered help in solving this and other challenges facing the design in order to turn it into a volume manufactured product. In the mean time, 'evolutionPro', all made in the UK, costs £3,295 plus £795 for electrics. A pressure die cast light alloy version 'evolutionSport', which will almost certainly be made in Taiwan, has a target price of £495 plus £295 for electrics.

Thorpe has had several years of experience as a composite engineer working on motorsport projects including LeMans, Indy and F1.

Karbon Kinetics

Pointers

* Use of modern high strength materials such as carbon fibre composite allow designers to make radical improvements in design

* Joints between carbon fibre composite components that make best use of the advanced properties are not always the same as those made with conventional materials

* Finite element analysis, fatigue and proof testing are all now regarded as essential parts of the development process

Author
Tom Shelley

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