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Bendy spokes: Independent developments from the US and the UK both combine composite materials and innovative designs to improve the performance of traditional spoked wheels.

Dating back to the late 19th century, spokes provide a rigid and lightweight wheel structure. They remain omnipresent in bicycles and motorcycles, although have been superseded by forged solid steel or aluminium alloy rims in motor vehicles. Spokes, of course, are wire rods that lace into a wheel's axle hub and run radially outward to the rim, the outer perimeter that holds a tyre. Screw threads cut into their far ends mate with headed nuts set in the interior of the rim. There are usually two sets of spokes coming off either end of the hub.

As the wheels turn, spokes experience alternating cycles of tension and compression, depending on their position relative to the hub that imposes part of the vehicle's weight on them. Spokes in the top hemisphere of the wheel are pulled taught, as the hub hangs from them; spokes In the bottom hemisphere are squeezed by the hub that tends to bear down on them.

Not that the rider feels it; these wheels themselves offer almost no give. For mountain bikers and motorcyclists, the work of softening bumps along the way may be done by various arrangements of shock absorbers mounted in the frame. But for others, the only cushioning is provided by the small pillow of air enclosed in the pneumatic tyre. That group includes users of wheelchairs, which also ride on spoked wheels, a feature that makes them almost impossible to ride comfortably off-road.

A UK innovation promises to change that by reducing vibration up to 70% compared to spoked wheels. Six years ago, Jelly Products launched a design that makes use of carbon fibre springs instead of spokes. That 'loopwheel' design provides a much greater hub movement, relative to the rim, than spokes. Its particularly rigid rim passes on shocks and bumps to the carbon fibre springs, which flex in response to minimize the shock on the hub. Deflection might be as much as 70mm on a 559mm-diameter wheel, according to inventor Sam Pearce (pictured, right, with partner Gemma Pearce). Arranged at 120[degrees] apart around the rim, they ensure that the hub is supported from above and below at any time (and, in fact, from any side, allowing the wheel to also counter horizontal bumps equally well as vertical ones.)

Pearce contends that the reduction in mechanical efficiency caused by the spring assembly--a physical inevitability--has a negligible effect on rolling resistance. However, weight can affect that. Its double-loop wheels are a lightweight 1.8kg without push rim or tyre. And in September, the company launched two even lighter-weight models, Loopwheels Extreme and Loopwheels Urban (pictured inset, above), which feature only three springs. Although they work in the same way, they are more than 400g lighter.

Pearce explains that the company faced two principal engineering challenges in creating the design. The first was reducing the width of the carbon-fibre springs to dimensions that are reasonable for a wheelchair, but retain sufficient lateral stiffness. The second has been fastening the bendy material to rigid metal hub and rim, for which both a mechanical fixture and adhesives are used.

The company, which only employs seven people, also assembles the wheels (production capacity: 100/month); the springs are also made nearby. It has benefited from 264,000 [pounds sterling] of public funding for product development and pay for underpinning research, carried out in 2017 with Strategic Simulation and Analysis (for spring development testing) and Composite Braiding, as well as two crowd-sourcing Kickstarter campaigns in 2013 and 2015.

Another cushioning wheel, intended for motor vehicles, also had some big news this year, after originally being brought to market in 2012. Michelin's Tweel (right) is made of a special rim, polyurethane spokes and a more or less conventional hub--but without a tyre.

In June, Michelin signed a major development agreement with General Motors of the USA to develop a version of the Tweel, to be called Uptis, for an autonomous car model from 2024 (main picture). "The rapid gains in autonomous electric vehicles are an interesting opportunity for Uptis," says Tweel product development engineer Chris Mast. Uptis promises never needing to be pumped up, or run the risk of a flat or a blowout. "The auto-makers want a maintenance-free unit. With electric cars, you don't have to change the oil; their brakes can last a lifetime. Tyres are the last maintenance wear item on the vehicle. If we can offer maintenance-free tyre, we round out the whole package."

Conventional spoked wheels don't bounce because the spokes tie the rim to the hub, keeping it rigid. Anyone who has played with a plastic hula hoop knows how a circular ring of material can act like a spring; throw it on the ground side-on, and it will bounce, oscillating between a horizontal and vertical oval. This is too bouncy to make a good car wheel.

But the way a Tweel is built enables it to perform in an entirely different way, even though it acts like a semi-rigid mechanical spring. Its rim consists of a layer of rubber sandwiched between more rigid materials on the inside and outside of the rim. Wound around the circumference of outer and inner rim is a so-called inextensible membrane that prevents the rim from stretching or compressing. When the hoop is deflected, it forces the interior rubber layer to shear, or stretch, transferring the load and bending moment throughout the hoop structure. Since the hub hangs from the top spokes, the lower spokes and contact patch are free to compress or deform. That reduces the contact pressure of the wheel on the ground, and over obstacles, similar to a pneumatic tyre (pictured, below).

The stiffness of this type of structure, known scientifically as a Timoshenko Beam, depends on the shear strength of the rubber and the thickness of the beam. Varying other design elements such as architecture, type of reinforcement and number of spokes can match the Tweel design to suit different applications. But, once designed, its behaviour does not change. Admits product development engineer John Duty: "That's the best and the worst thing about the Tweel. We design its air pressure for life. We can't alter it for more comfort." And 'life' really could be much of the operational use of the parent vehicle. Tweels have been found to last three times as long as pneumatic tyres in lawnmowing applications in the USA.

Other Tweel models are used for golf carts, skid steer construction vehicles. And they could be used in many other types as well. The biggest challenge, reports Chris Mast, is matching the performance and the price. Although the designs are long-lived, they also cost twice or three times as much to manufacture as pneumatic tyres. Work continues on ways to try to bring down costs.
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Title Annotation:THE WHEEL
Publication:Engineering Designer
Date:Nov 1, 2019
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