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Point out the time period “leaf spring” and there’s an inclination to consider old-school muscle vehicles with unsophisticated, cart-sprung, solid-axle rear ends or, in motorbike phrases, prewar bikes with leaf spring entrance suspension. Nevertheless, KTM is now reviving the concept for motocross bikes.
In actuality, whereas crude, previous suspension methods typically used leaf springs, the spring itself isn’t often the supply of their lack of sophistication. Chevrolet’s Corvette used transverse leaf springs on impartial suspension from the second era in 1963 proper till the launch of the eighth era in 2020, adopting composite plastic single-leaf springs within the ’80s. Much less famously, Volvo makes use of composite, transverse leaf springs in a number of of its newest fashions. Used appropriately, leaf springs made of recent supplies could be lighter than metal coils, and in some situations their lengthy, flat form is less complicated to bundle. Composite leaf springs, product of a single piece somewhat than the stacked leaves of conventional steel leaf springs, additionally keep away from the friction of the a number of leaves rubbing collectively, which was one of many important drawbacks of older designs.
Leaf springs have appeared on motocross bikes within the trendy period earlier than. Yamaha’s 1992–93 manufacturing facility ‘crosser, the YZM250 0WE4, used a single composite leaf on the rear, its entrance finish clamped beneath the engine and the rear bolted to a linkage beneath the swingarm, in order the rear wheel rose, the leaf flexed to offer springing. The concept was to clear the world the place the rear spring and damper would usually sit, permitting a straighter consumption path for the engine. A compact, rotary damper was additionally fitted and the bike was a race winner in each 1992 and 1993 within the All-Japan Championship.
KTM’s new design, revealed in a patent utility from the Austrian firm, refers back to the Yamaha and factors out related benefits when it comes to packaging, however adopts a special format. As proven within the photos, KTM places the leaf in a virtually vertical orientation, tight towards the rear of the engine to clear the area usually crammed by a coilover (the patent confirms that whereas its main picture exhibits the system overlaid on an image of a traditional motocrosser, the coil spring proven within the picture wouldn’t be current).
The highest and backside of the spring are every firmly clamped to the tip of linkages. The higher linkage is pivotally mounted on the bike’s important body, whereas the decrease linkage pivots from a bracket beneath the swingarm. The result’s that, because the swingarm strikes upward, a bend is launched to the composite leaf spring. So as to add adjustability, the higher linkage’s size is adjustable by way of a screw thread and an adjuster knob, making it simple to extend or cut back preload within the system.
The patent doesn’t present a damper for the rear finish however its textual content confirms {that a} standard damper can be used to manage the rear suspension. Nevertheless, it must be extra compact than a traditional rear shock, or mounted in another way, to permit KTM to make the most of the advantages of the leaf spring, that are largely associated to the area it frees up. The patent suggests this area could possibly be used to make components of the powertrain just like the airbox, consumption tract, or muffler, as an illustration, bigger or extra environment friendly. Moreover, the design may permit extra flexibility of format in future electrical-powered motocross bikes.
Past the packaging benefits, the system’s different profit is its adjustability. KTM’s patent exhibits how altering the size or form of the linkages holding both finish of the spring can alter the suspension’s habits. In a single illustration (Fig.7 within the patent), 4 completely different lever preparations are proven to alter the rear suspension’s habits: altering from rising fee (7a) to fixed fee (7b), and reducing spring charges (7c and 7d). These radically completely different behaviors are achieved with out altering the spring itself.
As ever, a patent utility is not any assure that an concept will attain manufacturing, however the packaging benefits of the leaf spring rear finish may develop into more and more precious, significantly in future as electrical powertrains power engineers to rethink the traditional layouts which were honed throughout a century of piston-engine bikes.
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