Alan Dowds outlines their significance…
Alongside engines, frames are proper up there by way of significance for bikes. Through the years we’ve seen a whole lot of various shapes, sorts and stiffnesses, constructed from a large number of various supplies. Which one’s greatest? Properly, that’s an excellent query. Right here’s why…
Many of the materials design decisions on a motorcycle are apparent these days. Engine crankcases are product of aluminium, as are pistons, blocks, and heads – solely a madman would attempt to make a forged iron head, as occurred 60 or 70 years in the past. Cranks are metal, as are gears and shafts. Valves are stainless-steel on regular bikes, or titanium on efficiency machines. Wheels and fork outers are nearly all aluminium, fork interior tubes are metal, fairings are plastic, and rubber tyres are, er, rubber.
There’s one main part the place there’s nonetheless a elementary alternative although – the principle body. Virtually each bike on the highway has a body constructed from both metal or aluminium, and the selection varies throughout the kind of machine and price. Flagship journey machines just like the KTM 1290 Tremendous Journey and Triumph Tiger 1200 plump for metal tubes, as do many of the least expensive 125 learner bikes on the market. Alternatively, nearly all present supersports machines have aluminium frames, and that’s progressively turning into true of most traditional equipment, too.
So why are there nonetheless two choices right here? The very fact is that each metal and aluminium are glorious decisions as body supplies – however there are such a lot of elements concerned that neither has taken over utterly in the way in which that solid aluminium has for engine circumstances and wheels, or cast metal for cranks and conrods.
Going again to fundamentals, aluminium is loads lighter than metal, and resists corrosion much better. Metal is stronger, although, and can also be extra robust: it has higher resistance to fatigue, shock masses and cracking. They’re each pretty cheap to purchase; metal is mostly cheaper and simpler to manufacture by welding, stamping or urgent. However aluminium is good for casting, with a a lot decrease melting level. Alloying aluminium with different components like magnesium, silicon, copper and zinc additionally tweaks its energy and different traits to swimsuit. It’s fiddly to weld, although, and wishes particular gas-shielded strategies.
Up till the Eighties. all motorbikes had metal frames. The know-how advanced from bicycle designs within the nineteenth century: pioneering engineers fitted small petrol motors to the pushbikes of the day, with their narrow-diameter metal tube frames. Over time, the tubes grew in dimension and energy, and the form of the frames advanced to deal with rising energy and efficiency. Pressed metal elements additionally appeared, made utilizing flat metal sheets stamped out on huge presses, then welded collectively.
The fee and complexity of fabricating aluminium dominated it out, till the Japanese started improvement on the finish of the Seventies. Suzuki’s 1983 RG250 Gamma was the primary to seem with a mass-produced aluminium body, utilizing square-section tubes in an identical dual-cradle structure to a metal tube design. The GSX-R750 launched in 1985 used the identical primary arrange, and by the top of the twentieth century, all of the Japanese companies have been promoting sportsbikes with giant, stiff aluminium frames, borrowing tech from the racetrack.
Aluminium’s lightness – it’s round a 3rd of the load of metal with a 3rd of the energy – meant designers may use extra metallic, in giant hole beam sections. These have been the identical weight and energy as a smaller metal tube, however a lot stiffer, as a result of stiffness is about form, not materials. Bear in mind the varsity science experiment with a sheet of A4 paper? Roll it right into a cylinder, and you’ll steadiness a heavy guide on high. Unrolled, it may well’t even assist its personal weight. The identical concept meant that giant aluminium sections, with skinny partitions, may out-perform metal designs.
Ducati caught with its metal tube designs, although. Right here, the precept is triangulation: use a collection of quick metal tubes, welded right into a community, or trellis structure, and you’ve got a really stiff form, with pretty mild weight. The draw back is elevated manufacturing prices: all these little tubes must be lower exactly to size then welded collectively, both by good artisan welders, or advanced robots. It’s less expensive to have a fabricated twin-spar aluminium body, and even higher, a single aluminium casting.
The additional prices of high-end metal tube frames have been okay for a premium model like Ducati, although, so basic superbikes just like the 851, 916 and 999 all used metal trellis frames. And Ducati saved utilizing it as a trademark design in different fashions just like the Monster, Multistrada, Diavel and Streetfighter. Lately, although, even the Bologna engineers have been dropping the trellis body. The Multistrada, Monster, Panigale, Streetfighter and Diavel have all switched to aluminium monocoque frames, with solely the DesertX, Hypermotard and retro Scrambler ranges sticking with metal tubes.
These days, metal frames have gotten restricted to some use circumstances. It’s nonetheless preferrred for small, low-cost bikes, the place efficiency and weight aren’t as essential as price and resilience. Huge journey and off-road bikes nonetheless work effectively with metal: its fatigue resistance means it may well take repeated exhausting impacts with out cracking or failing, in contrast to solid aluminium. And companies like Royal Enfield use metal frames to swimsuit the robust highway situations in its dwelling market.
Broadly, although, aluminium is successful out. Trendy design and manufacturing strategies are giving companies ever extra management on how a body will carry out: small, delicate modifications to wall thickness on a principal body spar can enhance dealing with and really feel drastically. One downside with frames which might be *too* stiff is chatter: at excessive lean angles, the suspension isn’t aligned with the wheel motion while you hit a bump. If a fork leg is leaning over at 45 levels, any bumps attempt to bend the fork leg up, slightly than compress the springs. Permitting a little bit of managed flex in the principle body can assist take in that motion.
Once more, although, there are exceptions. Bimota’s new KB998 has a hybrid body, with aluminium swingarm pivot plates, bolted to a metal tube trellis entrance part. That provides the agency the management to tweak stiffness to swimsuit – certainly, the WSBK race staff has been in a position to take a look at utterly totally different entrance trellis sections at totally different tracks, to seek out one of the best dealing with answer. It’s not an inexpensive, or apparent answer – however is smart for such a small-scale, unique bike. And it’s additional proof that each metal and aluminium are nonetheless one of the best supplies for body designs.
