It’s a bit of an unsung hero in the chassis stakes, the modern motorbike frame. Like the techs behind the scenes in a MotoGP team or the backroom designers at a big bike firm, it just gets on with its job, while the flashier members of the team get all the plaudits. So when a new superbike comes out – like last year’s GSX-R1000R, say, we all get het up about the gas forks, the Brembo brakes and the multi-adjustable rear shock.
Even swingarm design is far saucier than a frame, for some reason. Maybe it’s because it’s often hidden under plastic bodywork. Or maybe it’s because they’ve done such a good job for so long that we now largely take them for granted. Up until the 1970s, though, most bike frames were an afterthought at best. They were, literally, just metal brackets to hold the engine and suspension components together, with a rough stab at stability and stiffness. Most bikes made about 20bhp, and even with that asthmatic power output, they handled like clown cars which had dangerously failed an MOT test. Wobbles, weaves, tank-slappers, they did them all if you dared go at any sort of speed, plus they weighed a ton, being made from low-grade pig iron. If you were lucky.
Once upon a frame
Like everything else in bikes, the Japanese took it and sorted it right out – eventually. Even the likes of Kawasaki and Suzuki could make minging frames at first though – Kawasaki’s KH two-stroke triple range and Suzuki’s GT strokers struggled even in a straight line. Back then, a steering damper wasn’t there to stop tank-slappers from hard acceleration – they were there just to try and make the thing steer in a straight line.
It took until the mid-1980s before they had the job jobbed, with bikes like the GPz and GSX ranges – decent, stiff, steel tube cradle layouts which held the then-standard inline-four motors nicely, and did a good, steady job of handling the 100-odd bhp maximum output, and the increasing grip available from tubeless then radial tyres. The game really changed when they switched to aluminium. But to understand why aluminium improved matters, we need to have a look at what a frame has to do, and how we can make it better.
At its most basic level, a frame needs to hold the front and back wheels in line, it needs to be fairly light, and shouldn’t be distorted by the forces generated by the engine, brakes or rider. Obviously, we have suspension and steering setups too, and the frame needs to accommodate those and let them do their work, again, without flexing or bending. The problem with those early steel tube frames (borrowed initially from bicycle technology) was that they weren’t rigid enough to stay in shape as power levels increased. Greater speeds and acceleration distorted the frame, so wheels began to move out of line, steering geometry changed, and the bike lost stability and grip
The techy bit
It’s important to remember the difference between ‘stiffness’ and ‘strength’ here. Strength is how much force something can take before it breaks, while stiffness is how much force something can take before it distorts, or bends. A steel towing cable is very strong – you can’t break it easily – but it’s not at all stiff; it bends at the slightest force. Steel is stronger than aluminium – typically about three times stronger. But it’s also about three times as dense. So, for the same weight of metal, you can have an aluminium frame tube with three times more metal than steel.
Larger tubes are generally stiffer than smaller ones, so for the same overall mass, an aluminium tube frame will be made of larger sections, making it stiffer, more rigid, and less liable to flex than a steel tube design. Your kids have probably investigated this in school with the ‘rolled-up paper’ experiment. An A4 sheet of paper rolled into a cylinder will support an amazing amount of weight on its ends before collapsing. Compared to an unrolled sheet, which can’t support even its own weight, the simple change of shape gives a gargantuan increase in stiffness.The shape is vital to stiffness.
As every schoolkid also learns, triangles are the most rigid shape. And whether you’re building a toy bridge in class from spaghetti and marshmallows, or welding up a custom chassis from titanium tubing, triangulating the structure will add stiffness with minimal weight. That’s because a triangular structure needs to distort all its joints and at least one main strut to change shape. A square or rectangle, by comparison, only needs to flex at its joints to distort out of line. In addition, forces work in straight lines, so joining the areas of a frame with the greatest distorting forces (the steering head and the swingarm pivot, generally) with straight connecting beams, will direct those forces along the strongest possible path. Having bends and angles in the path that the forces take will add stress at those points, and mean you need a stronger – and heavier – frame to resist distortion.
Al the time
Back to the aluminium frames then. The first ones largely followed the steel designs, which wasn’t the ideal use of their different characteristics. The first production bike with an aluminium frame was the 1983 Suzuki RG250, which had a double aluminium cradle frame, using square-section tubes. Looking at it now, you can see how it’s broadly similar to the earlier GT250 frame, which used round steel tubes – a double cradle running down under the engine, braced tubing from the steering head, and a triangulated rear subframe.
The RG frame was lighter, but the shape of the frame wasn’t really optimised for the new material. Suzuki, and Yamaha, would do loads of good work here over the next decade or so, trying out various frame layouts, manufactured using extruded aluminium tubing and cast aluminium steering head and swingarm pivot plates, all welded together. The first GSX-R750 in 1984 had a ‘wraparound’ cradle frame, with top tubes that bent over and down behind the cylinder head – not ideal in terms of shape, but it worked well for the time.
Later GSX-Rs from the SRAD era changed to a twin-beam design, improving weight and handling massively. Yamaha’s Deltabox was probably the first beam aluminium frame, appearing on the 1985 TZR250 in Japan. It made for a light bike (around 125kg dry for the TZR), and fine handling, with that direct link between steering head and swingarm. The Deltabox moniker still exists. With Yamaha giving that name to the frame of the current R1M, despite its very different cast design.
By the early 2000s frame design seemed to be in a very good place. Materials, computer-aided design and experience was producing aluminium beam frames of incredible stiffness, able to deal with the 150bhp outputs of powerful engines, and hold super-stiff USD front forks and rear swingarms in line no matter what a rider could do. But at the very limits of handling, problems were starting to emerge. Top MotoGP and WorldSBK riders were beginning to complain about poor handling in bends – particularly a kind of front-end ‘chatter’ or front wheel/tyre vibration.
The problem took an age to identify but turned out to be a counter-intuitive one. Forks and frame had become incredibly stiff – which seems like the best situation, logically. Ride over a bump while upright, and the force pushes the wheel up against the springs and dampers, absorbing the bump and keeping the tyre in touch with the road. The hugely stiff frame and forks are so stiff they hardly move at all. But when the bike is on its ear mid-bend, with a 60°+ lean angle, any bumps, of course, are still trying to move the tyre and wheel upwards. But the direction of the force of that movement is now mostly across the forks, rather than pushing up against the fork springs.
Weird flex, not okay
In the old days, the wobbly skinny forks and limp steering head would flex a bit, absorbing the vertical component of the movement from the bumps, and the rider would hardly notice. Now, the forks and frame were so resistant to flex in all directions, that the vertical, upward force from the bump was being directed right into the chassis, upsetting the bike at the most crucial point, bouncing the tyre off the deck, losing grip, and causing the chatter upset.
By now though, engineers were understanding more about where stiffness was needed in a frame, and where some flex could be beneficial. This so-called ‘tuned’ flex was hard to incorporate into extruded aluminium beam frames – obviously, the extrusions are uniform in construction and have fairly constant stiffness along their length. But a new technology was coming – fully cast frames, made in one large piece, by pouring molten aluminium into a mould. Now, engineers could make certain sections thinner, allowing some flex, yet making the crucial components thicker, with stiffening ribs to direct extreme forces.
This makes it super-stiff in some directions – keeping the wheels and drivetrain in line under hard acceleration and braking – while still retaining flexibility in some other carefully chosen directions. So there are none of the bad flexings that made 1970s superbikes weave over 130kmph, while allowing the ‘good’ flex that stops wheel chatter, and actually improves the ‘feel’ and feedback a rider gets while riding.
The other things to consider in frame design are the practicalities of design. Stuff like triangulated space frames and monocoque designs sound great in theory – but if you need to take the engine out to change the spark plugs, or remove the swingarm to change the gearbox sprocket, then it quickly gets old. The twin-beam aluminium frame of the 1990s and 2000s allowed loads of space above the engine, for downdraft carbs, and a big airbox, with a fuel tank sat behind. That allowed ram-air scoops a straight shot, into the airbox, and then a short intake tract into the cylinder head.
By comparison, a cradle-type frame left little room for an airbox, and the intake air had to come all the way from the front of the bike, past the engine, then turn round and go back through long intake tracts and side-draft carbs, finally reaching the combustion chambers. You can see then, why the design of a frame has become an integral part of the overall design. Choices made here can dictate the whole performance of a bike, even down to the engine’s performance. And like the unsung heroes who are changing the brake pads and torquing the sump plugs in a MotoGP team, the frame will just get on with its job of making a winning bike work…