Rider feel and chassis flex are two of the most important factors in getting a rider around a racetrack as fast as possible
Japanese manufacturers have won the last 10 MotoGP world championships, but the most successful chassis engineer in the MotoGP paddock isn’t Japanese. Alex Baumgärtel comes from Augsburg in Germany. His Kalex motorcycles are the dominant force in the Moto2 world championship, with a clean sweep of eight rider and constructor titles over the past four seasons.
Baumgärtel is particularly interesting because he comes from a car-racing background. He is probably the first car engineer really to succeed in motorcycle grand prix racing; whereas André de Cortanze, Mario Illien and John Barnard all failed, for one reason or another.
The 50-year-old was working for the Holzer Group when he decided to build his first motorcycle chassis, for his own use on track days. That was 13 years ago. Soon after he established Kalex Engineering with fellow bike-mad car engineer Klaus Hirsekorn. What really attracted Baumgärtel to motorcycle racing, apart from his own interest in thrashing bikes around racetracks, is the nebulous concept of rider feel. The feeling that the rider gets through the seat of his pants is arguably the most important factor in bike racing.
“Car racing is the complete opposite – it’s all data, more or less,” he says. “I enjoy bikes more than cars because this black hole that the rider calls feeling is super-interesting. You read the rider’s eyes to know if the bike is working or not. The look he gives you is more important than what you get from the data-logger, because when a car racer gets into trouble he goes into the gravel and the mechanics clean the car, but when a bike racer gets into trouble he gets a lot of pain.”
Baumgärtel has yet to build a chassis for the MotoGP class, but his Moto2 creations are very similar in materials, dimensions and geometry. Like most MotoGP manufacturers, Kalex fabricates its frames and swinging arms from welded aluminium-alloy sections machined from billet.
The hazy concept of rider feel is inextricably linked to the tricky concept of chassis flex. One hundred per cent rigidity isn’t desirable in a racing motorcycle, for several reasons.
Grand Prix bikes ride through corners at more than 60deg of lean, which makes their suspension almost useless, because vertical forces are best looked after by something operating in the vertical plane. Frames and swingarms are therefore engineered to offer a modicum of lateral flex that helps the tyres soak up irregularities in the asphalt.
“All race bikes are softer than road bikes in the lateral plane, to achieve flex at high lean angles that absorbs bumps and increases grip,” says Baumgärtel. “But they also need to be stiff enough to allow fast changes of direction and to avoid wobbles that can reduce tyre contact. The task is to separate lateral stiffness from longitudinal stiffness, with the frame, swingarm and forks.”
This is the most talked-about aspect of chassis flex in MotoGP, but in fact it’s not the most important.
“Flex is also a tool of steering,” adds Baumgärtel. “How the chassis bends under certain loads gives a self-steering effect, which makes the bike turn more easily. Imagine the frame and swingarm curving like a banana. With the centre of gravity more or less in the centre the chassis makes a curving line through the corner. This helps turning because something curved will turn a corner better than something straight, which forces all the turning through the front tyre, which stresses the tyre more. Of course, you need to adapt stiffness to the tyres, so if the tyre construction changes you need to rework it.”
This no doubt explains why Ducati finally gave up using its desmodromic-valve V4 MotoGP engine as an integral part of the Desmosedici’s frame. Many years ago this was a popular technique in the design of race bikes, but lean angles and cornering loads have come a long way since then. Ducati changed its ways in 2012, once Valentino Rossi had failed to make the Desmosedici steer through corners properly. The company’s results have been on the up ever since.
Swingarm flex is another vital quality, both in the corner-entry and corner-exit phases, because most riders get crossed up when they’re braking and sideways when they’re on the throttle.
“You can make a big difference to the rider’s rear-tyre contact feeling during the corner-entry phase by adjusting lateral and torsional stiffness in the swingarm,” says Baumgärtel. “Flex is a big thing here, because if you’ve got a super-stiff swingarm the rear will get very snappy when it steps out, because there’s no damping in the side forces.
“We also need to make compromises with chassis stiffness to make the acceleration phase sweet. But getting the drive spin right when the rear is stepping out isn’t easy because it’s a non-steady state of loadings.”
The degree of flex involved in all these situations is very small; just a millimetre or two. But it is vital. Motorcycle chassis engineers only learned of its importance once they could build fully rigid frames and swingarms.
In 1990 Piero Ferrari fabricated a super-stiff carbon-fibre frame and swingarm for Cagiva’s 500cc Grand Prix team. The chassis was a nightmare, creating no feel and lots of chatter. Three years later Yamaha built a similarly stiff aluminium chassis for reigning world champion Wayne Rainey.
“At that time chassis stiffness was the black art, nobody knew what was too stiff and what was too soft,” recalls the American. “We ended up going way too stiff, which is why we really struggled in 1993, because the chassis put too much force through the tyres.”
Mat Oxley has covered premier-class motorcycle racing for many years – and also has the distinction of being an Isle of Man TT winner