MotoMatters.com is delighted to feature the work of iconic MotoGP writer Mat Oxley. Oxley is a former racer, TT winner and highly respected author of biographies of world champions Mick Doohan and Valentino Rossi, and currently writes for Motor Sport Magazine, where he is MotoGP correspondent. We are featuring sections from Oxley's blogs, which are posted in full on the Motor Sport Magazine website.
Why MotoGP has gone soft
Frame flex is the black art of MotoGP chassis design. Here’s why chassis are getting softer, not stronger, despite ever-increasing speeds and horsepower
Study this photo of the front end of a Repsol Honda RC213V, particularly the upper triple clamp. Notice how the underside (between the two HRC logos on the left and right) has been scalloped out, so that much of the triple clamp can only be a few millimetres thick.
What does this tell us? That Honda, like everyone else, is making its MotoGP chassis softer and softer.
There are a few reasons for this, but most of all it’s because Michelin’s MotoGP tyres are of a softer construction than the Bridgestones, which the championship used from 2009 to 2015. If the tyres are softer, the chassis must be made softer to match, because both must work together in harmony.
“Historically speaking, the Bridgestone tyres had a much harder construction, so everything was much harder: the triple clamps, the chassis and many other parts,” says Takeo Yokoyama, HRC technical manager. “Since Michelin tyres were introduced we are going softer and softer, to get more feedback, to get more feeling and to make the bike turn better. We are still going in this direction.”
There have been other changes since the Michelins arrived. The stiffer Bridgestone slicks required a lot of load to make them work, so chassis became shorter to increase load transfer to the front tyre on the way into corners and to the rear tyre on the way out of corners. The Michelins need less load, so some factories have slightly increased wheelbase since 2015, while also raising the centre of gravity to ensure the bike still drops into corners nicely.
However, chassis stiffness is the most fascinating area of chassis design and it’s still as much a black art as a science. Engineers want the frame and ancillaries to offer excellent longitudinal stiffness, so the bike stays straight and stable while pulling 1.5g during braking, and they also want enough stiffness so the bike reacts to rider steering inputs.
But once the bike is on its side the chassis needs to flex laterally, because the front forks and rear shock don’t really work once they are past 45 degrees from the vertical. A few millimetres of lateral (sideways) flex provided by a softer frame, swingarm, triple clamps, forks and so on allows the bike to track bumps and ripples better, so the tyres grip the asphalt more effectively, which, of course, is what racing is all about.
More grip can deliver more corner speed, but it also delivers more turning. This is currently one of the engineer’s biggest jobs in MotoGP – helping Michelin’s front slick make the bike turn quicker.
Ducati still struggles in this area – remember Álex Rins riding around the outside of Andrea Dovizioso at Jerez and Valentino Rossi doing the same at Termas? – and Gigi Dall’Igna knows that too little flex in the right places is part of the problem.
Read the rest of Mat Oxley's blog on the Motor Sport Magazine website.