MotoGP Observations

9-7-19: Flex matters

Carbon reinforcing around the engine mount hanger area of the Suzuki MotoGP bike

Modern MotoGP bikes have insane amounts of horsepower. No one wants to quote exact figures, but by now they must be approaching 300 bhp, and with a minimum weight of 157 kg, you're talking about some serious speed. A Formula One car has a minimum weight of 722 kg and around 1000 bhp. An F1 car would need 1,380 bhp to have the same power to weight ratio as a MotoGP bike. A 38% increase. In MotoGP, the rules of the game allow only one engine configuration for the year. Whilst engine development stops at the beginning of the year, the search for a competitive edge is a never ending process of chassis refinement and electronic tweaking.
Ducati, Honda, KTM and Aprilia have all gone down the V4 road. A configuration that is possibly easier to extract more power from than Yamaha and Suzuki's inline fours. The inline four though, does have its advantages, and the Yamaha and Suzuki's are known to be the sweetest handing bikes on the grid. One of the reasons for this is the swing arm. The inline four allows the bikes to run a longer swing arm for the same wheelbase length, thus giving the rear suspension an easier time of it.

You could be forgiven for thinking that the increased horsepower requires a stiffer chassis to cope, but this is not necessarily the case. The control Michelin tyres are softer than the previous Bridgestones, both in compound and carcass construction. The chassis must be tuned to suit the tyres. This had led to a development path that focuses on both rigidity and flex. A contradiction indeed. Fore and aft, or longitudinal rigidity must be maintained to provide stability under braking and that critical front end feel going in to corners, but a certain amount of lateral flex must be built in to make gains elsewhere and some are doing it better than others.

A full carbon fibre frame on the early Ducati GP9

The fluid dynamics of suspension systems work at their best when the bike is upright or close to it. The performance tapers off drastically when the bike is leaned over at more than 45°. Mark Marquez has been recently achieving lean angles of an incredible 66°, so this is important. A chassis with built in lateral flex is the best known solution for compensating for this drop off in suspension performance at high lean angles. A softer chassis at high lean offers many advantages, the main one being grip. The softness of the chassis at full lean also contributes to extended tyre life. Perhaps the biggest advantage though, is the riders ability to put the power on earlier and harder, thus ensuring a higher top speed at the end of the straight.

The Ohlins carbon fibre outer forks are slightly oval shaped to allow for some lateral flex

The teams have been achieving this lateral flex in a number of different ways. All of the chassis, with the exception of the KTM, employ twin spar aluminium beam frames with hanging plates to suspend the engine. Different thicknesses of these plates makes for different amounts of lateral flex. Aluminiun, and metal in general, tends to snap back into its original position quickly when slightly distorted under pressure. This is not desirable when trying to achieve some lateral flex and this is the reason some teams are experimenting with carbon fibre overlays in some areas of the frame. The carbon fibre dampens this springiness. It is not, as is often thought, to make the frame more rigid, it is to slow down the alloy's reaction.

Carbon fibre swing arm on the Honda

Honda seems to be at the forefront of this lateral flex development and it shows. When combined with Marquez's incredible ability, it is flooring the opposition. Yamaha and, to a lesser extent, Ducati, seem to be suffering the most and lagging development in this area may be one of the main reasons Yamaha has fared so poorly in recent years. This is not a new problem and Yamaha have some history with this. In the 1993 500 GP season, Yamaha produced an extremely stiff chassis for Wayne Rainey. Rainey had trouble riding it because it had no flex at all and all the stress was put into the tyres. When the team changed to the more flexible French made ROK chassis, Rainey got back to his winning ways.

Selective carbon reinforcing on the Ducati GP18

Honda has also used some other measures to combat this lateral stiffness. The Marquez bike has radically milled down triple clamps that are only a few millimeters thick. The other Honda riders are not using this. Suspension manufacturer Ohlins, are also playing their part. The carbon fibre outers on the forks are slightly oval shaped. This provides longitudinal stiffness under braking whilst still allowing for some sideways flexing when it is needed. All of the teams are now using this type of fork, including KTM, who use WP suspension.

The triple clamps on the Honda RC213V are only a few milometers thick

It is already well established that controlled lateral flex is a desirable trait in MotoGP bikes and the race will be on to see who can come up with the best solutions to get it. But lateral flex, in and of itself, is not the ultimate goal. It is the best way that designers can presently see the combat the real problem, which is, the suspension not functioning properly at the lean angles being obtained. The future may bring some sort of active suspension solution that allows the forks and rear shock to work properly at any angle. However, I feel that the near term solution will be a return to full carbon fibre frames that will differ from the ones that have been previously used. The technology used to produce alloy frames in MotoGP is at a high level, perhaps unrivalled anywhere, but carbon fibre technology is way behind Formula One where they have long since turned the black art into a quantifiable science. You just have to look at the time it has taken teams to get carbon swing arms, and some still don't have them. Carbon fibre can be used to precisely and minutely adjust torsional stiffness in any direction and MotoGP may have to look to F1 to gain the expertise.

Written by Paul Harmon, 9-7-19

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