JUST TWO RULES

1. Don’t Fall Off the Motorcycle.
2. Don’t Get Eaten by a Bear.

FLEX, BEND AND TWIST YOUR FRAME DOES WHAT!?
WORDS/PHOTOS MARK MCVEIGH – CEO motoDNA

FLEXIBLE CHASSIS, RIDERS AND MOTOGP ENGINEERS

Behind the scenes, MotoGP Engineers are continually refining and modifying their state of the art two wheeled missiles.

It’s only when you can’t hide those design revisions that the Journos’ tongues start wagging.

Chassis flex, for example, is in the spotlight again because of the updated swingarm clearly visible on Marc Marquez machine recently at Le Mans.

Designed to close the gap on Yamaha with improved tyre edge grip, acceleration, and corner entry, Honda has reduced stiffness and thus increased the fl ex of the swingarm on the RCV213V.

From the photo above its looks like HRC has simply cut a hole in the right had side of the swingarm to increase fl ex, but there’s a bit more to it.

How does this one modification improve so many aspects of the bikes performance? Well, let’s start by explaining some of this engineering terminology.

FLEXIBILITY

Flex is the chassis’ ability to absorb force, bend in different directions and then return to its original position.

Lateral means sideways, so lateral flex is bending to the side, or in the bikes vertical plane – namely when the bike is cornering at extreme lean angles.

Uncontrolled frame fl ex, whether it be lateral, torsional (twisting) or vertical can be thought of as the bike behaving in an elastic state. It has a clear influence on the bike’s handling, making it feel slow and heavy due to the dynamic geometry changes taking place with trail, wheel alignment, etc.

CHASSIS FLEX

The ideal amount of chassis fl ex allows an element of lateral fl ex to absorb the bumps whilst retaining stiffness in torsion so the wheels are in the same plane and to control braking and acceleration forces in the vertical plane.

Stiffness is the chassis’ resistance to deform under loads generated from cornering, acceleration and braking.

Chassis performance comes down to the combination of overall fl ex, stiffness and strength of the frame, swingarm, forks, engine mounts, triple clamps and other components working together in harmony without creating chatter.

Chassis material and its thickness, internal ribs and how they are welded together all influence stiffness and flex.

If one of those components’ stiffness changes, the overall balance and feel of the bike will change.

COMPLEX ENGINEERING

Motorcycles are notoriously among the most complex machines to engineer.

It’s very difficult to change one aspect of the bike independently without affecting An example of this chassis component interdependence is the recent trend of under slung swingarms in MotoGP and WSB, the bracing having moved from the top to underneath the swing arm.

This design is more efficient at reducing lateral tyre contact patch movement when the swing arm twists.

Also worth mentioning is a rule of thumb in any engineering design, which is to change stiffness gradually. Components often fail at the point where there is a sudden change of stiffness.

Ducati famously forgot this rule with their GP11 carbon fibre chassis, which was basically short and stiff sub-frames connected to a very stiff motor.

other aspects.

An example of this chassis component interdependence is the recent trend of under slung swing arms in MotoGP and WSB, the bracing having moved from the top to underneath the swing arm.

This design is more efficient at reducing lateral tyre contact patch movement when the swing arm twists.

Also worth mentioning is a rule of thumb in any engineering design, which is to change stiffness gradually. Components often fail at the point where there is a sudden change of stiffness.

Ducati famously forgot this rule with their GP11 carbon fibre chassis, which was basically short and stiff sub-frames connected to a very stiff motor.

BEND LIKE A TREE

The chassis should bend like a tree according to Yamaha’s legendary engineer Masao Furusawa.

A longer chassis creates a long lever, which allows fl ex to be designed more gradually, accurately and effectively.

So just how do MotoGP engineers work out the right amount of strength, stiffness, flex and in what plane or direction? Well, engineers increasingly evaluate huge amounts of data in MotoGP, which can be measured, quantified and related to the bike’s performance.

However it’s difficult to quantify what the rider feels and it’s this conundrum that ensures race engineering remains a balance between an empirical art and a science.

Subsequently the factories go through a huge amount of research and development to settle on the motorcycle’s overall handling performance.

R&D

Computer aided engineering tools such as Finite Element Analysis are used by engineers to test designs in the virtual space. The whole bike is modeled in 3D CAD, and deflection, stress, vibration, buckling, etc can be tested and validated before making physical parts.

In the end, though, racing is a head game and it really all comes down to what the rider feels.

ADAPTING

Relentless tyre performance gains and the ensuing extreme lean angles ensure that engineers are constantly refining the balance between stiffness and fl ex to increase the overall performance of the bike and more importantly give the rider that all important feel.

Riders too have to change and adapt; just look at Rossi who has modified his riding style massively to make it more modern and adapt to the new tyres.

It’s now Marc Marquez’ turn. The Spaniard is aware that his hallmark rear end corner entry slides are costing him time and he will adapt his riding style; substituting for the time consuming slides, an improved rear floating feeling on turn in courtesy of Honda’s latest flex enhanced swing arm design.

Mark McVeigh is a former International 250GP Road Racer and MotoGP Engineer who now works as a Moto Journalist and Director of Coaching at the Dorna accredited motoDNA Academy.

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