PERFORMANCEBIKES.CO.UK 41more important job to do, according to Baumgaertel.
“Flex is a tool of steering,” he says. “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 can overstress it.
Of course, you need to adapt stiffness to the tyres, so if
the constructions change you need to rework it.”
Flex is also a suspension component. “All race
bikes are softer than road bikes in the lateral
plane, to achieve flex at high lean angles,” adds
Baumgaertel. “But they also need to be stiff
enough to allow fast changes of direction and
avoid wobbles that can reduce tyre contact.
The task is to separate lateral stiffness
from longitudinal stiffness.”
Racers need flex to help damp out
some of the forces going through
the chassis, but it’s important that
flex never changes from a positive
to a negative motion.
“Ifyouhaveaflexforcethat’salwaysrising,thenit’s
predictable,sotheridercanfeelwhat’sgoingon,”says
Baumgaertel.“Butnothinginachassisisinfinitelystiff;
everything is in flex to a certain amount. Therefore
everypartofthechassishasaspringrate,andyoutry
to separate them out. We can simulate things, up to a
point, by putting certain lateral and longitudinal loads
on the front and rear contact patches, then we try to
manipulate things with different geometry, different
shapes and different materials. But there are always
different points of view about what works best; it comes
down to rider feeling.”
The amounts of flex aren’t huge. When a race chassis
bends like a banana through a corner, the bend is about
one millimetre. This isn’t much but it’s enough and it
explains why Ducati finally gave up its concept of using
the engine as a stressed part of the Desmosedici’s
frame. Frame flex at full lean is a couple of millimetres,
or about one millimetre in the swingarm.
All MotoGP and Moto2 chassis manufacturers (apart
from KTM, who use a tubular steel frame) fabricate
their frames and swingarms from aluminium alloys
machined from billet. Nothing is bent or pressed, as
this can cause inconsistencies. The billet sections are
hand-welded. Robots aren’t good enough. Yet.Tyres and suspension
Tyres are the biggest focus of chassis
design in all three MotoGP categories,
because any championship that uses
control tyres is governed by those
tyres. Manufacturers must adapt
their chassis to generate the correct
loads to make the tyres work; so
chassis flex, geometry and so on
are tuned to match the tyres, not
the other way around.
And tyres do more than just grip
the track. They are a vital part of a
race bike’s suspension, offering more
mid-corner cushioning than the forks
and shock and chassis flex, because‘FLEX IS A TOOL OF STEERING,
HOW THE CHASSIS BENDS UNDER
CERTAIN LOADS GIVES A SELF-
STEERING EFFECT, WHICH MAKES
THE BIKE TURN MORE EASILY’
Frame and swingarm
wall thickness varies
from 1mm to 5mm.
Eliminating weakness
at areas of stress is keyGP TECH / CHASSIS DESIGN
Corner entry dynamics
This is one area that is, to some degree,
affected by rider preference – whether a racer
wants a stiff front end when they haul the bike
up for a corner, or prefers a more progressive
feeling. Baumgaertel reckons: “You want a fair
amount of dive to change the trail, which helps
the transition into the corner.”