6 THINGS YOU NEED TO KNOW…
We are all familiar with the holy trinity of steering geometry; rake offset and trail. Rake, or castor angle, is fixed by the angle of the headstock. The yoke offset or the mounting of the front axle on the forks moves the wheel forward of a line drawn through the headstock to the road – trail is the dimension between the point where that line hits the road and the tyre contact patch directly below the wheel spindle. Weight distribution is another familiar concept along with its twin cousins C of G and mass centralisation. Anyone who has ridden a bicycle has a handle on countersteering, even if they don’t know it. So, cornering’s nothing a bit of trigonometry couldn’t resolve. But of course Pythagaros never rode a 250bhp bike over Lukey Heights.
At any given moment, a bit of trail braking into the bend will have compressed the forks and lifted the rear, altering the rake angle while the tyre contact patch is rolling off to one side and the invisible line is pointing the other way. The wheels are no longer in line so tyre slip angles, their misalignment with direction of travel, come into play.
Meanwhile the forces that are being generated as the machine’s mass is leant towards the centre of the bend to resist the centripetal force are being restrained by the tyres as they search for grip so the coefficient of friction at any particular point will have to be accounted for along with tyre profile, deflection and wear. These same forces are compressing the suspension, further altering the geometry and moving the centre of mass.
Engine and gears
Gear selection will determine forces being applied through the rear wheel and engine speed will need to be taken into account as the effects of gyroscopic precession generated by various rotating masses resist or enhance the bike’s willingness to comply.
Mass is always changing
The bike’s mass drops steadily as fuel load reduces; fore and aft weight distribution is only constant when the motorcycle is parked. Meanwhile, the motorcycle’s giros and ECU are conspiring to create their own solutions to throw into the mix. And then a bump on the track deflects the front wheel. We are going to need a bigger pencil.
What if the rear wheel is spinning up and drifting? Let’s call it the Stoner variation, which brings us to the key factor in our calculus. The rider can affect pretty much all of the above by making adjustments to the bike. Which is why, however, we analyse the specs and strive to find the optimum settings, getting a bike through a bend at speed will always be more art than science, more instinct than theorem and one the most challenging and satisfying things a human can undertake.
Tyre compresses, suspension compresses… the variables are huge