Motorcycle News (UK) - - Garage -

Ge­om­e­try les­son

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.

Front tyre

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.

Rear tyre

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.

Rider’s head

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

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