Is200/ 300/ Sc Geometry

[Tony-Bones]

I owe a lot to LOC and those who know what wim is will understand, but due to my work commitments i haven't been as visible here as i was in the past. So for the benefit of new members i would like to open a little geometry window so that new members get a better understanding of this geometry lark that has plagued the IS200/300/sc.

First thing we need to do is understand those angles..... What are they and surely they work on everyone's car and that's the manufacturers law?

The simple answer is that there is no such thing!

Seeking the perfect set-up is dependent on variables that exceed even the most experienced in the industry, F1 with millions invested each season still cannot deliver 'matter of fact' positions, so what do you do? how can anyone establish the perfect set-up.

First lets look at the manufacture

Geometry within new models tends to be an extension of previous established positions, minor tweaks to accommodate suspension travel, wider tyres and so on is often visible and do fall into the 'safe zone', but attempt to manufacture a completely new Geometry then things can go very wrong, as it did for the Nissan 350Z and the new Peugeot 1002.

So now your modified

Taken from the above the car had recognized Geometric positions and is now modified... suspension, tyres, turbo but now things just don't feel right, well maybe understanding the 'forces' involved within Geometry not just the name of the angles could help.

Toe: exerts no force unless violated to the maximum since the chosen position 'Dynamically' is 0

Camber: exerts a 'conical' force and will want to roll into the lowest point of the imaginary cone, the force adds security to straight line travel and compensates for body roll.... excessive camber will make the steering heavy and lazy (turn out) due to the 'compressive' force generated from the angular position of the imaginary cone.

Castor: exerts a non-reactive longitudinal force assuming the positions over the axle are within manufactured tolerances, on cornering the Castor contributes toward displacement of the steering axis and the position of the 'scrub radius' this force is very important.

KPI/SJI: exerts a very high force toward directional stability, this force is immediately detectable if any attempt is made to deviate from straight line travel... the force is generated by 'inclining' the pin during any steering action, this inclination lifts the vehicle and adds weight on the pin, in reply the equilibrium through the rack will return the steering so the KPI can relax.

How to develop your own positions

First consider your reasons, what do you expect from the car and to what end, is tyre wear an issue, are the modifications cosmetic or deliberate? The four examples shown reflect the most common consequences of modifying a car, in particular lowering, the forces displayed need to be examined in your own example and explored.

It could be easy to assume now that all things 'Geometrical have been covered?

Further reading:-

Scrub radius

Ackermann theory

Delta curve

Bump steer

Thrust angle

Camber curve

Over/under steer

Tyre slip angle/resistance/trail

Lock angles

Cradle symmetry

Memory steer

Castor trail