WIP
ASC Paper Notes
Aspects of stability
rear end stability in turning and crosswinds
high-speed straight-line ability; reducing steering corrections
resistance to tipping in turns and sudden changes in road surfaces (from slippery to grippy)
“swapping ends” under hard braking? not quite sure what this means
CG must be a design specification
Slip angle stuff
Slip angle is a function of the lateral load, as well as the vertical load (on that tire)
generally, automotive tires have a max slip angle of around 10 degrees. further than that, and the vehicle will lose grip and slide
The plot of bike tire slip angles, showcasing how increasing load increases the lateral load for an X slip angle
this increase in grip is not proportional, however, with doubling the load from 67-135lb only yielding a 66% increase in lateral load
this is known as “load sensitivity”
coefficient of lateral load is the ratio of lateral to the vertical load
Placement of the CG is very important when it comes to a side load being applied, which primarily occurs during turning. The following shows the scenarios that can occur when the CG is moved fore and aft of the Neutral Steer Point (NSP)
The distance of the NSP from the front axle is determined by the following equation
Assuming the tires in the front and back are the same model, and thus the cornering stiffness values are the same, the equation simplifies down to 1/3WB.
The location of the CG relative to the NSP determines the characteristic of the yaw response. This can be summarized in variables known as the static margin and the understeer coefficient
The Static Margin (SM) is defined as “the distance from the CG rearward to the NSP divided by the wheelbase and is expressed with the following equation (somehow…?)
In our case, with 3 wheels of the same cornering stiffness, this can be simplified down to
Neutral steer is when SM = 0, which means LG = WB/3 in our case.
Understeer is achieved when SM > 0, and the CG is ahead of the NSP which in turn means LG/WB < 1/3. This is considered the more stable setup.
Oversteer occurs when SM < 0 and the CG is behind the NSP. This means LG/WB > 1/3 and is considered unstable.
This will be elaborated on further. Typical American passenger cars have an SM of ~0.06 while sports cars typically come closer to neutral and oversteer characteristics (Ferrari Monza hits an SM of around 0.003).
Reading List
https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.633.5587&rep=rep1&type=pdf