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Project Goal
The goal for this project was to understand how the changes to adjustable parameters of the vehicle i.e. spring rates, ARB rates, roll center height, weight bias, track width, wheelbase, weight, and chassis torsional rigidity effect the handling of the vehicle. We use these parameters to calculate the lateral load transfer distribution of the vehicle which will give us a good baseline of the steady state handling characteristics of the vehicle.
Lateral Load Transfer Distribution (LLTD)
LLTD refers to the distribution of lateral load transfer on each axle (front and rear) in a steady state corner. For example, if you have an LLTD of 60% front that means that the front axle carries 60% of the total lateral load transfer due to cornering acceleration. In general, the higher the LLTD of the axle, the less grip this axle will have due to tires sensitivity to normal load. This is used tune the handling characteristics of the vehicle.
To the left is an example of a vehicle with a very high rear axle stiffness(Note the rear inside tire lifting off the ground). Since this vehicle is a FWD vehicle, the rear tires do not have a driving torque applied from the engine meaning the driver can not apply throttle to get the vehicle to turn in. So to assist with this, engineers will make the rear axle of a FWD vehicle stiff, and the front axle soft in order to assist with turn in under braking. This is an example of a vehicle with a high rear LLTD.
The Tool
Define Vehicle Constants.
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Define springs based on desired ride frequency (aero maps), and then make a *guess* for the total roll stiffness front and rear.
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Changing the total roll stiffness front and rear changes the LLTD. You can then change the total roll stiffness front and rear to fit your spring rate, and desired LLTD. For a baseline, the magic number method of 5% more LLTD than your static weight distribution works well. The desired LLTD is ultimately determined with test time as drivers have different preferences on what they want the vehicle to do.
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Once the desired LLTD is determined, the ARB can then be set. The ARB is used to tune the roll stiffness separately from the ride frequency. Hence the need for an ARB (unless you just want a crazy high ride frequency, boooo!) ARB parameters can be changed in the section in the bottom right, which then change the values to the left. Change these ARB parameters until they are as close as possible to the required ARB contribution.
Equations for LLTD with chassis stiffness included. For all other equations, please reference pages 680-685 in Race Car Vehicle Dynamics.
Thats it! Now go out there and test the baseline magic number setup. If this setup is not what the driver prefers then tune accordingly. Once an optimal LLTD value is achieved this can then be used on any vehicle in the future as an almost perfect baseline setup.
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