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TECHNOLOGY – SIMULATION


nature of this test, if there is a change it’s very easy to isolate what caused it. We can also easily quantify the change. For example, if we are dealing with a front tyre pressure change, the delta FFact will look like that shown in equation 5. Clearly, if you are measuring a change at the rear you simply


attach multipliers to that. This is the key to set-up referencing. While not perfect, it gets us into the ballpark, and that is our goal. The most effective way to


do this is to use your racecar simulator to match your expectations with what occurred with the car. Some of you think this is fudging. It isn’t. What you


"Some of you think this is fudging. It isn't"


substitute the wdf with weight distribution at the rear (wdr). Once this data is calculated, fi gure 4 can be readily populated. The next step is to match our


set up data with the tyre model. The critical thing to note here is that the more roll centre you apply, or the stiffer you make one end of the car, or the more static tyre pressure you apply, the more the hot core temperature and pressure of the tyre will increase. These adjustments are a race engineer’s primary tools when they are dialling in core tyre temps and pressures. What we are going to do here is match this to our baseline set up and then


are doing is using the racecar simulator to quantify what’s going on with the car, which is another reason why I maintain it is imperative that you do your simulation work yourself. So, to achieve this, we need


to assign the base set up and the pressure multipliers. As an example, fi gure 5 shows the sample controls in ChassisSim. As can be seen, the base set up is indicated in the fi rst column. Note: the tyre pressure you enter here is the warm tyre pressure from the base set up. This is very important. The next step in the process is to assign the pressure multipliers. Some baseline


Table 5 – some rough rules of thumb for the set up sensitivity parameters


Parameter


Tyre pressure Roll centre Pitch centre


Spring / third spring rate Bar rate


h


pccur pcref


kcur kref


TPref rccur rcref


Value


1*(TPcur – TPref)/TPref 30-40*(rccur – rcref)/h 30-40*(pccur – pcref)/h 10*(kcur – kref)/kref_total 10*(kcur – kref)/kref_total


The variables here are as follows: TPcur


= current roll centre = current pitch centre = c of g height


tyre pressure of the reference set up = roll centre of the reference set up = pitch centre of the reference set up = current spring, third spring or bar rate


= current tyre pressure =


= spring, third spring or bar rate of the reference set up kref_total = sum of all the spring and bar rates of the reference set up


suggestions (with numbers in psi) are illustrated in table 5. These numbers are subject to


car factors such as motion ratios and parameters. However, the key is to use these pressure multipliers to dial in some typical set up changes and dial this. Also, bear in mind that what we are presenting here is only an approximation. Before you turn your nose up in disgust, let me just remind you of some approximations that work pretty well – in most CFD analysis for aircraft, air viscosity is ignored,


but it gets you in the ballpark. And the calculation of damping ratios ignores tyre spring rate, but it's still a valuable tool. Remember, the key here is


we are using set-up referencing to quantify what’s going on with the car. From that we can make the appropriate determination of set up changes. Consider it the fi rst step toward quantifying how sensitive a set up change really is, and a valuable insight into determining how to make your racecar go faster.


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