Plotting Cn-Cy and Cn-Ay Graphs.

[Claude Rouelle]

OK that is better

3. The lateral forces are positive for the right hand turn. How do you define positive? Which direction? If possible please provide a sketch, not only a list, for all these sign convention.
Most to the people who can help you are too lazy or too busy (at least I am) to take the time to get back to RCVD to re-learn that specific sign convention.

Claude

[Claude Rouelle]

Can you show us a Cn Vs Ay at 8, 10 and 12 m/sec?
Can you tell what your wheelbase is?

[Claude Rouelle]

What is the tangent speed for your car?

[apoorv]

The earlier graphs I posted are at 27m/s not at 17m/s, which I typed mistakenly. Our tangent speed is at 19m/s. Wheelbase is 1.525m

Here I am attaching images at speed 18, 19, 20 m/s
Cn-Ay at 18.jpgCn-Ay at 19.jpgCn-Ay at 20.jpg

[apoorv]

To plot graphs below 15m/s I had to use relaxation parameter(0.7) in order to converge the solution but this changes my tangent speed to 11m/s. For 8m/s the solution does not converge unless I change my convergence criterion and and high relaxation parameter to around 0.9 (which again changes my graph behaviour).
Here are the imagesCn-Ay at 10.jpgCn-Ay at 12.jpg

[Claude Rouelle]

Labels for beta and delta values please!

[apoorv]

Its same as previous one!

[Claude Rouelle]

So to remember the beta and delta value you use, the reader has to look at the previous post? Who is trying to help who here? Just post these values and labels on your graphs!!

I know I act as a bit as teacher here (or a customer or a design judge for this kind of exercise, that is the same) but just try to be consistent!

Anyway I think something is quite inconsistent in your graphs

1. At 10 m/sec your control is negative, at 12 m/sec positive and at 18 ms/ negative and at 19 m/sec positive again..... I do not know how your calculations were made and I do not know your car, tires, weight distribution, roll stiffness distribution, aerodynamics properties etc...but I have serious doubts about these calculations.

2. At any speed it seems that the yaw moment at the limit (max G) is always zero: no understeer, no oversteer.... Too good to be true I think.

3. What is the relaxation parameter; how do you define it? What is the unit? What is the meaning of it?

4. Same question for your convergence criteria.

Many questions and a few criticisms from me but I have to say that not a lot of guys in this forum went that deep and that quickly. that is encouraging.

However, you are only at the beginning: the goal is not to make a simulation that works: it is to make a simulation that is pretty realistic and useful to design and tune your FS car.

[apoorv]

1. When I plot my Cn-Ay diagram it gives me tangent speed at around 19m/s. When I try to plot the graph below 15m/s the solution does not converge as my yaw rate values become very Large. Therefore to Converge the solution for speeds below 15m/s I use a relaxation parameter to calculate more stable value of yaw rate on each iteration.

w3=w2(1-Pr)+w1*Pr
Pr is relaxation parameter (0.5 for low speeds)
where, w1 = previous yaw rate estimate(initial value)
w2= new yaw rate estimate(after 1st iteration)
w3=more stable yaw rate estimate(value for 2nd iteration)

so when I don't use any relaxation parameter the tangent speed is 19m/s but when I try to plot for speed below 15m/s I have to use Relaxation parameter and it gives me different tangent speed.

2. The weight distribution is very close to 50:50 and same front and rear cornering stiffnesses and no aero.

3. Relaxation parameter is basically a mathematical tool that allows me to estimate value of Yaw rate on each iteration that allows to converge the solution at low speeds. No units just a number.

4. The convergence criteria is that for each iteration the AY values must be within 2% range
i.e. Ay(new)-Ay(old)<2%

[Adam Farabaugh]

I find it odd that you only need +/- 3 degrees of steer and body slip to saturate the tires

Edit:
A "relaxation parameter" as I understand it is just an IIR low pass filter applied to some of your converging values. This helps avoid gradient based (which basically is what we all use to solve problems like this) methods from diverging. It should not affect the actual result that much, just whether or not convergence is reached. Personally I generate all my plots with the same low pass cutoff, and now that I have a good one chosen, I never have to change it.