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First, you need to do a different SMS file for every wheel size because the timing of the flat spot noises depends on the circumference of the wheels.

So the process I use is like this:

1.) Create the clips. Count the number of flat spot hits in the clip and divide by the clip length. This gives the number of revolutions per second. Multiply by 60 to get the revolutions per minute.

2.) Figure out what speed you want to be the maximum. Use this website to calculate the RPM for that speed for the wheel size you are using. Common US sizes are 33 and 36 inches. I did SMS files for 28 inches, 33 inches, 36 inches, and 38 inches, and had to calculate the RPM for a given top speed for each one.

3.) Take that calculated RPM and divide it by the calculated RPM of your clip from step 1. Multiply by the sample rate of the audio sample. This is the frequency value for the top speed. Make sure your speed is converted to m/s for the frequency curve, because that's what OR uses.

4.) You can't divide by zero, so for the minimum speed in the frequency curve, I use 1 m/s. Divide 1 by the maximum speed you set and multiply by the frequency you calculated in step 3. This is the frequency at 1 m/s.

5.) The car needs to be able to move forward and backwards, so the resulting frequency curve in this sample needs to have both positive and negative values.

You'd also need to build a volume curve where the volume is relatively high at 29.0576 m/s and zero at 1 m/s.

So the process I use is like this:

1.) Create the clips. Count the number of flat spot hits in the clip and divide by the clip length. This gives the number of revolutions per second. Multiply by 60 to get the revolutions per minute.

2.) Figure out what speed you want to be the maximum. Use this website to calculate the RPM for that speed for the wheel size you are using. Common US sizes are 33 and 36 inches. I did SMS files for 28 inches, 33 inches, 36 inches, and 38 inches, and had to calculate the RPM for a given top speed for each one.

3.) Take that calculated RPM and divide it by the calculated RPM of your clip from step 1. Multiply by the sample rate of the audio sample. This is the frequency value for the top speed. Make sure your speed is converted to m/s for the frequency curve, because that's what OR uses.

4.) You can't divide by zero, so for the minimum speed in the frequency curve, I use 1 m/s. Divide 1 by the maximum speed you set and multiply by the frequency you calculated in step 3. This is the frequency at 1 m/s.

5.) The car needs to be able to move forward and backwards, so the resulting frequency curve in this sample needs to have both positive and negative values.

You'd also need to build a volume curve where the volume is relatively high at 29.0576 m/s and zero at 1 m/s.