[Traindude]

It's a fact of life that larger wheels rotate slower than smaller wheels. At first, I thought that this meant changing any speed-related parameters for volume and frequency curves for things like flange squeal, brake squeal and steam chuff sounds.

So, with that in mind, I had to calculate the difference in size:

D/W=F

D = The diameter of the wheel that I am reconfiguring the .sms file for.
W = The wheel diameter that the .sms file originally took into account.
F = Scaling factor

Example:

Intended diameter: 38 Inches, Original Diameter: 40 Inches

38/40 = 0.95

Thus, all speed-related parameters in the existing frequency and volume curves have to be multiplied by 0.95 to get the new values for the new wheel diameter. This also means that steam locomotives would need two or more flange and brake squeal streams, one for the driving wheels and one or more for the idler (non-powered leading/trailing truck/bogie) wheels.

However, when I thought about it more, I realized that even though the wheels of different diameters rotate at different speeds, the treads of the wheels are all moving at the same speed and therefore there is no need for scaling the volume and frequency curve speed variables in such a manner.

However, in the case of steam chuffs, this type of scaling would definitely need to be done for locomotives with different driving wheel diameters to sound "in sync", right?

[gpz]

Traindude, on 23 April 2023 - 04:44 PM, said:

40/38 = 0.95

I'm pretty sure this equation is wrong. ;-)

[Traindude]

gpz, on 24 April 2023 - 08:07 AM, said:

I'm pretty sure this equation is wrong. ;-)

38/40 = 0.95

Corrected.

[copperpen]

As the smaller wheel would rotate faster then the equation should be 40/38 = 1.052

[Traindude]

copperpen, on 25 April 2023 - 11:24 AM, said:

As the smaller wheel would rotate faster then the equation should be 40/38 = 1.052

Let's say the speed value in a volume or frequency curve is 30 meters per second. So, by using this equation, the new value would be 30*1.052 = 31.56 meters per second. So, it would appear that that this equation would cause the wheel to sound like it's rotating slower, not faster, since the trigger would be happening at a higher vehicle speed ("Speed" Variable).

It seems counterintuitive, but if we wanna make the wheel sound like it's rotating faster, we need to reduce the vehicle speed values. Thus, we would use 38/40 = 0.95, and then 30 m/s * 0.95 = 28.5 m/s.

[Weter]

Wheels, belonging the same vehicle, will have same surface of rolling speed, but different rotation speed.

[gpz]

Variable1 is proportional to the vehicle travelling speed, which equals to the circumferential speed of ALL wheels, regardless of the diameter. A wheel's angular speed (rotation speed) can be calculated by dividing this by Dπ.

[Traindude]

Weter, on 25 April 2023 - 01:46 PM, said:

Wheels, belonging the same vehicle, will have same surface of rolling speed, but different rotation speed.

Exactly! I knew that since the treads of the wheels all move at the same speed, regardless of the wheel diameter, then flange and brake squeal parameters DON'T need scaling.

On the other hand, scaling is still necessary for adjusting chuff rates on steam engines ("Speed" or "Variable1", whichever may apply to your particular *.sms file). To give another example:

Let's say you have a steam locomotive *.sms file, originally configured for 69-inch diameter driving wheels, and you want to adapt it to 80-inch diameter driving wheels. The formula would be:

80/69 = 1.15942029

After rounding, we've now defined 1.16 as our scaling factor, and it's all a matter of taking the Variable1 values and multiplying them by 1.16. For example, if, at one point, the Variable1 value in the original *.sms is 30, then:

30 * 1.16 = 34.8

So any time the number 30 appears as a Variable1 value, we replace it with 34.8.

It should be noted that the scaling factor should be greater than 1 if the wheels are to be bigger, and less than 1 if they are to be smaller, than the original diameter that the *.sms was designed for.