[Genma Saotome]

Q: Why are my results in calculating Davis A, B, and C so wildly different than what FCalc does?


Some background:

I recently read Peter Newall’s excellent write u p on the history of the Davis Formulas. First Class work. In it he recommends steam era rolling stock should use the original formula and not one of the more modern variants. This is:

R = A + BV + CDV^2

Where
A = 1.3 + (29/axle weight) <---US tons per axle
B = 0.045 <---a constant for freight cars
CD = (0.0005 * cross section area in sq ft) / (axles * axle weight)

The unit of measure for R is foot pounds per ton.

Here are the car specifications:
Gross weight in US tons: 23.25
Cross section in sq ft: 135.0
Nbr of Axles: 4
US Tons/Axle: 5.8125

Push those values thru the formulas returns:

• A = 6.2892
  
• B = 0.045
  
• C = 0.0029

When I put the data for the same car into FCalc I see these results:

• A: 664.15
  
• B = 11.4757
  
• C – 1.502394

WAAAAAY different. Hmmm… ok, perhaps I should multiple my values for A, B, and C by gross weight in US tons because of the unit of measure for R… and that gives:

• A = 146.23
  
• B = 1.05
  
• C = 0.068

Still WAAAAY different.

So what is wrong here?

[dajones]

I think the FCalc units are different. Newtons and meters per second instead of pounds and mph.

Doug

[Genma Saotome]

dajones, on 28 April 2018 - 03:38 PM, said:

I think the FCalc units are different. Newtons and meters per second instead of pounds and mph.

Doug

Hmmm. to convert the units to newtons is only a 30% increase... not enough to make much difference.

I have the spreadsheet that Joe used while developing Fcalc2. His formula for A is:
1.3*(29/axle weight) whereas Davis used 1.3+(29/axle weight).

On converting the action there was little difference in the result so that's not it either.


I guess what I should be asking for here is for someone to take the numbers I used and to plug them into the formulas I types to see if I made a mistake somewhere.

[steamer_ctn]

Hi Dave,

Genma Saotome, on 28 April 2018 - 11:38 AM, said:

WAAAAAY different. Hmmm… ok, perhaps I should multiple my values for A, B, and C by gross weight in US tons because of the unit of measure for R… and that gives:

• A = 146.23
• B = 1.05
• C = 0.068

This is the correct answer in US units.

R is the resistance for this wagon type, and usually it is quoted in lbs per ton wagon weight, ie as you have surmised it thus needs to be multiplied by the weight of the wagon.

FCalc can also get this set of values (see screenshot).

It thus becomes imperative to understand which units are being used, to get the input and output units of FCalc, and thus the input units into OR as the correct values. As your calculations show, an incorrect UoM put into OR can have a dramatic impact on how the train performs.

This page also has some information might be of assistance.

Attached thumbnail(s)

• 

[engmod]

Hi Peter,

I have fcalc.zip and I don't show the davis figures like yours?

Your page link points to "ctn.local" and therefore doesn't resolve from here.

[steamer_ctn]

engmod, on 28 April 2018 - 10:02 PM, said:

I have fcalc.zip and I don't show the davis figures like yours?

What units have you selected to display in FCalc? (See UoM under link below)

engmod, on 28 April 2018 - 10:02 PM, said:

Your page link points to "ctn.local" and therefore doesn't resolve from here.

Oops, sorry about that.

Try this.

Thanks

[espee]

After calculate, click View and select Davis Coefficients, should get the above screen.

[Hobo]

Where can one download " FCalc2 " ? It is not available at Steam4me .

EDIT !

The regular FCalc download is actually FCalc2 and is at TS .

[Genma Saotome]

Alright, I see where I made my mistake -- I did not notice the choice between metric and US standard units that's available in the File menu (why there and not under Edit?). Establishing the correct baseline and I now see the same results comparing spreadsheet to program.

Thank you!
=====================

As for why I was doing this, it relates to my ongoing exploration of how best to use .inc files -- in this case how to simplify t he creation of multiple loaded .wags for a given model where the variable is the lading -- it's weight specifically.

Here's the spreadsheet (click on image to enlarge for reading):


I'm using a simple vlookup function to retrieve a single lading name and weight from a list of 260 commodity classes used byu the US Interstate commerce Commission. All ladings were classified and all Class 1 railroads were required to report what they carried on a quarterly basis. In this case I'm using the average tons per load reported by the Western Pacific in 1950.

The spreadsheet allows me to pick a commodity class which in turn shoves the tonnage into the Davis Formula. As I've already record the other parameters (in blue) that job is done. The resulting .inc file will hold only those parameters specifically related to the weight of a loaded car. The net effect will be I craft a .wag for an .empty car, craft a number of .inc files for various ladings it might carry, and then in copies of the empty car .wag add a single line of "Include ( {path, if necessary}\\{model name}_[lading_name}.inc". Done.

I need to fiddle a bit more with the spreadsheet to deal with the other weight related parameters as well as create a place where the resulting data can be cleaning copied and pasted into the .inc file but all that is rather trivial.