I studied how this works at length, but the post was lost when the forum was upgraded. Anyway, I saved the original. so here's the "reprint":
I did some experimenting with the Adheasion parameter, and found some interesting results. First of all, the fourth value is not used at all, as mentioned in the tech docs. The third value is adhesion while sanding, according to the tech docs, but this is only partially true. It also turns out to be a multiplier for the first two values, which are adhesion under slipping and normal conditions, respectively. All three values can be altered simultaneously by changing the NumWheels parameter, which is really just a divider. The actual divisor used by the sim is NumWheels/2, so setting NumWheels equal to 2 means that the actual values in the Adheasion parameter are used by the sim.
For the purposes of the experiment I used the HHP locomotive with the MaxPower, MaxForce and MaxContinuousForce values set very high so that the tractive effort would be limited by the Adheasion parameter and NumWheels only. I also set NumWheels equal to 2 to effectively remove it's influence. When the third value of the Adheasion parameter is equal to 2(the default value), the maximum tractive effort equals the second value times the locomotive weight. The slipping tractive effort equals the first value times the locomotive weight. However, the tractive effort while sanding is 0.95 times the locomotive weight, which is clearly wrong. Unfortunately, reducing the third value also reduces the first two values by a proportional amount, so the third value is also effectively a multiplier for the first two values, and the value of the multiplier is equal to one-half of the third value. The net results of what I discovered are as follows: Tractive Effort(slipping) = {(first value)*(third value/2)*(locomotive weight)}/(NumWheels/2) = (first value)*(third value)*(locomotive weight)/(NumWheels) Tractive Effort(normal) = {(second value)*(third value/2)*(locomotive weight)}/(NumWheels/2) = (second value)*(third value)*(locomotive weight)/(NumWheels) Tractive Effort(sanding) = 0.95*(third value/2)*(locomotive weight)}/(NumWheels/2) = 0.95*(third value)*(locomotive weight)/(NumWheels) I also discovered that these same values are used while braking, but for the locomotives only. NumWheels and Adheasion have no effect whatsoever on the braking of wagons. The only ways to control the braking of wagons are to adjust the MaxBrakeForce and BrakeCylinderPressureForMaxBrakeBrakeForce parameters. The sim uses multipliers to account for reduced adhesion under rain and snow conditions(the values are 0.9 and 0.55, respectively, and can be adjusted in the default.wag file). These multipliers affect the tractive effort(and braking for locomotives) under slipping and normals conditions only. The tractive effort while sanding remains the same even in bad weather, and interestingly, you can also increase the maximum brake force exerted by the locomotive by sanding while braking. Once again, there is no change whatsoever in the braking behavior of wagons under bad weather conditions. This is dependent only upon the MaxBrakeForce and BrakeCylinderPressureForMaxBrakeBrakeForce parameters, and remains the same in all types of weather. The practical results of all this is that you can set both normal and sanding adhesion to reasonable values since the sim uses the NumWheels divider for all three Adheasion values. In the case of the HHP, I used Adheasion ( 0.35 0.8 2 0 ) and NumWheels( 5 ). According to my equations above: Tractive Effort(slipping) = 0.35*2*(locomotive weight)/5 = 0.14*(locomotive weight) Tractive Effort(normal) = 0.8*2*(locomotive weight)/5 = 0.32*(locomotive weight) Tractive Effort(sanding) = 0.95*2*(locomotive weight)/5 = 0.38*(locomotive weight) These all seem to be reasonable values for a locomotive equipped with an electronic antislip system. For a regular diesel locomotive without an antislip system I might use Adheasion ( 0.56 0.8 2 0 ) and NumWheels( 8 ), to give sliding, normal, and sanding coefficients of 0.14, 0.20, and 0.24, respectively. I'm not 100% sure what the sliding coefficient should be, but I think values between 0.1 and 0.15 are realistic for normal conditions. For a steam locomotive you can use the same value for Adheasion(to give the same ratios of sliding, normal, and sanding adhesion), but simply adjust the NumWheels parameter up to account for the reduced percentage of the weight on the drivers. The same can be done for diesel locomotives with a A-1-A A-1-A wheel arrangement(three axle trucks with the middle wheel unpowered). In that case, you would use NumWheels( 12 ) and the adhesion values would all be reduced by one-third which would be realistic since only two-thirds of the locomotive weight is on the driving axles. In all cases you must set the either the MaxForce or MaxContinuousForce parameter(or both) to be at least equal to the tractive effort while sanding, provided this is realistic for the particular locomotive, in order to have the tractive effort limited by the Adheasion and NumWheels parameter. If you set MaxForce and MaxContinuousForce to something smaller, then your maximum tractive effort will be limited to whatever those parameters are set for(at least under normal conditions). The Adheasion and NumWheels parameters might still limit your tractive effort further under rain and especially snow conditions. Please note that MaxForce is a value for maximum tractive effort that just seems to be used by the locomotive between about 13 and 26 mph in place of MaxContinuousForce, and at no other time. It can be more, less, or equal to MaxContinuousForce, so you can use it to further modify the tractive effort curve. For example, some EMUs experience a roll-off in tractive effort at 15 or 20 mph and then an increase a few mph later as the motors go from a series to a parallel arrangement(even some electronically controlled EMUs exhibit this behavior), so making MaxForce less than MaxContinuousForce can simulate this.
I couldn't have said it better myself. BTW, in the example you're describing you may be past the point where power is limited by tractive effort, so that would be why sanding has no effect. On most modern freight diesels this is under 15 mph. Electric locomotives, especially passenger, may be limited by tractive effort even at 60 mph due to their (relatively) low weights and high power output. In both cases, the adheasion parameter needs to be set up correctly or sanding will have no effect.
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