The recent investigation examining the impact of the motor on engine performance including grade & train length has raised several questions. The impact of the wheel material and diameter is the focus of this discussion.
In conjunction with a three engine/ five motor study, fifteen other engines are being prepared to be sold. Motors from my stock of fifty are being installed as an upgrade. These engines are being tested in DC without any control electronics and with a DCC module.
One of these engine/motor combinations blew all expectations for performance and train length. The engine is an Athearn BB Pennsylvania PA-1 with a Mashima 1833 can motor. The trucks are if the older outside frame & metal side frame variety.
Clearly thinking that this result was from the motor, it seemed natural that more focus should be placed on that motor. So it was added to the larger three engine five motor test series. This motor was not previously thought to be a good performer.
The first step was to test the motor on a BN U30B engine, one of the three in the series. This engine has a four axle drive system. The trucks are the later inside frame configuration. The wheels have been upgraded to the 42 inch Athearn NS material.
Realizing these differences existed between the new engine and the engine that showed the the spectacular pulling capability, the motor was thought to be the largest factor, so the testing proceeded expecting to see a similar result.
it was not to be, as shown below:
This was startling. There are four major differences:
- Engine weight,
- Truck frame design, outside vs inside
- Number of drive axles, 6 vs 4
- Wheel material, 42 in stock BB Sintered metal vs 42 in Ath NS
This obviously could be a result if all of these. Engine weight is known to have a contribution, but much less than shown here for the weight difference. The frame design and the number of axles require more change than a simple test, so they will be looked at later.
The wheel material along with diameter are something that can be easily examined to see the impact.
For this examination, the following wheel materials & diameters were tested:
- 42 in Ath NS- base for engine, Athearn replacement wheels
- 40 in NWSL SS- stainless steel from Northwest Short Lines
- 42 in stock BB wheels- standard Athearn blue box sintered metal wheels
- 40 in stock P2K wheels
- 42 in NWSL SS wheels
the results of these tests are shown below
These results certainly do no clear up the question. The measured draw bar force did vary by a significant percentage, the best being fifty percent better than the poorest. However the poorest was the 42 in stock BB which was in place for the exceptional engine. Interestingly, the next poorest was the base 42 in Ath NS wheels. The best result came from the 42 in NWSL SS wheels. Interestingly, the second best was the stock P2K wheels.
These results were verified by running three sets of 42 in Stock BB wheels, the results were close with the the best shown. The 40 in NWSL & the 42 in Ath NS were run twice with nearly the same result.
Interestingly, like the draw bar force, the 12 volt velocity also shows more variation than would be expected. The inference is that the wheels are impacting the voltage at the motor. The trend of velocity does not follow the same order as the draw bar force
The results of running the wheel combinations at 2.5% grade are shown above. Here the trend follows the draw bar force. The number or cars range varied from 8 cars to 19 cars. The 42 in Ath NS configuration was tested at grades up to 11 percent. This is shown as the green line. The average of the recent grade & train length tests is shown in black.
the previous results are recast in the above chart. Here the weight of the Maximum cars pulled is normalized by the engine weight. While the wheel comparison relationships do not change, their relationship to the average is better.
Finally, the impact on the performance coefficient is shown above. This shows that there is more than unproved pulling capacity going on in these tests. The best engine is six times better than the poorest engine.
While this does not answer the question about the differences between the two engines with the same motor, it does highlight that wheel material and diameter can have a significant impact on both the train length capacity and the overall performance of these model engines.
another factor that may be coming into play is the roughness of the wheel surface. That is likely the reason the NWSL products do so well. Need to figure out a way to measure this parameter.
In doing more research on this matter I came across this:
https://en.wikipedia.org/wiki/Frictional_contact_mechanics
Excellent work indicating the changes in contact area due to deformation in the wheel impact the amount of force created, both prior to wheel slip and after after slip occurs. I suspect that temperature also comes into play here. The problem is much more complex than a simple 1D friction equation would indicate.
