When I run road diesels I like to run them in multiple unit configuration, not just one loco by itself. Having said that, I have identical locos made by the same manufacturer that don’t run at the same speeds. (e.g. two Life-Like GP7s that run differently or two Athearn RS-3s that run differently)
What I would like to know is if two locos have identical mechanisms, why does one run faster than the other? Is it the motors? The gears? The driveshafts and couplings? Or the way they were put together?
Were the pairs bought at the same time or one at a time? Has each loco in the pair been lubricated the same? Have they been run-in the same amount of time? Wheels clean? That’s about all I can think of.[%-)]
It could be the preciseness of the assembly, of the gears, etc. Maybe one has gears with burrs on it, or even too much lubrication.
Many years ago I got two of the original Atlas RS-11’s, and put them on my pair of modules to test them. All I had was the 8 feet of track to use. Even in this short space, one of the diesels would always pull ahead of the other. Since I didn’t have any way to break them in, I put a push pin at the end of the main lines and ran the models touching the pin, they were running in place and not moving. I did this for about 15 minutes in each direction. After this short break in period, the models then did run at the same speed.
Two automobiles are built in the same factory, same day, same line, same shift. In fact they’re one behind the other. When they come off the line, one out-performs the other. Why? Nobody knows.
I’ve had this same loco question myself. I’ve even bought identical locos, broke them in the same, maintained them the same and one would always run faster than the other, even if by a small margin.
The small difference in running speed probably will not be significant enough to prevent double heading. Double heading may not be advisable if the faster engine still runs faster if it is connected to your train (assuming it can pull it without slipping) than the slower engine if it is running free. If this is the case, the slower engine will not pick up any of the load and the faster engine will be doing all of the work.
Tolerances in the engine are the main reason. If an engine is built on the tight side it will have more internal drag. This is not as common as it once was due to increased Q.C.
Here is an easy way to find out if the motor or gears are causing the speed differential: Swap the motors. If the formerly slow locomotive is now faster, the problem is in the engine. If no change check the slow locomotives gear boxes.
Minute differences in all the driveline components, and differences in motor timing with small DC motors are the reasons. No big deal; you can still run them together.
With Life-Like engines, I’ve noticed a sometimes very great difference in speeds between two engines that I would expect would run the same. If you’re in DCC, it’s pretty easy to adjust the CV’s of one engine to match the other so they’ll run together perfectly. If you’re in DC, not too much you can do except fiddle around with resistors and such trying to slow the faster one down a little.
MFG. TOLERANCES can vary somwhat between engines but the differences are slight - like 10%-5%. These, you can put the faster ones in front - just to keep the couplers from ‘pushing’ and opening.
PRODUCTION CHANGES can change parts in the drives. I HAD 2 kATO SD-45s from 2 different production runs - 2 years apart. Forget it! Going from a 12volt incandescent light to a Diode CV or an LED board can drop the voltage to the motor by several volts, alone.
BEST BET: BUY identical engines in pairs, of same production runs - or - spring for DCC.