Apologies in advance for the long post but I thought I'd do some more comprehensive testing over the weekend before commenting any further and part of that involved unboxing another 48. But before I go on can I just say, man a couple of 48s heading a train looks good. I don't know how much thought goes into aesthetics when designing diesel locos as opposed to cars or some classic planes and wooden runabouts and what-not but 48s have always been a favourite of mine. I'd accept arguments that bulldogs look better but I dunno, maybe I just saw 48s everywhere as a kid. I bought 4 during the Auscision $220 preorder and wish I bought more. I nearly unboxed the other two just to lash up the 4 hauling a wheatie...
But back to the surging thing. Thanks for the input fellas. MJW, I can't remember reading about AM 48s having motor issues but although some may have had problems I'd tend to agree with 2LaGrange that the way it behaves normally on a flat and uphill proves the motor is ok.
Regarding running in, 48158 has had a fair bit of running in and the biggest thing I noticed about the newly unboxed one, 48101, is how slower it was. Slower to start and slower top speed. Not by much, but I assume that will change given a chance to run it in too.
Now some test results. Consist is free running made up of fourteen carriages, mix of lima 12 wheelers, powerline and austrains bs/fs, casula EHO and CR and austrains LFXs and no pizza cutter wheels on the old stuff. It's just an odd heavy mix I've been using to test track reliability but the surging was first noticed on a smaller rake. Downhill grade is 1 in 40 over approx 4 metres with large radius bend near the end.
- The new 48101 doesn't exhibit the surge as much as 158. It's still there but not as bad. It may mimic 158 after it's been run in but that's not possible at the moment due to trackwork. Pffft, typical!
- When I put 158 with 4303 leading which is a bit faster than 158, the surging is noticeable
- 4303 and 101, some surging.
- 4303 heading the rake has no surging.
- If I put faster 42207 with 4303, there's no surging.
- 101 and 158 together almost eliminate the surging.
- No loco showed surging when running light down the grade.
- When surging does occur it coincides as the carriages start to run on into the loco and will often occur repeatedly until the concertina affect from coupler slack and engine/s speeding and slowing down settle after the bottom of the grade.
All locos seem to have similar free movement in the geartrain from backlash. When viewed from above and a stationary loco is pushed forwards and backwards the distance moved is approximately that between sleepers on peco track.
I think it's fair to agree with those commenting on the 48s being a light loco, the backlash in gears and accepting the way it is. Remedies are to run in pairs or more for the heavy loads with out of the box speed matched locos, ie locos of the same type for DC operation, like keep the 48s together, or run a 48 or two on light consists. That's all fine and what I intend to do.
However, there's still a bit of an unanswered mystery to all of this I'd like to answer as I make this mole hill into a mountain. When the carriages run into the loco and the concertina thing happens the speed changes which are visibly and audibly noticeable are like a square wave form. If you can imagine the pitch of the motor the run on will see the shunt cause a rise in RPM, but when it drops it goes below where it eventually settles which is what it was originally set at. The carriages bunch up then the RPM increases, couplers slacken off then they close up again, the RPM drops below set speed and so on. When it settles the RPM returns to where it was set which is higher than the lowest it goes during the surgeing. That's the bit I don't get. I'd mention things like back EMF and filtering on the main board or in line with the motor but I've no idea what I'm talking about there. I still may try direct wiring between wheels and motor to note any difference, or just get on with life, I dunno, we'll see.