jerky starts.

What kind of transformer?

Some transformers don't start to ramp up from 0 volts, they start at 5 volts.  This may be contributing to the "jerkiness"

Ed

What motors?  Be specific.

What kind of power supply? Again, be specific.

Which engines? What manufacturer or importer?

You gave us nothing to discuss.

Ed is correct, there has to be a specific data before you can get pointed info. Not all these locomotive run the same....they range from total crap to precise running jewels.

Bob

I agree with the original poster--nothing ruins the illusion like a jerky start.  It's a good, valid question and there are a LOT of variables...

A permag 12V motor having a high stall current, and without some kind of closed-loop feedback speed control will tend to give a jerky start; it may also stall after starting once the slack of the trailing consist has been stretched, because the increased current draw will tax the power supply and reduce the total voltage in the circuit.  Substituting a motor with 24V winding will start more smoothly and be more resistant to stalling.  But it will make fewer RPMs at the NMRA 12V reference point.  Using a 24-volt control system designed for G-gauge will restore the original top speed and give finer control at all speeds.  The use of a cut-wave or "pulse" power supply may also yield improved operation at low speeds.  The tradeoff is increased motor heating and/or electrical noise.

The gear ratio is important- nearly all of the locos with the so-called "China drive" are capable of more than 100 scale mph, which compromises their ability to maintain slow speeds without stalling.  In the case of steam locos with direct drive to a worm wheel on the driving axle, the worm wheel should be of the largest possible diameter to provide more pull-in torque for the worm.

Any loco with self-locking worm gears may stutter at very slow speeds if there's insufficient torque to overcome the internal friction.  Depending on the gearbox characteristics, it may also "buck" or surge going downhill.  Furthermore, a loco with two independent motors and two locking worm gears won't start until BOTH motors are making enough torque to overcome the static friction, and then the loco will tend to start abruptly.  The performance of *ANY* China-drive loco can be improved simply by removing one motor (whichever one has the "lazier" start characteristic when tested independently.)  Another technique is to wire both motors in series.  This is somewhat equivalent to substituting a 24-volt motor as described above.

Gears should be inspected carefully--burrs, malformed or chipped teeth, bent wheels or axles, misaligned bearings, dirty wheels or poor electrical pickup, pinch points in the gauge, kinks and dirty track will all contribute to stalling or jerking at slow speeds.

One more thing to think about--prototype locos are free-rolling.  Three men can push a loco on level track, and without brakes it would go downhill like a roller coaster!  Unfortunately physics don't scale down.  The ratio of total static friction (in the motor, gearbox, and trailing car axles) to rolling friction once underway is much higher in a model compared to the prototype.  Think about a real freight train starting on a cold day (especially before the widespread use of roller bearing trucks on freight cars.)  There's a lot of static friction in the journals, but even after that static friction has been overcome, the train won't suddenly accelerate due to its enormous mass.  Even with heavily weighted cars, there's a lot less mass to accelerate in a model (lower inertia), and this reduced mass also translates to relatively insignificant momentum compared to the prototype.

Closed loop speed control (which use an optical tach or back-EMF) helps to maintain a steady speed on curves and grades, and prevent stalling as the slack is being taken out.  But in my experience these electronic systems can't duplicate the delicious hysteresis, the precious "moments"  when a prototype loco strains from a dead stop to one... two... three mph.  Some experienced loco builders will disagree, but I'm a fan of heavy flywheels, non-self locking gears, and generally a lot of mass.  One of these days I would like to fabricate a Hobbytown-style clutch drive scaled up for O-gauge duty.  Taken together, all of these things help preserve the illusion, and make our trains more forgiving and fun to operate!  -Ted

Ted,

That is an interesting post.

Thanks.

Rick

http://www.youtube.com/watch?v=WsDs_Mi6YFo

http://www.youtube.com/watch?v=mhmaVf8dnyg

I tried to find a few youtube reviews that dem. the models actions that Iam talking about but most reviews don't show the starts from a stop or vs. ver. The camera catches action as the engine is moving slowly and not from a start. Reviews trying to hide something?

But I notice it on my trial runs with my MTH Zcontroller which I bought to just run until I really decide where to put the bucks and direction in powering my ever increasing fleet. Granted a lot of this jerkyness one can see in the HO line, but my MTH has the flaws too. I want to eliminate this as alot of ones model viewing is done at slow start up  speeds and stopping. Stopping when the power falls and there, I guess, is not enough flywheel to keepp the action smooth to a stop but a quick fall off and the cars jumble up behind. I haven't run my OMIs in DC yet as my transformer is DSC. and I bought my engines before MTH produced the proto 3. Proto 3 is where I may get too if MTH comes up with a board add on so one can run DCC as well. Bad paying near $500 and now have to reboot ones engines. My MTH engines are scale UP ES44ac,8-40CW, Sd70ace and all PS2.  

I want to thank you for adding any information to my inquiry.

Phill

Lets keep it simple .... try cleaning the track.

J

Two very big issues:

First ALL two motor locos I have seen tend be jerky.   The two motors never quite start at the exact same time.   The camera tends to emphasize this more, might have something to do with frame to frame making in the motion. 

Second, any motor with worm directly on axle gear tends to have low torque at low speeds which translates to jerky starts unless it is a very large high torque motor.   The solution is reduction gearing which has been used in two rail scale steamers since the Max Grey days.    The motor worm goes into a gear box which had one or two reduction gears between the worm and the axle.   This allows the motor to turn at a higher RPM for every speed which puts in a better torque range.

The top speeds mentioned above describe another aspect of the situation.   To run at such high speeds, requires high gearing which is not conducive to low speeds and smooth starts and stops.

Think about your car, you have a transmission with more and more gear ratios.  It used to be 3, then 4 and I hear mfg talking about 6 and 8 now.    You have a low gear for starting where the motor can rev up and use its torque, then you have higher gears for driving at each speed to allow the motor to stay in a good range and still get the speed.   

As Ted said," Another technique is to wire both motors in series."

Regards,

Ed