What gear ratio for DCC control?

[34theletterbetweenB&D]

Many of us will have been used to gear ratio selection for DC control, to deliver a speed range performance from a smooth start to a desired maximum speed at 12V.

 

However, it is somewhat different with DCC. All that really matters is that the reduction ratio is sufficient for enough torque to start the maximum load smoothly. What has become apparent to me is that on small wheels something in the range of 5 - 10:1 is enough if a large motor can be used, up to 15:1 for a smaller motor. On large wheeled (i.e.mixed traffic and faster steam) traction 20 - 25:1 is sufficient. So what if the thing gallops away at a scale 300mph at 12V? Set CV5 to a third or quarter of full range as required to trim maximum speed to a scale value.

 

The advantage is that a lower ratio gear train is typically easier to assemble and quieter.

[Kenton]

Isn't there more to this than simply the gearing? The motor has a more significant impact? Also is not "trimming" the maximum output of the motor at 12v (using CV5) to compensate for poor (too low) gear ratio simply failing to maximise the ideal performance of the motor?

 

I think the approach should be matching gear ratio and motor rev to match a prototypical speed at maximum/optimum output. This usually means a much higher gear ratio than 40:1 and certainly higher than 5 - 10:1.

 

I do not see any difference between DC and DCC in the mechanics - just because DCC gives you the ability to use CV5 to "trim" to me it seems sub-optimal and not ideal.

 

But have to agree with your last statement that the lower the ratio the easier it is to assemble. Perhaps sometimes an overriding factor. As for quieter, I believe that is more a mark of poor assembly/design of the box, rather than the ratio.

[Gordon H]

Many of us will have been used to gear ratio selection for DC control, to deliver a speed range performance from a smooth start to a desired maximum speed at 12V.

 

However, it is somewhat different with DCC. All that really matters is that the reduction ratio is sufficient for enough torque to start the maximum load smoothly. What has become apparent to me is that on small wheels something in the range of 5 - 10:1 is enough if a large motor can be used, up to 15:1 for a smaller motor. On large wheeled (i.e.mixed traffic and faster steam) traction 20 - 25:1 is sufficient. So what if the thing gallops away at a scale 300mph at 12V? Set CV5 to a third or quarter of full range as required to trim maximum speed to a scale value.

 

The advantage is that a lower ratio gear train is typically easier to assemble and quieter.

 

The main disadvantage of this approach is the loss of resolution in speed steps it creates - but that may not be much of an issue to you in practice.

For example, if your loco runs 4 times as fast as it needs to, you can restrict its maximum speed via CV5 to a quarter of this - but you will have reduced the number of available speed steps accordingly, from (say) 128 down to 32.

This might not seem too bad, but the same would not apply for a 28 step decoder - and would probably be unusable with a 14 step decoder.

[34theletterbetweenB&D]

That doesn't happen in my experience. The full speed step range automatically remaps onto the motor output range from between whatever CV2 and CV5 values are set.

 

I presume there is some limit to resolution, but not run into it with a CV5 set at 70 (from 255) which is run in 28 step speed mode, more than good enough control for an 08.

[Debs.]

Mechanical advantage and torque must also be part of a successful drivetrain`s equation, not just control-resolution or maximum speed.....I`d rather have a drivetrain geared to provide low-range torque, upto a scale/appropriate maximum-speed coinciding with the upper working speed of the motor.....then the DCC aspect can truly work it`s magic.

[Gruffalo]

The High Level website provides a calculator in which you can enter Motor RPM, Gear Ratio and Driving Wheel Diameter with the theoretical maximum speed being displayed for you. It seems that the 6'8 1/2" dia wheels (large passenger express steam locos) with a 40:1 reduction and 15,000 rpm (Mashima 1020) will give a scale speed of 90mph.

 

In days of old when using Romford gears with Bulldog motors, I used the 60:1 ratio to get what appeared to be a sensible speed. After all, how many of us have a couple of miles of track over which our motive power can achieve its full potential and then more room for maintaining such maxima? Personally, I want to see good traction on 1:50 gradients, steady pulling away from a standing start without much (if any) slip and probably no more than a scale 30mph through station approaches to be, in my eyes, credible. That leads me towards a greater rather than lesser gear reduction, 1:54 is I think the simplest maximum available from High Level's Roadrunner series which equates to a 67mph max with 15,000rpm input whilst the Loadhauler would give an 80:1 option and that equates to 45mph for the same drivers.

 

I am tempted to go down that latter route but I would like to know if others have positive experiences and why so many people mention the High Level range. 

[Gordon A]

I would use the same gear ratios for DCC as I use for DC. My usual range is 50:1 or 60:1 for most locos.

 

Some of my small industrials have in excess of 100:1.

 

The gear ratio should be what is appropiate for the motor and wheel size combination to give the performance that you require.

The job of the DCC chip is to enable you to run multiple locos without section switching, and to have lights and sound if required.

The DCC chip can fine tune the loco's performance such as the rate of acceleration and top speed.

 

Gordon A

Bristol

[Gordon H]

That doesn't happen in my experience. The full speed step range automatically remaps onto the motor output range from between whatever CV2 and CV5 values are set.

 

I presume there is some limit to resolution, but not run into it with a CV5 set at 70 (from 255) which is run in 28 step speed mode, more than good enough control for an 08.

 

Interesting. Changing CV5 is not something I have experimented with much myself as I have had no real need to do so.

I was thinking more in practical terms of how decoders typically produce PWM drive using standard 8 bit microcontroller timer facilities. To be able to map a lower maximum duty cycle with the same number of speed steps whilst maintaining the same basic PWM period (or frequency) would likely require a 16 bit PWM facility and some extra arithmetic to work out the step values in between.

[The Bigbee Line]

An interesting thread, but I got a bit lost on some of the hi-tec lingo.  What is the heating effect when comparing high and low gear ratios.  I ask because (dare I say it) most models never get a good run with a load on the open road.  I've used 2 x Kato diesels at a scale 20 mph (roughly) and after a while you can feel the motors are warming up.  I did once consider putting fans on the back of the flywheels to create a slight air flow?

 

Serious comments appreciated

[Gruffalo]

Low levels of gear reduction would mean the motor may not run at higher speeds so the integral cooling ducts would perhaps be less effective. I suspect therefore that the more reduction you have in the gear train, the cooler the motor will run as its inbuilt cooling will be more effective. Motors should be rated for a certain period of running at full speed but we do not know what characteristics apply when they are used at different parts of their speed curve. As I have stated, my requirements are for a locomotive to draw away from a station and up an incline before reaching a fiddle arrangement rather than endless circuits at high speeds. The likelihood of a motor running for more than 5 minutes without a significant break in my case is pretty rare, except maybe when shunting.

 

Nice thought about fans though Ernie. How about something produced in FUD? That could be a simple answer, as long as the balance was pretty good. It would move air, not be too heavy (probably less than the weight of the current flywheels and shouldn't be too expensive to produce in a range of diameters / shaft sizes. It could even be made to sit over a small brass flywheel.

[Gruffalo]

Well, after the various contributions on this and the Resurrecting Portescaps threads, I have taken the plunge and spoken to Chris at High Level. New boxes for my 2251 and Hall are on their way (the Comet 2251 isn't as smooth as I think it should be). Any other suggestions for a chassis for the Hall - an SE Finecast kit that I do not wish to build with SEF's own white-metal chassis? I have noted that Comet do an etched one, anyone else that can be recommended?