Double Heading Locos

[Tdm]

Below is a quote taken from the current Farish 4F thread

"Hi,
I have been double heading on DC for years. The only thing I don't do is match Farish or Dapol with Union Mills locos and you hardly ever need to double head those but when you do the haulage capacity is enormous.
Keith"

I have never tried it - but what is the advantage of double heading - especially steam locomotives - does it really increase the pulling power and reduce slippage when climbing a slope?

I have a figure of 8 track as part of my layout with a considerable slope that starts on a curved section, and some locos have trouble pulling a load up it - in such an instance would double heading help? (I'm purely DC by the way).

[PLD]

We'll it worked on the Prototype!!

In the real world a pair of 4Fs could haul a train roughly equivalent to a solo 7F.

Unfortunately horsepower and equivalence between locos does not scale down to the model, but as a rough guide on the model I would reckon on a pair of 4Fs handling around 1.5 times the train length/weight of a single one...

[CarriageShed]

It certainly does increase the pulling power. I've been experimenting with double heading combinations, and I have a test train of six Farish Pullmans, three of which are heavily weighted with slabs of lead. When one of my Farish 4F 0-6-0s was pulling this train alone up a 1-in 65ish incline it was struggling and slipping all over the place (mostly due to a metal tender that had sticky wheels). When I added a Farish 4-4-0 on the front, the two of them managed to sail up the incline, working in perfect harmony together.

[ScottyStitch]

My understanding of DC double heading is (from earlier OO experiments) that the two motors wplit the voltage and so the train itself will be slower for a given throttle setting. Logic would dictate, half as fast?

[Rabs]

Not quite.  The two motors are connected in parallel, so both see the same voltage.  If your power supply can provide enough current for both then the total electrical power will be doubled (assuming the same motor in both locos).  Unfortunately, that doesn't necessarily lead to a doubling of mechanical pulling power because the two motors will naturally run at slightly different speeds for the same voltage because of slightly different levels of friction in the drive mechanism.  As a result the faster one will spend some of its power trying to drag the slower one up to the same speed.  In practice you can probably expect the pulling power of between 1.5x and 2x if the locos the same model. 

Of course, if the locos aren't the same model then the speed differences can be much larger and the benefit might be substantially less.

[port perran]

I often run double headers but it seems to me that not all locomotives are compatible. I think you have to ensure that one or other isn't significantly pulling or pushing the other.
I find the Dapol pannier and GW prarie (45XX series) make a very nice well balanced combination.

[ScottyStitch]

Not quite.  The two motors are connected in parallel, so both see the same voltage.  If your power supply can provide enough current for both then the total electrical power will be doubled (assuming the same motor in both locos).  Unfortunately, that doesn't necessarily lead to a doubling of mechanical pulling power because the two motors will naturally run at slightly different speeds for the same voltage because of slightly different levels of friction in the drive mechanism.  As a result the faster one will spend some of its power trying to drag the slower one up to the same speed.  In practice you can probably expect the pulling power of between 1.5x and 2x if the locos the same model. 

Of course, if the locos aren't the same model then the speed differences can be much larger and the benefit might be substantially less.

Of course they are in parallel.....

Thanks for the clarification.