Doubleheading, why does it work?

Hopefully you selected the proper tach tape for the size of the flywheel on the motor.  Using the wrong tape will not help with pulling power.  It is not a matter of the 'best' tape but the proper one.  

Thanks for not using the 'L' word!!!!

Pennsydave. Sorry, poor word choice. I meant prettiest, but should have written, the closest tape to the measurement. It was a while back, and I don't remember the specific diameter, but I don't think it was a perfect match.

AMCDave

I beg your pardon. Some of my closest friends run Lionel trains and I will use the word proudly. Why, if I had a friend name Lionel, I'd buy him a Lionel train. I even have Lionel trains. Or did you mean *ash-up? I can't use that word. It gets censored.

Did you lube the cars?  The prewar and post war cars need to be lubed to roll easily.  I have a 2353 which will pull more cars than will stay on O-31 track.  It will pull more than 20 cars, a mix of post war and MPC cars.

Your analogy of you pulling the rope with one hand while another person grabs your other hand to help pull is a good analogy of double heading.  The other person's force that he applies to your hand is transmitted directly through your body to the rope.  If you stop pulling, the other guy's pull will be transmitted to the rope.  Suppose you were on ice, and the other guy was off the ice on dry pavement.  Do you think he could pull you off the ice along with whatever load you were pulling?

For what it's worth, I have an MTH Berk with the original Protosound. Running conventional. It'll easily pull 23 or 24 Lionel die-cast hoppers (most of them the 2-bay offsets, but a few 3-bays in the mix), on O-72 curves. Only other power draw is the lighted caboose.

I'd second the lubing of cars.

David

Marty,

     I think AMCDave meant "Lash Up" not Lionel as the L word.

JohnB

Marty R, it's not that the word "lash-up" gets censored but it's like fingernails on a chalk board to some.  Which I think is LOL.

Rick

Am I missing the point, or, has NO ONE actually addressed Marty's original question?  I don't know the answer, does anyone?

OK.....why does multiple locomotives work??? Horsepower.

Think of it this way. You have a house trailer that weighs 3000 pounds. You might move it with a Honda Civic....but not far or long.

Same trailer with a diesel powered heavy duty pick-up. No problem because of the HP and torque the motor provides. 

Two locomotives coupled pull as  one...or like having a bigger engine.

If he is inquiring about REAL locomotives, then he probably should be inquiring over on the Real Trains Forum.

If the two engines are matched correctly for power then double heading should be no problem.

I had to add a set of motors to my Williams F-7 ABA set to pull the six car passenger set at a speed above snail pace. After adding the extra motors I needed to add two more passenger cars to slow the engines down just a little.

Sometimes double heading engines is a hit & miss deal. Do some work and see what matches up well. Most MTH engines with PS-2 should run decently when double headed. Two TMCC engines should work together as well.

Problems usually happen when you mix & match different brands of engines, just like in the real world of trains.

Lee Fritz

Ah, vectors!

Using your example, first person pulls on rope with 50 lbs of force. Second, person pulls on first person with 40 lbs. As long as both keep pulling (and the first person keeps all body parts intact and attached firmly to the rope), the rope has 50 lbs plus 40 lbs = 90 lbs exerted on it.

Same as if you stand in a convertible doing 50 mph and throw a baseball forward at 40 mph, someone on the ground trying to catch it sees a 90 mph baseball coming toward him.

Jim

I have doubleheaded my Lionel Milwaukee Road S-3s and I did it the real way. i.e. two seperate engines, two seperate TMCC controls. You get a better feel for having two locomotive engineers at their respective throttles of their locomotives. When you start out you can actually have out-of-beat exhausts until you get them in sinc, just like a couple of hoggers doubleheading. Imagine how much fun it would be to put in a third engine in mid-train or as a rear helper with a third tmcc. Think about it. To heck with diesels.

It is very difficult to get a pusher engine to help from the rear on model trains.  They either push the cars off the track to the outside of a curve, or stringline them off the track to the inside of a curve.  I watched the try to get it right for a half hour on Tony Lash's layout with a long coal drag..  Didn't work.

Conversely, I watched a video of a Forum member pull a long 100+ car freight with 8 engines at the head.

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Dennis

Thanks folks, this is a great thread. I will play, in the evenings, with cleaning and lubing a few wheel sets. I get all the examples, especially that using a pusher car can't work. Also, about moving a camper with a Civic. It's just hard to picture transferring the energy through the couplers.

Without letting the science get in the way here, it work because it looks great, Especially with steam engines

The ball would be traveling initially at 90 mph until the drag from the air slowed it down.  

Credentials:  BS Math 1963, MSEE 1965, and 50 years of modeling and analyzing dynamic systems on analog and digital computers.  

Does Galileo count as a physics major?

Thank you Jim. You brought me right back to High school physics which is what you taught as I recall.

I loved physics class. Not because I understood it but because it was like a magic show to me.

I enjoyed your article in OGR run 275, January 2015 by the way!

TJ

I wrote to Myth Busters from the Discovery channel to test this theory out about the baseball...

by the way..hello everyone...I finally retired from the Military and look forward to get more involved as family permits..

Marty,

it it is the same as adding horses to pull a wagon. The more you add the more weight you can pull.

JohnB

Cdtluna,

Thank you for your service. Enjoy retirement.

JohnB

I guess old physics teachers just can't quit!

The example of the convertible and baseball is accurate. To be precise, I should say a 90 mph baseball relative to the ground.

The Mythbusters took on the problem of whether the damage caused by two autos each travelling at 50 mph toward each other head-on was really the same a single auto hitting a brickwall at 100 mph.

They found that the effect wasn't really quite as much as the single car at 100 mph. But, that was due to the way materials deform, etc. The relative speed was still 100 mph between the two - just didn't produce the damage.

Now - back to trains!

Jim

Sorry Jim but that is wrong.  Two cars traveling toward each other colliding at 50mph is the same as one automobile hitting a wall at 50mph.  They don't add together to make the impact 100 mph.

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Dennis

Interesting we brought Galileo into this thread.  I just finished reading the book: "Galileo, Watcher of the Skies".  An interesting but difficult read.

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Dennis

Ok, In conventional, one engine(& tender if applicable) is faster than the other at no load speed , even if only slightly. I lead with the engine that jumps first from a dead stop. It stops it from pushing on the lead engine or tender causing jerks till they sync.

But what if the ball was in a box with a cat? 

4-4-0+4-4-0=ƒµ∩³ , Those are my credentials.

Dennis,

I was the "pusher" engineer on a friend's HO layout.  Model pusher engines worked just as well as the prototype.  You just have to maintain coupler slack somewhere in the middle of the train.  

Ed

In other words to clarify further:  two automobile hitting each other head on at 50 mph is an impact of 50mph for each car.  Each car acts like an immoveable brick wall to the other.

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Dennis

Whatever is correct, the outcome will be the same, R.I.P.  

Dennis,

Interesting that you used the word "immoveable".  It would seem to me that each car acts more like a brick wall moving toward the other at 50mph.

Hmmmmm.....

Ed

ED, It is true though.  You can maybe look it up somewhere.  I'm also talking about two identical cars.  It gets more complicated if one car outweighs the other.

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Dennis

In a head on collision where both cars are abruptly stopped, momentum is conserved.  A 2000 lb car has half the momentum of a 4000 lb car, and so if both are moving at 60 mph when they hit, the total momentum is 4000 lb * 60 mph - 2000 lb * 60 mph = 2000 lb * 60 mph = 120,000 lb-mph.  The cars are not going 60 mph after the collision, but rather 120,000 lb-mph / 6000 lb = 20 mph.  Both are going in the same direction and the heavy car won the battle as its speed dropped from 60 mph to 20 mph in the same direction, but the light car had it speed changed from 60 mph to 20 mph in the opposite direction, a change of 80 mph.  

The units I have chosen to use are not what a physics major would use.  He would use slugs for the mass of each car and ft/second for the velocity, but he would get the same answer.

This is a sophomore physics problem.

Here is a example of the reason why I double head engines...

The first 9 cars are railking die cast cars.

I do not use traction tires. The wheels with the groove have been replaced by a standard wheel and the grooved wheel replaces the ridiculous false wheel on the rear axle. 

The horse-team analogy is a good discussion tool.  In above photo from Wikipedia, all horses directly pull on the drawbar.  So unlike MU coupled engines, the force applied to the wagon is not all on the last horse closest to the load.  A "clever" lazy horse could walk along at the exact speed the wagon (consist) is moving providing just enough tension on the drawbar but not provide any useful pulling power on the drawbar.

Like the horse team example, if the engine closest to the load in a MU is pulling the consist, a "clever" lazy lead engine going at exactly the speed of the working engine can provide no effective work and it's drawbar-coupler would be slack.  The term "slacker" comes to mind...just going along for the ride.  It could be worse with the lead engine actually retarding the consist by not even keeping up so the working engine has to push it up to speed with the drawbar-coupler in compression.

So back to trains.  Say we have engines that can safely (without burning up the motors) pull up to 1 lb at the drawbar at 20 sMPH.  Let's say cars at 20 sMPH require 0.1 lbs-force to pull them so each engine can pull 10 cars.  Ideally 2 engines can then pull 20 cars, 3 engines can pull 30 cars, and so on.  This requires very careful matching/synchronization.  With 3 engines, the drawbar between the lead and 2nd engine would have 1 lb on it, the drawbar between the 2nd and 3rd engine will have 2 lbs on it, and the drawbar between the 3rd engine and the 30 cars would have 3 lbs on it.  By the way, the last coupler between the 29th and 30th car will have 0.1 lbs on it.  Each engine only provides 1 lb of force but they all sum together (vectors from high-school physics).  But this is theoretical.

In practice model engines will be slightly speed mis-matched even with the "digital" precision of matched tach-tapes.  Of course the engines move at the same average speed but there is a complex interaction of pushing/pulling with the force on the couplers changing, with wheels intermittently spinning/skidding to equalize engine speeds, and so on.  The net effect will be 3 engines will pull more cars than 1 engine but not 3 times.  Complex electronics could be added to synchronize multiple engines but you couldn't afford it and why bother...just run your MU's and have fun.

There's a similar on-again, off-again discussion about the motors in dual-motor diesels.  I'm not aware of any model train manufacturer that has made the complex electronics to sync the efforts of both motors to maximize efficiency or effective pulling power.  I know some guys have hand-selected motors to get better matching.  In any event, a dual-motor chassis with the simple electrical hookup will pull more than a single-motor chassis and that's good enough to have fun at an affordable price.

At the risk of sounding like a rivet-counting, metaphor-mixing physicist, there is a difference between the effects of impact and the relative speed. The relative speed of one car to the other is definitely 100 mph.

But, the impact, or more precisely the transfer of energy and momentum, is not the same as a 100 mph collision as described by servoguy.

I won't bring up the coefficient of elasticity which is also necessary to accurately describe the energy transfer of a collision (think colliding steel balls vs. colliding lumps of clay)!!!

Come to think of it - it's nice being a retired physics teacher!

Jim

Jim, you don't need to consider energy to determine the effects of a head on collision.  It is conservation of momentum that is important.  Energy is not conserved and you should know that.

Now if you want to do a detailed analysis of the damage to each car, then you must consider where the energy of each car is dissipated.  This is a difficult problem and is way beyond this forum.

My analysis of two cars of different weights hitting head on is exactly correct, assuming the cars hit directly head on.  

I never described a 100 mph collision.  I described a collision where both cars were going 60 mph.  The cars had equal speed but not the same momentum.