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Author Topic: Weight that an a Gauge loco can/should pull  (Read 1243 times)

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Offline Josiebenn

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Weight that an a Gauge loco can/should pull
« on: July 08, 2017, 08:45:57 am »
Hi.   As each loco, say a Farish 37, 66, 70 or a Kato RE 460, has a motor with a certain power, there must be an optimum weight of train behind it that can be pulled within its power range.   Why should finding it be trial and error?   I've asked one of the makers, only to be ignored.  Don't they like questions like this?  Too sensitive?  Whether we accept or ignore the 'torque' limitations is our business, but the engineering mind tends to want the best for its machinery, while getting the most out of it.   

So, any non-speculative knowledge out there?

Offline austinbob

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Re: Weight that an a Gauge loco can/should pull
« Reply #1 on: July 08, 2017, 08:54:22 am »
Unfortunately its not just about power. Its about how much power you can get on the rails without the wheels slipping. This depends not just on the power of the motor but also the weight of the loco and the friction coefficient between the wheels and the track. For instance there can be a big difference between the pulling power of the same loco with and without traction tyres.
In addition the rolling resistance of each coach or wagon may be different even for the same make and type. I've taken brand new coaches out of the box which will roll away just by looking at them, yet others need a to be pushed to make them move.
So, ultimately it is going to be about trial and error in my opinion. Each loco and set of rolling stock has so many variables that this is bound to be the case.
 :beers:
Size matters - especially if you don't have a lot of space - and N gauge is the answer!

Bob Austin

Online NeMo

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Re: Weight that an a Gauge loco can/should pull
« Reply #2 on: July 08, 2017, 09:18:53 am »
@austinbob is absolutely right.

To give you some context, real locomotives suffer these constraints, too. The horsepower rating of the diesel engine inside a loco isn't the same thing as the force applied to the rail. Indeed, it's frequently a lot less because energy is lost all the way through the mechanical parts of the locomotive as heat and sound. So the design of the loco, particularly its transmission, was important.

To give one example, O S Nock, a rail engineer, if a somewhat biassed one, describes a run behind a Class 52 'Western' that was, of course, nominally rated as a 2700 HP 'Type 4' diesel. The performance of the loco was consistent with a drawbar horsepower around 1500 HP. By  contrast he was more impressed with the Class 47s, which had diesel engines of similar power but delivered drawbar ratings around the 1700 HP mark.

(Just to be clear, Nock wasn't a fan of the diesel hydraulics, and may well have cherry-picked his data here. But whether or not the numbers he quotes fairly reflect their respective diesel locomotive classes accurately, they do show that the nominal 'strength' of its diesel engine is very much greater than the amount of 'pull' the locomotive actually has when it's working.)

The amount of weight over the driving wheels is crucial. Tank engines for example lost 'pull' the longer they were running because they were using up coal and water. So as they got lighter, they couldn't apply quite as much force to the rail. Similarly, adding non-powered, load-carrying wheels to steam locos always compromised pulling power. Whereas all the weight of an 0-6-0 engine was applied to its driving wheels, increasing adhesion, a 4-6-2 design of similar weight would have less adhesion, even though the non-powered wheels might be improving other traits, such as stability at high speed.

I do agree with you @Josiebenn that model locos should pull something like prototypical loads at realistic speeds and up reasonable slopes. That is something Dapol and Farish should guarantee. While getting 100 wagons behind a 9F might be asking too much, certainly such a model loco should comfortably handle 20 or 30 of them, and up a typical model railway slope too. If it's slipping with a dozen wagons, then really Dapol need to think more carefully about its design and where they're adding weight or traction tyres.

Cheers, NeMo

Online Bealman

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Re: Weight that an a Gauge loco can/should pull
« Reply #3 on: July 08, 2017, 09:36:46 am »
With respect to the OP, is it really important? Unless it is a really big layout with steep gradients running 70 bogie coal wagons as on some US layouts, then it doesn't really matter to most modellers' layouts.

An out of the box British loco is quite capable of pulling a representative train.
Vision over visibility. Bono, U2.

Offline austinbob

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Re: Weight that an a Gauge loco can/should pull
« Reply #4 on: July 08, 2017, 09:48:08 am »
Must agree with that. Its never been an issue for me. Some smaller locos don't pull much but they didn't in real life either.
There are occasionally complaints about pulling long trains up inclines, usually unprototypically steep like 1 in 50 or worse. Again, not an issue for most people.
 :beers:
Size matters - especially if you don't have a lot of space - and N gauge is the answer!

Bob Austin

Offline njee20

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Re: Weight that an a Gauge loco can/should pull
« Reply #5 on: July 08, 2017, 09:49:08 am »
I disagree, I've got Dapol 66s that won't manage 28 PCA tankers on the flat, a full rake is 36. On a gradient, no chance! Farish Are much better.

Austinbob is spot on though, there's huge variation in stock, plus your tracklaying, curve radius etc. There's no way a manufacturer can say "it'll haul X", unless they were so conservative it would be meaningless.

Offline Yet_Another

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Re: Weight that an a Gauge loco can/should pull
« Reply #6 on: July 08, 2017, 10:26:09 am »
Also, while the calculations can be fairly consistent with a reasonable error margin for a full scale loco, as the scale goes down, effects like friction become more significant, making the margins larger, and also increasing the variability of results. Track/wheel cleanliness and the quality of tracklaying have a much bigger influence in N gauge.

Just look at the arguments over what the new Graham Farish Castle will pull.
Tony

'Things are not done by those who count the cost of every thought or deed.'

Online Bealman

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Re: Weight that an a Gauge loco can/should pull
« Reply #7 on: July 08, 2017, 10:28:57 am »
Yes, I think it's down to well maintained track and electrical in this scale.  :thumbsup:
Vision over visibility. Bono, U2.

Offline Dorsetmike

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Re: Weight that an a Gauge loco can/should pull
« Reply #8 on: July 08, 2017, 11:03:34 am »
Quote
An out of the box British loco is quite capable of pulling a representative train.

Not always the case, the original Dapol M7's out of the box could never manage a 12 coach "empty stock" job, most struggle with more than 2 coaches, at the other end of the scale most Union Mills locos can pull far more than the prototype.
Cheers MIKE


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Online NeMo

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Re: Weight that an a Gauge loco can/should pull
« Reply #9 on: July 08, 2017, 11:05:16 am »
I disagree, I've got Dapol 66s that won't manage 28 PCA tankers on the flat, a full rake is 36. On a gradient, no chance! Farish Are much better.


 :sorrysign: but I think you're being slightly unreasonable here. 28 wagons is a lot more than the "average" modeller will use, let alone 36! Not saying that sort of haulage wouldn't be nice, but ultimately what we're playing with are toys, not scaled down versions of real working machines.

For the average modeller, a long rake might be a dozen or so wagons, or 6-8 coaches for a long passenger train. A heavier loco might have greater haulage, but at the cost of faster wearing gears, greater electricity consumption, and more heat inside the bodywork.

That said, there are some Dapol locos that really do have very poor haulage by any standards, and as I said earlier, even something like their 0-4-2T should be able to haul 2-3 coaches up a typical model railway incline without fuss. The fact that their existing GWR 14xx struggles to do that is disappointing, and perhaps indicative of poor design.

Cheers, NeMo

PS, let's put the Union Mills paradigm to bed, @Dorsetmike. These locos have solid metal bodies, and much bigger motors, both of which increase adhesion considerably. The compromise here is that their external detailing is basic, at best, and the motor size limits the range of tender designs, even compromising them in some cases. I, for one, would not want Farish or Dapol going this route.

Online Bealman

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Re: Weight that an a Gauge loco can/should pull
« Reply #10 on: July 08, 2017, 11:14:46 am »
Exactly the point I'm trying to make. Haulage is not an issue, as layouts are a huge exercise in compression.

Hence the trains are suitably compressed.

We are lucky in N, having the advantage that we can run longer trains, for which you need a huge run for in OO.

I think that on most UK outline layouts, haulage is not an issue.

I just visited Webbo's Canadian layout with a triple diesel lashup hauling 28 bogie freight wagons!
« Last Edit: July 08, 2017, 11:16:28 am by Bealman »
Vision over visibility. Bono, U2.

Offline MJKERR

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Re: Weight that an a Gauge loco can/should pull
« Reply #11 on: July 08, 2017, 11:44:53 am »
I disagree, I've got Dapol 66s that won't manage 28 PCA tankers on the flat, a full rake is 36. On a gradient, no chance! Farish Are much better.
Which Dapol version of the Class 66?

The early DC version could easily haul the same rake length as the Farish version
The later DCC version could only haul about one third of that rake, due to that version being lighter
That is one of the reasons I had to change my layout as that version was unsuitable for my layout

Offline martyn

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Re: Weight that an a Gauge loco can/should pull
« Reply #12 on: July 08, 2017, 11:56:08 am »
As has been said on the Forum, look at the messages about the Castle hauling powers, and 'real life' gradients.

Mass, friction, rolling resistance, wind resistance and other such factors do not scale down, and, as I said, I'm not aware of an N gauge loco with working sanders......... :) Don't forget that the humble class 08 only had 350 engine horsepower, but was geared down for maximum traction at very low speeds. Having said that, I'm happy with most of my locos, steam and diesel, able to haul 8+ coaches or 30 or so 4-wheel wagons; I'm also well aware than certain models haul well over and some well under what we might expect, based on the prototypical performance. I know the 'real' railway has, and had, much longer trains in service, but it is a case of what one can reasonably haul in the relatively limited space most of us have available, in unscale curves and gradients.

I would also say that 'modern' rolling stock models from Dapol and Farish has a much lower rolling resistance than Poole Farish or Peco.

See also on the Castle thread my notes about gradient-Shap and such banks were regarded 'steep' at 1;75. I'm also well aware that modern diesel and electrics can climb such grades with greater ease than steam locos could.

Martyn
« Last Edit: July 08, 2017, 11:59:04 am by martyn, Reason: extra info »

Offline njee20

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Re: Weight that an a Gauge loco can/should pull
« Reply #13 on: July 08, 2017, 03:52:50 pm »
I think you're being slightly unreasonable here. 28 wagons is a lot more than the "average" modeller will use, let alone 36! Not saying that sort of haulage wouldn't be nice, but ultimately what we're playing with are toys, not scaled down versions of real working machines.

Farish 66 manage it. Older Dapol 66s manage it. Most modern exhibition layouts can run trains that long, it's about 6'. Not a crazy length at all, particularly as a lot of people are drawn to N because of the ability to run long trains in comparatively modest spaces.

My point was people are saying "it's irrelevant, everything will haul a reasonable train", I'm saying that's not necessarily true. I could run longer trains, but the limiting factor is the loco (in that case). These are not longer than scale length trains at all, they are still compressed. If your layout is a 3' BLT then no, it almost certainly isn't an issue, but that doesn't mean it's not an issue universally.

 
Which Dapol version of the Class 66?

The early DC version could easily haul the same rake length as the Farish version
The later DCC version could only haul about one third of that rake, due to that version being lighter

Yes, there have been some subtle changes along the way too, newest ones are worst ironically, early DCC ready ones are better, the oldest are the best, but a bugger to convert to DCC.

Offline PLD

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Re: Weight that an a Gauge loco can/should pull
« Reply #14 on: July 08, 2017, 05:13:06 pm »
As many have alluded to, judging a model on the performance of the prototype is not a fair comparison...

The model wagon or coach is a much higher proportion of the prototype weight than the model loco is of the prototype loco and it's rolling resistance is more variable and generally
Our model gradients and curves are much steeper and sharper than the prototype.
We expect the model to accelerate from zero to a scale 120mph in its own length - the prototype takes 2 miles or more to reach 60 mph (and who has a straight, level 2 miles on their layout - even in N gauge??)

In other words, if a loco doesn't haul as much as you hoped for, don't just blame the locos; look at what you are asking it to haul and in what conditions...

 

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