Weight and CSBs

Discussion of model and prototype wheel/rail interaction.
Lindsay G
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Weight and CSBs

Postby Lindsay G » Fri Feb 07, 2014 12:59 pm

I'm now at the stage of considering final positioning of decoders and other bits and bobs in a tank engine (subject of another thread just started). Position of these will have a knock-on effect on positioning of lead but there's plenty of room to accommodate everything in a number of configurations. This then has my wee brain going into overdrive, resulting in another couple of questions :

Weight. Is there an optimum weight that should be aimed for in a loco, in the same way as there is a recommended weight per axle on stock? The more weight the better for pulling power, but can you get to the stage that there is so much weight that it has a detrimental affect on the motor?

CSB's. The gauge of wire used will depend on the weight of the engine, the greater the weight, the thicker the wire. However, there must come a point where the gauge of wire needed to have the loco riding at the correct level, is so thick that there is insufficient deflection. I realise that I'll have to spread the weight correctly and don't anticipate any problems in this respect.

Of course, everything can be assessed by trial and error, but many others may have the answers based on experience or more scientific means of getting there.

Lindsay

David Knight
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Re: Weight and CSBs

Postby David Knight » Fri Feb 07, 2014 2:39 pm

Lindsay,

Positioning should give even distribution over the wheelbase so as to achieve the deflections used in the calculations for the positioning of your CSB anchors. In fact all the numbers should be on the spreadsheet. I would use the tanks for the bulk of the weight and the bunker and smokebox for balance and any extra weight to be added. I'm sure others will weigh in in due course.

Cheers,

David

Alan Turner
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Re: Weight and CSBs

Postby Alan Turner » Fri Feb 07, 2014 3:37 pm

All the information is on the spread sheet, including the CG of the weight and the wire gauge to achieve the required deflection with the required weight.

regards

Alan

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Will L
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Re: Weight and CSBs

Postby Will L » Fri Feb 07, 2014 3:42 pm

Lindsay G wrote:.. The gauge of wire used will depend on the weight of the engine, the greater the weight, the thicker the wire. However, there must come a point where the gauge of wire needed to have the loco riding at the correct level, is so thick that there is insufficient deflection. I realise that I'll have to spread the weight correctly and don't anticipate any problems in this respect.


Sorry no, think about that again. The spread sheet works out the deflection you will get with a given size of wire and given weight of loco. For that weight of loco you will get that amount of defection. When you know the loco weight feed it back into the spread sheet and see what size of wire gives the required deflection.

I always build my chassis on 20 thou wire so that effectively there is very little deflection under the chassis own weight, so I can check that I have actual built the chassis resonantly square. Even then there is some deflection on the 20 thou wire, though not enough to hide the sort of faults I'm looking for. I then change down to the right size wire when the the loco is finished. Nothing I've built to date has needed anything close to 20thou at full weight. The biggest so far was 14 thou on my C12 which is a good heavy loco well able to pull heavy trains up hill. To get .5 mm defection on your average 0-6-0 on 20thou wires the spread sheet say it is going to have to weigh about 1.25 kilo's.

One last though, correct distribution is as important as absolute weight in getting good haulage from the loco.

Will

allanferguson
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Re: Weight and CSBs

Postby allanferguson » Fri Feb 07, 2014 6:07 pm

There was a thread somewhere anent the effect, on adhesion and roadholding, of putting a couple of corpulent white metal crew on the footplate, thereby shifting the C of G by a considerable amount.

Lindsay G
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Re: Weight and CSBs

Postby Lindsay G » Fri Feb 07, 2014 7:33 pm

Hi, and thanks for the replies to date. However, they're missing the points of the questions I was posing or, possibly and more likely, I'm not making the points clear enough. So, here goes again with some examples (including a rather extreme example for which I apologies in advance). I used Alan's spreadsheet first time around and I've opened it back again for everything that follows - but the spreadsheet can't factor in the ability of a motor to cope with the various weights.

Weight. My initial estimate of the finished weight was made by comparison to a class 104 which is of comparable size and weighed in at 250g. This in itself feels heavish, heavier than the small locos on Burntisland 1883, so might be perfectly adequate weight-wise for the loads envisaged. The 104 could easily take more lead, and the class 782 could take even more because of the longer and higher tanks and larger bunker, so I estimate I could easily more than double the weight, let's say to 650g. However, with that much more weight, do I get more benefits in adhesion or whatever, or might the motor start to toil a bit under the weight without anything hitched up?

Now, if I were to replace the lead with osmium (but at over £2000 for 100g, I don't think I'll bother. I just love Ebay - you can buy about anything there. Think I'll sell my air guitar - always in tune, and never bounced off the nearest Marshall amp), I can more or less double the weight of the lead on board, let's say to 1250g (that's just under the weight of 3 large tins of baked beans, and by pure coincidence (honest!) the weight quoted by Will), would the poor motor just start to smoke and smell funny?

So, between the extremes of 250g and 1250g, how does one assess the optimum weight?

CSB's. At 250g, 14-15 gauge wire does the business, and at 650g we're up to 18 gauge. Are the ride characteristics of these 2 combinations the same, or does one provide better performance than the other? At 1250g we're up to a whopping 22 gauge per the spreadsheet. I've got some 22 gauge here but I won't experiment because it's way over the size of the holes in the Markits knobs. However so much girth and resistance there it's hard to imagine, even under all that weight but with so little distance between the fulcrums , the wheels deflecting to the same extent and the engine performing as well over dodgy track or ply sleepers laid there to demonstrate the beauty of CSBs. However, I gave up physics at a very early stage at school, so it might be that everything can be scaled and calculated.

My brain's starting to hurt.

Lindsay

P.S. Will - great tip to initially run things with a stiffer set up, hadn't heard or though of that.

P.P.S. Allan, the portly crew caused a problem in one of Will's threads if my memory stands up.

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grovenor-2685
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Re: Weight and CSBs

Postby grovenor-2685 » Fri Feb 07, 2014 10:29 pm

Not so far mentioned but weight is also very beneficial for maintaining good electrical contact, you are asking whether we think your motor might be overstretched, but I don't think you have mentioned anywhere what sort of motor you are fitting or what gear ratio. Both of which are relevant.
My own view is the heavier the better, but I do have a layout with steep hills. I would think your loco could comfortable go up to somehere between 350 and 400g.
What you do need to check is that the loco will spin its wheels when up against a buffer stop, what kills motors is stalling on a high voltage. With a modern motor of reasonable size and a gear ratio suited to slow/medium speeds you will be surprised how much weight it needs to stall it.

Coreless motors are a bit different and with one of those use an ammeter to measure the current with the wheels spinning and don't put in more weight than needed to achieve its rated current.

Regards
Keith
Regards
Keith
Grovenor Sidings

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Will L
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Re: Weight and CSBs

Postby Will L » Sat Feb 08, 2014 12:15 am

Hi Lindsay

Have to say your original post said nothing about you concerns for the motor. I'm with Keith on this, if your loco can't spin it wheels when faced with an unmovable object, be it a buffer stop or a heavy train on a hill, the you may need a bigger motor. But don't be fooled, motors that burn out usually do so because the chassis is stiff and uses a huge proportion of the motor output fighting internal resistance. The aforementioned C12 (a 4-4-2 remember, although its a 4-4-0 in disguise really) is fitted with a Mashima 1024 which some would consider quite a small motor, has a 80:1 gearbox and weighs in at 335 grams (with about 10% carried by the bogie). This will spin it wheels as required when faced with the immovable object and will shift a train weighing just under 2 kilos up a 1 in 60 slope. That's 35 4 wheel wagons at 25 gram a shot, or 9 to10 heavy brass coaches at around 200 grams a go. The prototype would not have got anywhere close to this, so as a P4 modeller who is supposed to care about fidelity to the prototype, perhaps I should take some weight out!

So I think your worrying about a non problem. If you must fill your loco with spent uranium, you may need a rather bigger motor if you ever find a layout big and bumpy enough to present any real challenge. Assuming, that is, you've built a descent free running chassis, and given the apparent quality of your build to date I would assume nothing less.

Will

billbedford
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Re: Weight and CSBs

Postby billbedford » Sat Feb 08, 2014 12:54 pm

Lindsay G wrote:Weight. Is there an optimum weight that should be aimed for in a loco, in the same way as there is a recommended weight per axle on stock?


It has been suggested that people use 4gm per ton of adhesive weight of the full sized loco. To pluck a figure from the air.

Or if you want to work from the model:

The starting drawbar resistance of a wagon is about 0.5 gm and for a coach about 2gm, the adhesive factor* for your loco should be about 4. If you wanted your loco pull, say, 10 more wagons you need: 10 x 0.5 x 4 = 20 gm more weight on the loco, and for each extra coach 1 x 2 x 4 = 8 gm more weight.

So I would suggest that you build your loco and test it's hauling capacity, then slowly add ballast (and beef up the CSB springs) until it gives performance that you would wish.

Of course all the above assumes that your layout is going to be flat. If you are intending to model something like the Lickey Bank, as Keith has, you will need to take the ruling gradient into account.

*The adhesive factor is defined as the adhesive weight divided by the starting tractive force. The 4 represent the reciprocal of the coefficient of friction of a steel** wheel on steel** rail. Locos with an adhesive factor of less than 4 will be more prone to slipping on starting and ones with more than 4 are either under powered or over weight.

**OK, I'm going to ignore nickel silver here.
Bill Bedford
Mousa Models
http://www.mousa.biz

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Russ Elliott
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Re: Weight and CSBs

Postby Russ Elliott » Sat Feb 08, 2014 6:24 pm

Lindsay G wrote:At 250g, 14-15 gauge wire does the business, and at 650g we're up to 18 gauge. Are the ride characteristics of these 2 combinations the same, or does one provide better performance than the other?

Theoretically, on dodgy track, the heavier loco will rock, pitch and bounce at a lower frequency, but I doubt the frequency difference will be discernible (or measurable, for that matter).

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Will L
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Re: Weight and CSBs

Postby Will L » Sat Feb 08, 2014 7:56 pm

Russ Elliott wrote:Theoretically, on dodgy track, the heavier loco will rock, pitch and bounce at a lower frequency, but I doubt the frequency difference will be discernible (or measurable, for that matter).


I think we can overplay this theory. While I accept that they must be some movement on the springs if they are to work, I don't think that on any reasonable well set up chassis, no movements is readily discernible at all. The C12 has a big front overhang with only 10% of the weight on the bogie which you would think would be inclined to move about on it springs if anything does, and while I have practical evidence of weight transfer under load, its still nothing you would spot by eye. An 0-6-0 should have absolutely no issues.

Will

David Knight
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Re: Weight and CSBs

Postby David Knight » Sat Feb 08, 2014 10:04 pm

FWIW my J72 weighs in at 145g on .012" wire. The main difference between it and my compensated engines is that it glides over dodgy track rather than clunks and will pull 29 wagons, average weight 45g on straight and level track. I have a Mashima 1224 powering an HighLevel Roadrunner + with 54:1 gearing. The test was done with Gibson wheels on N/S track.

I mention this only because the J72 looks to be a similar size to the 782.

HTH

David

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Russ Elliott
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Re: Weight and CSBs

Postby Russ Elliott » Sun Feb 09, 2014 1:28 pm

Will L wrote:I think we can overplay this theory. While I accept that they must be some movement on the springs if they are to work, I don't think that on any reasonable well set up chassis, no movements is readily discernible at all.

I agree. I was merely trying to answer that particular aspect of Lindsay's question, and as others have pointed out, there are more pertinent considerations when deciding about weight.


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