A theoretical weight question

[TheDukeOfWellingtons]

There I was idly thinking that a real Peak diesel with full tanks scaled down to 1/76 would weigh almost two tonnes ( 147 divided by 76 ) when a learned friend pointed out that the scaling factor should be 76 cubed. "Alright" says I, "thats still a third of a tonne!" but he says a third of a kilogram is the real answer. I'm not happy. My latest Bachmann Peak weighs 562 grammes so is massively overweight. Before I start weighing all my stock, has anybody else ever wondered about 'scale weight' or is it just me?

[Oldddudders]

There I was idly thinking that a real Peak diesel with full tanks scaled down to 1/76 would weigh almost two tonnes ( 147 divided by 76 ) when a learned friend pointed out that the scaling factor should be 76 cubed. "Alright" says I, "thats still a third of a tonne!" but he says a third of a kilogram is the real answer. I'm not happy. My latest Bachmann Peak weighs 562 grammes so is massively overweight. Before I start weighing all my stock, has anybody else ever wondered about 'scale weight' or is it just me?

THe NMRA, US-based modelling standards etc group, do not much mention scale weight, but do have lots of empirically-derived data on how much model coaches and wagons should weigh to give them a chance to stay on the track during normal operations. Most RTR stuff is too light. On the other hand, while the weight of your Peak may not accurately reflect its scale, your interest might be in whether it can haul a scale load - however implausible that is in the average domestic situation!

[TheDukeOfWellingtons]

Whenever non-modellers visit my layout and pick up a loco they usually comment " Blimey! Thats heavy!" and I have always said " it should weigh about 2 Tonnes if it were to scale" so I'm just a bit disappointed now that I've got to say " Yes, it is a bit......." . I am resisting the urge to weigh everything else ( Well OK, an old Hornby Mk 1 weighs 118 grammes but I haven't looked up the weight of a real one yet ). The next concern is the decreased gravitational pull of a 1/76 scale world.

[Flood]

Coaches are normally classed as weighing 35 tonnes in B.R Working Timetables so that equates to about 80 grammes.

 

Don't forget that r.t.r. models have bodies made from plastic that scales to at least 3 inches (76.2 mm) thick. That's about the thickness of some WW2 tank armour.

[10800]

The next concern is the decreased gravitational pull of a 1/76 scale world.

 

Noooo, please don't go there otherwise we'll be getting into scale time again, and that way madness lies ...

 

As your friend advised, weight is a function of volume which scales down as a cube function when comparing real to model. But as Ian alluded, what you should be asking is "is my model loco heavy enough to pull the model loads I want it to on my model track" and "are my wagons heavy enough to stay on the track". Don't worry about correctly scaled down weight - there are loads of things like friction and strength which don't scale down exactly, and why should they? Just think about the model and what it needs to do, not whether every law of mechanics and physics is scaled down in the same way as the linear dimensions.

[Baby Deltic]

When it comes to hauling a scale load, bear in mind that a real Peak has six axle drive, whereas the Bachy one doesn't, so adhesion would come into it....... and the bogies and axles on a real Peak are sprung with proper suspension.

[Gordon H]

Whenever non-modellers visit my layout and pick up a loco they usually comment " Blimey! Thats heavy!" and I have always said " it should weigh about 2 Tonnes if it were to scale" so I'm just a bit disappointed now that I've got to say " Yes, it is a bit......." .

 

From experience, weight is a major factor in reliable running. The heavier the better every time.

If you want to see some evidence of how the use of heavy locos improves reliability, if you can, come to the MERG stand at the Peterborough show this weekend and look at the state of the rails on the 'Hysteresis Loop' demo track.

As a kind of very long running experiment, the track on it has not been cleaned for what must be 8 years or more. Last known cleaning was at a Model Engineering show at Ally Pally, and it ran terribly for the rest of that day!

In places the rail surfaces are now completely black, but at each show all I do is put the loco on and set it running again. On average the loco performs about 500 laps of the track per day (I know because the software that runs it has a lap counter). Total must be well into the several tens of thousands in the dozen or so years since it was first constructed. It has been run using both DC and DCC during that time.

Current loco of choice is an American Atlas S-4 switcher, which is extremely heavy for its size.

[298]

It's not just the weight that bothers me, just the other day my layout suffered a serious shunting incident and yet all the vehicles involved were undamaged. No drooping buffers or crushed vestibules, and the re-railment was done in seconds without a full HSE investigation or the Police sealing off the crime scene for evidence.

 

It's about time the manufacturers built scale, crashable models. I'd buy a Peak if it disintegrated and the motor flew out when crashed into a Nuclear Flask at the scale ton...

[TheDukeOfWellingtons]

I've never been too concerned about the ability to pull scale loads, it was just the theoretical question about how much a real diesel loco would weigh if it could be shrunk to 1/76 scale. Surprisingly, I suppose, some diesel models may be fairly accurate in this regard purely by chance. I have pinched the kitchen scales and will find out which models are the closest to their '00' scale weights but for now I have to go to work again.

[Barry Ten]

There I was idly thinking that a real Peak diesel with full tanks scaled down to 1/76 would weigh almost two tonnes ( 147 divided by 76 ) when a learned friend pointed out that the scaling factor should be 76 cubed. "Alright" says I, "thats still a third of a tonne!" but he says a third of a kilogram is the real answer. I'm not happy. My latest Bachmann Peak weighs 562 grammes so is massively overweight. Before I start weighing all my stock, has anybody else ever wondered about 'scale weight' or is it just me?

 

76 cubed is 76 x 76 x 76 = 438976, so the "correct" scale weight should be 147 divided by that figure, which is indeed about a third of a kilogram. Your Peak is therefore not that far off its scale weight, all things considered.

[Jenny Emily]

I had a conversation with my Father about similar scalings with regard to the 4" scale Garrett road locomotive (one third the size of the real one) he had built. I pointed out that whilst it was indeed a third of the size, the fact that the atoms within the material it was made of were still at 1:1 scale meant any comparisons on scale weight/strength/performance were meaningless. It was also why a scale one third size injector would not work well - the water molecules are still the same size, whilst the passageways they are expected to pass through are one third size.

[shortliner]

I think that, rather than the size of the molecules having an effect, the problem lies with surface tension causing a "bunging-up" of small holes when the water tries to pass through, rather like crowds trying to pass throug pinch points or turnstiles - if you can get them to pass through in line astern, hefalump-style, the flow is far greater than when they are all fighting each other. Think big shops and New Year sales

[Oldddudders]

I think that, rather than the size of the molecules having an effect, the problem lies with surface tension causing a "bunging-up" of small holes when the water tries to pass through, rather like crowds trying to pass throug pinch points or turnstiles - if you can get them to pass through in line astern, hefalump-style, the flow is far greater than when they are all fighting each other. Think big shops and New Year sales

If in those sales you bought a bargain Gore-Tex jacket, then how about the behaviour of molecules there? ISTR that the membrane has 9 billion holes per square inch, each just big enough for molecules of water vapour to pass out, but too small for water molecules to enter.

 

Cool - as they say!

[Worsdell forever]

I had a conversation with my Father about similar scalings with regard to the 4" scale Garrett road locomotive (one third the size of the real one) he had built. I pointed out that whilst it was indeed a third of the size, the fact that the atoms within the material it was made of were still at 1:1 scale meant any comparisons on scale weight/strength/performance were meaningless. It was also why a scale one third size injector would not work well - the water molecules are still the same size, whilst the passageways they are expected to pass through are one third size.

 

Most model live steam locos have far more power for it's scale than the real thing, a 5' gauge A1 (Gresley) may have a boiler pressure of 90psi, half of the pressure of the real thing but in a 12th scale model. A lot of this power will be lost in inefficient steam passages as mentioned above re injectors.

[modfather]

mind you, is that scaled from the dry or tare weight... hence taking into account all the fluids on board

 

*ducks for cover*

[Baby Deltic]

THe NMRA, US-based modelling standards etc group, do not much mention scale weight, but do have lots of empirically-derived data on how much model coaches and wagons should weigh to give them a chance to stay on the track during normal operations. Most RTR stuff is too light. On the other hand, while the weight of your Peak may not accurately reflect its scale, your interest might be in whether it can haul a scale load - however implausible that is in the average domestic situation!

 

I suppose you could use the 76 cubed route calculation to work out the scale draw bar pull that a Bachy Peak would need to have the same scale haulage power of a real Peak.

 

So class 45 and 46 have a maximum tractive effort of 55,000lb.

 

What if we said 76 x 76 x 76 = 438,976.

 

55,000 / 438,976 = 0.125lb?

[M1AYM]

It's not just the weight that bothers me, just the other day my layout suffered a serious shunting incident and yet all the vehicles involved were undamaged. No drooping buffers or crushed vestibules, and the re-railment was done in seconds without a full HSE investigation or the Police sealing off the crime scene for evidence.

 

It's about time the manufacturers built scale, crashable models. I'd buy a Peak if it disintegrated and the motor flew out when crashed into a Nuclear Flask at the scale ton...

 

Like it – Although the scene wouldn’t be limited to HSE investigations and police SOCOs running around. The half life of the breached Nuclear flask might just prevent return to the layout for 45 million years + !

 

Assuming you simply stuffed the peak into the buffers though, the scaling factor of 76 would probably also apply to any model repair compared to the real thing spending a week at Donny works whilst the welders & metal bashers got to play.

 

 

[M1AYM]

Whenever non-modellers visit my layout and pick up a loco they usually comment " Blimey! Thats heavy!" and I have always said " it should weigh about 2 Tonnes if it were to scale" so I'm just a bit disappointed now that I've got to say " Yes, it is a bit......." . I am resisting the urge to weigh everything else ( Well OK, an old Hornby Mk 1 weighs 118 grammes but I haven't looked up the weight of a real one yet ). The next concern is the decreased gravitational pull of a 1/76 scale world.

 

Your peek scaled up is 99.5 Tonnes over weight.

 

 

According to Issac Newton, g=GM/r squared, therefore unless anyone can tell us otherwise – the factor of 76 cancels out and your peek would still be 99.5 Tonnes over weight relative to the real thing on a 1/76 scale planet.

 

The difference in weight of your scaled down peek to the equivalent weight of a real one can only be due to material densities. What this implies though is that the density of all the materials in your peek relative to its volume is more than the density of a real peek in relation to its volume.

 

 

Now this is getting scary.I suggest adding a column to your weight results table with factors of over or under relative scale weight . e.g your peek is relatively overweight by a factor of 1.68.I wonder what a Mk1 coach will be as most of the real thing is wood ?. Results eagerly anticipated on the assumption that head office doesn’t require the scales back in the kitchen for production work.

 

 

[Platform 6]

Before I start weighing all my stock, has anybody else ever wondered about 'scale weight' or is it just me?

I never got that far. I stopped idly thinking when I realised I was looking at my OO layout with eyes a scale 18 inches apart.

 

This enhanced 3D effect seems to make my modelling look crap   

[TheDukeOfWellingtons]

Hmmm, yes, I think I'll leave it there. No more weighing for now. There lies madness indeed.

 

My layout looks the same with one eye closed, thus counteracting the strange effect but now I'm worried about the size of my right eye, the pupil of which is 3 feet across in 00.

 

( Your eyes must be a lot closer together than mine which are about 25 feet apart )

[34theletterbetweenB&D]

I suppose you could use the 76 cubed route calculation to work out the scale draw bar pull ...

Well you could, but I find it rather more useful and interesting to adjust the weight on the driven wheels of a model (and thus for equivalent materials the tractive force that may be exerted) to the maximum power output of the loco. The actual weighting applied has no direct relationship to the cube root weight of the prototype, but is derived empirically from the maximum load a loco of a given power class has to handle, in the condtions my layout set up imposes (1 in 80 gradients). The end effect is that a loco (or locos) of sufficient power class have to be allocated to work any particular train. Put a 4F or 5F unit on a train requiring 8F power, and it will slip to a standstill, requiring an assisting engine to get moving again. A starting point of 250g on the driven wheels for a class 3 six coupled, with increments of 50g per power class is generally about right for my operation.

[bertiedog]

Forget the scale weight and haulage, make the loco weight as much as the motor and gears will take. Most stock is relatively light, and free rolling compared to the real thing.

 

 

Most RTR locos are a bit light on weight, the axle bearing designs will not take much more, unlike metal chassis etc., A big factor is the power available from the motor / gear combination, with today's small motors struggling to provide the power on too low a ratio gear train.

 

Most model locomotives will out haul the real thing, as long as the track is scale curved etc. Tighter curves have the greatest of all effect on the haulage.

[Platform 6]

( Your eyes must be a lot closer together than mine which are about 25 feet apart )

 

Oops, you're correct Dave .. should've been feet and not inches. Which brings to mind one of my favourite quotes (source forgotten):

 

 

"I'm not saying he looked shifty but his eyes were in single file".   

[BernardTPM]

I wonder what a Mk1 coach will be as most of the real thing is wood ?.

 

The Mk.1 body and underframe is steel; only some of the furnishing and panelling is wood.

[StaffsOatcake]

The Mk.1 body and underframe is steel; only some of the furnishing and panelling is wood.

 

 

 

The floors in mark ones are actually reinforced concrete sections to give the coaches some weight