TT Car Weight "Standards"

TT Car Weight "Standards"

Postby gerhard_k » Tue Jan 13, 2015 12:11 pm

Caution: Verbosity and possibly excess information ahead! :dance:

Some months ago, I looked at the recommended weights for TT cars. First of all, the US NMRA RP 20.1 and European NEM 302 are wildly different – for a 40-foot TT car, NEM says (after the necessary unit conversions) 0.88 ounces, the NMRA says 2.25 oz! That's over a 2:1 difference, which btw holds for N and HO as well.
That caused me to look at the weight scaling across the modeling scales – here's the write-up I made for myself:

Weight – NMRA RP 20.1, in oz: ________________________________________ NEM 302: (+30% tol.) ???
N: __ 0.5 + 0.15 *L (in) ______________ 0.5 + 3*0.15 = 0.95 oz _________ 0,17 g/mm => 0.15 *3 = 0.45 oz
TT: _ 0.75 + 0.375*L - so 40' car => 0.75 + 4*0.375 = 2.25 ___________ 0,25 __________ 0.22 *4 = 0.88 !!
HO: _ 1.0 + 0.50 *L ___________________ 1 + 5.5*0.5 = 3.75 ____________ 0,40 __________ 0.36*5.5=1.98
S: __ 2.0 + 0.50(?!)*L __________________ 2 + 7.5*0.5 = 5.75 ____________ 0,60 __________ 0.54*7.5=4.05
O: __ 5.0 + 1.0 *L _____________________ 5 + 10*1.0 = 15 ______________ 1,00 __________ 0.90*10 = 9.0

Note that the European (NEM) weights are 'way lighter than the NMRA weights, and especially for TT (note to the calculations: 0,17 g/mm = 0.15 oz/in, etc); and NEM does not have a "starting" weight.
From one scale to the next, scale ratio > cubed = volume ratio: N>TT: 160/120 = 1.33 > 2.36; TT>HO: 120/87 = 1.38 > 2.63; HO>S: 87/64 = 1.36 > 2.51; S>O: 64/48 = 1.33 > 2.36
So, since the scale and volume ratio steps are almost equal from one scale to the next, and the weights should be proportional to volume, it looks (from the second column above) like the TT car is a bit heavy compared to N and HO, should be ~1.9 oz; and the S car is 'way light, should be ~7.8 oz (which it would be, if NMRA's per-inch multiplier were 0.75, which seems logical). And maybe the N-scale multiplier should be 0.25? That would bring the N car up to 1.25oz, making the NMRA TT weight look more nearly "in-line".
But comparing volumes to weights, from N to O = 36.8 by vol., 15.8 by wt., i.e., the smaller scales are proportionately twice as heavy? Example: scaling-by-volume a 40-foot prototype car, loaded wt = 140K lb => 0.55 oz for N, 20.3 oz for O! (and 1.30 oz for TT, 3.39 oz for HO, 8.54 oz for S) –
Makes you wonder how the early rule-makers came up with their formulas… possibly, using strict scaling, the larger-scale cars just seemed too heavy, and being worried about staying-on-track for the small scales, they decided to compress the range?
(End of my previous musings)

So: what "standard" to adopt? Personally, I feel heavier cars have better operating reliability and are less easily upset during uncoupling (assuming Kadee couplers), so my cars will follow the NMRA RP, at least as a goal. :dance:
Now - What are your preferences or (actual operating) experiences?
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Re: TT Car Weight "Standards"

Postby railtwister » Tue Jan 13, 2015 2:15 pm

I've recently been checking coupler height, wheels and car weight of my HO rolling stock and testing it out on my modules while they were on active display for over five months in 2014, and I've brought many of them up to the NMRA weight RP, usually by using penny's to increase the weight (cheapest way I've found). They seem to operate well at that weight, though it might be a bit on the heavy side, since some of my lighter weight rolling stock has also been performing pretty well, too. My modules have an old Roco double-slip switch in a small yard which has always caused tracking problems, but since it is in the yard rather than on the mainline, it hadn't been a big priority to fix, because little running at a show involves switching. Finally, I checked it with an NMRA gauge and found the molded-in guardrail flangeways to be over .020" too wide (that's a lot). After gluing in some strips of .020x.100 styrene most cars tracked though the switch much better, except for those equipped with equalized trucks, which for some reason still derail reliably, even when using the same wheelsets as rigid trucks which track through just fine. Weight doesn't seem to be an issue here, at least in this case. I would think that equalization would be something that would improve tracking rather than hinder it, but who knows?

I do think the European weight for an HO car is a bit too light at 2 oz., but I don't have enough experience with TT scale to suggest what would be a good operational weight for rolling stock in that size. As for N scale, it seems to me that MicroTrains has done a pretty good job of figuring out the optimum weight in that scale, though I've never actually compared it to the NMRA RP.

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Re: TT Car Weight "Standards"

Postby milwrd1 » Wed Jan 14, 2015 7:20 pm

I've always used the NMRA RP guidelines with good operational results. Much of the older, American made TT was underweight per the NMRA guidelines, and I've found that adding weight improved operational performance.
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Re: TT Car Weight "Standards"

Postby modorney » Thu Jul 30, 2015 12:47 pm

I've talked around, and even talked to some who weight HO cars to twice NMRA, which is a bit much, but they do operate well.

Consistency is as important as increased weight.

My conclusion is to weight TT to a standard that is 90 percent of HO.
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Re: TT Car Weight "Standards"

Postby MacG » Thu Jul 30, 2015 1:35 pm

I also use the NMRA RP guidelines. Some of my cars, e.g. Gold Coast, are too lightly. But I have no problems with. I have built a calculator with the spreadsheet of OpenOffice, it's with millimeter accuracy.

For my pre-production tests I always check the weight. Is it lighter than NMRA RP 20.1 then I check the functionality with this car direct behind a locomotive and a 20+ cars train.
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Re: TT Car Weight "Standards"

Postby oldtrainguy » Thu Jul 30, 2015 4:59 pm

Just wondering where do you prefer to put the weight? Some of the gold coast cars I have are glued together so I'm not able to put any thing inside the car.
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Re: TT Car Weight "Standards"

Postby MacG » Fri Jul 31, 2015 1:10 pm

I haven't added some weight to the lighter Gold Coast cars.
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Re: TT Car Weight "Standards"

Postby Tom Dempsey » Fri Jul 31, 2015 3:43 pm

Tungsten BB's and two part epoxy resin are your friends Don. Just place them up inside the plastic representation of the frame.
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Re: TT Car Weight "Standards"

Postby railtwister » Fri Jul 31, 2015 5:28 pm

I frequently use pennies to add weight to my HO cars, they ase easy to find, and cheaper than lead mag wheel weights, even if you can find them wholesale. I try to add weight at the car ends, over the trucks. Avoid putting the weights in the middle of the floor so the frame doesn't sag. Also, I don't recommend Goo for use on a plastic car, because it releases it's solvents slowly, and can cause sink marks in the opposite surface from where the Goo is applied. Epoxy, ACC/CA, or good quality caulk will also work.

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Re: TT Car Weight "Standards"

Postby WillYart » Wed Nov 04, 2015 5:09 am

Followed NMRA weights. Like the operational results. I used Bismuth shot, in some cases glued to undercarriages, in other cases melted and molded. It's almost as heavy as lead but nontoxic.

Here is a thread on the process of making bismuth weights: custom-molded-weights-t1848.html?hilit=Bismuth#p21359
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