Car stability paper

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Glenn Brooks
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Car stability paper

Post by Glenn Brooks » Sat Dec 30, 2017 3:24 pm

A few weeks ago I found a paper that described several methods to measure car stability and gave some simple math examples about how moving the center of gravity for live steam rolling stock can cause derailing and rollovers. written in easy to understand language.

Of course I didn’t bookmark it. Now can’t find it.

Anybody point me to a link where it might be?

Thanks much,
Glenn
Moderator - Grand Scale Forum

Motive power : 1902 A.S.Campbell 4-4-0 American - 12 5/8" gauge, 1955 Ottaway 4-4-0 American 12" gauge

Ahaha, Retirement: the good life - drifting endlessly on a Sea of projects....

STRR
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Location: Westminster, CO

Re: Car stability paper

Post by STRR » Sat Dec 30, 2017 8:07 pm

Glenn,

I have one answer to improve/guarantee car stability: Lower the center of gravity.

Many of the smaller railroads weight their cars to lower the center of gravity. I have seen these derail and stay completely up right due to the c of g being so low. Yes, it does add to the train weight being pulled but most railroading is a compromise. Hedge your bets to the direction of what you want or need.

I would love to see that paper as well. If you find it, or maybe I should say; when you find it, please post a link here so I can read it.

Thank You,
Terry

Glenn Brooks
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Re: Car stability paper

Post by Glenn Brooks » Sun Dec 31, 2017 1:41 am

Thanks Terry, good advice. Yes, I’ll post a link, or send you a copy when I find it again. Iam almost certain it exists here on Chaski, but can’t find it now, for what ever reason. However, maybe I read it somewhere else- it was written by a live steam guy, so hopefully someone remembers where it is.

Glenn
Moderator - Grand Scale Forum

Motive power : 1902 A.S.Campbell 4-4-0 American - 12 5/8" gauge, 1955 Ottaway 4-4-0 American 12" gauge

Ahaha, Retirement: the good life - drifting endlessly on a Sea of projects....

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aopagary
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Re: Car stability paper

Post by aopagary » Sun Dec 31, 2017 9:48 am

it's not that hard to calculate... when the center of gravity (same as the center of mass on the earth's surface) moves outside the wheel track, a car will tip over. i'll take back the "ridiculously" adjective in the previous post now that i see the CG/CM of a seated person seems to be just above their lap. still high, but likely the reason for reasonable stability in 1.5" scale riding cars. as long as passengers take on the responsibility of remaining reasonably centered, a well built car shouldn't be a problem.

frankly a bigger problem i usually see is a car which is unbalanced end-to-end. with my passenger gondola, a single person needs to sit in the front seat or the decreased load on the front truck will likely cause a derail before long.

cheers...gary
cog.jpg
CG seated person.jpg
passenger gondola.jpg

STRR
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Re: Car stability paper

Post by STRR » Sun Dec 31, 2017 10:31 am

Glenn,

I'll do some looking to see if I can locate that paper. If I find it, I'll be sure to let you know.

Garry,

Your comments are totally accurate. You even used your own experience to illustrate the concept; seating in front or back. You illustrate the CG concept in a direction many people tend to ignore or don't even think of: front to back. Most always think of side to side tip over but when the CG moves outside the center of ground contact (either side to side OR front to back) you WILL tip over in that direction, just as your gondola get tippy when someone is seated in the rear seat, as the only weight in/on the car. If you were to add, say, 100 lbs of weight between the frame underneath your gondola, you would significantly alter the CG and make the car more stable both front to back and side to side.

I stated in another thread, Railroading is a series of compromises. In this case, the compromise would be total train weight, difficulty in manually handling the gondola, and possibly additional wear on the truck springs and bearings. Each one of us has to make the decision as to which will be best in their own situation.

Cheers,
Terry

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NP317
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Re: Car stability paper

Post by NP317 » Sun Dec 31, 2017 10:31 am

CG stability is a reason for the increasing adoption of straddle cars for passenger hauling, especially on a public railroads.
Not having feet trapped inside car walls contributes to safe riding.
~RN

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Marty_Knox
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Re: Car stability paper

Post by Marty_Knox » Sun Dec 31, 2017 11:50 am

The 7 1/4 News has had articles related to this.

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aopagary
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Re: Car stability paper

Post by aopagary » Sun Dec 31, 2017 2:37 pm

math (old math) is fun!...
tipping angle (degrees) = (atan ( G / (2*CG)))* 57.3

the Excel arc tangent (atan) function returns radians, the 57.3 turns radians into degrees.
track gauge (G) & center of mass/ gravity (CG) are in inches.
here is what Excel spit out for 7.5" gauge track...
tipping angle.jpg
some of the lower CG angles may be moot.
without a seatbelt, most people would likely fall out of the car before it tipped over.

as a point of reference, in 7.5" gauge, a wheel lifting 1" off (above) the rail is a 7.6° angle.
cheers...gary

Liesegang
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Re: Car stability paper

Post by Liesegang » Mon Jan 01, 2018 4:53 pm

At MLS we found that beam cars with a floor about 1" off of the tracks are very stable. If they derail, they don't tip over.

Terry L

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aopagary
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Re: Car stability paper

Post by aopagary » Tue Jan 02, 2018 6:56 am

there is no doubt that straddle cars are the safest cars for passenger transport...
RLS straddle cars.jpg
not only do they provide an extremely low CG, but the bench seating allows for the best weight distribution end to end.

but again, when constructed properly, there is no reason that a modified freight car cannot safely transport passengers. doing a rough calculation on my passenger gondola, i would estimate the CG of a loaded car to be somewhere around 12-13 inches above the rail head. certainly well above a straddle car, but by my calculations, it would still take a huge tipping force to topple that car.

for someone like me who only occasionally invites passengers for a ride, along with providing a comfortable ride for a brakeman...
RLS conductor car.jpg
you have to admit that prototype cars, especially when unloaded, provide a much better visual.

cheers...gary

tornitore45
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Location: USA Texas, Austin

Re: Car stability paper

Post by tornitore45 » Tue Jan 02, 2018 7:10 am

I do not know if these cars have springs, if they do is also a factor. Center of Gravity is a static thing but in a moving car the springs define the natural oscillation response and that can amplify the swing up to the static tipping point.
Mauro Gaetano
in Austin TX

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pat1027
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Re: Car stability paper

Post by pat1027 » Tue Jan 02, 2018 1:26 pm

There is a piece on the IBLS website though it doesn't sound like it's what your looking for. http://ibls.org/mediawiki/index.php?tit ... -On_Trains

What straddle cars do vs a gondola is better control where the rider sits. If you look at Gary's two pictures the passengers on the straddle car don't set appreciably lower. Their feet are but their seats are not (if they aren't a tad higher). We look at the center of gravity as being static when in reality it is moving through the entire trip. Passengers don't sit still. The lean out to look, turn to talk to the people behind them and get their phones out to shoot video. While they are doing this they shift around on their seat combined with the the car doing it's own rocking back and forth. With a seat on a gondola they can shift their weight closer to the side of the car. A straddle car by design keep them seated closer to the center of the car.

Tom Bee has written a few things.
http://www.islandpondrailroad.com/tombe ... frames.htm
http://www.islandpondrailroad.com/tombee/track/77.html

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