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Best strategy/practice to plan transition from parallel tangent to parallel curved track? Dream and design
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New member 3 posts GtLiving New member 3 posts
GtLiving

I use 3dplanit and have a design I started to implement. It was based on 32" min radius for mainline. It is single track but has passing siding and other track in parallel around curves. I had settled in 2 1/8" center to center standard (from practical testing, John Armstrong's planning book, and car length to curve nX). Then I read the NMRA RP-7 and thought what if I need more spacing for future equipment.

Based on the tool and gear I have, I am confident 2 1/2" distance on curves is good. The question is how to get there in planning. If I use 2" tangent spacing then another 1/2" of separation has to be gained. My standard for easments was 1/2".

  • Making the inner curve use an easment distance of 1" substantially increases length of that easment which may affect planned turnout location. Instead of outer 1/2" and inner 1", a 1/4" and 3/4" change would lessen both easment lengths, but at what cost? Is this a reasonable idea?
  • Would it be advisable to always force the mainline easement to be at least 1/2". That means whenever the mainline is outer curve the other track uses 1" easment,

When two straight tracks approach each other at wide angle (> 120 degrees), I've tended to connect these w/o drawing a separate circle section> minimum radius. 3dplanit makes the circle and easments though I've since learned with less distance than my 1/2" standard. If you make the mainline first and it ends up being the inner curve, how best to approach adding the outer curve (.e.g. passing track)?

  • Using parallel tool to make the distance of circle 2 1/2", 3dplanit has choices for connecting tighter tangent track like a quintic easment which will make a long S curve. Thoughts about this? Anyone do it?
  • Could make the entire siding  center to center the 2 1/2 inches, except I think it affects the look. Thoughts?
  • Can easment straight to circle center distances be less (than your standard) when arc is smaller (less than 120, 130, 140 degrees)?

What do others do to manage connect track of different center to center differences between straight and cuved track?

Greg
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ctxmf74 Super User 15,955 posts
ctxmf74

"What do others do to manage

Quote:

"What do others do to manage connect track of different center to center differences between straight and cuved track?"

    For odd situations I draw in the curves and tangents then use a flexible batten to draw in a connecting line. I'm a surveyor so could calculate the line but I hate to waste time on something that doesn't really show up in the end results. In fact I don't routinely calculate any easements , I just draw the plan with curves tangent to the straights and then when laying track relax it a bit where they meet, flextrack wants to ease it's self if given the chance.....DaveB

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dwtrains Regular poster 115 posts
dwtrains

Flexible Batten

As Dave B  said - I also use a flexible batten.

My straight parallel tracks are on 2-inch spacing and my curves are at 2 1/2-inch.

Just as you would lay out an easement decide on where on the straight track to begin then pick a point on the curve where the easemnt is to end. Layout the batten so that it is aligned with the centerline on the straight track. Then align the other end with the radius of the curve at the chosen point. Let the batten create it's own easement between the two points.

I always (my preference, no basis for it) start with the inside (smaller) radius curve. After you've laid out both easments - look at it and see it looks ok. Your eye will tell you alot.

Don
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janreimers Active Member 69 posts
janreimers

In 3rd Planit you could

In 3rd Planit you could try the following:

  1. Set up the 2 pairs of tangent tracks with 2" spacing (use parallel tool if you like to get exectly 2") 
  2. Select the connect tool and set the locked radius to 32"
  3. Connect the inner two tangent tracks with "Connect with easements fixed"
  4. Keep the locked radius at 32"
  5. Connect the outter two tangent tracks with "Connect with easements fixed"

This will give you two curves which consume the minimum amount to tangent for your chosen radius and easment settings.  The curves will have the widest separation in the middle of the curve, which is where side swiping is most likely.  Unfortunately the non-constant separation in the curves might look a little unrealistic to you.  You might be able to hide this with super elevation.  If  the non-constant separation in the curves bothers you then I think your only option is the have 2 1/4 " separation on the tangents going into the curve from each end.  I suspect this is a constraint imposed by Euclidian geometry, and is not a shortfall of 3rd Planit. (I am qualifiying my statements in anticipation of an expert contradicting me

Regards

Jan
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GtLiving New member 3 posts
GtLiving

Figured out how to make it work

Thanks for batten and 3rd Planit suggestions.

I resigned to longer easements, but no longer than necessary. I hadn't used the fixed radius connection tool. I'm using it now to keep spiral length to a minimum.

The challenge the offset between curve and straight track. In John Armstrong's book Fig 8-8 pg 73, the suggest for broad curves is an offset of 1/2". For sharper curves the offset is less, 3/8" for narrow. I am thinking the suggested larger offset for broad curves is more about looks. If I use 3/8" offset on the outer track then I can use 7/8" instead of an 1" for the inner track offset (that solves connection to existing track issue).

I've finally realizied that 1/2 the spiral is in the curve, so straight tracks approacing at wide angle need broader curves with more curve length to fit 1/2 spiral. If broad enough one can also reduce the separation between parallel curve tracks (The NMRA tool with my equipment indicates a 40"R curve only needs 2 3/8" and 60"R - 2 1/4").

I'm moving forward.

Greg
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