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PostPosted: Sun Jan 11, 2009 9:59 pm 
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Joined: Mon Jul 02, 2007 10:39 am
Posts: 37
Location: Columbia, SC
Hi everybody,

I'm an old, retired mechanical enginer seeking some advice.

I've had a request from a potential customer who is planning an industrial layout and wants to use #4 turnouts with inner radii of 20 in. and maybe some that go down as low as 12 in. He says short cars and small 4 wheel tank type engines are planned for his roster.

I have never made up a curved switch using Twist Ties, but I have some #6's on hand.

So ...... I drew a 20 in. radius on a piece of balsa plywood and pinned the #6 down to it. It looks like it will be alright. I've put in an order for #4's

Now the question ...............

Given that the inner radius is 20 in. what is the outer radius?

I used some curved, track laying templates to determine that it is close to 26 in. How close am I?

Can some one steer me to a table of information (or give me a formula) that would define the inner and out radii of the various possibilities (#5's, #6's, #8's, etc.)? I've forgotten most of what used to know about circle segments, chords, etc.

Thank You,

Chuck Lobaugh

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"oldsnowtop" ... A seller of switches on eBay


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PostPosted: Mon Jan 12, 2009 12:48 am 
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Joined: Sat Aug 13, 2005 10:48 am
Posts: 365
Location: East Texas - USA
Chuck,

Most "rules of thumb" suggest about a 1:1.25 relationship of curved turnout radius offsets. Because of the differences in the point lengths, and the straight portion of the frogs, there is no actual formula for reliable radius of curved turnouts. Doing the minimal as the diverging side is a FAR better approach than using the broader radius as the control radius and attempting to determine the diverging minimum radius.

A number 4 straight turnout (NMRA geometry) has a 15" (in HO) radius - so only those radius smaller that 15" will result in a curved turnout. Curving a number 6 (with normal diverging radius of 43") down to 20" forces a longer turnout than is strictly necessary. Using a number 5 with a normal of 26" radius would be a more likely candidate for the 20" minimum. The ideal is to use the straight turnout number that's normal diverging radius is within about a closest to the desired radius to find the optimal shortest turnout. Reliability will always be best with the longest turnout one can accommodate. Radius alone is not the controlling factor. The relationship of the frog (either as a straight section or actually curved), and the geometry of the points, together with the wheel bases of the equipment operated are all interrelated. Judicious gauge widening and controlling of the check gauge also plays a part in the degree of compression allowed and still remain reliable.

Note that if the broader radius needs to accommodate a larger class of equipment and only the diverging shorter radius will be constrained to the shorter equipment, then the larger numbered frogs will certainly be a far better approach. It's not a simple answer for curved turnout geometry.

-ed-

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COSLAR RR - http://www.coslar.us/
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PostPosted: Mon Jan 12, 2009 11:10 am 
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Joined: Fri May 07, 2004 11:22 pm
Posts: 311
Location: Port Dover, Ontario
Hi,

Using a #4 TwistTie and curving the inner radius to as close as 20" as possible, the outer radius will be close to 55"

The high ratio of a #4 turnout results in a large difference between the inner and outer routes when curved. Since the inner route is a compound arc, it can only be approximated and not precisely set to a 20" radius.

Using software, I am able to take a straight turnout and bend it, then measure the resulting drawing to get dimensions. When a #4 Fast Tracks TwistTie is curved to about 20" inside, the outside radius falls around 55" or so.

Hope this helps.

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Tim Warris
Fast Tracks
http://www.fast-tracks.net
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PostPosted: Tue Jan 13, 2009 1:08 am 
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Joined: Mon Jul 02, 2007 10:39 am
Posts: 37
Location: Columbia, SC
Ed, Tim,

Thank you for your replies. I'm going ahead and making up a switch jig using a #6 Left Twist Tie.

I plan on using the Twist Tie ties (say that 4 times fast) to locate retaining pins so that I can place PC ties in position to solder the rail.

I'll take some pics as I go along and share them later.

Tim, your 55 in. answer seems to be too shallow.

The pinning of the Twist tie to the substrate (on a 20 in. radius) does seem to make a 25-26 in. outer radius.

Chuck Lobaugh

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"oldsnowtop" ... A seller of switches on eBay


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PostPosted: Tue Jan 13, 2009 1:15 am 
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Joined: Mon Jul 02, 2007 10:39 am
Posts: 37
Location: Columbia, SC
Tim,

Disregard my comment about the 55 in. in the previous post.

I finally tumbled to the fact that we are talking about apples and oranges.

I will have to wait until youse [sic] guys send me the #4 (which I plan to order) to make any judgement.

Chuck

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Chuck Lobaugh
cfl9280@sc.rr.com
"oldsnowtop" ... A seller of switches on eBay


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PostPosted: Tue Jan 13, 2009 10:05 am 
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Joined: Fri May 07, 2004 11:22 pm
Posts: 311
Location: Port Dover, Ontario
Hi,

Take a look at this newsletter from a while back....

viewtopic.php?f=2&t=278

It documents a technique to use TwistTies to build a curved turnout that also uses PC board ties.

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Tim Warris
Fast Tracks
http://www.fast-tracks.net
service@fast-tracks.net


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PostPosted: Tue Jan 13, 2009 11:13 am 
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Joined: Mon Jul 02, 2007 10:39 am
Posts: 37
Location: Columbia, SC
Tim,

Right on target! Thank you.

More food for thought.

Chuck

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"oldsnowtop" ... A seller of switches on eBay


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