Let's say I want to make a 28" radius loop in HO. When I come out of the loop into straight track is a transition that can cause problems for the train. A video I watched shows how to do this in a 20" segment. Can I just use a 32" radius at the beginning and of the loop as a transition?
What you're talking about is called an "easement curve", or just, an "easement." The name tells what it does, ease the train's transition from curved, to straight, track, or vice versa. Since you have a generous curve of 28" radius, you may not need an easement at all. Easements are generally used on much tighter radii curves commonly used on model railroads, like 18" radius curves. However, if you just want easements, it certainly won't hurt anything to have them.
There are several methods of creating an easement. One "crude but effective" way is the one you suggest, simply add a section of broader (32") radius curve at each end of your 28" curve.
There is also the "bent yardstick method '. To use this, you draw your 28' radius curve accurately with a tramel, or stick radius tool. Then draw a straight line slightly outside where it looks like that straight line might meet the curve. The straight line should stop a least a foot short of the curve however.
Now pin a yardstick right along the straight line. Let the yardstick protrude well beyond the curve. Then gently bend the end of the yardstick toward the curve until some middle part of the stick meets the curve. Trace along the yardstick between the straight line and the 28" radius curve. That connecting line you just drew is the easement curve.
There is also an engineering math formula for determining an ideal easement curve. I've never bothered with it, but both the math formula, and drawings & text for the bent yardstick method, are in John Armstrong's classic book "Track Planning for Realistic Operation." It's a great book to have, as it explains a whole lot about building a model railroad that can be operated like a real one. If you want the book, it's available through www.amazon.com
Hope that answers your question;
Traction Fan :smilie_daumenpos:
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