Automobile turns are necessarily eased due to the decreasing radius from tangent movement to the best-fit until the radius begins to increase again to the next movement along a tangent path. IOW, you are driving straight, you commence a turn, even a tight street or driveway entrance, and you are turning your steering wheel. With each arc of steering wheel movement, your tire path radius begins to tighten, meaning it is essentially an eased curve.
Good discussion on Automobiles vs Train easements. That’s something new I had not thought about.
Yes, thats right out of John Armstrongs Track Planning For Realistic Operation book - I still have mine which I bought in the 1980’s and it’s well worn. I dug it out of storage boxes to refresh my memory when starting my layout last year.
Basically I use an offset of approx. half inch when making easements on 30 (or in my case 32) inch curves - it’s not 100% dead on per his table but pretty close. One reason I like using Atlas flex track is it is springy and it naturally springs to the spriral easement if I use a few points marked as references at the start-point (from the straight tangent), mid-point (off-set) and end-point (where the spiral meets the fixed curve radii).
These in-progress photo’s of the staging yard show the straight to curve areas - all were laid with easements:
Whether or not a highway has spiral transitions or not varies from one state department of transportation to the next. Almost all interstate highways would use spiral transitions. For secondary roads some states use spiral transitions, some use constant radius, and some use compound curves (consecutive curves in the same direction with different radius).
For railroads, AREA recommends spiral transitions for all mainline tracks between tangent and curves and between compound curves. The only constant radius on a modern railroad would be on industrial trackage.
Many years ago in my civil engineering coursework I had to layout all those curves by hand. Today our AutoCAD and/or InRoads software programs automate the process. Makes you really appreciate the talent of the old time railway engineers.
Ray
And current modelers … even if I used a tool to design the easments, I don’t have a GPS guided follow the dot machine to lay track. Oh … but I do have a template made from 1/2" plywood.
Never mind, back to appreciating the talent of old time engineers.
I used to think 30" was pretty broad but then I read The V&O Story about Allan McClelland’s famouts V&O layout. He showed how much better his full length passenger trains looked on broad curves (48" if I remember) than on the tighter curves. For full length passenger cars I think 36" is the bare minimum for good appearance and even that is less than ideal. Unfortunately few of us have the space for the broad curves so we make compromises. I have 36" minimum radius on my mainline and have 3 corner curves. One goes through a tunnel, one a deep cut and the other behind structures so it hides what is somewhat unrealistic appearance wise.
Yep, and because John Allen was not pleased with the appearance of long cars on the 30" curves, he even went as far as to “kib bash” a Walthers 89’ Auto Rack car to shorten it a full panel, to improve it’s appearance on those curves.
I had a 36" curve on a former layout and I know those curves looked sharp under my Walthers 89’ autoracks. It would probably take curves well in excess of 40 inches to improve the appearance significantly. But most of us don’t have an air craft hanger or other large space and have to cope with what we have.