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Three Kinds of Banking

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  • Three Kinds of Banking

    I'm in the process of building a routed 1/32nd scale track which will include two elevated corners. One of those corners I am considering banking.

    Which got me thinking about the ways slot track corners can be banked. I figure there are at least three ways you can bank a curve:

    1) A true bank -- where the surface of the track is a conic surface
    2) A pulled bank -- where the surface of the track changes its bank angle due to mechanical stress
    3) A planar bank -- where the surface of the track is flat, but tilted

    Let me explain...

    What I call a true bank is a surface that is mathematically part of a cone. The illustration below -- pulled for Wikipedia -- shows it well.

    Untitled.jpg

    The angle of the bank stays constant around the entire curve. It is relatively easy to create a true bank as long as the angle of the bank is shallow. You simply insert wedges under the track and fasten it down. This puts a lot of stress in the track, but it will work. Trying to create a high bank this way, however, can overstress the track, breaking it.

    Also, transitioning from a straight onto a true bank can be an issue. The straight has to tilt, and that can cause cars to slide before they get to the corner. Also the cars in the outside lanes get pushed up going into the corner, then dropped as they leave. That dropping makes it more likely the cars will slide downward on the straight leaving the corner.

    All of the above make a true bank best limited to shallow angles of bank.

    The second kind of banking -- what I call a pulled bank -- is a much more complex surface from a mathematical viewpoint, but a much simpler curve to create mechanically. The most practical way to create a pulled bank is to take a flat sheet of MDF (Medium Density Fiberboard -- the traditional material for routing slot tracks) and cut out a curve with attached straights. Something that looks like a 'U". If you pull the two ends of the U towards each other the curved section of the U will naturally bend into a banked surface. The straight sections of the U will twist and form smooth transitions onto and off of the banking. The photo below shows a pulled bank.

    When you create a pulled bank the included angle of the curve changes. To create a 180 degree banked curve you need to start with a flat curve of maybe 178 or 176 degrees. The curve as you cut it out of the MDF looks like something between a 'V' and a 'U'. The trick in creating a pulled bank is figuring out just what that included angle needs to be to get the degree of banking you want. You can work it out mathematically but it is possible to make scale models out of poster board and figure it out experimentally. I've done that making 1/12th scale models, and it works well.

    IMG_3137.jpeg
    On a pulled bank the bank angle is not constant. It is highest at the center of the curve. This is a result of the mechanical forces twisting the track. But the track itself is much less stressed than a true bank would be.

    Because the bank angle of a pulled bank starts shallow, becomes steep, and then becomes shallow on exit cars transition into and out of the curve with much less tendency to slide on the straights. The curve in the photo above has a bank angle of 20 degrees in the center, but closer to 10 degrees on entry and exit.

    The third kind of bank is what I call a planar bank. Planar as in plane. Or to use a more common term, flat. It is simply a flat curve that has been tilted. It is the simplest type of banking to create, and one that does not stress the track, at least not in the corner itself. Like a pulled bank, cars going around a flat bank will see the most effective banking at the center of the curve. I say 'effective' because the bank angle does not change, but the direction of the cars changes along the banking. When the cars are in the center of the curve they are tilted in the 'roll' direction. If we are talking about a 180 degree curve, on entry and exit they are tilted only in the 'pitch' direction -- which doesn't help in cornering. A 90 degree corner should work better with a planar bank.

    On my track I am considering using a planar bank on that one elevated curve. It could make it especially tricky because the included angle of that corner is greater than 180 degrees. On exit it would have reverse banking! Horror!

    Oh I am cruel...

    So, three kinds of banking -- each with their advantages and disadvantages. My own preference is for pulled banked curves -- they are easy to make once you have worked out the included angle, and cars perform well on them. But others may have different preferences with their own reasons.

    Ed Bianchi

    PS - Jeff Stillwell of Stillwell Racing Enterprises used to have a huge HO track with a 180 degree curve that had a reverse planar bank. The banking didn't help the cars corner, it helped them spin out! Worse yet, the corner was located so that folks at the drivers' stations could not see it! You drove blind into that corner and prayed your car would come out the other side! Demonic!





    Last edited by HO RacePro; 06-24-2021, 05:36 AM.

  • #2
    I used to race on a track that was not banked anywhere, but it did have one corner with an off camber exit and that was bad enough. That particular track was built like a commercial raceway and the off camber portion happened by accident. Eventually the builder dropped out of the hobby and the track was moved to a new location. After the track was re-assembled the off camber was less pronounced.
    I recall Jeff (Homer) Stillwell's posts on another slot BB, I would not have thought he was capable of such a nasty trick.
    Here is a dandy chart for doing banked turns.

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    • #3
      There is one more type of banked curve I've seen -- not on slot tracks, but in the real world. There is one at Brooklands in the UK, and one at Monza. Neither is used for racing anymore. Both are in dilapidated condition. (Well, actually, the Brooklands curve was used for a slot race some years ago. For real.)

      I'd call this type of banking "toroidal", in that the racing surface looks like part of a torus. (And what is a torus? That's the shape of a doughnut!)

      Untitled.jpg

      Here is a photo of the toroidal banked curve on a test track owned by Daimler-Benz.

      Untitled.jpg

      Not what you'd call a GP car, what? Some crazy things happen at Mercedes!

      And not really the kind of curve you'd want for a slot track. The outer lanes would have a great advantage over the inner lanes. Besides that, it would be very difficult to construct.

      But yeah, that is a fourth type of banked curve.

      Ed Bianchi

      PS - Jeff Stillwell is still extant and entertaining us. I had a long phone conversation with him just a bit ago. Good man Homer!
      Last edited by HO RacePro; 06-24-2021, 08:00 AM.

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