Tuesday, September 16, 2014

What's Wrong With This Picture? Part 2 Answer

A few days back I posted this picture from the 1973 Estes catalog - What's wrong with it?

Lonnie responded:
If the inside and outside diameters are correct, the tube thickness should be .04" instead of 0.021.
From RichsRockets :
No, because inside and outside diameters differ by TWICE the tube thickness... think about it.
Anonymous wrote:
I remember seeing this ad as a kid and wondering what parallel winding meant and why was a line drawn around the center of the tube. Still doesn't make sense to me.

Here's how the drawing should have looked:

The illustration in the 1973 catalog looked like two smaller length tubes joined by an internal coupler. Parallel wound BT-30 tubes had a wrap joint that ran down the length of the tube.

The inside and outside tube dimensions don't add up!
Outside Diameter           .765" 
Inside Diameter              .725
                                         .40 ?

BT-30 tubes were thicker walled, more like a BT-55 or BT-60, but not that thick!

The listed wall thickness is listed as .021". That's probably close to the truth.
I would guess the printed inside diameter was incorrect.

TRIVIA: I read somewhere the original BT-30 sized tubes were made from file folder manila material. Gleda Estes hand rolled them on the kitchen floor. 


  1. If the inner and outer diameters were exactly 0.725" and 0.765" then the wall thickness would be exactly 0.020". HALF the difference between the inner and outer diameters (because the outer diameter includes two wall thicknesses).

    If the numbers were rounded the inside diameter could be as low as 0.7245" and the outside as large as 0.76549999..." Then the difference would be just under 0.0410" and the wall thickness would be 0.0205", which rounds down to 0.020". So 0.021" is indeed wrong, because it's too large... but only by something between 0.0005" and 0.0015".

    1. I should have said "the wall thickness would be JUST UNDER 0.0205" " — hence rounded down to 0.020" and not up to 0.021".

  2. This is why I never succeeded as an engineer.