Square Wheels and Catenary Roads

5 August 2007 | 2 Minute Read | Academic, Play

The other day Paul told me that square wheels (and, indeed, any regular polygon except a triangle) can ride on a road constructed of inverted catenaries. A catenary is the curve made by a telegraph wire when stretched between two poles — a cable hanging under its own weight.

Catenary

A little bit of thinking about the road makes you realise that the length of the catenary s must be the same as the length of the side of the square, and the height h must be

h = s(√2 − 1) / 2

This is because the centre of the square, the axle, must remain at a constant height from the ground for a smooth ride.

Anyway, armed with this information, I spent this afternoon making a real-life version of this. I used some string to mark out a catenary of the right size, and used it as a template on two bits of plywood. A fair bit of jigsawing and filing later, I had this:

Model of a catenary road

I was really surprised how well it worked! Check the video to see how smoothly the “wheel” runs along the “road”!

2 Comments

  1. Ivor Biggun

    8 August 2007 19:51

    erm..how is your masters work going? have much spare time? How have I gone so long without knowing what a catenary was, Until till now I thought it was the love child of Sylvester and Tweety Pie. thank you for testing that we can recognise blurred numbers

  2. cmbuckley

    8 August 2007 20:10

    Why are you on AOL?

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