I'm stuck in understanding size tables of a bike.

For example this size chart: enter image description here

Am I right in my thoughts? Effective top tube (ETT) is horizontal value so, it's B1 on the chart. BS is the length of line parallel to the frame. But then why BS is less then B1? (if B1 is horizontal, than it's cathetus of the triangle, it should be less then hypotenuse)

Or ETT is not horizontal line, but the line parallel to the frame?

And also, what is A1 on the picture? It looks like the main value (size of those bikes are called by A1 values), but what is the difference with AS value?

  • B1 is the hippopotamus, not BS. Jun 27, 2014 at 17:57
  • (Don't ask me, though, to define the other two hides.) Jun 27, 2014 at 17:58
  • (But the net is that the manufacturer can define any measurement they wish, any way that they wish. Only a few are even halfway standardized.) Jun 27, 2014 at 18:01

1 Answer 1


If you look at the full resolution it is clear. BS is center to center (head to seat) along the (sloped) top tube. Since the seat tube is slanted that is a shorter distance. Geomemtry C2C

Cathetus and hypotenuse apply to a right (90 degree) triangle. There is not a 90 degree triangle on the BS.

enter image description here

  • Well, there is a 90 triangle involved but BS is the hypotenuse, not a side. B1 is the ETT dimension. A1 is "effective seat tube", ie. how long the seat tube would be on a bike with horizontal top tube. ( A very odd measurement btw ). Jun 27, 2014 at 15:38
  • @FredtheMagicWonderDog That inner triangle is not necessarily 90 degrees.
    – paparazzo
    Jun 27, 2014 at 15:49

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