What is the best girth to have?

[montage]

I heard that the wider the girth, the stiffer the ride, but some people find a thinner tube gives them more pleasure

 

[Headgardener]

Montage I am trying to fathom out exactly what you are talking about:?:.

 

[PpPete]

I'm a thin tube man myself.

 

[gavintc]

My best has a really thick one and a massive bottom bracket.

 

[Garz]

My wife has always preferred girth, it offers a very stiff ride!

 

[PpPete]

That's what she tells YOU!

 

[02GF74]

I heard that the wider the girth, the stiffer the ride.

where did you hear that?

take a minute and think about it before realising it is nonsense.

not only is the tube diameter a factor but just as importantly the tube wall thickness and material it is made of.

think of a bike made from baking foil - makes no difference to tube diameter, big or small, it is not going to be stiff.

now if we keep the cross section constant, then the larger diameter tubes will have thinner walls.

Let's start with a solid rod then gradually make the tube larger diameter so it has to have thinner walls ending up with a large diameter tube and foil thin walls.

the tubes will be "stiffer" (again it depends on you definition of stiiff and how you measure it) than the solid rod but there comes a point when the wall thickness gets too thin it becomes less "stiff". there is data out there that you can use to figure out the tube dimension you need for the particular application.

 

[Crankarm]

I don't like the vibration a stiff ride gives so cushion myself with a suspension seat post .

 

[RecordAceFromNew]

Mmmm all else being equal (say for the same material, length and weight), it is theoretically true that the wider the girth the stiffer a tube will be under bending load. It is because the stiffness of a beam (whatever the shape be it an i-beam or a tube) being bent is dependent firstly on how it is supported at the ends, and secondly the "second moment of inertia" of its cross section, see http://en.wikipedia.org/wiki/Beam_(structure)

As a tube's diameter increases (and the wall gets corresponding thinner), it will buckle under a certain load as someone mentioned above. Buckling however is a catastrophic failure sensitive to many parameters.

In practice, frame joint/material/design/manufacturing etc. obviously also have important influence on its stiffness. Some interesting (and perhaps surprising...) stiffness test results of actual bike frames can be seen at http://materials.open.ac.uk/bikeframes/bikeframe.htm