I’m looking at buying a Specialized Carve or Scott Scale 950.

These bikes have different seat tube and seat post diameters and it got me thinking does the diameter of a tube affect the strength and flex of the tube?

Please enlighten me……

It does, but do you really need to out-think the designers?

If you are going to try then a good place to start would be the elastic section modulus equation for a pipe.

r2 is ext radius, r1 is internal radius (d2, d1 being ext dia and int dia respectively)

This gives a geometric property of tubes that is used in strength calcs.
For example the yield moment (the moment to make the tube yield) will be calculated by simply multiplying the section modulus by the yield stress of the material.

Ok. So if the materials are the same and then it does come down to the modulus/diameter?

I ask because I have meant a seat post on the similar Scott model but with the tube being less on the Carve I’m worried more.

I don’t understand is “meant” autocorrect for broke?

If its the same material then yes it comes down to the differences in section modulus. A bigger section modulus will resist a bigger bending moment, and feel stiffer. But I feel sure that you may not notice these things and perhaps that the designers will have considered this.

Unless- are you a Clydesdale? Have you experiences seat tube failures and perceived that they may be due to your volumetric advantage?

If so go for the burly bike..

that they may be due to your volumetric advantage?

very well put so much better than “are you a biffer”

and the equation above says wall thickness is the key issue on strength (assuming material selection is not pink marshmallows) – diameter has little impact – but and that is a big big big BUT how it all goes together – design is the key

There is a noticeable difference in ride quality between say 27.2mm and 30.9mm. I find the narrower ones have an off-putting amount of spring to them. I know someone who found the ride of his recent Santa Cruz Chameleon too harsh, so shimmed down to a 27.2mm (hence the comparison).
There are so many other factors in the mix, that it’s rarely as black and white as that though… Ride them both.

You’re not qualified to comment until you have digested and understood the second moment of area.

very well put so much better than “are you a biffer”

Umm size may be down to enormous bones and massive muscles rather than assumed fat bifferness. I’m not going to wind up a potential bezerking leviathan, are you calling him a biffer?

and the equation above says wall thickness is the key issue on strength (assuming material selection is not pink marshmallows) – diameter has little impact

In the realm of bike tube thicknesses and diameters most engineers consider it the other way round, ie tube radius gives the biggest increase (you need to be careful with increasing radius as if you keep thickness the same you start to get buckling risk).
I find a visual representation helps.
The graph below shows tube thickness vs section modulus for various thicknesses. So, at 0.8mm wall thickness, if you double radius from 10mm to 20mm you go from 222 mm3 to 946 mm3, but if you double thickness at 10mm from 0.8 to 1.6 you from 222 mm3 to 394 mm3, or at 20mm you get 946mm3 to 1781mm3.

So doubling radius gives approx 4 fold increase in elastic section modulus, but doubling thickness only gives a double increase in section modulus.

So despite thickness being cubed and radius only multiplied by 4, you have to remember that the thickness is normally only a small fraction of the radius, which is why the radius dominates.

now I’ve posted this, after 16 minutes when I cant edit, I’ll notice an enormous error, so feel free to pick holes..

PS if you want to check my sums there is an online calc here
http://www.novanumeric.com/samples.php?CalcName=SectionModulus

The graph below shows tube thickness radius vs section modulus for various thicknesses

idiot..

Took longer than 16 minutes 😉

Other factor o’course is that all other things being equal, greater wall thickness means greater dent resistance. And also when tubes get fatter they also often get thinner walled to avoid them getting massively heavy.

(there’s probably a very complicated relationship between tube radius and dent resistance, due to different areas of contact point etc, but let’s not get into that in case Giant decide to sell us slightly thicker tubes to improve rock roll-over)

Took longer than 16 minutes

The children distracted me.

That’s the theory, in practice there are many more practical reasons that will determine why certain manufacturers choose certain tube diameters, such as what’s available as stock material, they’ll buy in bulk so might want to share/standardise tube’s across their model range, and compatibility with components such as seat posts, so it is unlikely that they will be optimising the tube dimensions to suit the theoretical requirements.

not sure if it’s been mentioned but you also have to consider stability.

A wider thinner wall may well be just as stiff or stiffer than a narrower thicker tube but the side walls can get so think that that the tube buckles and dents from clamping and impacts etc too easily.

Oh, also weld area- wide tubes means more weld and broader intersections.

yield moment (the moment to make the tube yield)

Thanks for clarifying that by changing the order of the words 🙂 Now can you clarify absolutely everything else as I haven’t got a clue what anyone’s talking about.

I like the pretty colours though.

I went to art school.

Thanks for clarifying that by changing the order of the words

I’m a natural pedagogue..