"a larger diameter tube is stronger than a smaller diameter one of the same weight. "

I thought this was false. Is that a cake?

in which direction?

Stiffer, yes (up to the point where the tube wall is too thin and dents/collapses too easily).

Mmmm…. parkin.

it'll be stiffer but more liable to damage

OK kids let's try again!

"a larger diameter tube is stronger than a smaller diameter one of the same weight. "

I thought it was pretty much common knowledge. Hence massive tubed but thin walled aluminium frames.

if you make a tube twice as big (diameter), you square the strength. if you make a tube twice as thick, you only double the strength.

(more or less)

edited 😉

OK kids let's try again!

"a larger diameter tube is stronger than a smaller diameter one of the same weight."

Then it depends what you mean by stronger. And putting it in bold doesn't make that any clearer.

Does anyone know the difference between strength and stiffness?

EDIT – I'm quoting someone else – I'd assume it to mean "resistance to bending and breaking" or similar.

Under what loading? The strength, in terms of yield/UTS will be material dependant, but the load to failure will be dependant on whether it's tensile/compressive/bending/torsion/mixed-mode.

The question is either ill-conditioned, or the answer is 'maybe' 😉

Edit:

EDIT – I'm quoting someone else – I'd assume it to mean "resistance to bending and breaking" or similar.

Well, which is it?? Resistance to bending OR breaking??

i think that it has the same strength as strength is a material property and related to cross sectional area. no?

Depends on the application, generally for the same volume of material a larger diameter tube is more rigid torsionally as well as in tension/compression. However this does reduce the wall thickness and increases the chance of denting and crushing.

The walls of some roadbike tubes are ridiculously thin for this reason!

As bristolbiker says depends on the loading. The term strength can relate to alot of properties!

It's a reference to a top tube.

BB I'd say bending.

I = ? (do4 – di4) / 64

work it out.

Well then, if the loading is quasi-static and you assume the geometry is PERFECT, then that may be true….. but as the wall thickness gets v thin and the diameter increases, the tendancy to buckling becomes much greater and will occur at a much lower load than if the stresses were calculated purely based on bending theory (My/I as per Wors above). So as the diameter increases/wall thickness reduces the bending LOAD to cause failure will rise and then start to fall as tube buckling becomes the dominant failure mode.

M'be 😉

Trying to discredit a certain calderdale based bike designer?

Just trying to identify the BS wors!

which direction is the F***ing cake? tard.

Like BB says. As stress = My/I, the formula suggests that material furthest from the Neutral Axis (distance 'y') sees the most stress, making it larger means it should be able to handle more. However as 'I' (basically area of particular section) stays constant, the tube gets thinner and then other modes come into play.

i think that it has the same strength as strength is a material property and related to cross sectional area. no?

no, the further the material is from the axis, the greater its resistance to bending and twisting. The same reason girders are made in I-beam sections, often with holes in the web (the vertical part of the 'I')

simon what unit is strength measured in?

edit; for clarity tensile/ compressive strength.

KPa usually and it's obvious where you're heading but simple, linear tensile/compressive strength often isn't all that relevant to bike frames or indeed many real life designs of mechanical items.

If your asking the question you woulsn't understand the answer……

if you make a tube twice as big (diameter), you square the strength. if you make a tube twice as thick, you only double the strength.

(more or less)

Is broadly correct in compression allong the axis of the tube. Strength increases aproximatley with the square of diameter, to a point where the walls are too thin and buckle (which isn't an exact point, but can be predicted fairly accurately).

In tension it would have no effect.

In bending it's more complicated, half* the tubes in compression, halfs* in tension.

*lies, dam lies, and engineering judgement

Does no one like my "is that a cake" joke?

sfb – it depends how you interpret strength. You are interpreting it here as the failure load of the member, but in the engineering sense it would be more commonly interpretted as the specific failure load of a material – i'e failure stress, which is material dependant. Either approach is valid and depends on the framing of the question.

The I-beam analogy is relevant in that the sizes of the I-beams are chosen such that they're failure loads are similar to the their flange or web buckling loads under the primary loading regimes, such that they are 'usable' in most applications (failure under all applied loadings and possible failure loads for each applications needs to be checked though).

Spoon – In the limit, tensile buckling modes are as much a problem as compressive ones, esp for mixed-mode loading, such as that seen by a top tube. I agree that this is unlikely to be an issue for any 'real world' wall thicknesses though.

Does no one like my "is that a cake" joke?

It was a joke?

I'm still waiting for my parkin… *drums fingers*

cynic-al – Member
Does no one like my "is that a cake" joke?

cynic-al – Member
Does no one get my "is that a cake" joke?

cynic-al – Member
Does no one like me?

now stop winding me up and gimme the cake.
and if this is a joke and there isnt any cake, i suggest you make amends quick smart and find a damn cake. pronto.

*googles parkin to see if its actually cakey cake*

Northern speciality. Sort of dense sticky dark ginger cake. Goes very well on night rides 🙂

elliptic – it's a Glaswegian joke, try google.

Ah, I see. There is no actual cake on offer.

*leaves thread in disappointment*

would it be fair to say that it lost something in the explanation?

Yes, it was always going to though so I left it to you losers to do the work to maximise disappointment 8)

al in delivery of disappointment shocker.

Heh. You should speak to my GF!!! 8) OMG etc

sfb – it depends how you interpret strength. You are interpreting it here as the failure load of the member

no I'm not! Where do you think I said that ?

I'm confused, are we talking about stiffness or strength? I thought strength was the stress at which a material begins to yield?

Axially isn't the strength the same?
In bending won't the larger diameter tube yield at a lower degree of deformation?
I would have thought most bike designers don't expect stress in the tubing to get too near the yield point.

I asked for parking in Afan café once, turns out it was a chocolate banana cake, they'd never heard of parkin. I was sad.

That is all.