1. If you have a cantilevered tube that constantly varies in wall thickness (but not diameter) in "perfect" profile – i.e. tapered so that when a load is placed on it every part of the tube is subject to the same stress, is it stronger than a tube which has the wall thickness of the thickest part all the way along? (because the stress is concentrated at the support end).

I am thinking of a seatpost, or s/g handlebars btw.

2. If you have a hrdroformes tube, say a downtube, which started life as a s/g round tube, it is no stronger at its wider end (or so I am told). But does its increased stiffness transfer load to further down the tube thereby increasing strength of the tube overall?

Armchair engineer comments welcome 😉

you tube 🙄

😡

1) It's not *stronger* no, it just has less wasted material – meaning all parts of the beam are equally close to failure, rather than wasted section where some of it wouldnt ever fail before the another. What tapering/butting allows you to do is reduce weight. It might be considered stronger if you had to calculate involving self-weight (very large structures or complex loading). For bars etc, tapering/butting just allows weight reduction for the same strength. But you will sacrifice rigidity by tapering, as all the material is now loaded more greatly.

2) Again, far too complex to give definitive answers without knowing the items in question and the loading mechanism, but ultimately if you take a hydroformed tube you thin the walls to make a larger diam, this'll be more rigid and lighter (than a smaller diam tube with the same rigidity) but if loaded properly will not be weaker and will indeed transfer some of the load down the tube to where it is thicker (top of the head thoughts, might require a re-think). But that'll all be variable based on things like what are the other tubes like, what are the proportions of things, where are they welded to a suspension pivot…etc etc

1) the one that doesnt vary but is the thickness of the varying ones thickest point.

2) tricky one, the force will move down the tube.

That bent and broken kona from the other day looked a good example, extreme force will find the weakest point, it was hard to say what went wrong with that bike, did it bend first then break or break first then bend??

Tinsy, it bent first, if it had snapped at the welds first there would have been no way to apply the forces required to bend the tubes from that point on.

will it blend?

Cheers chaps. I like a good technical discussion 😀

FWIW the kona bent and (gradually) broke at the same time IMO…the tubes could not have bent without remaining attached to the seat tube, but in bending they gradually broke the welds as they pulled away from teh seat tube.

thicker tubes are stronger and stiffer (for a given weight) than their thiner walled counterparts.

However you move from one mode of failiure to another. If you stand perfectly still on an upright coke can it will hold your weight, apply slightly more weight (i.e. concetrate it on one side of the can) and it crumples.

If you make a tube from paper (or even better start off with several A5 sheets and make various diameter/wall thickness tubes) you can crumple them between your hands like a concertina.

Fishing rods are tapered and butted, and they tend to snap at a fialry random point (usualy an eyelet)

Indeed. In fact you could say its an indicator of a fairly well designed frame – all areas took equal damage while absorbing the energy – if the head tube had sheared right off you could say the rest was overly strong and thus a waste of material.

thisisnotaspoon – buckling of the compressed side of the tube could indeed occur if the tube was too thin. This is why its a complex analysis (to the OP). Thicker larger diam tubes are stronger than thinner, but not necessarily stronger than butted/tapered ones, it depends on your loading arrangement.