The Youngs modulus of steel is 210GPa aluminium is only 75GPa.
I reckon that the max diameter of the stays and size of the rear triangle are constrained such that an Aluminium alloy frame of a similar weight to a Steel one will be more flexible. It's not possible to oversize the aluminum tubes enough to get the same stiffness without the wall thickness being so thin you get end up with a crushed cola can bike.
Aluminium is less dense than steel, so the wall thicknesses don't have to be stupidly thin, although there are bikes that aren't very knock resistant around.
Early Alu bikes such as Vitus and Alan were very flexy compared to Steel bikes of the era, 531 etc. But modern steel bikes are oversized and are stiffer than they were. Alu is more oversized and still stiffer. The recent use of Hydro forming is putting material where it is needed so making frames still stiffer in certain directions rather than in all.
you are right of course. a lump of steel is 3 times stiffer than a lump of aluminium. but it's what we do with it that's important;
Aluminium has very poor fatigue properties, if it flexes, it will fail
(everything flexes).
Steel is great for making springs, at stress levels aproximately half the yield strength a steel spring will last forever
(not even titanium does this).
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Steel frames can be built from skinny tubes which are flexible.
Aluminium frames have to be built from large diameter tubes which are stiffer.
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the stiffness of a tube has as much to do with it's shape/size as it's material.
if you built 2 identical frames, one from steel, the other from ally, the steel frame would be 3 times stiffer, but also 3 times heavier.
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We've decided that a bicycle frame should weigh 2ish Kg, and with that amount of material, An ally frame is either oversized/stiff, or normal/skinny/flexy and broken through fatigue. whereas a steel frame is either normal/skinny/flexy, or oversized/stiff and paper-thin, and broken though terminal denting.
An aluminium frame WILL fail, eventually. A steel frame might just last forever.
personally, i love steel bikes.
Read this article about recent frame breaks:
[url] http://brightonmtb.org/2009/05/13/bad-luck-or/#more-1240 [/url]
The recent use of Hydro forming is putting material where it is needed so making frames still stiffer in certain directions rather than in all.
Hydro forming only changes the shape of the tube - does not move material along it's lenght. Therefore it affects stiffness but not strength.
steel has a higher modulous than alu, but halso has comparable increaces in density and strength. So you can use half to 1/3 of the volume of steel and build a frame to the same weight, stiffness and strength as an alu one. Thats why comaprable frames weigh about the same (stiffee Vs PA, both arround 5lb for example).
But as there's more alu to work with you can make the tube diameter bigger, without danger of the tubes buckling or being pinched. Steel is already well under 1mm thick when formed into tubes.
Hydro forming only changes the shape of the tube - does not move material along it's length
True Al but they're getting very clever with it now and starting with tubes that aren't one thickness to start so that in combination with hydroforming, it does exactly what you quoted...
Steel and aluminium have about the same specific modulus, but very roughly speaking, the stiffness of a thin walled tube increases with the major dimension to the power of three, where as mass only increases with the same dim to the power of one (approximately). So the stiffness goes up much faster than the mass as you make the tube bigger. As long as you’re not (too) constrained by overall dimensions, ally tube can be stiffer for the same mass or lighter for the same stiffness (40%ish).
Then of course you need to think about buckling, fatigue, impact, shaping, welding, threading etc… which is where it gets complicated I think.
rob harold is on the money it gets complicated
hydrofrming question marks remain????
I reckon that the max diameter of the stays and size of the rear triangle are constrained such that an Aluminium alloy frame of a similar weight to a Steel one will be more flexible. It's
The rear triangle is fairly well triangulated in in all three planes so I don't think that its hard to make rigid in either material. Its the front end where AL gets the big gains as the tubes can be so big.
As has been said a 1mm wall thickness Al tube made three times the diameter of a 1mm steel tube will have the about the same mass, but be about 6 times stiffer in bending. I don't think you could do that on a bike but it illustrates the point
"The recent use of Hydro forming is putting material where it is needed so making frames still stiffer in certain directions rather than in all"
it can shape tubes with this as an aim but hydroforming often means little control over actual wall thickness and i've seen variances of 0.5mm in the same tubes at the head tube end - not good at all. manufacturing quality varies from factory to factory but HF is risky unless done to a very high tolerance standard. it also means that thinner walls can't be done so HF frames are usually hevier than required. fashion over function..