[rideit]

Q here.
If two bike frames were made, one of steel, and one of aluminum, with the exact same geometry and wall thickness, which would be stiffer?
Would the aluminum one snap like a twig in the first dynamic ride?
What other properties would they exhibit, comparatively?
Just wondering.

 

[ALEXIS_DH]

same tube diameter and same wall thickness, the steel frame would be much stiffer, and much heavier as well.

 

[stevew]

same tube diameter and same wall thickness, the steel frame would be much stiffer, and much heavier as well.

i was always under the assumption that steel had a more forgiving ride than aluminum...

steel is real and all that....

 

[rideit]

So, I guess if the frames both exhibited reasonable weights (say, 3.5-4 pounds), would the aluminum one be strong enough to use?
Isnt steel technically more flexible than steel in this application?

 

[ALEXIS_DH]

i was always under the assumption that steel had a more forgiving ride than aluminum...

steel is real and all that....

Simplifyng, there are 2 "strenghts" for materials.
Ultimate (which is the max tensile force before it breaks) and yield (the max force before it deforms permanently).

Steel is more "elastic"... as in the difference between yield and ultimate isnt as large as it is in aluminum.
I think the whole "steel is real" comes from that. You can engineer more flex in steel than in aluminum, with less risk of failure/fatigue.

 

[canadmos]

Make them both light enough and they'll both fold like a paper air plane.

 

[ALEXIS_DH]

So, I guess if the frames both exhibited reasonable weights (say, 3.5-4 pounds), would the aluminum one be strong enough to use?
Isnt steel technically more flexible than steel in this application?

at the same weight, the aluminum one would be stronger.

 

[rideit]

Thank you, that is a very helpful explanation.

 

[rideit]

at the same weight, the aluminum one would be stronger.

But at the exact same wall thickness?

 

[canadmos]

Also no ChatGPT answer?

 

[Full Trucker]

At the same weight, the aluminum one would be heavier.

FTFY.

 

[chuffer]

The problem with your question is that it makes no sense to build a bike frame with steel and aluminum tubes of the same diameters and wall thicknesses. The "feel" of the frame comes from the second moment of inertia of the tube size selected *in combination* with the modulus of elasticity of the material.

Very simply put: A steel frame built to "alu dimensions" will be absurdly heavy and stiff. Conversely, an alu frame built to "steel dimesions" will be a wet noodle...a light one...

 

[6thElement]

Which one is red? That will be the faster one.

 

[Westy]

i was always under the assumption that steel had a more forgiving ride than aluminum...

steel is real and all that....

With a stronger material you can use a smaller diameter tube, which will have more flex than a larger diameter tube.

 

[AngryMetalsmith]

Could you define "stiffer" ? And is this a hypothetical question or are you researching hardtail options ?

 

[rideit]

The problem with your question is that it makes no sense to build a bike frame with steel and aluminum tubes of the same diameters and wall thicknesses. The "feel" of the frame comes from the second moment of inertia of the tube size selected *in combination* with the modulus of elasticity of the material.

Very simply put: A steel frame built to "alu dimensions" will be absurdly heavy and stiff. Conversely, an alu frame built to "steel dimesions" will be a wet noodle...a light one...

Agree, but this is purely hypothetical.
The debate was/is whether frame material choice was irrelevant, as it was posited that the differences in stiffness and ride quality were ‘all in my head’.
Which I think can’t be true.

 

[Westy]

Modulus of elasticity Bigger number is stiffer for the exact same physical dimensions.

Aluminum 69Gpa
Steel 210 Gpa

*does not specify alloys

 

[AngryMetalsmith]

Ride quality will dimmish over time, but more noticeably with aluminum as it fatigues quicker than steel.

 

[johnbryanpeters]

This whole discussion remains close to meaningless.

 

[rideit]

What discussions aren’t, in the grand scale of time, the ultimate death of ourselves, our species, and ultimately, our planet?

 

[Westy]

Agree, but this is purely hypothetical.
The debate was/is whether frame material choice was irrelevant, as it was posited that the differences in stiffness and ride quality were ‘all in my head’.
Which I think can’t be true.

It is a complex question. There are a variety of design parameters in a tube set that will affect ride quality. Material is certainly one of them, but so is tube diameter thickness, butting, and shape. Most aluminum bikes these days have complex hydroformed tubes. You can also affect the ride quality of a bike by the bend in a tube and the bike geometry. A classical full frame will be stiffer than a more modern compact frame. No one parameter will determine the overall ride quality. All else being equal a bike with extended chainstays like a Mullet will have more flex.

 

[johnbryanpeters]

Material a and material b with the exact same dimensions is inane to start with. One decides what one wants for function, Calculate it out for a, for b, discover differences, select material(s).

 

[Westy]

a black and white world is easier to understand for simple minds.

 

[rideit]

Ride quality will dimmish over time, but more noticeably with aluminum as it fatigues quicker than steel.

Well, if it got more compliant, couldn’t that be perceived as an improvement in ride quality to some people looking for that?

 

[Westy]

A composite tube can be stiff in one direction and flexy in another, can also have variable stiffness along the length, all depending on the direction the fibers are laid up.

 

[rideit]

Tom Ritchey said that Carbon is basically in every way a superior frame building material than steel, except for cost.
(my .02, perhaps not in durability, and end of life recycling and disposal?)
But these days, even high end steel tubing comes close (outside of mold costs).

 

[chuffer]

Agree, but this is purely hypothetical.
The debate was/is whether frame material choice was irrelevant, as it was posited that the differences in stiffness and ride quality were ‘all in my head’.
Which I think can’t be true.

Right, but that isn't what you asked in your OP. You literally asked about two frames of the exact same dimensions with the only difference being the material. I accurately answered the question you posed. You didn't ask the question you really wanted to. ;-)

I think Westy and the Mustache answered your actual question pretty well.

 

[Westy]

Ride quality will dimmish over time, but more noticeably with aluminum as it fatigues quicker than steel.

I think that is a bit of an old wives tale. Metals can be work hardened, I am sure you are familiar with that, copper work hardens quickly. But requires plastic deformation, permanent deformation. If you are work hardening a frame it is fucked.

 

[rideit]

True, it was even unclear in my mind what the actual question was until it had been broken down a bit more.

 

[Westy]

Tom Ritchey said that Carbon is basically in every way a superior frame building material than steel, except for cost.
But these days, even high end steel tubing comes close (outside of mold costs).

If cost weren't an issue a Berrylium frame would probably be superior, fuck it a single carbon isometric crystal.

It is silly to try to isolate or remove any one variable.

 

[johnbryanpeters]

If cost weren't an issue a Berrylium frame would probably be superior, fuck it a single carbon isometric crystal.

It is silly to try to isolate or remove any one variable.

Just don't file beryllium to fit...

https://en.wikipedia.org/wiki/Beryllium_poisoning

 

[Full Trucker]

A composite tube can be stiff in one direction and flexy in another, can also have variable stiffness along the length, all depending on the direction the fibers are laid up.

Ah yes, the elusive horse wearing a party hat: vertically compliant yet laterally rigid.

 

[Westy]

Just don't file beryllium to fit...

https://en.wikipedia.org/wiki/Beryllium_poisoning

Why I use the grinder

 

[Full Trucker]

Why I use grindr

I’ll see myself out.

 

[chuffer]

Fatigue limit is indeed a real thing, but probably not relevant here. Fatigue limit is essentially the stress (the actual engineering definition) level that material can withstand to infinite cycles without failing. For most (as I recall) aluminum alloys the fatigue limit is zero. Meaning that even at very low stress loads aluminum will eventually fail, but "eventually" is not generally relevant to modern bike designs. Probably talking millions or billions of cycles.

I think more so than fundamental material properties like modulus, strength and fatigue limits, proper design is key to how a bike rides these days.

On a side note - I think that my old (1990) lugged cromo paramout frame became more compliant in the first 12 months of riding and then kind of plateaued until I finally killed it in 2009. My much more modern (~2009?) tig welded salsa cromo frame never really seemed to change much in compliance. Every now and then I wonder why that is. I suspect it is probably the silver brazing on the paramout vs. the tig on the salsa.

 

[johnbryanpeters]

Why I use the grinder

The guidance systems I worked on in Cambridge had beryllium frames. Still wish I had one as a souvenier.

 

[AngryMetalsmith]

Well, if it got more compliant, couldn’t that be perceived as an improvement in ride quality to some people looking for that?

No, it does not become more compliant, aluminum begins to become work hardened much faster than steel which results in brittle material prone to failure. Steel, by nature, is more compliant than aluminum.

 

[rideit]

Aha.

 

[jdcamb]

The Pirates look pretty dismal again. Jeez.

 

[AngryMetalsmith]

I think that is a bit of an old wives tale. Metals can be work hardened, I am sure you are familiar with that, copper work hardens quickly. But requires plastic deformation, permanent deformation. If you are work hardening a frame it is fucked.

Riding is work hardening.