General Cycling Discussion - Material properties of steel vs. aluminum

I'm just a bit baffled and was curious if anyone could shed some light on a particular matter:

I'm currently taking an advanced Materials Science course, and have learned that steel has a higher modulus of elasticity than aluminum. Also aluminum as a material has better vibration dampening than steel. What this directly corresponds to is that steel is supposed to be stiffer and more rigid than aluminum. This goes completely counter to all my experience with bicycle frames.

Does anyone have a physical explanation for this? Is it simply that good steel bikes are made with thinner walled tubes, and aluminum bikes are generally made with larger cross sections? I realize there are a multitude of alloys and tempers for both materials, but my professor seems to believe steel will almost always be more rigid than aluminum.

I'm just a bit baffled and was curious if anyone could shed some light on a particular matter:

I'm currently taking an advanced Materials Science course, and have learned that steel has a higher modulus of elasticity than aluminum. Also aluminum as a material has better vibration dampening than steel. What this directly corresponds to is that steel is supposed to be stiffer and more rigid than aluminum. This goes completely counter to all my experience with bicycle frames.

Does anyone have a physical explanation for this? Is it simply that good steel bikes are made with thinner walled tubes, and aluminum bikes are generally made with larger cross sections? I realize there are a multitude of alloys and tempers for both materials, but my professor seems to believe steel will almost always be more rigid than aluminum.

Someone has something backwards. Steel frames usually turn out more flexible than Al.

You should be able to estimate the force required to produce a certain bend (deflection of one end with the other end in a fixed support, and keep it elastic) in a tube based on typical steel and aluminum frame tubes and published material properties. The effect of diameter and of wall thickness are considerable, with if I recall diameter being dominant.

Is stiffness what you always want in a frame? Personally I don't think so, but that's really a different homework problem, isn't it?

If you looked at maximum tensile and compressive strain in the two tubes (large diameter Al and normal diameter steel CrMo) you'd find larger values in the aluminum for the same deflection. But the Al tube would be closer to the limit of its flexibility than the steel, and would not have as many such cycles before serious degradation due to fatigue set in.


Road Fan

I think heavier wall and larger diameter in the aluminum would be the reason. The prof is correct, the modulus of elasticity is going to be higher with steel, but that doesn't mean the finished structure is going to be stiffer unless the geometery is identical.

Look at their fatigue limits.

Aluminum handles fatigue poorly and aluminum frames have to be designed stiffer to account for it.

Steel has an excellent fatigue limit so steel frames are designed closer to its ultimate strength.

Steel furthermore has a yield point well below its ultimate strength and is therefore less prone to catastrophic failure when pushed close to its limit. You are more likely to notice or feel the problem before the crack gets big enough for the frame to break. Catastrophic failure is bad. Ask any corporate attorney.

The end result is the "feel of steel".

Metallurgy for Cyclists (http://spokesmanbicycles.com/page.cfm?pageID=328)

How can you be taking a materials science course without having passed a course with basic beam theory?

Other than that aluminium and steel have a similar specific modulus of tensile elasticity. Steel frames can't be made both light and with large diameter tubes(stiff) because the walls would need to be foil thin leading to dent and crush failures.
And what they said about aluminium needing to be kept well below it's yield strain to avoid fatigue, steel is not known to fatigue in most cases, and steel is far cheaper than aluminium, by the pound, and generally easier to work with, so there is no point in making a small diameter, thick wall, bike frame from aluminium.
The difference in vibration dampening of the two metals really isn't significant, compared to something like wood, plastic, rubber, structural foam, or even lead

Years since I worked on tubing- but I used to make Kart Frames (Go-Karts) All of these were made of mild steel but there are grades of steel aswell. Taking it that one gauge of steel is used- just changing the quality of the steel would affect handling and the life of the tubing before it broke/cracked/split.

Now using the same wall thickness of tubing in steel and aly-you will get different spring and flex in the two materials. Only thing is that aly would break well before steel would.

The way to overcome this is to use aly of a larger diameter and different wall thickness. This would give it a longer life and also affect the handling characteristics.

So you would not be comparing like for like with the usage of either material.

But by experience- A top rate double butted steel frame made the purpose and with a proven track record by a reputable builder- Would give a far more comfortable ride than a similarly made aly one.

And I do have a top rate aly bike that is stiff- gives a very comfortable ride and works exceptionally well but I think this more down to the CF forks and seat post and the handbuilt wheels that are fitted to it.

Aluminum is lighter than steel but not as strong. So in order to make a frame with aluminum, one has to use larger diameter tubes. Large diameter tubes are much stiffer than narrow diameter tubes. This is why the early aluminum bikes had a reputation for having harsh rides. They did! Steel was seen as more flexible and more compliant because it was.

I understand that engineers now know how to design around the limits of the materials. So a good bike frame can be made out of aluminum, steel, titanium or carbon fiber. Right now, all the buzz gives carbon fiber the clear edge.

Oh great , now the debate is turning scientific..

I think if you took two otherwise identical frames and wrote "stiff" on one and "flexy" on the other, 95% of all people riding it would tell you that the "stiff" frame was a much rougher ride.

Use psychology, not materials science. :)

Does anyone have a physical explanation for this? Is it simply that good steel bikes are made with thinner walled tubes, and aluminum bikes are generally made with larger cross sections? I realize there are a multitude of alloys and tempers for both materials, but my professor seems to believe steel will almost always be more rigid than aluminum.

That's it. Aluminum bikes made of the same diameter tubes are incredibly flexible (old Alan frames or Vitus frames come to mind) and not terribly strong (you don't find a lot of the old Alan or Vitus frames around:rolleyes:). Increase the diameter and the tubes can be made thinner and the frame stronger and stiffer.

Steel is always more rigid than aluminum. That's why they make springs out of the stuff:rolleyes:

Steel is always more rigid than aluminum. That's why they make springs out of the stuff:rolleyes:

That and steel has an infinite fatigue life below it's yield point, aluminium does not.
Titanium alloys also have infinite fatigue life but are much more moola and not as stiff or strong as good steel in direct tension size for size, but its lighter, so more can be used, this is why Ti spokes never really caught on(bigger=less aerodynamic).

Magnets won't stick to an aluminum bike.

That's why they don't make Refrigerators out of aluminum.

I'm currently taking an advanced Materials Science course, and have learned that steel has a higher modulus of elasticity than aluminum. Also aluminum as a material has better vibration dampening than steel. What this directly corresponds to is that steel is supposed to be stiffer and more rigid than aluminum. This goes completely counter to all my experience with bicycle frames.

Why that's easy. It's because you don't ride on steel or on aluminum. You ride on a bike frame that was made of either steel or aluminum.

Try to find somebody who owns an Alan or Vitus aluminum bicycle. They were built with tubes that were roughly the same size as the steel tubes that had been used at that time. The French aluminum bikes were very light but very noodly. Exactly what your materials course has taught you to expect.

Magnets won't stick to an aluminum bike.

And I thought I got a bad batch of magnets.

Steel is always more rigid than aluminum. That's why they make springs out of the stuff. No, steel is more elastic...that's why they make springs out of steel.

It isn't just the material that matters. The shape and size are very important in determining strength and stiffness of a structure.

No, steel is more elastic...that's why they make springs out of steel.

Steel has a higher elastic modulus. This means that it is stiffer and resists deformation when bent. Aluminum is more elastic and deforms when it bends. That's the reason you don't want to make springs out of it.

That and steel has an infinite fatigue life below it's yield point, aluminium does not.
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Not quite correct. Steel does indeed have a fatigue point, where if its alternating stress is less than the point, will have infinite life. This does not mean that alternating stress below the yield point will give infinite life. The point varies upon alloy but is generally in 20-40% of its yield.

Aluminum has no such point, and any alternating stress no matter how little reduces its life span.

Not quite correct. Steel does indeed have a fatigue point, where if its alternating stress is less than the point, will have infinite life. This does not mean that alternating stress below the yield point will give infinite life. The point varies upon alloy but is generally in 20-40% of its yield.

Aluminum has no such point, and any alternating stress no matter how little reduces its life span.

Bingo!
cyclic stress fatigue - that's why you make springs out of steel and not aluminum, and why you want the stiffest possible aluminum bike frame.

Ride a steel frame then ride an AL frame. The steel one will feel like a wet noodle. Steel like they say, is real. Real ****ing slow.

Steel has a higher elastic modulus. This means that it is stiffer and resists deformation when bent. Aluminum is more elastic and deforms when it bends. That's the reason you don't want to make springs out of it.Elastic means that it springs back when deformed. What you're describing for aluminum is plastic deformation.

Does anyone have a physical explanation for this? Is it simply that good steel bikes are made with thinner walled tubes, and aluminum bikes are generally made with larger cross sections?

I never hear it discussed much in these material discussions, but frame geometry makes a big difference in feel. A lot of steel bikes (old and new) have longer chainstays, longer wheelbases, and thin seat stays - all contribute to a more compliant ride.

Bob

If steel has more flexibility than aluminum(modulus of elasticity) how can the teacher say that it is stiffer? Aluminum does not soak up vibrations better than steel.Maybe by weight,but not be volume.In solid objects,mass or the ability to give,soaks up vibrations.Aluminum is not know for either of those qualities.

There's a reason they make most racing car frames from steel and it's not because they don't have the money to make it out of aluminum.They have major stress related problems after racing it.

As a materials science and engineering major graduating in less than a month, I love these discussions. Its the shape of the tube, not the actual material in this case. That's why there are more jobs out there for mechanical engineers than materials engineers. The mechanical engineers compensated for the shortcomings of aluminum by making sure that it never flexed enough to cause significant fatiguing. They did this with giant tubes.

I think we should get into the precipitate hardening properties of copper aluminum alloys vs copper, magnesium zinc alloys. Or perhaps how the Hall-Petch relation applies to the grain refining effect of niobium in high strength steel.

PS...is anyone out there looking to hire an entry level materials engineer in the next few months or so? :D

Elastic means that it springs back when deformed. What you're describing for aluminum is plastic deformation.

You have it backwards. Look here (http://en.wikipedia.org/wiki/Young%27s_modulus). A lower Young's modulus (aka elastic modulus) means that the material is more elastic. Rubber has a Young's modulus of 0.1 GPa, aluminum has one of 69 Gpa and steel has one of 190 GPa. I certainly wouldn't call rubber nonelastic. The rubber doesn't resist deformation. Hit it with a hammer and it will spring back. Hit a piece of steel with a hammer and it will deform permanently. But it takes much more force to deform it.

I'm currently taking an advanced Materials Science course, and have learned that steel has a higher modulus of elasticity than aluminum. Also aluminum as a material has better vibration dampening than steel. What this directly corresponds to is that steel is supposed to be stiffer and more rigid than aluminum. This goes completely counter to all my experience with bicycle frames.



Steel "rings" while aluminum doesn't. That "ringing" damps vibration by converting it to mechanical
energy which disapates the pulses of the vibration. Sure, you can't hear the ringing but if you put a
vibration probe on the frame while riding you can see it in orders of amplitude.

You have it backwards. Look here (http://en.wikipedia.org/wiki/Young%27s_modulus). A lower Young's modulus (aka elastic modulus) means that the material is more elastic. Rubber has a Young's modulus of 0.1 GPa, aluminum has one of 69 Gpa and steel has one of 190 GPa. I certainly wouldn't call rubber nonelastic. The rubber doesn't resist deformation. Hit it with a hammer and it will spring back. Hit a piece of steel with a hammer and it will deform permanently. But it takes much more force to deform it.

Also not quite right.

Elasticity is the ability to deform or stretch under load and return to it original shape when unloaded.
Modulus is the stiffness for a given size, specific modulus is the modulus divided by the specific gravity.

Also not quite right.

Elasticity is the ability to deform or stretch under load and return to it original shape when unloaded.
Modulus is the stiffness for a given size, specific modulus is the modulus divided by the specific gravity.

From Wikipedia

modulus of elasticity, is the mathematical description of an object or substance's tendency to be deformed elastically (i.e., non-permanently) when a force is applied to it. The elastic modulus of an object is defined as the slope of its stress-strain curve in the elastic deformation region:

http://upload.wikimedia.org/math/f/e/1/fe1e14b874cd4dc2824350ca69376acc.png

where λ (lambda) is the elastic modulus; stress is the force causing the deformation divided by the area to which the force is applied; and strain is the ratio of the change caused by the stress to the original state of the object. If stress is measured in pascals, since strain is a unitless ratio, then the units of λ are pascals as well.

I'll agree that elasticity is the tendency of a substance to return to its original form after deformation. However, there is nothing in the above definition that would be related to the size of the object being tested.

The Webster's definition is

Main Entry:
mod·u·lus
Pronunciation:
\ˈmä-jə-ləs\
Function:
noun
Inflected Form(s):
plural mod·u·li
Etymology:
New Latin, from Latin, small measure
Date:
1753

1 a: the factor by which a logarithm of a number to one base is multiplied to obtain the logarithm of the number to a new base b: absolute value 2 c (1): the number (as a positive integer) or other mathematical entity (as a polynomial) in a congruence that divides the difference of the two congruent members without leaving a remainder — compare residue b (2): the number of different numbers used in a system of modular arithmetic
2: a constant or coefficient that expresses usually numerically the degree to which a body or substance possesses a particular property ([such] as elasticity)


In this case, definition 2 would be the most appropriate. The elastic modulus could also be stated as the elastic coefficient.

Steel "rings" while aluminum doesn't.

I believe you have hit the nail (steel one:rolleyes:) on the head.

What do you mean it doesn't ring,what is that suppose to mean?You've never seen aluminum wind chimes? If it's some kind of shape that should ring and doesn't,I'd be looking for a crack!Hanging a crankshaft,hitting it with a hammer is a great way to pretest for cracks.Good if it rings,you better be looking closely for cracks if it doesn't.Dead soft aluminum won't "ring" as well as tempered will,but it will ring unless it's cracked or has some casting flaw in it.Get an aluminum tube,cut off a piece,hang it from a string,hit it,it will ring.You can probably get lead to ring if you try hard enough.The only metal I can think of off the top off my head that won't ring is mercury,it probably will if you can get it cold enough.You can get water vapor to ring if you freeze it in the shape of a bell.You can probably get all kinds of things to ring if you can get them in a solid enough state to do so.

I do stress relieving at work on all types of metals with vibration and sound waves.Harmonics is one of the main reasons it works.

There's a reason they make most racing car frames from steel and it's not because they don't have the money to make it out of aluminum.They have major stress related problems after racing it.Racing car frames (pure racing, specially constructed racing cars that aren't constrained by rules) use carbon fiber these days.

Generally an aluminium-honeycomb cored carbon-fiber/epoxy composite sandwich, based monocock chassis. Non repairable, but if made right it absorbs a lot of impact in a crash and it's close to the lightest stiffest fanciest thing out there. (Burt Rutan's Voyager airplane was made very similarly. Went non stop round the world, without refueling.)

You have it backwards. Look here (http://en.wikipedia.org/wiki/Young%27s_modulus). A lower Young's modulus (aka elastic modulus) means that the material is more elastic. Rubber has a Young's modulus of 0.1 GPa, aluminum has one of 69 Gpa and steel has one of 190 GPa. I certainly wouldn't call rubber nonelastic. The rubber doesn't resist deformation. Hit it with a hammer and it will spring back. Hit a piece of steel with a hammer and it will deform permanently. But it takes much more force to deform it.

I'm an electrical engineer who simply evaluates a lot of motors, vibration, and audible noise, as well as the dampening associated with them so excuse me if I'm wrong but doesn't plastic deformation imply more hysterisis?

Yes, there a modulus of elasticity established for aluminum. However, there is, more or less, enough hysteresis that "below the yield point" is a vague concept when you're dealing with aluminum. When you design a bike frame with it, you have to limit the yield to where the hysterisis doesn't result in metal fatigue within a reasonable lifespan.

Below the yield point works well when you're working with steel. You therefore design around low cycle fatigue instead of high cycle fatigue.


Ride a steel frame then ride an AL frame. The steel one will feel like a wet noodle. Steel like they say, is real. Real ****ing slow.

There are a lot of riders out here wishing I was real ****ing slow on my double butted Reynolds 631 frame.

I once went fishing out by San Clemente Island. We dropped anchor and I must have cast out bait for an hour with out a bite. So we decided to troll for Yellowtail. Even when they were running we didn't get as many bites as this. I have to wonder what kind of saddle material the OPs professor says is best? Sorry I just felt we were getting off the track here. The question was if the professor was correct or did we get the correct statement made by that professor. Neither matters if the OP is taking a test, the answer is always what the professor said in class.

The OP's original question indicated he was taking an "Advanced Materials Science" course. I wonder what they teach in the "Introductory Materials Science" course.

I think if you took two otherwise identical frames and wrote "stiff" on one and "flexy" on the other, 95% of all people riding it would tell you that the "stiff" frame was a much rougher ride.

Use psychology, not materials science. :)

I agree completely!

I couldn't tell different frames and frame materials apart if my life depended on it, and I dare say that most people are seriously deluding themselves, either consciously or subconsciously.