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.

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".

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. :)

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:

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.

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.

And I thought I got a bad batch of magnets.

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.

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.

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.

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

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.