WHY STEEL?

This question is often asked, because major companies of the cycling industry opted for carbon and aluminum alloy for their frames.

Their main sales arguments have always been the stiffness and lightness of their products.

Why? Because marketing-wise, these are notions easily explainable and understandable for the common cyclist.

But it’s not because a bike is light and stiff that the dynamic behaviour of the bike is up to the the task of the riders needs.

By the way, In other mechanical sports, especially in motorcycle, brands praise first and foremost a controlled stiffness, just like Husqvarna with their FC 450 motocross chassis:

“…the engineers in Research and Development designed a chromium molybdenum steel frame that redefines every structural aspect that benefits handling, ergonomics and packaging. With carefully calculated torsional and longitudinal flex, the frame works in harmony with the suspension to offer sharp handling and superior comfort.

That’s exactly what we are doing with our frames. Production Privee’s goal is to offer a coherent stiffness for the targeted riding style in order to achieve grip, tolerance, comfort and reliability.

The riding style doesn’t dictate the weight of a bike. Yes the weight is an important topic, but there are more important factors when designing a bike frame:

Geometry,
Tolerance and comfort,
Dynamism
Strength
Reliability
Maintenance

And to be fair, the formulation of bike frame requirements wouldn’t be complete without the notion of cost, an other key factor in the search of solutions.

And so - without talking about aesthetics, brand image, etc…- to the famous question “what’s best?” (question I personally hate), we would answer back:

What’s the purpose? and at what cost?

At this game, steel is a winner, and you can notice it in motorsport, steel chassis are doing podiums every week-end.

The 208 T16 driven by Sébastien Loeb at Pikes Peak is built around a tubular steel chassis.

Why? Because steel is a material which fabrication and use totalise 150 years of experience. The wide range of tools, fabrication processes and heat treatments at the service of the frame designers makes steel a wonderful material with powerful mechanical performance for a reasonable cost of implementation.

Want to know more about our chrome-molybdenum steel and our tubes? Click here https://www.productionprivee.com/en/productions/article/mcs-steel-ktp-flex-system

Want to keep on reading and know even more? Let’s continue!

For the next part of this article we’re going to present you some technical arguments justifying the use of steel for mountain bike frame construction.

The purpose of this part is to present some notions and give some keys in material choice to a broader audience. I had to take some shortcuts and I had to do a few scientific approximations that will hurt the eyes of readers with more specific knowledge. These ones are cordialy invited to a beer to talk about it! (A few ones are already on the way I’m sure).

For the ones that want to know more or have questions, feel free to contact us!

1/MECHANICAL AND PHYSICAL PROPERTIES INFLUENCE

To illustrate this informative article, I need to introduce at least 4 essentials properties:

The Modulus of Elasticity (or Young’s modulus)
The Yield Strength
The Density
The Fatigue strength

1.1/ What’s the modulus of Elasticity, or Young modulus?:
To make it simple, it’s a constant, a number that describes the material “stiffness”.
Let’s take a very simple example: a plastic knife is flexible and can be bent easily. Now take the same knife but made out of steel, it’s hard to bend, it’s stiffer (but it will cut easily a bad quality steak)
The modulus of Elasticity (or Young Modulus) is generally represented in the scientific literature by the letter E. In ascending order (from the more flexible to the stiffest), the Young modulus of an Aluminum alloy is 70, titanium 110, and steel 200. Carbone composites’ Young modulus can vary according to several parameters but in the frame of this informative article we can assume it’s at least the same as steel’s Young modulus.

1.2/ The Yield strength:
It’s the admissible maximum strength before the material becomes unable to get back to its initial shape. f you pass this level of strength, the material deformation is irreversible. Let’s take the knife example, this time made out of aluminum alloy, if you bend it, the knife can take its shape until a certain level of stress, if you bend it too hard, it will remain bent.

Quick fact, the yield strength is almost non-existent with carbon composites. We can approximate the yield strength equals the ultimate strength. The failure is nearly immediate when a certain level of stress is reached. In material science, this behaviour is called “fragile”, because it breaks like a glass.

Chart A. Typical STRESS-STRAIN curve from a tensile test for a metal.

Chart B. Stress-Strain curves for several materials. The bigger the Young modulus E is, the steeper is the slope of the curve, the "stiffer" is the material.

What you can see on these white boards are stress-strain curves coming from tensile test. A tensile test consists in pulling on a normalised material sample till it brakes and record the stress and strains. The picture A is a general stress-strain curve for a metal (for ductile materials). The first part of the curve is straight, this is the domain of elastic strains: if we stop to pull on the material sample, it goes back to its initial shape. If you pull hard enough, we’re going to pass the elastic limit. It means that from there, all the deformations endured by the material sample are irreversible, this is the plastic strain domain. If we continue to pull on the sample it will brake, we can “measure” the stress when that happens, it’s the ultimate strength.

1.3/ The density:
It’s easy, the density is physical value describing the weight of material for a given volume. A cubic meter of water weights 1000kg, of steel 7850kg , of titanium 4500kg, of aluminum 2700kg.

1.4/ The fatigue strength:
The fatigue strength is the ability to resist failure while undergoing cyclic loads, in another word, it’s a property describing the lifespan, the reliability.
At this game, steel and titanium are materials of choice because under a certain stress level, their lifespan can be considered as unlimited. And that is not the case of an aluminum alloy which have a limited lifespan, shorter or longer according to the stress levels of the cyclic loads. Carbon composites have an exceptional lifespan only if their manufacturing processes are from an outstanding level (aerospace and defense industry levels).

2/ GEOMETRICAL INFLUENCE:

In the material science field we refer to moment of inertia or quadratic moment. It’s simply the influence of shape and thickness of a solid on the strength of a structure (just like a bike frame and its tubes).

It’s an important notion that influences the strength of a structure according to the direction of the load. It can be easily represented by a diving board at the swimming pool. The thicker the board the stiffer it is, it’s as simple as that.

3/ MATERIALS COMPARISON:

Now that you are more familiar with these few notions, we can take a look at the performances of aluminum, steel and titanium.

Here’s a small chart:

When comparing materials, it’s quite interesting to compare their density: by dividing their mechanical properties by their density we can get a sense of “performance” compared to their “weight”.

We can notice steel is “stiffer” than aluminum and titanium. It is interesting enough to point out that steel and aluminum have a similar “stiffness” when compared by their density.

Let’s see what’s happening with the yield strength:

Aluminum has the lowest yield strength and steel the biggest.
When compared by their respective density, we can notice Titanium is 12% stronger than steel and aluminum 15% less strong than steel.

Remember the fatigue strength notion? Below a certain stress level, steel and titanium have an unlimited “lifespan”:

It’s important to point out that not only aluminum has a limited fatigue strength but the strength level leading to a premature failure is quite low.

We can also notice that steel has a higher fatigue strength than titanium but to be complete let’s compare that to their respective density:

We can see, that steel is way more performant than aluminum alloy, and titanium is the most performant on the fatigue/density side of things.

And last but not the least, it’s essential to consider the cost of these materials:

A steel frame costs almost twice as much as an alloy frame, and a titanium frame doubles the price of a steel frame.


KTM builds their Grand Prix bikes around a tubular steel chassis.

To sum up, steel offers a good compromise between performance, reliability and cost.

Titanium is an outstanding material but expensive, and “difficult” to deal with when it comes to manufacturing. The aluminum advantages are its low density, a good stiffness/weight ratio, it’s one of the cheapest, and that partly explain its large diffusion on the market. However, the mechanical performances of an aluminum alloy is way lower than steel and titanium.

4/ INTERPRETATION:

At this stage a small example is useful to understand how the designer can play with materials. We will keep the same diving board example, the cantilever beam.

Let’s take a tube made of 6061 T6 alloy and another made of our PP MCS 4130 CRMO steel, with 100kg at their end:

The aim is to calculate the stress level at the basis of the beam, its deflection at the end of the beam and compare the results.


What should we notice about the results?
1/ Beforehand we can note the tubes weight nearly the same despite their geometrical characteristics difference. The steel tube is 11 grams heavier than the alloy one.

2/ The deflection difference: we can notice the steel tube has a bigger deflection, 10mm for the MCS 4130, 6mm for the 6061 T6.

3/ The stress difference between the 2 tubes, 126 MPa on the alloy one and 462 MPa on the PP MCS 4130 one. This difference is only due to the geometrical characteristics of the tubes, the moment of inertia or quadratic moment.
Let’s compare the stress levels against the yield strength of each material.
6061 T6 tube: 6061 T6 yield strength is 255MPa. With 126MPa at the basis of the tube, the tube can withstand the load.
MCS 4130 tube: MCS 4130 yield strength is 900 MPa. With 462 MPa at the basis of the tube, the tube can withstand the load.
Let’s compare the fatigue strength. That’s where it starts to be interesting:
At 126 MPa in the 6061 T6 tube, we already passed the fatigue strength value:
It means that if the tube undergoes a cyclic load of 100kg, the aluminum alloy tube will end up breaking.
For the MCS 4130, at 462MPa we’re still far from the fatigue strength value:
It means that if the tube undergoes a cyclic load of 100kg, the tube WILL NEVER BREAK.

That’s a small example to give an insight on what influences the design and strength. Designing and manufacturing a frame is subject to choices and compromises. It all depends on what the frame requirements are, in terms of riding style, performance, strength, cost, etc…

At PP, according to the riding style, chassis type and kinematic, we work on sections, thickness, butted lengths and heat treatments of our steel MCS 4130 tubes to give our frames a controlled and optimised stiffness, a perfect dynamic behaviour and and awesome reliability.

Want to know more about our chrome-molybdenum steel and our tubes? Click here https://www.productionprivee.com/en/productions/article/mcs-steel-ktp-flex-system

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