BMX Bikes / C / Comparison of Frame Materials

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When trying to pick a material for a bike frame, there are many really important material considerations; weight, stiffness, strength, toughness, impact resistance, ride quality, corrosion, manufacturability. Yes, you actually need to consider them all, which can make it very confusing. So we’ll discuss all of these issues in the context of choosing a bike.

Every mechanical design its very nature a combination of material and design. Therefore it is important to consider the design as well as the material. For instance, I could design a bike out of lead that was as strong as a bike out of aluminum, but the penalty would be weight. The frame designs would also look extremely different. So for the following discussion on how to pick a material, we’ll assume that the actual design is exactly the same and we’re only interested in what material it’s built out of. In other words, pretend you just walked in to a bike shop and they have the same exact bike made out of 6061 series aluminum, steel, carbon fiber composite, Ti 3-2.5, Ti 6-4, Scandium1, and Magnesium2. As a quick note, you can find frames that are made out of both “scandium” and “magnesium”, but in reality these are both just ALUMINUM alloys that have scandium and magnesium added. The amounts will depend upon the manufacturer and what material they choose to use, but it’ll be something with MORE than 90% aluminum, a bunch of other alloying elements (Copper, Iron, etc….) and then maybe 5% of magnesium or scandium. This is a common misconception!

Back to the material choice. To understand how to choose which material, you have to understand what each of the properties (weight, stiffness, strength, toughness, impact resistance, ride quality, corrosion, manufacturability) listed above is and how they impact the whole design process.

Weight:
This one is self-explanatory. To compare across materials you need to have some measure, so we’ll use density. The denser the material, the more it will weight for a given volume. Since we’re saying that our frames are all the same design, then the frame made out of the material with the lowest density will be the lightest.

Stiffness:
For materials, stiffness is measured through the elastic response of the material. What does this mean? Elastic response is the material response that is totally recoverable when you deform a material. Suppose we take a bar of material, grip it on both ends, and pull on it. The material will stretch, get longer and skinnier. (Figure 1.) If we simply elastically deform the material, then when we release the bar it returns to its original shape. If we measure the force it takes to pull the bar, and plot this force against the amount the bar “stretched”, then we’ll see the plot in Figure 2. This plot is stress vs strain, where in VERY simple terms stress is a way of scaling the force applied, and strain is a way of normalizing the material extension. The stiffer the material, the more force it takes to stretch a given amount. (Figure 3.)

Strength:
Strength is a little more complicated. Normally when we say strength we’re talking about tensile strength for most engineering designs. This includes bike frames. Take that same bar of material, but if this time we plastically deform it then we permanently change the shape. See Figure 4. What happens is that it will recover elastically a little from the deformed shape, but it will remain taller and skinnier. You NEVER really want plastic deformation in an engineering design. Therefore the tensile strength is a design variable that engineers use, and it is the strength (stress) at which point the material begins to deform plastically. Figure 5. The curved lines in Figure 5 are where the material deforms plastically.

Toughness + Impact Resistance:
Now to something that is really important in mountain bikes, and maybe even road bikes if you take a dive. Impact resistance and toughness are really a measure how much the material can absorb before failure. In an impact the bike may deform plastically, or permanently, as in the ding that results when you lay the mountain bike down on a rock. However, that ding is better than a crack in the tube! To avoid the ding, you need as high strength a material as possible, stiffness doesn’t really matter. To stop the frame from breaking after the ding happens when you hit a rock you need a tough material. Toughness is really a measure of the energy the material absorbs before failure, which is directly proportional to the area under the stress-strain curve in Figure 5. Therefore strength, stiffness, and the elongation before failure of the material all play a role in this regard. Figure 6 shows three curves, where the blue curve is like carbon fiber being deformed, the red curve is like aluminum, and the green curve is like titanium. Which one do you think is tougher? I’d say Ti. The Carbon fiber doesn’t really deform plastically, but rather breaks right at the tensile strength. Therefore a tough carbon fiber frame fill be one that has a high strength or low stiffness, or both. The aluminum and the Ti both deform to about 10-13% before they break, i.e. the same level, but the Ti is stiffer and stronger. How can Ti be stiffer? Most people think of aluminum bikes as much stiffer than just about anything with the exception of maybe carbon fiber. That’s true, but that’s normally because they make the tubes VERY large to increase the moment of inertia and therefore “geometrically” stiffen the frame. This is not a stiffness derived from the material. So toughness is really a measure of the stiffness, strength, and the ductility (elongation before failure) of a material.

Ride Quality:
I don’t really know how you rank this very quantitatively. It’ll be a combination between the weight, the vibration absorption, the stiffness of the frame design, and the material. You could have two different Aluminum frames and they could be very different by nature of their designs. So, this may really be bike dependent.

Corrosion:
This is an important thing to consider, especially if you live anywhere where it rains, or where they put salt on the ground. Steel frames oxidize (rust) with water. Salt will help accelerate this process. Aluminum frames also oxidize, and they HATE rock salt. It’ll eat at the aluminum, and eventually cause environmental damage leading to failure. An interesting point is that most cleaners that you find at the grocery store (simple green, formula 401, etc…) are too acidic for aluminum, and will eat at the frame and components. The US Air Force learned this the hard way, and nearly lost a few F-15s in the process. Carbon fiber is fairly inert in the environment, but most carbon frames will have aluminum inserts. Finally Titanium probably beats the other materials hands down on corrosion. It is very inert at normal operating temperatures. Consider that it’s the main material used in the body when bones need to be mended or joints replaced. It really doesn’t corrode to badly.

Figure 1Figure 2Figure 3Figure 4Figure 5Figure 6
1 Scandium alloys are actually 5xxx or 7xxx series aluminums with scandium added up to 2% scandium. This is done for high strength and for some advantages over Al-Mg and Al-Li alloys in terms of producability.
2 For Magnesium, you won’t find a wrought alloy with more than 5.5% Mg added, and it’s done to increase strength, weldability, and corrosion resistance. Magnesium is the major alloying element in the 5xxx series aluminum alloys.

Manufacturability:
Most bikes are made from tubes that have to be attached together. How those tubes are made and how they are fastened together is important. First of all, some companies will make tubes by rolling up a piece of sheet stick and welding a seam into the tube. They then grind and polish so you won’t be able to tell, but this is not a good way to make a tube. You’ve now put in another joint that can fail and needed to be welded. This is often the case in Ti6-4 frames. How you weld a standard metal frame together is very important. For steel you can use lugs, TIG welding, or fillet brazing. Lugs add weight, TIG welding is good on weight but it actually melts the tubes that are being put together and creates a heat affected zone (HAZ) in the tubes that may eventually lead to a failure. The fillet brazing uses a brass-silver “glue” that gets melted in to hold the tubes together. This is actually the best system, since the tubes don’t form this HAZ, and the braze is strong enough to hold it all together. For aluminum, the most common practice is TIG welding, and most Aluminum frames will fail at the welds due to the HAZ in the tubes. Ti is also TIG welded, and has the same problems as Aluminum. However, especially in Ti 6-4 you can find frames that are welded with commercially pure Ti (CP) instead of Ti6-4, and therefore this acts like a fillet braze”. In theory these frames are stronger since the HAZ is smaller in the Ti tubes as the CP Ti brazing material melts at a lower temperature than the 6-4 Ti tubes, so the tubes are never melted in the welding process.

 
 

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