Do Motorcycles Have Steel Or Aluminum Frames?
A few years ago, steel was the champion of motorcycle frames. It is strong, long-lasting, and widely available. However, things have changed a bit recently. Many motorcycle manufacturers have switched to using aluminum to make some of their motorcycle frames. The reason for this is technological advancements.
Aluminum castings used to be a lot more expensive and labor-intensive than steel castings. However, aluminum alloys are now just as easy to cast as steel. The result is the increased popularity of using aluminum frames. Why is this, you might ask?
Well, there are several advantages of aluminum. Let’s compare steel and aluminum in terms of price, strength, durability, and other features to determine why motorcycle manufacturers are now more inclined to use aluminum frames.
Steel Vs. Aluminum - Price
Steel is cheaper than aluminum, so it was the preferred material for bike frames for many years. Apart from the price of the raw materials, aluminum had a labor-intensive method to create the bike frame. On the other hand, steel is cheaper to purchase and requires less labor to work with.
However, due to technological advancements, the process for pressing frames with aluminum has simplified, meaning that the production costs are less. Aluminum can now be alloyed and heat-treated to make the manufacturing process more straightforward. So, although steel is still cheaper to work with, aluminum isn't that much more expensive.
In addition, as you will see later, you can get more product from a piece of aluminum than you can from the same size piece of steel. Therefore, although aluminum is more expensive based on weight, it isn't a significant consideration because the price per volume is much the same as steel.
Steel Vs. Aluminum - Weight
When designing a motorcycle, the power to weight ratio always comes into play. Most users want a powerful motorcycle that weighs as little as possible, giving it a higher top speed. Therefore, the weight of steel compared to aluminum is a significant consideration for motorcycle manufacturers. This is also where aluminum has a definite upper hand.
Steel weighs 490 pounds per cubic foot, whereas aluminum weighs 169 pounds per cubic foot. That makes aluminum weighs only 35% of what steel weighs. This is a significant advantage, as a motorcycle with the same engine and size will have a far better power to weight ratio. This is likely one of the reasons some superbikes are now manufactured with aluminum frames.
Steel Vs. Aluminum – Manufacturing
It was much easier to create motorcycle frames from steel than aluminum a few years ago. This is because aluminum was difficult to mold and required a lot of labor and different welding materials. On the other hand, steel was easy to mold and required much less time and effort, resulting in lower production costs.
However, recent aluminum alloys have made it possible to produce aluminum motorcycle frames for far less expense and effort. It is now possible to manufacture an aluminum motorcycle frame in the same amount of time as a steel one, lowering the production costs at the same time.
In addition, aluminum alloy is much stronger than pure aluminum. As a result, the frame doesn't have a significant disadvantage over steel in terms of strength. While steel frames are still cheaper to produce than aluminum, aluminum's weight to strength ratio results in a better motorcycle frame.
Steel Vs. Aluminum - Strength
Steel can bend under pressure, meaning that if you drop the motorcycle, the steel frame will likely bend upon impact. Aluminum, on the other hand, doesn't bend. Therefore, when dropped, the bike's frame may break upon impact. However, the force required to break an aluminum motorcycle frame is severe.
The big difference between the strength of aluminum and the strength of steel lies with the weight. Aluminum weights 65% less than steel, but you need more aluminum to make the aluminum frame as strong as a steel frame.
However, if you compare a ¼ inch thick piece of aluminum and steel, the aluminum will hold up better than the steel. The steel, because it can bend, will buckle first. Therefore, aluminum is the better material for the pipes and tubes required to make a motorcycle frame in a side-by-side comparison.
Another big difference between steel and aluminum is the fatigue limit. Steel has a practically infinite lifespan if it isn't exposed to a pressure of more than half its Ultimate Tensile Strength (the breaking point). Therefore, if you don't exert more pressure on the steel than half of the force required to break it, your steel frame should last indefinitely. 304 Grade stainless steel has a UTS of 590 Mpa.
Aluminum, on the other hand, doesn't have a fatigue limit. Therefore, any dents or weak points in the aluminum will cause it to break over time. This, in addition to aluminum's inability to bend, means that your aluminum frame is at greater risk of breaking than a steel frame. For comparison, aluminum has a UTS of 570 Mpa.
Therefore, even though aluminum isn’t quite as strong as steel, the real difference lies with the breaking point and fatigue limit.
Steel Vs. Aluminum - Durability
Apart from the strength and fatigue limit, the durability of a motorcycle frame is determined by one more factor. That is rust. Motorcycles have so many parts that can rust that it needs to be discussed as part of the durability of the motorcycle frame.
Steel frames are coated to prevent them from rusting. However, as the motorcycle has so many moving parts, the coating can chip away from the frame, leaving weak spots on the frame that can start to erode. In addition, whenever you drop or dent your bike, the coating is further chipped, making the motorcycle more prone to rust.
Aluminum cannot rust. Therefore, regardless of the moving parts and the amount of physical damage you inflict on the motorcycle, it will not make the aluminum frame more prone to rust. Aluminum has an aluminum oxide coating that repairs itself when damaged. This coating also protects the frame from corrosion.
Therefore, although aluminum cannot bend and doesn’t have a fatigue limit, it cannot rust. Depending on your location, this is a significant advantage of an aluminum frame.
Why Are Motorcycle Frames Made From Aluminum?
Now that we have discussed how steel and aluminum compare in terms of price, manufacturing, weight, strength, and durability, you may be wondering why motorcycle manufacturers are using aluminum to make their motorcycle frames. You will have this question because, from the discussion above, you can see that steel is cheaper, stronger, and more durable than aluminum.
While this is true, aluminum has one main advantage – it is lighter than steel. This is such a significant contributor that it outways the advantages of steel in many cases. However, I'm not convinced that motorcycle manufacturers will replace the frames of all their motorcycles with aluminum ones. Where speed is concerned, an aluminum frame will give you a better power to weight ratio, resulting in a faster motorcycle.
However, where durability is involved, and if you drop your motorcycle often, a steel frame that can bend and move is a better alternative. So, the answer to the question of what the frames of motorcycles are made of is both. Currently, motorcycle frames can be made of either steel or aluminum.
About THE AUTHOR
Russ currently owns a Yamaha FZ6N and KTM RC 390. When it comes to vintage bikes, his favorite motorcycle is the feisty BMW R32. He also holds a particular interest in the LAMS segment and triple cylinders. Himself a riding enthusiast, Russ has had experience with racetracks from around the world including Willow Springs Raceway in California and the Imola Circuit in Italy.Read More About Russ Crowley