Bike frames can be made of Steel, Aluminum, Titanium, or Carbon Fiber. They are then covered with paint. So when you have a bike in front of you, it can be quite difficult to know for sure which material the frame is made of. This is why I am writing this guide, to help you figure out which material is a bike made of, which can be pretty useful when buying second-hand bikes.
Definition of alloy
In this article, we will look at different materials but focus a bit more on alloy. Alloy in the cycling world refers to Aluminum alloy. But in scientific terms, it means a material made of a mix of metals. So it’s much broaden than Aluminum.
For this article, I will refer to alloy as Aluminum.
How to tell if your bike is made of alloy
It’s not always easy to tell what a bike frame is made of. This is why I recommend following the steps below to try to identify if your bike is made of Aluminum or not.
Steps to tell if a bike is made of alloy:
1. Look for a marking on the bike frame
It’s quite common for manufacturers to print the type of material on the bike frame. It’s generally a sticker or a decal you should search for. I have seen it on the seatstays, on the chainstays, or like on my bike at the bottom of the down tube. What’s written is not always directly the material but sometimes it’s just a brand name of a grade of material. For instance, if you read “ALUXX” on a bike made by Giant, it means the frame is made of Aluminum.
2. Check with a magnet
Take a magnet (you probably have one on your fridge) and try to stick it on the bike frame. If it sticks, it means that the frame is made of Steel, unless you are confronted to high-end steel bike frame, where they could have used stainless steel. This is quite unlikely though. So, if the magnet doesn’t stick, it could be made of Aluminum, Titanium or Carbon Fiber.
3. Listen to the sound of the bike frame
Tap with your fingernails the top tube of the frame and listen to the sound it makes. If you are looking at a recent bike, it’s extremely common for the fork to be made of composite, so compare the sound of the top tube to the sound of the fork against your fingernail.
If the bike is made of composite material (carbon fiber), the sound is quite dull and sounds like plastic, while metallic material will have a metal sound that rings longer after tapping.
If you are still unsure after doing this step, just look at the bike to eliminate if it’s a composite bike. The frame will be very light, with no signs of a junction between parts. The chainstays (and seatstays) generally look like a hollow square profile rather than a tube or an oval.
4. Look for signs of welding
If you find signs of welding at the junction between the tubes on the bike frame, it means that the frame is not made of carbon fiber. It’s not possible to weld composite materials.
However, different methods can be used to join together metallic parts of a bike frame. It’s quite common to have welding, using TIG (Tungsten Inert Gas) welding which produces clean and precise junctions on the bike frame with marks easy to identify. TIG welding is most commonly used in the bike industry on Aluminium, Steel, and Titanium bike frames.
Another tube joining method used on bike frames is called fillet brazing. It’s quite different from TIG welding, as it will use a brass or silver filler to make the junction between the tubes without actually melting the tubes. It looks a bit different from a welded joint after painting. You generally don’t see wave marks and the excess material is much thinner at the junction, or almost invisible when well done. Indeed, the junction is usually sanded to look nice and clean afterward.
Fillet brazing is very used on higher-end Steel bike frames as it looks much better. It’s also used on Titanium bike frames. It’s rarely used on Aluminum frames as it’s much more complex to do. Therefore if you recognize fillet-brazing on a bike frame, it is definitely unlikely to be made of Aluminum.
If you look at a vintage bike, you might also find bike frames made of special-shaped lugs to fit all the tubes. In this case, you are looking at a bike frame made of steel.
5. Check the color of the bare metal
Generally, you can tell by removing the seat post and looking down at the seat tube. This area is not painted. Also, on second-hand bikes, they are likely to have some scratches and a bit of paint damage somewhere on the frame where you can see the bare metal in a much better light than down the seat tube.
You can tell from the color of the bare metal the type of material. Aluminum looks like silver, it’s very shiny also. Titanium has a dull silver look. It’s not as bright as Aluminum. For Steel, it will look darker and less shiny, but with the magnet test, you should already know if it’s Steel or not.
Can you differentiate a bike frame made of Chromoly from High Ten Steel?
Chromoly is a high grade of steel used on higher-end bikes. It’s stronger and lighter than the most common High-Ten steel used on bike frames. It’s easy to tell by looking at the bike which grade of Steel was used. Generally, Chromoly bike frames have a sticker on the seat tube, like in the picture below. Also, not many bikes were made of Chromoly and generally they do not come with pre-assembled bikes. So it should not be too easy to guess.
Chromoly frames are also significantly lighter than High-Ten Steel frames. So this could also help you distinguish the type of Steel on your bike.
If you liked this article, you could also be interested in this other article I wrote about where Trek bikes are really made: