 The frame is literally the part that keeps a dirt bike together, but it seems like no one ever took the time to explain it properly or even explain why there are steel and aluminium frames. That's exactly what I'm talking about today. Before we get into the pros and cons of steel and aluminium frames, we first have to understand what manufacturers expect of the frame. Although we see a frame as a stiff and unbendable part of the bike, it actually bends and flexes at every corner and bump you encounter and works in combination with the suspensions to provide stability to the rider. There are three kinds of stiffness that greatly affect the performance of a dirt bike and are really important to understand. Lateral, torsional and longitudinal. Let's take a look at each one. Longitudinal stiffness affects the way the frame flexes from front to back and has the greatest influence on how much the rider will feel the vertical inputs of the terrain. If the frame has little longitudinal stiffness, the rider won't be able to get a good feeling from the front wheel and the bike will feel uneven during braking or on a whoop section. This happens because the frame will absorb part of the inputs transmitted by the suspensions. Ideally, the longitudinal stiffness should be as high as possible to concentrate all damping on the suspensions instead of the frame. A suspension is easily tunable, but a frame isn't. Torsional stiffness affects the way the frame flexes from top to bottom and greatly affects how nimble a bike feels. If a frame has little torsional stiffness, when trying to change direction, the rider will feel the bike as a big boat instead of a fast and sharp turning go-kart. This happens because when the rider tries to change direction, the frame will absorb part of the rider's inputs, making it more cumbersome to lean the bike over to the desired direction. So, torsional stiffness as high as possible, please. However, the lateral stiffness isn't as simple as the other two. Lateral stiffness affects the way the frame flexes from front to back, but like an affordable bicycle. This stiffness has a huge impact on how comfortable and stable a bike feels while cornering, since there's no suspension travel available to absorb the bumps. A frame with really high lateral stiffness will suit motorcross and supercross, since the dirt is relatively even compared to off-road, where every little rock and root can deflect the bike. If a frame has less lateral stiffness, it will be more suited for off-road, since the frame will be able to use its elastic properties to more easily absorb small rocks, thus keeping the wheels planted and keeping the rider stable. For supercross, the rider might lose some feeling from the wheels, since the frame will absorb part of the inputs. That's why Enduro manufacturers tend to choose steel frames, while motocross manufacturers usually choose aluminium frames. And how do manufacturers change the stiffness of a frame? There are two ways of doing it. The simplest option is to add more material on specific areas of the frame, such as reinforcement plates or simply increasing the thickness of the parts. The second option is to use shape factors to increase the stiffness without necessarily increasing the overall weight. The benefits of a shape factor are quite easy to understand. A flat beam is relatively easy to bend. However, a u-beam is unbelievably harder to bend for the same dimensions. It's not surprising that you can spot the same u-beams on aluminium frames. However, aluminium frames have not become the benchmark of the industry until fairly recently. If you think aluminium is used only for weight savings, think again. In 1997 Honda launched to market the first motocross bike with an aluminium frame, although they quickly figured out it was too stiff and improved the design on further iterations. But this begs the question, why would Honda even keep aluminium frames after that bust if steel frames were the industry standard? Short answer to reduce manufacturing costs. Steel is a lot cheaper, but is almost three times heavier than aluminium. However, the best mechanical property of steel is by far its ability to absorb energy. This is a tensile strength curve of steel and aluminium. This curve tells us how much load a material can withstand according to its deformation. In this case, we only care about the maximum load each material can withstand without becoming permanently deformed, which is highlighted in red. We can clearly see that steel can withstand a load over 1.5 times greater than aluminium. No wonder why aluminium frames feel so much stiffer than steel ones. Aluminium simply can't flex as much as steel. So, considering the mechanical properties alone, it's no surprise that all dirt bike frames are made of steel until 1997. But in terms of manufacturing, steel has no match to aluminium. First, it's a lot quicker to produce aluminium casted parts and it also allows you to cast reliably more complex parts than steel. Second, aluminium is a lot quicker to weld compared to steel and can easily be automated. Third, aluminium is unbelievably quicker and easier to machine compared to steel. And lastly, usually steel is only cheap if bought in standardized shapes, such as tubes, rods and sheets. And from an R&D standpoint, every change the engineers make can have a huge impact on the factory floor and the end consumer. With aluminium frames, engineers only have to change the cast mold of one of the handful casted pieces of the frame and keep the manufacturing process exactly the same. On a steel frame, every added reinforcement, such as tubes or plates, will increase the manufacturing time of every frame increasing costs. That's why all Japanese manufacturers shifted to aluminium. They could reduce the manufacturing time of a frame while also increasing the quality control. The chair is on top. The hardening treatment of aluminium frames happens at ambient temperature while the bikes are being shipped across the world and don't require any surface treatments. Steel frames, on the other hand, not only need time consuming surface treatments before the paint jobs but can also require heat treatments. Don't get me wrong, a well-designed steel frame can be as stiff as an aluminium one. And even if you use every shape factor from the engineering book on an aluminium frame, in certain circumstances, such as in off-road, nothing beats the spring like behaviour and forgiving nature of a steel frame. If this wasn't true, a bike with a steel frame would never stand on the podium of an enduro GP, MXGP or supercross race. Knowledge has power. Hope you liked it, don't forget to subscribe and if you have any questions, leave them on the comments below. Thank you for watching.