4130 steel is a chromium-molybdenum alloy that is often used for bicycle frames. It has a high strength-to-weight ratio and is relatively inexpensive, making it a popular choice for many cyclists. However, 4130 steel is not without its drawbacks. One of the biggest concerns with using this type of steel for bike frames is its susceptibility to rust. 4130 steel can also be difficult to weld, meaning that repairing a frame made from this material can be challenging. Despite these potential issues, 4130 steel remains a popular choice for many cyclists due to its affordability and strength.