Ask HN:Can you advise any tool for 3D design

Blender is terrific, free, and has lots of community support. It also has a famously steep learning curve, a complex UI, and poor documentation.

If you're new to 3D tools, I'd recommend finding a Discord where you can converse with other users live. The help will come in handy!

Do you just need to model them aesthetically, or do you need to calculate their physics and material properties too? Like are you actually going to manufacture these bikes, and how will you ensure they can properly support the rider and withstand all the dynamic forces?

Blender and Fusion 360 are quite popular. I suggest you watch some beginner's tutorials to see which option would be better for your use case.

3ds Max is a great tool that offers a lot of possibilities in 3D design.

Paper and something to draw with - like a pencil, pen, or marker - is the simplest thing that might work. Maybe splurge on a scale if you need accurate plans and a compass if the wheels you draw need to be round.

[Caveat: I'm assuming you actually want custom bikes and not custom designs (assuming that the goal is to make bikes and not plans or models) and I believe that the best way to start building bikes is building bikes not learning to use software. YMMV. Good luck.]