Rolling Chassis and bending miniature tubes

Today I am going back in time with the chopper project and I want to show you the 3D model of the rolling chassis and share some thoughts about the fabrication process of the frame.

As you can see in the photos, the rolling chassis is almost ready from design point of view. The remaining things to add is the top motor mount, the forward control mounts and rear brake mounts. The fender was redesigned in Inventor and now it includes mount holes to attach it to the frame. The fender clears the rear wheel at 1 mm and I am planning to use 0.5 x2 mm miniature hex bolts from Scale Hardware to attach it to the frame. The bolt head is only 0.46 mm in height so it should clear the wheel.

Back to the frame, it will be made mainly in 3 mm brass tube and the sections will be silver soldered together. All the bends have a 10 mm radius and to bend the tubes I started building a miniature tube bender. Fortunately, I found a nice CAD plan on the internet and only had to adjust the dimensions, use nuts & bolts I have around and remove some parts that I feel I don't need.

With the Inventor parts exported to CAD format, I went to a local water jet cutting workshop and they cut the basic shapes in no time (it took them 7 mins to cut all the parts). Unfortunately I found that due to the small dimension of the parts, the accuracy of the cut pieces is not that good. The water jet leaves a rough and uneven edge, which has to be milled or filed flat. This will reduce the dimension of the finished part with about 0.2 - 0.3 mm on each edge. So I had to enter the new dimensions in the Inventor solid model to get the actual size. So do take into account this aspect if you plan to use water jet cutting on small parts - you may need to rescale the parts a bit. However, the benefit in time is worth it.

Now it's time to finish the parts...

Chain drive design

It's time for the final drive design. For this project, I decided to use a roller chain instead of the "traditional" belt drive. I intend to use a 1/12 scale chain & sprockets kit from Top Studio, even though they are designed for Moto GP bike kits from Tamiya or Fujimi. But before ordering such a kit I need to design a sort of a mock chain and sprockets, just to assess the frame width and wheel/fender clearances.

After few days of gathering information and learning about chains mathematics (e.g. calculate the number of teeths, sprocket diameters), I finally managed to finalize the design and dimensions. I don't intend to have accurate sprocket ratio or pitch, just to have something to help getting the clearances right.

Just to give you an idea of the scale, here are the chain and sprockets data I came up with:
Pitch = 0.955 mm
Roller diameter = 0.5 mm
Roller width = 0.6 mm
Sprockets thickness = 0.5 mm
Driver sprocket outside diameter = 6 mm
Number of teeth on the transmission side sprocket = 18
Driven sprocket outside diameter = 18.843 mm
Number of teeth on the wheel side sprocket = 60

Of course these will change once I get the Top Studio chain kit, but it was a good learning exercise on how to design a chain and sprockets in Inventor. Plus that due to this exercise I discovered that the primary transmission plate has to be adjusted a bit to clear the chain. Also the oil tank's diameter had to be reduced from 13 mm to 12 mm, to clear the transmission plate. The rear fender will have a cut off on the left side, but I will not model that on the virtual model but do it directly on the fender part.

Later I will have to decide how to attach the sprocket on the transmission axle - I would like to be able to turn the rear wheel and both sprockets on the final/real scale model.

Here are some photos:

An introduction to the current project

The main project I am working currently is to design and build a 1/12 scale custom chopper motorcycle. I intend to build almost all the parts, with few exceptions being the tires, which are taken from a Revell motorcycle kit. The engine and gearbox will probably be 3D printed as is too intricate and complex to use conventional methods like milling. The 3D printed part will be used as a master pattern and they will be cast in resin. All the other parts will be made in metal, mainly brass and aluminum.

I have started this project approximately one year ago, working slowly on the design. Since it's an ambitious and complex project, I decided I will do the design in a CAD tool first and only then start building the physical parts. So I started learning various CAD tools and designing the bike in parallel. Actually the bike was the main motivator to learn the CAD tools as they are quite complex and have had no previous training in this domain.

Here are some photos of the current design

Welcome to my new blog

Welcome to my new blog, dedicated to scale model cars, motorcycles and connected hobbies. I started this blog so my hobbyist friends can follow my model builds easier.