One of the things I really wanted to try and avoid was using springs in the tensioning design. My plan was to keep the design as simple as possible and use as few components as possible but after much thought about how I could make the two bolt design work I came to the conclusion that spring tensioning was really the only way to do it. Which led to design number two shown below.
The red shafts initially started out life as threaded bar with nuts for the springs to push against but I soon scrapped that idea. As I had to buy some bar for the main shafts I decided to go for some straight 8mm round bar and shaft collars instead. I felt this was worth it to avoid the possibility of the springs catching on the thread.
In this design I’ve also dropped the size of the wheels down to 70mm diameter which looks a lot more realistic. Overall I’m pretty happy with this design but there are still a lot of questions to be answered regarding how large various components are.