Chain or Belt Drive (Modeling)

In BikeCAD 9.0, all of the drive train components have been consolidated into a single dialog box accessible from the Drivetrain icon icon. The first tab inside this dialog box is used to configure the cranks. The next tab configures, the chain, rear cassette and rear derailleur and the third tab configures the pedals. Moving back to the second tab, we see that the option to model a chain and a derailleur are new in BikeCAD 9.0.

Also new in version 9.0 is the Drivetrain dimensions tab where we can display a couple of dimensions characterizing the rear derailleur. The first dimension shows the gap between the top derailleur pulley and the currently engaged rear cog. The second dimension shows the length of the derailleur cage.

We can also show the number of links of chain required and the specs on the rear cassette and front chainrings. BikeCAD determines the length of chain required by calculating the minimum length of chain that could be wrapped around the largest cog and largest chainring. We can show the chain in this scenario by going to the derailleur section and choosing cog 1 and chainring 2. We call the largest cog in the back Cog 1 because this is the lowest gear available in the back. We call the largest of two chainrings Chainring 2 because this is the highest gear available in the front. When looking at this arrangement, it would seem from the way the chain meanders through the derailleur pulleys that this is not the absolute shortest length of chain possible. The reason for this is that in the chain section we've specified that 1 extra link be used. If we change this field to 0, we can see the chain is even tighter now. Any slack that still remains is only due to the fact that a full chain link is one inch long and it's not an option to add partial links.

If we're curious to see how things would look if we took any more links out, we can enter -1 in the Extra links field. It is generally recommended that 1 extra link is used over the minimum because not all rear derailleurs can handle the kind of contortion required of them when the chain is strung so tight between the largest cog and largest chainring. Of course, ideally the chain would never be engaged in such a cog and chainring combination, so for aesthetic reasons we'll shift the chain into a smaller cog for our default display.

Also in this section of the drivetrain dialog box, we can switch from a chain drive to a belt drive. Since bicycle belt drives such as the Gates carbon drive do not employ derailleurs, the derailleur section is removed when Belt drive is chosen.

Also, regardless of what rear cassette or crankset is chosen, only the smallest rear cog and the largest front chainring will appear. Ideally, we would go into the sprockets section and specify the exact single rear cog we want for our belt drive and do the same for the crankset. Also, since derailleurs are not being used, we should specify zero extra links. While the distance between each pin in the chain is one half inch or 12.7mm, the distance between teeth on the belt drive is 11mm. If we switch back to a chain drive and want to model a fixed gear configuration, we'll start by selecting the specific rear cog we want along with the single front chainring of our choice. The derailleur is still shown under these circumstances. We can get rid of it, by going to the Rear dropouts dialog box (Rear dropouts icon) and unchecking the box for the derailleur hanger.

While we're here, we might also want to change the orientation of the dropout slot by changing the angle from -90° to 180°. At the same time, we'll make our slot longer by changing the D field to a larger value. The next video in this series will further discuss tensioning the chain when using track dropouts or the newly added sliding rear dropout option.