PVD Two-by-Two

What is Two-by-Two?

Some call it double-double or dos-dos.

This system consists of two single speed gearings mounted side by side. To change gears, the wheel is removed, the chain moved latterally front and rear, and the wheel is replaced. Typically, the chain needs to be re-tentioned.

This is not a system for racing. It is great for people that want a fun single speed with just a bit more functionality.

What makes PVD's special

With the PVD setup, the chain does not have to be re-tentioned when the gear is changed. This is accomplished by having near identical center to center distances on the gears. This can be done by going through a bunch of calculations. see: Length Calculation and Single Speed Gearing

Example

This is a 29er townie that I built. I wanted to be able to ride it around Fairfax, but be able to jump onto single track when the moment struck.

The gearing is 34-15 (street) and 30-19 (singletrack). The street gearing is 206.5 inches of development and separated by 16.807" and the singletrack geaing is 143.9 inches of development and separated by 16.852", both with a 92 link chain. This difference of 0.045" is within acceptable use, but best to set the chain up with the shorter spacing.