YourDyno needs as a minimum one brake RPM sensor per independent brake. So if you have 1 brake you need 1 RPM sensor that measures the speed of that brake. If you have two brakes that are physically interlinked, you need only 1 RPM brake sensor and if you have 2 independent brakes you need 2 brake RPM sensors. For an inertia dyno you need 1 roller RPM sensor.
In addition (but not instead) the Engine RPM can be read. This will provide automatic calculation of gear ratio. Options for Engine RPM are described here.
Brake RPM sensor considerations
YourDyno RPM sensor is recommended as a low cost, proven sensor with shielded cable.
But any sensor that provides a 0-5V square wave signal will work. VR sensors do not work directly, but can be used with an interface board that converts the signal to 0-5V. For example this one (select the 5V version).
YourDyno needs 200 trigger pulses per second as a minimum to work optimally. This means a trigger wheel is necessary. The RPM sensor senses the teeth on the trigger wheel, and produces one pulse per tooth. There are a few rules for the trigger wheel.
Trigger wheel mechanical/material considerations
The trigger wheel must be of a ferrous metal (iron or steel).
The distance between the teeth on your trigger wheel must be perfectly regular, otherwise you will have noise. You must also avoid eccentricity in the trigger wheel, as it will cause the sensor to trigger at slightly different times as the wheel spins.
Here is the spec for the mechanical dimensions of the wheel.
How many teeth do you need
As a general recommendation, choose a trigger wheel with enough teeth to give at least 200 pulses per second at the RPM you care about. This is because YourDyno needs 2 pulses to produce 1 result, so unless you have at least 200 pulses per second you will not get 100 updates per second. More importantly brake performance and inertia accuracy will be reduced.
There is no hard limit to the maximum number of pulses per second, but it is recommended to stay below 6000.
How to get hold of a trigger wheel
YourDyno sells a 30 tooth trigger wheel, suitable for most roller and hub based dynos. Some modifications will typically be needed to fit it to your brake/roller axle.
NOTE: A typical crankshaft ignition/ECU timing wheel does not work! They are made with one or more gaps in the teeth in order for the ECU to know the absolute position of rotation of the crankshaft. YourDyno expects no gaps, and such a wheel will cause a large noise in the RPM readings.
Using an Encoder
Using an encoder for the brake RPM is also possible if you do not want to use the standard YourDyno RPM sensor. Keep the pulses per revolution below 250 or so and choose a open collector type encoder (i.e. an encoder that needs a pull-up resistor to work). YourDyno has a built in pull-up resistor to 5V.
YourDyno has an internal pull-up resistor to 5V for the Signal input. Some setups with long RPM wires may need an extra pullup on previous hardware versions (Rev AD and earlier). Add in that case a 1kOhm resistor can be added between 5V and the RPM Signal input (ensure with a multimeter that you really connect to 5V, it MUST not be higher).