RPM sensors
Intro
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.
In addition (but not instead) the Engine RPM can be read in the following ways. This will provide automatic calculation of gear ratio:
- Spark plug wire signal pickup via a suitable adaptor, for example from DTec. See below how to connect it.
- RPM signal from ECU, see below
- OBD2 via Bluetooth or USB using an ELM327 unit
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.
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.
More teeth is better up to a certain point. If you have say 2000 pulses per second, YourDyno will make an average of 10 samples per calculation, which automatically filters noise without any downside to accuracy.
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
You can use a steel or iron timing belt wheel as a trigger wheel (not aluminium). Machine or grind off for example every other tooth to make it fit the minimum spacing requirement, if necessary. Just search for “Steel timing belt pulley” on Google. Here is one from RS-Online, they have many.
Or you can have one fabricated. Here is a dxf drawing of a 20 teeth wheel. Adjust the center hole size as needed. Thanks to Paul Alfred!
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.
Engine RPM
It is completely ok to not have an engine RPM measurement. In this case you use the brake RPM sensor + gear ratio to know the Engine RPM. Gear ratio can either be entered directly if you know it, or you can use the engine’s tacho and hold the RPM at a set point (for example 4000RPM) then press a button. YourDyno will calculate the gear ratio.
Alternatively you can add a direct Engine RPM reading in one of the following ways:
Spark plug wire signal pickup
DTec sells a suitable RPM adaptor. It can pick up the signal from the spark plug wires, from the coil or a VR sensor. Here is how to connect it:
You need to program “Pulses per revolution” to either 1 or 0.5, depending on your engine (1 for 2 stroke or 4 stroke with wasted spark, 0.5 otherwise).
RPM signal from ECU or coil
If you go this route, make sure to let the signal from the ECU or the coil trigger an optocoupler. No pullup is necessary, this is included inside the YourDyno box.
OBD2
RPM reading from OBD2 is supported via Bluetooth or USB using a ELM327 unit. These units are very low cost and works well with YourDyno. They can also be used if both RPM inputs are used on the YourDyno box. WiFi version is not supported.
