I have another little project that I have been thinking of making, but am having some trouble with the circuit design. In a nutshell, here is what I am envisioning:

Objective: Create an electronic solution to indicate the currently selected gear of a 5-speed transmission in a V-twin motorcycle.

Background: There are some aftermarket products on the market that claim to be able to do this for my year/make/model of motorcycle. However, I have read numerous forum reviews on these off-the-shelf products and they have been generally hit or miss in terms of the reliability with which they operate. Furthermore, these products are $$$. I figured that with some of your help, I could make my own for much cheaper and learn a lot in the process. One aspect of these products that I would like to use is the 7-segment LED display to indicate the gear 1 through 5. In terms of a housing, I may have access to a machine shop and some scrap materials to make a decent looking housing and handlebar mount for it.

Technical Background: In chatting on some motorcycle forums, some members mentioned that their off-the-shelf gear indicator works by using the ratio of the engine speed (RPM) to the vehicle speed (wheel speed).

On my motorcycle, I have access to these two signals. The motorcycle is well out of warranty and therefore I am not concerned about delving in and cutting/splicing wires.

My ECM has a data output for a tachometer signal and it currently goes to an aftermarket tach (that the previous owner added) to display engine speed in RPM. The tach is an analog face and needle display. I am fairly certain that the tach signal is some form of (if not duplicate) of the crank position sensor. The crank position sensor (CPS) is a variable reluctance (VR) sensor that generates an AC signal.

The vehicle speed sensor (VSS) is a Hall-effect sensor that takes readings off 4th gear of the output shaft in the transmission. Preliminary research into Hall-effect sensors leads me to believe that their output is some form of square wave, but I am not certain.

Note I understand that it may be difficult for you to help me since I have not measured either of those signals and only have basic electrical tools (2 DMMS) to do so. I do have a micro-controller with which I may be able to do some datalogging... I will attempt to provide as much info in this initial post to get an idea of this project's complexity and am willing to tap into those wires to get some data for those signals.

Main Questions:

  1. What type of circuit is required to take the "frequency ratio" of two periodic signals? One signal being perhaps sinusoidal and the other being maybe a square wave.
  2. I am assuming that the frequency ratio of the two signals will correspond to the gear ratio and thus the currently selected gear in the transmission.
  3. What circuitry would be required once the frequency ratio has been obtained to "translate" this into a digit 1, 2, 3, 4, or 5 and then display that digit on a small 7-segment LED display?
  4. What type of "resolution" would be required in determining the frequency ratio? The info I can provide is related to the RPM readings off the tach. The engine idles fairly steady around 1000 rpm. Max RPM before rev-limiting kicks in is around 6k-7k rpm. My main concern is that changing gears only changes the rpm by a small amount it seems (at least with the resolution I have based on the analog display face)...Note After riding yesterday, I got a better idea of the RPM change when I change gears, but keep the same road speed: For each gear change, I get a 500 RPM decrease (or increase depending on whether up or downshifting) while maintaining the same road speed.

Thanks for reading this and I hope it wasn't too much text! I have some more info that I can post as followup too if there is discussion in this thread.

  • \$\begingroup\$ Does your bike have a gear position indicator (GPI) signal wire? Tapping into that would be the obvious way to go. If not, I would try measuring the frequency of the sensors you mentioned. Can your DMM measure frequency? Once you figure out what you're measuring, a microcontroller can probably do everything you need. \$\endgroup\$
    – curtis
    May 5 '15 at 16:08
  • \$\begingroup\$ @curtis My bike does not have a GPI output. I was really hoping it did when I first began thinking about this project, but it's an old enough model to not have it... The DMMs I have can measure frequency. I will probably get around to those measurements in a few days. \$\endgroup\$ May 6 '15 at 23:47

A simpler solution which I have seen in an older Suzuki motorcycle was by sensing the position of the gear shifter. I am not sure how feasible it might be but you could probably achieve it using a few reed switches.

  • \$\begingroup\$ Thank you for mentioning this alternative approach. I had preliminarily stumbled across descriptions of this method during my internet research and may still puruse it if the initial method detailed in this thread is not feasible. However, there are a few concerns I have about the sensing foot shifter position. One concern is that of the durability of the sensors mounted on the shifter itself and nearby on the bracket. They could be damaged by heat, water ingress, grime, etc. Another concern is how to tell an upshift from a downshift. The first way that comes to my mind (contd. \$\endgroup\$ May 6 '15 at 23:54
  • \$\begingroup\$ Is to use 3 reed switches total. One below the shifter "rest" position for detecting motion in its lower arc (a downshift) and 2 above its rest position. When the 2 are activated, then the downstream microcontroller (or circuit) will know it is an upshift. \$\endgroup\$ May 6 '15 at 23:56

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