FREE Tach for your MC! - Star Motorcycle Forums: Star Raider, V-Max, V-Star, Road-Star Forum
 3Likes
  • 4 Post By KCW
  • 2 Post By KCW
 
LinkBack Thread Tools Display Modes
post #1 of 2 (permalink) Old 08-04-2017, 03:57 PM Thread Starter
KCW
Senior Member
 
Join Date: Sep 2016
Posts: 5,069
Garage
FREE Tach for your MC!

Last year I was playing with the idle on my bike, and used my cell phone to record the audio of the exhaust sound, played it back on an Oscilloscope, and measured the time interval to see what my idle RPM speed was.

Tedious and not very useful.

There is an ap you can get for your smart phone (android) called Spectroid that makes it very easy to see your engine rpms live in real time in living color!

Its a spectrum analyzer program, it captures sound and shows you the frequency components that are present, from 1 Hz to 10,000 Hz.

Without getting too technical, you run the program, hold it near one of your exhaust pipes, and look at the graph for the lowest frequency peak. Its showing you the frequency of the exhaust puff, which is the revs per second of one cylinder.

For example: my 650 idle speed should be 1200 RPM. Divide by 60 and you get 20 RPS (Hz). Since each cylinder fires on every other rev, you get one fire per rev coming out the exhaust pipes: put put put put put...

If you have two mufflers, each one will be half that frequency: 10Hz

So if you hold the phone mic right next to one exhaust, and your engine is running at 1200 rpm, you will see a peak on the spectrum graph at 10 Hz.

If you have a 4 cylinder road star at 1200 rpm you will see 20Hz, because cylinders are firing twice on each revolution. Road star idle might be a lower RPM - check your user manual.

Spectroid is a free ap, its easy to use, and you can hold your phone by your bike and adjust the idle speed while you watch it. Very cool!
2woofs, 2woofs, Keith Post and 1 others like this.

Last edited by KCW; 08-04-2017 at 10:19 PM.
KCW is online now  
Sponsored Links
Advertisement
 
post #2 of 2 (permalink) Old 08-04-2017, 04:08 PM Thread Starter
KCW
Senior Member
 
Join Date: Sep 2016
Posts: 5,069
Garage
If you want to do it the other way,

RPM = 2 X the lowest frequency peak * 60 for a 2 cylinder bike

RPM = 1X the lowest frequency peak * 60 for a 4 cylinder bike.

There are some settings on the program that will make it have better resolution at the low frequencies:

Under audio:
Leave the audio sampling rate set at the default: 48000 Hz
set the FFT size to 8192 bins (5.9Hz/bin)
set the decimation to 5 (0.18 Hz/[email protected])
set the window function to Hamming
Desired transform interval to 20mS (50Hz)
exponential smoothing factor to 0.3 (default)


under display
Frequency axis scale = logarithmic
Waterfall off (dont go chasing waterfalls! )
Max-hold trace = OFF

when you run the program two finger zoom in so you only see from about 1 Hz to 50Hz or so.

It will show you the frequency on the bottom scale. If you touch the screen a cursor will show up you can put on the peak to read the frequency.

Its also useful for other audio applications, if you zoom it back out to 1 to 10,000 hz, you can see the frequency response of music that is playing, you could tune a guitar, hold it by a fan and see how fast the blades are spinning...
Keith Post and Keith Post like this.

Last edited by KCW; 08-04-2017 at 05:57 PM.
KCW is online now  
Sponsored Links
Advertisement
 
Reply

Quick Reply
Message:
Options

Register Now



In order to be able to post messages on the Star Motorcycle Forums: Star Raider, V-Max, V-Star, Road-Star Forum forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.

Password:


Confirm Password:
Email Address
Please enter a valid email address for yourself.

Email Address:
OR

Log-in










Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page
Display Modes
Linear Mode Linear Mode



Posting Rules  
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

 
For the best viewing experience please update your browser to Google Chrome