Harley Davidson V-Rod Forum banner

1 - 8 of 8 Posts

·
Registered
Joined
·
62 Posts
Discussion Starter #1
I have just changed out the front sprocket on 2012 Nightrod Special (international version) from a 30 tooth to 28 tooth sprocket. I have read some blogs which say that the speedometer does not need to be recalibrated as the input for the speedometer is taken from a point where the gear ratios do not effect the speedometer. I live in Malaysia, so I called Harley in Singapore, and they say that the speedometer does not need recalibration when changing the front sprocket.
This seems odd to me. Where is the speedometer taking its information, which is unaffected by the gear ratios?
Can someone give me a definitive answer? Also, I would like to know where the speedometer is getting its input if the answer is "no recalibration required'.

Thanks again to all!
 

·
Retired, Not SLOW!
Joined
·
6,000 Posts
... changing the sprocket will be 7% off
^^^ What he said for the 2-tooth change you posit. Speedo input comes off the xmsn output shaft so changing drive sprockets affects # of revolutions of the wheels for a given engine RPM; decreasing tooth count decreases the number of wheel revolutions per (*pick a unit of time*) thus decreasing speed. You also just decreased theoretical top speed by 7% (at redline, if your bike will pull to redline in top gear) and affected fuel mileage.
 

·
Super Moderator
Joined
·
6,248 Posts
I have just changed out the front sprocket on 2012 Nightrod Special (international version) from a 30 tooth to 28 tooth sprocket. I have read some blogs which say that the speedometer does not need to be recalibrated as the input for the speedometer is taken from a point where the gear ratios do not effect the speedometer. I live in Malaysia, so I called Harley in Singapore, and they say that the speedometer does not need recalibration when changing the front sprocket.
...
...
Your dealer and the others that think this don't know how the VRSC speedometer works.

A VSS (vehicle speed sensor) reads the 5th gear teeth (there are 26 teeth on this gear) on the output shaft of the transmission producing an AC voltage with a frequency corresponding to the speed of this gear wheel. This AC voltage is read by the ECM (electronic control module) and converted to ground speed using a stored parameter called the PPU (pulses per unit distance). The ECM sends the ground speed to the IM (instrument module) where its displayed on the speedometer.

Any change to the bike that affects the final drive ratio or rolling diameter of the rear tire will change the number of pulses per unit distance seen by the VSS. These include changes to either the front or rear drive sprockets as well as the diameter of the rear tire (tire brand, tire inflation, tire wear, etc.). When you change the front sprocket as you did, you can either alter the VSS pulse stream electronically as mentioned by tunderdemon or you'll need to re-calibrate the ECM PPU parameter. If not the speedometer will show speed as if you had a 30 tooth front sprocket so:
actual speed = indicated speed X 28 ÷ 30
= indicated speed X 0.93333
= indicted speed – 6.6666%.

The dealer is probably not allowed to change the ECM due to legal or Harley imposed restrictions but flash tuners like MasterTune or Dynojet Power Vision allow you to alter the ECM PPU parameter.
 

·
Registered
Joined
·
62 Posts
Discussion Starter #5
Stever975,
Thanks for that! It didn't make sense to me that you could change something in the components of the drivetrain and still have the speedometer in calibration. Your technical description is appreciated as well.

Thanks to everyone for the input!
 

·
Registered
Joined
·
80 Posts
@rbabos, @stever975
Question... how do these calculations translate to the pulses/km setting on the map?

I'm moving from 30T to 28T as well... currently, for my 30T map, I've got 1215 in the "Pulses per km" setting. Does the pulses/km also get affected in the SAME manner as the indicated field?

i.e. as below... and also, which of the 2 is right?
1215 x 0.93 = 1129 pulses/km
or
1215 - 6.67% = 1134 pulses/km
 

·
Registered
Joined
·
3,984 Posts
@rbabos, @stever975
Question... how do these calculations translate to the pulses/km setting on the map?

I'm moving from 30T to 28T as well... currently, for my 30T map, I've got 1215 in the "Pulses per km" setting. Does the pulses/km also get affected in the SAME manner as the indicated field?

i.e. as below... and also, which of the 2 is right?
1215 x 0.93 = 1129 pulses/km
or
1215 - 6.67% = 1134 pulses/km
I have mine set to 1341 pulses/km with the 28 tooth in the PV calibration.
1341 - 28T
1245 - 30T
1437 - 26T
Ron
 

·
Registered
Joined
·
80 Posts
Bam! You're a legend Ron... love you brother! Thank you for your help ONCE AGAIN!
 
1 - 8 of 8 Posts
Top