Triumph Rat Motorcycle Forums banner

Will a different front sprocket affect your tachometer?

5K views 13 replies 11 participants last post by  Steve McKing 
#1 ·
I guess a better question is how the tach is driven on efi Bonnies with the digital odometer? I recently put a 17 tooth front sprocket on which has significantly thrown the speedometer off. Wondering if it's thrown off the tach also. It seems to be close but I'm not sure. By the way has anyone corrected their Bonnie speedo for a 17 tooth front sprocket with tuneecu?
 
#2 ·
Tachometer will NOT be affected by your gearing.
 
#7 ·
On my carb'd bike, the speedometer sensor is driven off of the rotations at the front tire. A change in the size of the tire will affect your speed. I don't think efi bikes do it another way... As for RPM, the sensor is driven by spark events. Those occur before the gears, so gearing doesn't affect tachometers.

RPM stands for rotations per minute of the pistons in the engine, not the wheel.

You'll see higher RPMs at 60mph with a 17t than a 19t. Hold the speed steady and your RPM will be different. Just like a bicycle, when you switch front a large front sprocket to a smaller one, the pedaling is easier... But you have to pedal faster to maintain the same speed.
 
#10 ·
.

RPM stands for rotations per minute of the pistons in the engine, not the wheel.
The sharing of information is a wonderful thing and internet forums can be a great place to do it. Regardless of the type of information being shared, it needs to be correct.

RPM represents Revolutions Per Minute and refers to the revolutions of the crankshaft. Pistons don't rotate.
 
#8 ·
A sensor called the crank position indicator keeps track of the alternator shell. Each time it comes around, the engine has made another revolution. The crank position indicator doesn't count sparks. On an EFI bike the information the crank position indicator provides helps the ECU decide when to make the spark and when to open the fuel injectors. The crank position indicator doesn't know you changed your sprocket so it is unaffected by the change.

Because Triumph's Bonneville chain kit comes with a 17 tooth front sprocket, I've been running with a 17 tooth sprocket since my last chain change -- 13,000 miles ago. I have a 2010 T100 and used TuenECU to re-calibrate my speedo. I think I set it to -9%.
 
#11 · (Edited)
Since you are talking about a digital odometer Bonnie, I should add that changing the sprockets will not affect the relationship between the speedo and the tach. Because your speedometer reads at the transmission, it also doesn't know you changed the sprocket. On Bonnies with a front wheel reading speedo, you will notice a difference with a sprocket change.

Where you will notice a difference on a digital odometer Bonneville is when you compare your bike speed to a GPS or one of those speed check signs that tells you how fast you are going.

As I said, I re-calibrated my speedo for the 17 tooth sprocket with TuneECU. Now, when I am traveling at 65 mph in fourth gear, my tachometer says the engine is turning 5000 rpm. If I went back to an 18 tooth sprocket, the tach would still read 5000 rpm when the speedo reads 65 mph. My GPS, however, would tell me that I'm actually going faster than that.

Here is something I am not sure about but I think it is right. When Triumph builds the Bonnevilles, it sets the speedometers to read a little high, but I think they set the odometers to read correctly. They read from the same sensors but might have different programs in the ECU.

When I changed to a 19 tooth sprocket for a while, I found that my speedometer agreed with the GPS, but the odometer said I was going further than the GPS said. Now, with the 17 tooth sprocket and re-calibrated speedometer, the GPS says I'm going further than the odometer says. This leads me to believe the tach and speedo are running on different programs. That makes sense to me because Triumph has a good reason for wanting the speedo to read optimistically and it also has good reason for wanting the odometer to read correctly. I'm not sure of this, but it is my observation.

So in order to re-calibrate your speedo with TuneECU, you will need some way to accurately check you actual -- not indicated -- speed. As far as the odometer goes, I think it will only read correctly when you have the stock tires, sprockets, and the optimistically reading speedometer.
 
#13 ·
There is a very near 1-to-1 correlation between the number of times the crankshaft rotates and the number of time the piston moves up and down. The correlation is so closes that it almost seems as if there is something connecting those two pieces. I know, correlation is not causation and all that, but that is how it seems.

So, IWannaGoFast saying the rpms is a count of the revolutions of the pistons is pretty damn close, plenty damn close, and Colin was being a pedantic arse.

And Forchetto, for all his faults (which are obviously legion), has never crucified anyone. Even figuratively.
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top