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Twins Technical Talk Technical Talk for Hinckley Triumph Twins: Bonneville, T100, Speedmaster, America, Thruxton, and Scrambler.

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Old 11-29-2013, 05:29 PM   #1 (permalink)
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Will a different front sprocket affect your tachometer?

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?
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Old 11-29-2013, 06:37 PM   #2 (permalink)
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Tachometer will NOT be affected by your gearing.
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Old 11-29-2013, 07:19 PM   #3 (permalink)
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How is your speedo driven ? Must be driven at the gear box or a sensor there. That method doesn't work well with gear changes. Doesn't effect the tach , though.
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Old 11-29-2013, 07:31 PM   #4 (permalink)
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Cool. So does anyone know how the tach is driven?
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Old 11-29-2013, 08:32 PM   #5 (permalink)
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rpms come off the spark events...
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Old 11-29-2013, 10:38 PM   #6 (permalink)
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Quote:
Originally Posted by beemerrich View Post
rpms come off the spark events...
It's electiconical.
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Old 11-30-2013, 07:11 AM   #7 (permalink)
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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.
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Old 11-30-2013, 08:41 AM   #8 (permalink)
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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%.
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Old 11-30-2013, 09:20 AM   #9 (permalink)
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Thanks Charles. Exactly what I was looking for. Like I said, the tach seemed right but there's no way I was doing 70mph in 2nd gear
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Old 11-30-2013, 02:43 PM   #10 (permalink)
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Quote:
Originally Posted by IwannaGoFast View Post
.

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.
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