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Instrument conctant k for mechanical speedometer
What is the constant k?
The constan k indicates the number of revolutions of the square of the speedometer shaft per kilometer. A constant k of 1.4 ,for example, means: with 1400 input revolutions the tachometer's counter shows 1km driving distance.

What influences the the constant k when I customize  my motorcycle?
  • a changed rolling circumference (different rim size, different tyres)
  • a modified sprocket (especially with speedometer drive on the motor housing)

Can I find the constant k on my old speedometer?
You will find information about the constant k of your speedometer in the descriptive literature of your vehicle or on the backside of the original speedometer.
Common constants are: 0.7 , 0.8 , 0.9 , 1.0 , 1.4 and 1.6 (respectively 700,800,900,1000,1400 and 1600).

Motorcycles and their constant k (original tires required!)
k= 0,7: BMW from R65 up
k= 1.0: BMW R45, Moto Guzzi, Harley Davidson, Simson, MZ
k= 1.4: BMW 1100R, BMW 1150GS, Harley Davidson, Ducati, Honda, Kawasaki, Suzuki, Yamaha
How can I determine the constant k myself?
If the value is not known, it can be determined independently according to the following instructions. These instructions apply to two-wheeled vehicles with an installed hub drive or with an original hub drive in the front wheel or Motorcycles where the  speed is measured at the gearbox or rear wheel.

Make sure to determine the instrument constant k carefully! An incorrect value of k will result in an inaccurate indication by the speedometer compared with the true driven speed.
1. Wheel circumference u [cm]
  • Measure the circumference of the front wheel (in cm) with a tape measure or by rolling along a straight line.

2. Transmission ratio x of your speedometer drive unit

  1. unscrew the speedometer drive cable from the original speedometer
  2. make a mark on the now visible square neck of the speedometer drive cable as a counting aid
  3. count the number of revolutions of the square neck of the speedometer drive cable by rotating the front wheel 10 times. The best way to do this is to jack up the Motorcycle.

3. calculation of the k-value by means of the two determined quantities u and x:

k= 10 * x / u
The result, rounded to one place after the decimal point, gives the K-value of the tachometer.




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