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1Back to top Go down    Changing FD ratio - speedo still correct? on Sun Jun 17, 2018 11:57 am

Snod Blatter

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Hi all,

I'm about to spring for a standard 32/11 final drive with the glorious 16 spline setup but just before I do so can someone tell me if my RS speedo will still read correctly afterwards? Which bit of the FD does the sensor read from? I'm pretty sure people swap these about without a second thought but I'd like to be clear on it..

Thanks!


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1989 K100RS SE ABS 8v VIN: 0149214
Others: 1.5 x Honda CBX250RS-E, '94 CB250, '95 TRX850, '16 Z250SL
http://justbikethings.blogspot.co.uk/
    

robmack

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Yes, it will read properly. The speed sensor still puts out 6 pulses per wheel revolution for any given FD ratio, and your speedo is calibrated to 6 pulses / rotation and the size of tire you install. The speedo doesn't care how many engine revs were required to get you to that indicated speed. Changing the tire size will have an effect on speed accuracy, not FD ratio.


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Robert
1987 K75 @k75retro.blogspot.ca
2011 Moto Guzzi V7 Racer
http://k75retro.blogspot.ca/
    

Snod Blatter

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Excellent, I didn't know if it perhaps read from the crown wheel teeth or something. Thanks cheers


__________________________________________________
1989 K100RS SE ABS 8v VIN: 0149214
Others: 1.5 x Honda CBX250RS-E, '94 CB250, '95 TRX850, '16 Z250SL
http://justbikethings.blogspot.co.uk/
    

Avenger GT

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Yes, the sensor reads from the crown wheel, so it doesn't matter what the gearing is it won't change the accuracy of the speedo. Putting on a larger or smaller diameter wheel however will make a difference.

    

Snod Blatter

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Are there six magnetised spots on the crown wheel then? What kind of German magic is going on?


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1989 K100RS SE ABS 8v VIN: 0149214
Others: 1.5 x Honda CBX250RS-E, '94 CB250, '95 TRX850, '16 Z250SL
http://justbikethings.blogspot.co.uk/
    

Inge K.

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Part # 2.



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Inge K.
K100RS -86. (first owner), K1100LTSE -94.
    

Snod Blatter

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Ohhh so that's what that bit does! I thought it was a bit fancy for a spacer Laughing

Now, does anyone want an RS final drive with a new crown wheel bearing and 20 worn out splines? No? Come on.. I'll include a heavily worn shaft into the bargain!


__________________________________________________
1989 K100RS SE ABS 8v VIN: 0149214
Others: 1.5 x Honda CBX250RS-E, '94 CB250, '95 TRX850, '16 Z250SL
http://justbikethings.blogspot.co.uk/
    

Avenger GT

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No magic, the speedo sensor is counting the revolutions of the crown wheel, so it doesn't matter what the gearing is. I have a RS final drive on my RT, and it doesn't change the speedo accuracy. However it does change the engine revs at which a given speed occur, ie if 60 mph is at 4000 rpm with standard gearing and you use a lower geared final drive, then 60 mph will be at higher revs. If you use a higher geared final drive, then 60 mph will be at lower revs. The speed will still be the same.

    

robmack

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The speed sensor uses the property of variable reluctance to generate a signal.
The toothed wheel in the diagram that Inge posted is the wheel that transmits the current reotational position of the rear wheel.

http://fullfunctioneng.com/info/Hall%20vs%20VR.pdf
Variable reluctance (VR) sensors, also known as mag (magnetic) sensors are built with a wire wound around a permanent magnet (pole) similar to a solenoid or DC motor. They do not require external power (considered passive) and have two leads, a signal and ground.

As the ferrous tooth approaches the sensor, changing magnetic flux induces a current which produces a voltage. When the tooth leaves the area, the magnetic field rapidly collapses producing an opposite voltage. This cycle produces an alternating current as seen by the blue line in Figure 2.

The ECU conditions this signal into a digital signal shown by the red line in Figure 2. This one cycle is interpreted as one tooth count. The magnitude of the voltage produced is directly proportional to how fast the magnetic field changes, therefore, the faster the engine spins the higher the peak voltage.


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Robert
1987 K75 @k75retro.blogspot.ca
2011 Moto Guzzi V7 Racer
http://k75retro.blogspot.ca/
    

10Back to top Go down    speedo still correct ? on Mon Jun 18, 2018 11:39 am

audibob

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I was rather hoping you had fixed your FD snod, giving hope to others that it can be done rather than a trip to the

a specialist.

Me?

I chickened out, and like you now have a 16 spline set up

33/11 on the back, which probably negates any advantage with K1100 TB,s fitted, but I wanted long term reliability.

The choice of a second bike in the stable is being clouded by stories of premature FD failures on several newish

BMW models.

Bob


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Yamaha 90
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Honda CB 360
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Triumph T150 Trident
Honda CB750 F1
Cz 175
Yamaha XS 750
R 100/7
R 80
K100 LT
K100 LT current bike
    

Snod Blatter

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Sorry Bob, I can just about cope with replacing bearings but when the splines are gone it's well beyond what I can do. I picked up the new drive last night, 150 for the FD, shaft, swingarm, disc and caliper with ABS sensor still attached. I didn't want anything other than the FD and shaft but he wasn't moving, just wanted it all gone. Fair enough. The splines are not quite like new but compared to what I'm currently running they are very meaty.

I still maintain that, for now, there is no point in fixing a final drive unit. It can be done but I wish I hadn't bothered! I hope the crown wheel bearing in this new unit is good though..

Oh, and good luck with your cam cover bolt saga!


__________________________________________________
1989 K100RS SE ABS 8v VIN: 0149214
Others: 1.5 x Honda CBX250RS-E, '94 CB250, '95 TRX850, '16 Z250SL
http://justbikethings.blogspot.co.uk/
    

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