Why does it do this???

[scott02464]

Im just trying to get me head round this.....

Iv been reading other threads but still none the wiser.

If I change the front sprocket to say a 14t, why does it alter the speedo readings?

I can understand it changing the revs, but why the speed reading?

Regards Scott

[Wicky]

Speedo doesn't run of the front wheel with a cable but electronically via a sensor on the back of the engine and is calibrated to a stock geared system to provide indicated mph. So when you alter the gearing the resulting indicated mph goes out of wack.

[scott02464]

Speedo doesn't run of the front wheel with a cable but electronically via a sensor on the back of the engine and is calibrated to a stock geared system to provide indicated mph. So when you alter the gearing the resulting indicated mph goes out of wack.

Cheers Wicky,

This is what i cant get round in me head.... If the sensor is at the back of the engine, and the engine hasnt been altered so it should still give the same reading as before (speed wise)??

Ie, the sensor if bfore the sprocket and not after the socket (In relation to the drivetrain?)

[MJONESY]

the indicated speed is altered because you get a different number of revolutions of the rear wheel to the number of revolutions of the front sprocket if that makes sense

[scott02464]

the indicated speed is altered because you get a different number of revolutions of the rear wheel to the number of revolutions of the front sprocket if that makes sense

When you say it like that, it makes sense lol Thanks MJONESY. Its kinda obvious now come to think of it.....

Looking forward to trying the 14t sprocket though he he

[Wicky]

so it should still give the same reading as before (speed wise)??

The speedo is a reflection of a preprogrammed ideal (i.e stock) system (gearbox and stock gearing), alter the gearing of the sprockets to something different then the bike doesn't know that and will report what it thinks the bikes speed. Hence why you need a speedo healer to make an electronic correction based on sprocket changes.

A digital bicycle speedo with a magnetic sensor has to be programmed with the circumference of the front wheel - say 27", if you then changed the wheel to a 26" wheel would you expect the bicycle computer to correctly indicate mph by automatically correcting for the change?

[scott02464]

so it should still give the same reading as before (speed wise)??

The speedo is a reflection of a preprogrammed ideal (i.e stock) system (gearbox and stock gearing), alter the gearing of the sprockets to something different then the bike doesn't know that and will report what it thinks the bikes speed. Hence why you need a speedo healer to make an electronic correction based on sprocket changes.

A digital bicycle speedo with a magnetic sensor has to be programmed with the circumference of the front wheel - say 27", if you then changed the wheel to a 26" wheel would you expect the bicycle computer to correctly indicate mph by automatically correcting for the change?

Cheers Wicky the penny has dropped lol, I knew that would be different but couldnt picture why.

Thanks for the explaination guys