How does a mechanical speedometer work?

[80GS1000]

Thinking about trying to get a GS1000 (19" front wheel stock) mechanical speedometer to work with the 17" front wheel on my GSXR 1000 forks....

A mechanical speedometer has two parts as far as I know, the speedometer itself and the speedo drive which is mounted on the front axle.

If you were to change front wheel diameter and keep all parts (speedo + speedo drive) the same, would the margin of error be proportionate to the change in diameter? So going from a 110/90-19 front tire to a 120/70-17 front wheel would cause the speedo to be fast by 12%?

Is the critical gearing in the speedo drive or in the speedo itself? Would fitting speedo drive from a 17" wheel bike fix this error? Or is the gearing ratio in the speedo itself also relevant?

I realize this won't be a bolt on...if it's even possible you'd have to make a part to bolt the speedo drive to the forks and then have a mechanical pickup going into the front axle to sense axle RPM.

[tkent02]

Thinking about trying to get a GS1000 (19" front wheel stock) mechanical speedometer to work with the 17" front wheel on my GSXR 1000 forks....

A mechanical speedometer has two parts as far as I know, the speedometer itself and the speedo drive which is mounted on the front axle.

If you were to change front wheel diameter and keep all parts (speedo + speedo drive) the same, would the margin of error be proportionate to the change in diameter? So going from a 110/90-19 front tire to a 120/70-17 front wheel (84 mm rolling diameter) would cause the speedo to be fast by 12%?

Is the critical gearing in the speedo drive or in the speedo itself? Would fitting speedo drive from a 17" wheel bike fix this error? Or is the gearing ratio in the speedo itself also relevant?

I realize this won't be a bolt on...

A 12% smaller rolling diameter would probably make it read about right, all my GSes have read fast about that much.

[chef1366]

The speedo gear. Yes a 17" speedo gear will fix this problem.

[80GS1000]

The speedo gear. Yes a 17" speedo gear will fix this problem.

Thanks Bill.

[posplayr]

What I heard

from Katman is that for mechanical speedo GS's and GSXR's the indicated Guage MPH indicated per revolution of the speedo cable is the same. So in other words the gearing in the speedo drive on the front axle is designed for the wheel/tire size to give the correct reading.

If you have a 24 mm axle (??) It might be possible to get a 2nd gen (20mm axles) drive and bore it out to accept a 24 mm axle? The speedo drive does engage into the wheel so you would also have to sort that out.

Pos

[Steve]

If you were to change front wheel diameter and keep all parts (speedo + speedo drive) the same, would the margin of error be proportionate to the change in diameter? So going from a 110/90-19 front tire to a 120/70-17 front wheel would cause the speedo to be fast by 12%?

No, it would only read faster by 5%. I just went through the math and found that if all the sizes are exact as the stated measurements, it is 5.004% smaller. Now ... the stock speedos are off somewhere in the range of 5-10%, so you might end up being about 12% off with that setup.

A 12% smaller rolling diameter would probably make it read about right, all my GSes have read fast about that much.

Actually, you need a larger rolling diameter to slow down the inherent error in GS speedometers. Having a smaller rolling diameter will speed up the wheel that much more, adding to the error. However, as noted above, the rolling diameter is only 5% smaller, not 12%.

I believe the stock tire size for my wife's 850L is 90/90-10. We have installed a larger 100/90-10 tire to slow down the rotation speed. Comparison against a GPS shows that 60 mph indicated is about 59.5 on the GPS, an error we can certainly live with.

.

[tkent02]

No, it would only read faster by 5%. I just went through the math and found that if all the sizes are exact as the stated measurements, it is 5.004% smaller. Now ... the stock speedos are off somewhere in the range of 5-10%, so you might end up being about 12% off with that setup.




Actually, you need a larger rolling diameter to slow down the inherent error in GS speedometers. Having a smaller rolling diameter will speed up the wheel that much more, adding to the error. However, as noted above, the rolling diameter is only 5% smaller, not 12%.

Doh! Backwards!

[isleoman]

With a 94 GSXR1100 front wheel on my 82 GS1100ez my speedometer is almost exactly 10% optimistic. I look just about every time I go past one of those stationary radars. I'm using the stock GSXR drive in the wheel mated to a 140 mph 1983 GS speedo.

No math involved. If I had to do math it would be wrong.

[Steve]

Doh! Backwards!

Is that by chance the story of your life?

I look just about every time I go past one of those stationary radars.

My son has started calling those "high score signs".
Seems that some of his friends find them and keep trying to get higher numbers.

.

[trippivot]

ride at an indicated 60 MPH and time your self passing mile markers on the highway.

I have a little round digital clock in the middle of my tachometer.

[Steve]

ride at an indicated 60 MPH and time your self passing mile markers on the highway...

That's nice, but then what do you do?

Since you asked, the formula is 3600 / seconds = mph.

For quick figuring purposes, from 54 to 66 seconds it is about 1 mph for each second difference. As you get farther away from 60, the error gets larger than that.
Most of our bikes read a bit optimistic, so riding at an indicated 60 will actually be about 55 or so, and will therefore take about 65 seconds to cover the measured mile. Also be aware that not all miles are equal on the highway. Any particular mile might be a few feet longer or shorter because placing the marker where it really belonged would involve putting it on a bridge or some other inconvenience, so they just put it where they can.

.

[posplayr]

Steve

Quote:
Originally Posted by 80GS1000
If you were to change front wheel diameter and keep all parts (speedo + speedo drive) the same, would the margin of error be proportionate to the change in diameter? So going from a 110/90-19 front tire to a 120/70-17 front wheel would cause the speedo to be fast by 12%?

No, it would only read faster by 5%. I just went through the math and found that if all the sizes are exact as the stated measurements, it is 5.004% smaller. Now ... the stock speedos are off somewhere in the range of 5-10%, so you might end up being about 12% off with that setup.

I'm coming up with 12% faster

Overall Wheel Diameter 110/90-19 = 2*Tire height (in) + rim size (in)
= 2*.9*110/25.4 + 19 = 26.79"
Overall Wheel Diameter 120/70-17 = 2*Tire height (in) + rim size (in)
= 2*.7*120/25.4 + 17 = 23.61"

1-23.62/26.79 = 11.87%

The speedo (with 120/70-17) will show 12% higher because the smaller wheel has to turn more revs per distance traveled.

As I mentioned earlier if the speedo drive that belongs to the wheel is used then there shoudl be no effect on indicated speed.

Pos

[Steve]

Well, I have to admit that you are right.

Not sure where I missed the calculations, but it's close if I accidentally use 19" wheels for both.

.

[chef1366]

A magnetic dish spins from the cable action and a spring loaded metal piece sits in the dish. (not touching) The drag from the magnetic field causes the indicator to rise.

[1980GS1000E]

Keep in mind that any change will result in the odometer being off as well.. Just because the needle reads 10% fast does not mean that the odometer is equally off.