Stewart Model 100 speedometer adjustment.

[Steve Jelf]

Rather than send the question directly to Russ, I'll post it here in case others are interested too. After working on the head and checking whether it works with a reverse drill, I reinstalled it in the car and tried it out today when I drove to town for groceries. It works, but reads low. With GPS showing 32 mph the speedometer shows about 20. Going 37 to 38 gets the speedometer up to 25. In a drive around the block, a known 2½ miles, the odometer ran up 2.45. If my figuring is right, that works out to a reading of 98 miles per actual 100. I call that close enough for gummint work. The odometer is the part that's important to me, because I want a running count of total miles. An electrical odometer resets to 0 if you have to change the battery, so this is better.

Though the odometer is more important to me, it would be nice to have the speedometer show closer to the actual speed. I'm guessing that the adjustment built into the Model 100 is not enough to correct the current under-reading, and that this head needs a change of spring. Is that right?

[TRDxB2]

Here is a link to an explanation Russ provided in 2015 about calibration
https://www.mtfca.com/discus/messages/5 ... 1422581728

[Russ_Furstnow]

Steve,
There are only three things that impact the speed on a Stewart speedometer:
1) the strength of the magnet
2) the strength of the return hair spring
3) the placement of the metal square tab found below the magnetic drum.
The metal square tab is probably what you need to move to increase he speed of your speedometer, but don't discount numbers one and two. Generally, moving the square tab to he left, or counterclockwise will increase the speed of the unit.
I hope this helps,
Russ Furstnow

[Steve Jelf]

Thanks, Russ. I'll try the tab adjustment and see how much that does.

[Bruce Compton]

Can you actually replace the hair spring? I have two 1913 Stewarts, one with a perfect hair spring and terrible drum while the other has a messed up spring but nearly perfect drum.

[Russ_Furstnow]

The hair spring and the magnetic drum can both be replaced. The magnetic drum can be also re-magnetized. The big issue is being able to calibrate the speed once any changes have been made. I use a Stewart calibration machine, but these machines are somewhat rare. I hope this helps. Russ Furstnow

[Bruce Compton]

Thanks Russ. What's the trick to replace the hair spring?? Bruce

[dykker5502]

Steve,
There are only three things that impact the speed on a Stewart speedometer:
1) the strength of the magnet
2) the strength of the return hair spring
3) the placement of the metal square tab found below the magnetic drum.
The metal square tab is probably what you need to move to increase he speed of your speedometer, but don't discount numbers one and two. Generally, moving the square tab to he left, or counterclockwise will increase the speed of the unit.
I hope this helps,
Russ Furstnow

But will that also fix the odometer issue? I doubt it and that was what Steve wrote was most important?

[Russ_Furstnow]

The accuracy of the odometer is driven by the road and pinion gears. To determine the road/wheel gear that will make the odometer most accurate, determine the diameter of the tire and mulitply times two to get the number of teeth necessary for the road/wheel gear. The pinion gear will ALWAYS be 16 teeth. Steve's gearing must be pretty good to show 98 miles for an actual 100 miles traveled! I hope this helps. Russ Furstnow

[dykker5502]

My point exactly, Russ - so Steve will have to live with 2% error. Or experiment with various tires as some may just hit the nail spot on.

[Wayne Sheldon]

Tire circumference varies by minor size variations due to brand, batch, and wear. A two percent error is about as close as it gets.