Mechanical Speedometer Calibration

[Tomcatfixer]

Ron was correct - the nut above the adjustment lever is not a coarse adjustment, but merely a retainer for the needle shaft and tensioner for the lever.

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HOWEVER, I did discover that I could hold the needle shaft in place with one hand and, with a flat-blade screwdriver in the slot at the attachment base of the spiral spring, rotate the spring's orientation on the shaft.

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Now that I've figured this out, I used the same technique on the Mobile Chernobyl's speedometer. Fortunately, I didn't need to remove the tripmeter and odometer wheels to facilitate this. I'm dying to take a spin on the bike to gauge where this moved the needle, but it's pouring outside at the moment.
Soon...

[Tomcatfixer]

Huzzah! A break in the rain allowed me to get out for a test ride. Before reinstalling the speedometer, I had set the adjustment lever to the mid point. My first ride revealed that the speedo was still too optimistic, so I adjusted the lever back down towards the odometer wheels again. On the second run, when I held the Mobile Chernobyl's throttle so that the speedometer showed a steady 60mph, the GPS indicated a true 58mph. This 2mph discrepancy stayed about the same up to 70mph, though, curiosly, the speedometer is a bit pessimistic around 35mph. I'll just be careful in school zones.

Now, to open up the Retro Solo's speedometer again and repeat this process.

It's very satisfying to have worked this out.

[n3303j]

Rotating the brass collar on the pointer shaft is the same as relocating the pointer. Moving the adjustable end of the spring at the taper pin to shorten the spring changes its response curve. You wanted to change its rate (response curve)

[aduthie]

This is fantastic. As a point of clarification, this adjustment doesn't change how the odometer works at all, right? It's just changing how the needle responds to the input from the cable and the gearing of the odometer.

[Tomcatfixer]

This is fantastic. As a point of clarification, this adjustment doesn't change how the odometer works at all, right? It's just changing how the needle responds to the input from the cable and the gearing of the odometer.

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[Tomcatfixer]

Rotating the brass collar on the pointer shaft is the same as relocating the pointer. Moving the adjustable end of the spring at the taper pin to shorten the spring changes its response curve. You wanted to change its rate (response curve)

I dunno, Ron. Relocating the ponter definitely has no effect on spring tension. Rotating the point where the spring grips the needle, as I did with a screwdriver, most definitely affects spring tension, as evidenced by my now (mostly) calibrated speedometer.

[jaybird]

Years ago there were a couple of, what I recall, pretty good threads on Ural speedometer calibration. With the number of posts on Soviet Steeds and the shortcomings of the forums search, it could take some digging to find them, might be easier to figure it out on your own as you are. Certainly more satisfying.
Nice write up so far anyway.

Happy trails,
Jaybird

[Snakeoil]

This is fantastic. As a point of clarification, this adjustment doesn't change how the odometer works at all, right? It's just changing how the needle responds to the input from the cable and the gearing of the odometer.

The spinning magnet imparts a force on the steel cup. If there was no spring, the cup would spin and eventually match the speed of the spinning magnet. The faster the magnet spins, the greater the force on the cup. The tighter the spring is wound, the more counter-force it puts on the cup. So the higher vehicle speed created higher magnet speed and the additional force overcomes the spring force at that point and the needle moves up the scale until the magnet's force and the spring force are equal and the needle stops at that speed.

[aduthie]

I had a vague sense of this; your description is clearer than what I had in mind. And I can infer the answer to my question is indeed "no, this has absolutely no effect on the odometer." (And in fact there is no way to recalibrate the odometer short of changing the output yoke in the gearbox, which is where the worm gear for the gauge gets its input.)

[Snakeoil]

Right, the odo is gear driven and the ratios of the gear train determine the calibration of the instrument. The drive gears in the tranny are part of the ratio determination. The precision of the gears determines the accuracy of the instrument. The final determinator of the accuracy of the reading on the odo is tire circumference. The tire will also affect the accuracy of the speedo as well. But I'm sure you know all that stuff, Andrew.