Tachometer accuracy

[Mr. Rick]

The other day, Dave was over and we did some strobe work on the new Altair.
I was on the strobe and Dave was watching the tach and working the throttle. I had hooked up a Fluke meter with an inductive pickup to get a second opinion on the RPM's, and we found considerable disagreement (almost as bad as the one between Fullauto and Worntorn) between the Fluke and the Smiths.

Common sense tells me the Fluke can be trusted more than the stock instrument, so the question is why the difference? One can imagine that the gearing was pretty well thought out for the stock parts, so do the Smiths tachs need calibration from time to time? If so, can I do it myself?

 

[baz]

How different were the readings?

 

[o0norton0o]

the smiths is not gear driven. The cable drives a magnet to spin which induces spin in an aluminum disc which is attached to the pointer. That disc also has a very fine spring on it to return the needle to zero and balance against the induced spin on the aluminum disc. Did you know that aluminum was slightly magnetic??

 

[Interbak]

Smiths magnetic instruments can go out of adjustment. I compared my speed to GPS and found it was reading about 12% high. I used the electric drill method and compared it to several others I had on the shelf, the rest were all within about 1-2% of where they should be. I swapped it out for the best of my spares. This wasn't the original speedo that came on the bike, so indicated mileage was never correct anyway. You can send them out to be recalibarted, there are several places that perform the service.

B

 

[oldbeezer]

drill a small hole to get access to the brass adjuster screw .

Where does one drill this whole?
Thanks Jim

 

[oldbeezer]

Thanks Ludwig

 

[acotrel]

Do we need a calibrated electric drill to check the calibration of the tachometer ? Or can we use a calibrated stop-watch and count the revs ?

 

[o0norton0o]

I've messed around with both my magnetic smiths intruments. I have made a test rig to drive them with a right angle drill and I also use a GPS mounted centrally on my handlebars to check my speeds and my RPM's on the bike (using math... yes, math)

I bought the book on ebay that explains the workings and repair of smiths magnetic instruments. It stesses that a good cleaning can often restore an undamaged instrument to working order. I've found that there are also a few variables that are unfixable, such as a worn out spring or possibly a magnet that has weakened over time. You just have to clean it good, being carefull not to damage anything, then see how it responds to the adjustment screw. If you can't get it to work reasonably well, you might need some new parts... Where you get them, I'm not sure. My tach was working badly and I bought a donor off ebay and made one good one out of them.

My speedo which I've modified to run off the front wheel works perfectly up to about 80mph, then it goes crazy and says I'm doing 95mph, when my GPS is saying 85mph.

 

[texasSlick]

Unless one has a strobotach to measure the actual rpm of a drill, driving the tach with a drill leaves a great deal of uncertainty as to the drills' actual rpm vs label. The label rpm is at the full load condition, and the load to drive a typical tach cable is nowhere near the full load.

In the absence of a strobotach, one should use a synchronous motor to drive the tach. Synchronous motor rpm is an exact multiple of the line frequency .... e.g. nominally 1800 or 3600 rpm for 60 Hz line. Be aware that the line frequency can vary +/- 10% at any instant in time ..... The power company is obliged to average 60 Hz over a 24 hour period. So even with a synchronous motor, there is still some degree of uncertainty,.

Slick

 

[Mr. Rick]

Baz: I didn't write it down, but will check again tomorrow

Thanks, Ludwig, for the tip and the photos.
Do you recall which way the adjustment screw must be turned, to get the readings higher?

Slick : Thanks. My plan is to move the screw, check it against the Fluke meter, see if I can get it close. No strobotach or synchronous motors laying around my garage...

Frank: PM sent

Interbak: Thanks for the info. It's the tachometer I'm fussing over...

 

[edgefinder]

In Phil Irvings Tune for Speed book he says to use a single fluorescent shop light in a dark shop to synchronize with a tape or paint mark on crank rotor, 3600 rpm at 60 hz. Slick i'll ask a friend who routes power in Oklahoma City what variation in hz we have

 

[Mr. Rick]

Looks like I might be able to get my iphone light to flash at 60 HZ. This wd be 3600 RPM?

 

[DogT]

Strobotachs are relatively cheap on ebay.

 

[Interbak]

Baz: I didn't write it down, but will check again tomorrow

Thanks, Ludwig, for the tip and the photos.
Do you recall which way the adjustment screw must be turned, to get the readings higher?

Slick : Thanks. My plan is to move the screw, check it against the Fluke meter, see if I can get it close. No strobotach or synchronous motors laying around my garage...

Frank: PM sent

Interbak: Thanks for the info. It's the tachometer I'm fussing over...

Tach and speed work the same internally...

 

[acotrel]

When calibrating any instrument there are two things to be considered - accuracy and precision. The first is how far away the instrument reads from the TRUE value. The other is repeatability. When you get mechanics playing with scientific instruments in tuning shops, the might not always know the basics. That is why most dyno figures for horsepower do not probably mean much. I don't know about torque readings off a dyno when all you are seeking is an improvement in your mid-range power. AS I understand it, the Dynotech inertia dynos have an inbuilt fiddle factor to make them give a reasonable horsepower reading for a certain Yamaha superbike. It usually probably does not matter as the error in the zero and shape of the calibration curve, is common to all of them. So all of that brand read the same. If you used a Heenan and Froud test brake, after using a Dynotech, you would probably find a big difference in the value you get for horsepower.

 

[RoadScholar]

I use a drill to see if the instrument in question works and responds, but keep in mind that the speedo drive unit on the wheel has a ratio that makes a direct correlation between the drill speed and the speedo more of a calculation. The tach also has a set of gears between the crank and the instrument; someone on this forum undoubtedly knows what the relationship is.

 

[RoadScholar]

When calibrating any instrument there are two things to be considered - accuracy and precision. The first is how far away the instrument reads from the TRUE value. The other is repeatability. When you get mechanics playing with scientific instruments in tuning shops, the might not always know the basics. That is why most dyno figures for horsepower do not probably mean much. I don't know about torque readings off a dyno when all you are seeking is an improvement in your mid-range power. AS I understand it, the Dynotech inertia dynos have an inbuilt fiddle factor to make them give a reasonable horsepower reading for a certain Yamaha superbike. It usually probably does not matter as the error in the zero and shape of the calibration curve, is common to all of them. So all of that brand read the same. If you used a Heenan and Froud test brake, after using a Dynotech, you would probably find a big difference in the value you get for horsepower.

My sense is that x number of dynos will give x number of readings, like oil pressure gauges. So you stick to one dyno and see what engine parameters change the readings, then you know if you have increased or decreased the parameters you are seeking to influence.

 

[Mr. Rick]

I use a drill to see if the instrument in question works and responds, but keep in mind that the speedo drive unit on the wheel has a ratio that makes a direct correlation between the drill speed and the speedo more of a calculation. The tach also has a set of gears between the crank and the instrument; someone on this forum undoubtedly knows what the relationship is.

On the face of the tach, we can see 4:1
Does this mean that turning the cable at 250 produces a reading of 1000 on the dial? Or is it the other way around?

 

[RoadScholar]

On the face of the tach, we can see 4:1
Does this mean that turning the cable at 250 produces a reading of 1000 on the dial? Or is it the other way around?

Someone more knowledgeable than me can offer a more illuminating answer to your question.

As far as I'm concerned the factory issued tach and speedo are more bling then information displays; their indications are RELATIVE and need to be regarded as "advisors" not gospel. The '72 Roadster I have listed in the For Sale section has electric instruments that are exceedingly accurate; I thought it would be good to know, but I still don't look at them (much). The engine lets me know when it's off its torque and would like the next gear. The traffic and road conditions tell me how fast I can go, and when I find a road with no traffic I travel at a speed that offers some exhilaration.

 

[acotrel]

My sense is that x number of dynos will give x number of readings, like oil pressure gauges. So you stick to one dyno and see what engine parameters change the readings, then you know if you have increased or decreased the parameters you are seeking to influence.

Most of the Dynotech inertia dynos are probably consistent on the readings they give, if you test bikes on several of them. So if you are only looking for IMPROVEMENTS, they are good. For bragging rights, they don't mean much. It is the calibration in the absolute sense which is suspect. Same with tachometers. Most of us set an arbitrary rev limit for our motors, and use the tacho as a guide. If the motor does not self-destruct the first time we ride the bike, we continue to use the tacho and simply rely on it, regardless of whether or not it is accurate. As long as it is repeatable, there is no problem. In that respect magnetic tachos are better than chronometric. If they are out by a couple of hundred revs, it probably does not matter.