• By
  • On 18th July 2015

As you may have read already, the minimalist approach was the main principle followed when building BLACKSQUARE and consequently the bike has no instruments such as speedometer or tachometer, signal lights, warning lights, etc.  However, when the original 1977 HONDA CB550, which later became BLACKSQUARE, came to the workshop back in 2012, I noticed that there was a dead fly on the tachometer’s dial.  How it got there has been bugging me ever since.  And I decided that the time has come for me to find out.  In the process, I hoped to discover how the tachometer works.

How did this fly get inside a tachometer that is supposed to be waterproof?

There is a fly in my tachometer closeup

There appears to be no apparent entry at the front.

Looking at the back of the tachometer,

it seems that there are three possible entries:   through the hole the electrical harness comes out of, through the hole around the tachometer cable’s connection and through the small vent hole at the bottom. For being waterproof, this thing is pretty open!

Okay, so far so good.  The fly could have entered from any of the holes in the back.  But how did it then get all the way to the front?  To find out, we need to remove the back cover.  All that needs to be done here is to undo the two acorn nuts and pull the cover away.

Once the back cover has been taken off, here is what we see:

Tachometer back side without cover

Note the two small holes at the periphery at about 11 o’clock and 5 o’clock positions.  They are probably vent holes.  This seems to be the only possible entry into the rest of the tachometer’s body.  So, it looks like we now have to take the two halves apart in order to see how the fly could have gotten all the way to the front.

Removing the two visible screws (one is clearly visible, the other is under the wires) will accomplish nothing at this point.  All they do is secure the tachometer’s mechanism to the lower half of the tachometer’s body.  To take the two halves apart, the metal band that holds them together must be pried open.

Tachometer pried edge of connecting metl band

To pry the band initially, a really small screwdriver could be used just to provide enough opening for a bigger screwdriver to finish the job.

Tachometer view showing how to pry the connecting metal band using a screwdriver

Once the metal band has been pried all the way around, it is possible to separate the two halves.

The two halves of the tachometer

By removing the two screws mentioned above,  the tachometer’s mechanism could be separated from the tachometer’s lower body.

Tachmeter mechanism and cover

The dial sits a small distance away from the tachometer’s upper body providing a tiny gap for the fly to squeeze through.  In my case, the dial was bent a little making the gap bigger.  So, that is how the fly got to the front: it must have entered through the tachometer cable hole or electrical harness hole in the back cover and then through either of the two holes in the lower body and then through the gap between the dial and the upper body or up the needle’s shaft and through the small hole in the center of the dial. Mystery solved.

But now that we have almost everything apart, let’s try to figure out how it works.

The tachometer’s needle is connected to a shaft.  A spring is wound around the shaft and secured to a post on the mechanism’s frame. At the bottom of  the needle’s assembly, there is a bell-shaped metal piece.

Tachometer's mechanism SPRING and BELL identified

A magnet positioned at an angle to the bell is connected via a shaft to the terminal that the tachometer cable is screwed into. There is no mechanical connection between the bell and the magnet.

Tachometer's mechanism BELL and MAGNET

Now, here is how I think the tachmeter works.

By definition, the tachometer displays the crankshaft’s speed of rotation in revolutions per minute (RPM).

The tachometer drive (which spins the tachometer cable, which spins the magnet in the tachometer’s mechanism, which in turn spins the bell and the needle attached to it) is not connected to the crankshaft.  It is connected to the camshaft.  The crankshaft and camshaft are connected via a chain.   When the crankshaft makes one full revolution, the camshaft makes half a revolution. For every 10 revolution of the camshaft (20 revolutions of the crankshaft), the tachometer drive makes 3 revolutions. (This is why the tachometer ratio for CB550 motorcycles is defined as 20:3 or more popularly known as 7:1.)

When we say the engine is idling at 1,000 RPM, that means the crankshaft is rotating at 1,000 RPM.  Consequently, the camshaft is rotating at 500 RPM and the tachometer’s cable inside the housing is rotating at [500 x 3]/10 = 150 RPM, which means the magnet is rotating inside the bell at 150 RPM.  As the magnet rotates, it spins the metal bell (the rotating magnetic force is trying to pull the metal bell with it, pretty much like a wind can move a propeller).  The metal bell begins to spin and as it does so it tries to unwind the spring.  It can only unwind the spring so much before the spring’s tension becomes equal to the force trying to unwind it and so the bell reaches a stationary “equilibrium” having turned only 10 degrees or so and the needle stays at the 1,000 RPM position.  When the crankshaft’s speed is increased to 2,000 RPM, the magnet turns at 300 RPM and now this causes the bell to spin a little further until the spring’s tension stop’s it and the needle again.  And so on and so on all the way up to the maximum RPM.  When the engine’s RPM decrease, the magnetic force spinning the bell decreases and the spring’s tension brings the needle back.

Pretty neat, isn’t it?

P.S. If you wanted to stop flies or other bugs from getting into your tachometer, a rubber grommet around the electrical cable’s hole should be able to do the trick.  Those can be easily found at just about any hardware store. The hole around the tachometer’s cable connection presents a slightly bigger challenge, but closing it sufficiently should be doable.  Plugging the vent hole is not a very good idea, in my mind. It is perhaps better to glue  a couple of very small screens on the inner side of holes in the tachometer’s lower body.

And if your tachometer is a little erratic, it is unlikely that anything inside is the culprit. The mechanism is just so beautifully simple. Lubricating the tachometer cable and dropping a drop of oil into the tachometer’s terminal that the cable connects to are the first remedies to try.