Parallax error – Mike V did you ever quantify this

[Barry]

One thing common to both static and strobe timing is the possibility of parallax error when making an observation through the timing hole. The instinct is not to look down from above but to ensure that the observation is as far as possible square on to the timing hole. I now believe that to be a wrong assumption as the outer casting is not tangential to the flywheel. You can actually observe this by looking carefully through the timing hole at the angle of the flywheel teeth. The attached image attempts to quantify the error between observation of the timing markings square on with the outer casting (yellow line) and a true line passing through the timing hole and crank centre (blue line). The magnitude surprised me 1/2 a tooth error or almost 2 Deg on this image of a full sized 93 tooth airhead  flywheel. As the timing marks on R45/65 flywheel are much deeper the potential error will be greater probably getting on for 4 Deg. I couldn't’t find a similar good quality image of an R65 flywheel so if anyone has one I will do the same measurement (This image a modified part of a very high quality photo by Stephen Bottcher).

The whole issue of accurate observation is made more difficult on my bike by the timing notch being quite a bit less than half way up the timing hole so I set about making a simple tool to insert in the timing hole with an indicator long enough to eliminate most of the parallax error in observing the timing mark. I used a full scale life size print of this image to calibrate the angle of the pointer on my tool.  

Mike V, I remember that sometime ago you were working on precise means of eliminating parallax error in the timing hole. Did you ever quantify the error ?

[Teo]

Great observation, Barry.  I too try to ensure I've eliminated parallax.  

Your tool is ingenious.  What is it made of?  It appears to be an old "top hat" wheel spacer with the pointer welded thereto.  Is that so?

Teo

[Barry]

What is it made of?It appears to be an old "top hat" wheel spacer with the pointer welded thereto.Is that so?
 

I suppose it does look like a top hat spacer. Never thought of using one. I think they would be too big as that picture is about 2 or 3 times real size. It's a turned brass boss with a 15mm dia thin wall tube and pointer soldered on. The flat on one side is to ensure I get the tool oriented exactly the same way every time.

[Mike V]

Barry,

Let me try to relay what my findings were in as short a thread as possible.  While I had the engine out of the bike during the restoration and after the top end job was completed. I was in search for absolute top dead center so I could reference the flywheel stamping position in relationship to the case notch and determine any error if in fact, there was any.  With the use of a piston stop and large radius timing wheel attached to the flywheel I found the case and flywheel marks to be surprisingly accurate. Barry, you're correct - the center of the crank, flywheel marking and case mark do not line up radially in a straight plane.  My intension was to first determine the relationship of these two positions by way of absolute TDC and then determine a corresponding sight-line so my reference during timing was acceptable. The markings were surprisingly close.  I ended up painting a white line along the tooth of the ring gear in-line with the case mark which illuminates quite well while the engine is running with a timing light.

The picture shows the position of absolute TDC of my flywheel in relationship to the case marking. I painted a white mark in a line between the case mark and flywheel mark along the flywheel face.  It gives me a third reference (of sorts) to develop a plane, or general angle of view.  In the end this helped me with my angle of view.  But we're really splitting hairs here Barry, as long as you position your sight line (eyeball) relatively close to the case and radially the degree of error is very little in my opinion.  I'm not sure if I went to the trouble of determing all of this analytically, if I could even make the timing adjustments to that type of precision.  It's an Accuracy vs. Precision thing in my mind.  It was a fun test to do but my guess would be if we all were within ±3° we may never feel the difference in performance.  In order to use a "third" point you MUST know where Absolute Top Dead Center is on your flywheel.

One full tooth spacing on the flywheel should equal about 4° (03°52'15") of angle.

[Barry]

Thanks Mike.

I also painted lines on the flywheel teeth which work well with strobe timing. My tool is better with static timing.

I fully agree with your comments about not needing absolute accuracy in ignition timing. Every graph of torque vs Ignition timing I've ever seen shows imperceptible changes to torque a few degrees either side of optimum.

My reason for doing it besides my own entertainment was pursuit of repeatability. Even though accuracy to + or - 3 deg is not that important I do want to be able to make repeatable changes for avoidance of detonation.

[Justin B.]

I have always been told by old wrenches that on cars, at least, once an engine has a few miles on it ( think timing chain stretch) timing with a light is pretty much a matter of luck.  I was told by several to keep advancing the timing until there was a bit of spark-knock under heavy load then back it off until it stopped.  We called this "power-timing" back in the day and it resulted in optimum ignition advance for your particular situation.  At this point you could then look where your timing fell with the strobe and use it for a couple of tune-ups.

[Barry]

I suppose "Power timing" or ping timing is effectively what I've been doing.

Running on 95 RON fuel my engine will ping (detonation) if the timing is too advanced. Hence my desire for a repeatable setting. I had been running deliberately a couple of deg retarded to be on the safe side but decided to push it closer to the ping setting.

Airheads are very audible when they ping under load and a little bit of pinging for a few seconds does no harm.  I have a hill on the way home that is my repeatable ping test. The bike will accelerate up nicely in top gear at 3000 + RPM but will ping below 3000 RPM if I lugg it.  

An argument can even be made that if an engine doesn't ping under extreme provocation (which you wouldn't normally use) then the timing is set too retarded. That's particularly valid on a bike where the ignition advance is very crude even compared with contemporary car points ignitions. We lack any vacuum advance for light load running where at least another 5 deg advance is desirable. Our ignition is therefore not really optimised for light load and for that reason it's not very efficient to be deliberately setting the timing retarded to avoid pinging under conditions which you shouldn't be using anyway.