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Dukester1Sun Jun-19-05 06:47 PM

  
"Crankpin Location at Top Dead Center"


  

          

Hello to All!

I have a question about crankshaft rotation, specifically the reference point of the crankpin (timing/right side, #1 piston) when the piston is at top dead center.

As the barrel is not directly above the crank/case at 12 o' clock like earlier singles/twins, what is the crankpin location when the piston is at top dead center?

When piston #1 fires what is the location of the crankpin?

  

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andersonWed Jul-06-05 06:51 PM

  
#1. "RE: Crankpin Location at Top Dead Center"
In response to Reply # 0


  

          

Hi,

No one has tried a reply to this. Possible because, like me, Im finding it difficult to understand.

You seem to be saying the motor has the barrels off set (forward or back) to the centre of rotation of the crankshaft.

I must admit I had never comtemplated a Norton motor like this. I have never measured it but I guess one could have the situation on a motor where the manufacturing tolerances were such that the barrels were slightly for or aft of the crankshaft.

Are you really sure your motor is like this ?

But anyway. If you want to know what the angle is you could solve it by trig.

The piston is going to be at maximum height when the gudeon pin centre, the crank pin centre and the crankshaft centre form a perfect straight line. So add the lenght of the con rod (centre to centre) to the distance from crankshaft centre to crankpin centre.

This gives you the hypotenuse of the right angle triangle (H) .

Then you need to measure or calculate the off set from crankshaft centre of rotation to the centre line of the barrel. This gives you the distance (O)the gudgeon pin centre is off set to the crankshaft.

You now have the hypotenuse and opposite side of a right angle triangle and you can calculate the angles.

If your trig is a bit rusty in NZ this is about first/ second year high school trig so if you can find a sensible 14 year old (Do they exist??? - my youngest turned 15 3 days ago) they should be able to do the sums.

But Im still a bit mistified as to what is happening. I guess if the motor geometry was really like this then setting igniton timing would be still OK. You could find TDC off the piston and then use a degree wheel to set the timing. But valve timing would need to be adjusted. You couldnt use the normal key way settings.

I hope I have understood this properly and my suggestion is correct.

John

  

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RohanSat Jul-09-05 12:56 AM

  
#2. "RE: Crankpin Location at Top Dead Center"
In response to Reply # 1


  

          

Having the cylinder vertical axis forward or rearward of vertically above the crankshaft centreline is referred to as "Desaxe" or "reverse deSaxe". Was all the rage in engine design - many decades back. As was some rod angularity at "TDC".

This very subject was discussed on the NOC chat list a while back. I recall it was suggested that there was no evidence that Norton intended the Commando cylinders to be anything other than directly above the crank centreline ??. Factory machining errors or poor cylinder reboring could possibly account for anything different out there ?
Having said that, I haven't actually looked at this - been meaning to measure up a spare engine kicking around someplace.

And its been said that in proddie bike racing, some smart thinking saw folk reboring and resleeving some engines back to std - with the cylinders some thou forward/behind the std position. This was effectively advancing or retarding the cam timing a few degrees. Kwacka 4s in particular responded well to this mod, and a modded engine would beat a std engine every time. Until the scrutineers knew to look for it... (With a suitable cam, this mod is useless of course, but proddy racing required everything stock std).

Trll us more if there is more to this story.

  

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Dukester1Mon Jul-25-05 05:11 PM

  
#3. "RE: Crankpin Location at Top Dead Center"
In response to Reply # 2


  

          

Thanks to John and Rohan for your replies.

My question was poorly written, I apologize.

Here is the story:

I have a fundamental understanding of crankpin placement in regard to the 360 degree rotation of parallel twins. I understand the relationship of where the crankpin is in each stroke, etc.

I am trying to get a reference for what the position is when either piston is ATDC and I believe Rohan answered this in his response.

Now, the real reason I am asking is that I am having a 90-degree rephased crankshaft made for my 850. Of course the camshaft has to be altered as well. I assumed that a 45 degree shift in the camshaft would be required, but was told later that 45 degrees would not be correct. I am trying to figure out the correct phase shift for the cam, thus the questions about crank reference.

  

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RohanMon Jul-25-05 07:43 PM

  
#4. "RE: Crankpin Location at Top Dead Center"
In response to Reply # 3


  

          

Mmm, unless there is some major complexity in Commando engines I'm not aware of then a 90 degree shift in one set of lobes should be precisely what is required for a 90 degree rephased crank. Don't forget that the cam spins "backwards" (in relation to the crank which spins in a forwards direction) so make certain the 90 degree shift is in the correct direction. And obviously your ignition has to know about this too...

Ignition setting in a std commando is simply 28 degrees (fully advanced) ahead of when the pistion is at TDC, regardless of any of the complexities above...

opethiselps,
Rohan.

  

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RohanMon Jul-25-05 07:55 PM

  
#5. "RE: Crankpin Location at Top Dead Center"
In response to Reply # 4


  

          

Err, speaking of poor wording. A 90 degree rephased crank needs the cam timing also to be altered to be rephased to the same 90 degrees.

Since the cam spins at half engine speed, this translates to cutting the cam and rewelding one set of lobes rephased 45 degrees - in the correct direction. Course, you could have a new cam made up, or the existing cam welded up and reground to suit.

  

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Dukester1Sun Jul-31-05 08:35 AM

  
#6. "RE: Crankpin Location at Top Dead Center"
In response to Reply # 5


  

          

Thanks Rohan!

Drew it all out and arrived at same conclusion as yours:

Camshaft inlet lobe for Piston #1 at 12' o'clock with camshaft inlet lobe for piston #2 trailing 135 degrees. This is where the 45 degree compensation occurs. Exhaust valve lobe location change needed as well.

You mentioned the timing neededs to be altered. Can't use standard Boyer I supppose. Any suggestions?

  

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RohanWed Aug-03-05 09:53 PM

  
#7. "RE: Crankpin Location at Top Dead Center"
In response to Reply # 6


  

          

If you alter the crank to 90 degrees in the appropriate direction, it will have the same firing interval as a Duc bevel drive motor (or a Pantah for that matter). Anyone looked to see how big a task it is to bolt that ignition setup into a Norton ? (Don't forget to take into account that bevel drive motors spin backwards if you come across an ignition set)

I recall some chat about Boyer maybe producing something suitable, can't recall the details. be worth checking with them before altering your crank ? And I seem to recall someone in Canada had done a few Nortons and Triumphs like this, be worth tracking him down to see what was what ?

opethiselps.

  

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Dukester1Sat Aug-06-05 09:13 AM

  
#8. "RE: Crankpin Location at Top Dead Center"
In response to Reply # 7


  

          

Thanks Rohan!

Found a site called " Brit Iron" which had an article describing how to modify Boyers for 76, 90 and 180 offsets for British twins Article called "Geoff Collins Boyer Files" (http://www.users.interport.net/c/s/cstringe/boyer.htm).

Pretty good stuff, but pictures aren't embedded in article. Wrote webmaster/contact point early in week but no return mail.

Will keep searching!

  

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Tom LonglandFri Sep-16-05 08:28 AM

  
#9. "RE: Crankpin Location at Top Dead Center"
In response to Reply # 8


  

          

If you e-mail me here : [email protected]
I'll send that article with the pics.
I don't know how to post to the forum, but, if you could, it might help others.
Tom

  

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Dukester1Thu Oct-06-05 08:19 PM

  
#10. "RE: Crankpin Location at Top Dead Center"
In response to Reply # 9


  

          

Will do!

Thanks!

  

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