Finding the con-rod length

[Rx7man]

Wondering how you folks find the con rod length...

I found the deck height fairly easy to measure.. stroke can usually be found in references... piston height from pin centerline is easy.., but con rod length on an assembled bottom end is pretty tough.

so I measure squish with no base gasket
Add the piston crown to wrist pin centerline distance
and half the stroke...

From that, I subtract the deck height and the distance from the squish band to the mounting flange of the jug

I'm left with the con rod length.

Worded differently, The height the piston is at at the top of the travel is the (squish clearance) + (crown to pin height) + (Stroke/2) +( Con rod length) must equal (deck height) + (cylinder depth).

On a Jonsered 920 I come out to 68.5mm no matter how many times I do it.. which is a little funny to have a .5mm in there.. but oh well!

Anyone else have a better way that doesn't involve splitting the crankcase and crank?

 

[NewToStihl]

I am absolutely no help to you. I am curious though - why would you be looking for the conrod length on a saw? Does it have something to do with modifying it?

 

[Rx7man]

When you're trying to find how far to move a port to get a certain duration, it becomes important... a long con-rod engine will have a very close to pure sinusoidal motion, while one with a short con-rod will have much more less piston motion near BDC per degree of crank revolution than a similar motion at TDC

 

[Rx7man]

I'd like to know how you get in there to do it... I don't have anything that'll even get close to fitting in there!

 

[Rx7man]

I guess easy is in the mind of the beholder!.. because trying to measure "a" in an assembled engine isn't as easy as on a cutaway

Yes, with your sketch, (a+b)^2 + c^2 = d^2,
or d = sqrt( (a+b)^2 + c^2)


Thinking more about it.. if you knew or were able to accurately measure the crank pin, you could probably cut a piece of light cardboard close to the length, and on a bit of a taper, and wedge it between the wrist pin and crank pin
Then you'd add half of the crank pin, half of the wrist pin, and the length of the cardboard where it touches..

 

[old 040]

When you're trying to find how far to move a port to get a certain duration, it becomes important... a long con-rod engine will have a very close to pure sinusoidal motion, while one with a short con-rod will have much more less piston motion near BDC per degree of crank revolution than a similar motion at TDC

smokey yunick built his race car engines using the longest rod he could fit in the engine, piston pin height has to be moved up to accommodate a longer rod, theory is that with a longer rod, there's more time for burn/expansion on combustion, in other words the piston remains at TDC longer............

 

[Chris-PA]

Funny because I just had that same issue when I was raising the crank on my Poulan clamshell. I have a CAD drawing that made up so I could see where the ports would end up when I moved the crank to help me decide what I wanted to do. But of course I needed the con rod length. I kinda eyeballed it on a loose crank assembly with a calipers at 59mm, which was good enough for my purposes since +/- a mm or so wasn't going to change the angles much.

 

[jughead500]

Smart asses.

 

[Marshy]

When you're trying to find how far to move a port to get a certain duration, it becomes important... a long con-rod engine will have a very close to pure sinusoidal motion, while one with a short con-rod will have much more less piston motion near BDC per degree of crank revolution than a similar motion at TDC

I don't agree but, hey I'm no engine builder either. Your explanation doesn't make any sense to me.

Length of con rod has no effect on distance of piston travel relative to crank duration nearing BDC. That is directly related to the stroke the crank was built with.

 

[Chris-PA]

I don't agree but, hey I'm no engine builder either. Your explanation doesn't make any sense to me.

Length of con rod has no effect on distance of piston travel relative to crank duration nearing BDC. That is directly related to the stroke the crank was built with.

If you think of it at the extremes you can see how it works. Picture a really absurdly long con rod - there will never be much of any angle relative to the cylinder bore, so the movement of the top of the piston is just the vertical movement of the rod journal (i.e. a sine function).

If you picture the shortest possible rod, there is considerable angle between the rod centerline and the cylinder bore axis when the crank is at 90deg. That angle has to straighten out as the piston gets to TDC, so the piston moves faster as it approaches and leaves TDC and BDC compared to a longer rod.

I think that a longer rod will put more force on the bearings as it spends more time near TDC with combustion pressure pushing down, but not much angle.

On these engines I doubt there is all that much difference, and packaging issues will probably always push for more compact engines and shorter rods.

 

[Marshy]

If you think of it at the extremes you can see how it works. Picture a really absurdly long con rod - there will never be much of any angle relative to the cylinder bore, so the movement of the top of the piston is just the vertical movement of the rod journal (i.e. a sine function).

If you picture the shortest possible rod, there is considerable angle between the rod centerline and the cylinder bore axis when the crank is at 90deg. That angle has to straighten out as the piston gets to TDC, so the piston moves faster as it approaches and leaves TDC and BDC compared to a longer rod.

I think that a longer rod will put more force on the bearings as it spends more time near TDC with combustion pressure pushing down, but not much angle.

On these engines I doubt there is all that much difference, and packaging issues will probably always push for more compact engines and shorter rods.

Considering a fixed RPM, piston velocity is a function of the stroke of the crank and not con rod length... Changing con rod length will not change the velocity or the traveled length of the piston at a given crank duration unless you change the stroke of the crank.

 

[Chris-PA]

For a fixed RPM, piston velocity is a function of the stroke of the crank and not con rod length... Changing con rod length will not change the velocity or the traveled length of the piston at a given crank duration unless you change the stroke of the crank.

It won't change the average velocity over over the stroke length (it must travel 2 x the stroke every revolution), but it does change the instantaneous velocity throughout the revolution.

 

[Rx7man]

Chris beat me to it.

It's not a mater of "agreeing" with it.. that's just the way it is..

Shorter conrods have the disagreeable side effect of putting high side loads on the piston and cylinder because of the steep angles.. Part of the reason diesels, with cranking compressions over 500 PSI all have long rods to minimize this side load.. they also don't care too much about weight

 

[Marshy]

this illustrates it. since the piston on the shorter rod travels farther in the same degree of crank rotation its maximum FPS velocity at any given rpm must be higher.
View attachment 407298

Thanks for taking me back to school. I was having a hard time of visualizing that. I buy it.

 

[srcarr52]

smokey yunick built his race car engines using the longest rod he could fit in the engine, piston pin height has to be moved up to accommodate a longer rod, theory is that with a longer rod, there's more time for burn/expansion on combustion, in other words the piston remains at TDC longer............

Longer rods increase the dwell time at TDC, allowing the initial expansion of the gasses to happen a little slower, allowing it to get to a full burn before the crank gets to an angle where it can utilize the expansion. This is especially beneficial with alcohol fuels.

But all this is really and added bonus on the top of the reduced side force on the piston.

 

[Rx7man]

2 strokes benefit from short rods in 2 ways though.. 1 is decreased crankcase volume = high primary compression ratios, the second is it gives you lots of time to keep the ports open nearly all the way at the bottom of the stroke

 

[one.man.band]

a few thoughts?

- there is bit of piston pin offset......the crankpin is not lined up w/the rod at TDC...so it's not exactly a right triangle at 90
- above may be the margin of error the way you do it Rx? that's the way i'd go. or spin a piece down of suitable length in the rod little end and measure direct.
- R/S ratio ........would think either rod or stroke

 

[Rx7man]

I just thought you should be able to fandangle a way to measure from the crank centerline to wrist pin centerline, and subtract half the stroke.. the problem is still getting the accuracy

 

[one.man.band]

turning down a suitable length rod ....to fit in the wristpin
measure from the to shaft
then subtract stroke......
but even then the offset will get ya a hair.
closest you may be able to get without removing the crank?

look about..there is something out there mathwise that may help. 0.5mm is about nuts there anyway, measuring with calipers. you would need a mic for anything closer.

 

[Rx7man]

I know that being off .5mm on your conrod length isn't going to drastically change anything in the port timing math.. it's just nice to know you're on the money!.. Calipers should do fine (mine go to .005mm), the trouble is finding reliable points to measure to/from

I guess you could also measure piston lift at certain intervals.. lets say 10*, and you'd see which one is the best fit mathematically.. again, probably only to .5mm!