Testing Crank Balance

[HeadSmess]

it isnt as clear cut as that... maybe do more reading before making definite claims...

http://victorylibrary.com/mopar/rod-tech-c.htm

https://www.highpowermedia.com/blog/3332/the-effect-of-con-rod-length

http://www.hemmings.com/magazine/hcc/2009/06/The-Mechanical-Advantage/1827793.html


blah blah blah...most race engine builders aim for the longest rods possible.

the increased side wall friction and wear from the use of a short rod more than outweigh any advantages in torque increase...

they do make for a taller engine, which is a drawback.

particularly interesting that the first link mentions that short rods INCREASE vibration.

i had an oddball lifan 1p52fmi a few years ago, aka, pitbike engine, with a 10mm longer rod than any other similar engine, for some strange reason... but it revved a lot harder, and seemed to produce more power...enough power to shred the splines from within the clutch housing one day. it also hit 140km/h with a racing cam fitted.

but, it was so oddball, i could never find a new cylinder stud for it when i accidently snapped one.. i once spent half a day at a pitbike shop, going through the several hundred engines he had out the back...not one matched, they were all the standard rod length... my cylinder and studs were both 10mm longer than standard. therefore the camchain was a different length as well, etc etc... other than that, it was a pretty darn good engine...

 

[Deleted member 12676]

well I was just going by what I've read. It made sense to me though.
But without having experimented in that area myself I shouldn't take any sides.

 

[Wolfshoes]

If you look at the upper right screw in the clutch cover you can see the spoke attached ready for a vibration test. I attempted to upload a .MOV video clip from an I pad but this format was not accepted by the forum site. I suppose a yah tube video link would work. This picture was taken while the motor was running. Not bad for an I pad.

 

[Deleted member 12676]

I uploaded your two test videos to my site.
Both engines show vibration, especially the one without the wooden support structure.
www.dragonfly75.com/motorbike/Delmar vibrate test-2.mov
www.dragonfly75.com/motorbike/Delmar vibrate test.mov

 

[Deleted member 12676]

even though you are using the lighter weight titanium wrist pins it still looks like both engines are lacking balance. But it is obvious that the makeshift engine mounting of the Skyhawk engine is allowing it to move more.
Here is his data on the engine of the 1st video:
Gasbike 66cc purchased April 2014
NT Carb
stock compression and head
titanium wrist pin (lighter)
stock CDI
All steel Schwinn beach cruiser
Wood motor mount
length of spoke subject to vibration 8 15/16"

Here is his data on the engine of the 2nd video:
Grubee 66cc Skyhawk GT5 date of mfg 6-15-2012
YD EPA carb
stock compression and head
titanium wrist pin
Jaguar CDI
Landrider bike

 

[Wolfshoes]

Makeshift engine mounting! Thats yankee ingenuity. At least you did not insinuate some kind of rig. If you want to see engine imbalance, let the engine move.

 

[Wolfshoes]

Using a spoke to test for out of balance vibration appear to be a valid test. The question it does not answer, at least to a degree of going into the engine to correct the imbalance, is the possibility the top of the crank assembly is already over cut, making the bottom of the crank the heavier situation. I have the rough idea of how to do a similar test that would show the circular pattern in a measurable way that would confirm the pattern demonstrated by the spoke. To do this test, a short stud, using an existing clutch cover bolt hole, would extend a short distance from the engine. Around this round stud would be 4 spring contactor clips as found in circuit board plugs for light wiring. The clips would mounted to be parallel to or 90 degrees from the piston stroke. The contact clips would be attached to the frame, in this case, the wood subframe. The contactor clips of these four points would be adjustable to allow a setting to be locked in at the point where electrical current, perhaps indicated by a light, would be barely detectable while the engine is running at a desired rpm where the vibration is apparent. After the engine is shut off, a measurement of the small distance between the stud and the clips could be made. If the crank is running top heavy as expected, the distance between the piston side of the stud and clip should be greater than the distance of the opposing stud to clip.

In running the spoke test for the video, I did something with the motorbikes I normally do not do; which is rev them up to the max without the normal drive load on the engine. In most situations, doing this on a stock motor, the inability of the provided spark plug to keep up would prevent the engine from exceeding a red line beyond it construction capability. In this case, I was doing it with iridium spark plugs that were able to keep up with the two cycle engines. Doing this also exposed another situation. It appears in 2014 Gasbike had cleaned up the porting, ignition and perhaps provided the higher volume NT carb version in their race bike rated offering. The benefits of this are not visible until you also empower them with a spark plug that can keep up; such as the iridium plug. With this configuration, rpms of the engine running without a load may be rivalling those found on engines with performance upgrades like high compression and reed valves. My point is that in doing static vibration tests with high performance plugs, it may be possible to damage the engine by over revving it.
It also may be possible that in revving the engine unusually high for the static vibration test, whatever is being done as a two cycle lubricant may not be adequate. In a 125cc Suzuki hop up kit sold in about 1971 to raise the engine rpm from 7000 to 10000, there was provided a replacement for their two cycle oil injection system. The replacement system had a smaller tank that only held about 4 oz of oil. The point is that the lubrication requirements of a two cycle engine running at the higher rpms could be different than running normal rpms; and even running the engine for a short time at the higher rpms could cause damage.

 

[Deleted member 12676]

the direction of the spoke vibration tells which way it is out of balance.
not enough counterbalance --- vertical vibration
too much counterbalance --- horizontal vibration

an engine would have to be jetted very lean or be too skimpy on the oil to gas ratio for it to seize during a few moments of having peak revs.
Here is the normal ratios for high performance engines:
20:1 regular mineral oil
25:1 semi-synthetic oil
30:1 full synthetic oil

I think that something plastic that is long and skinny would be even better than a metal spoke because of its lack of inertia due to its lack of weight.
Or also some kinda wooden skewer since it is also light weight. My thinking on this is the heavier it is, the more likely it will have frequencies at which it wants to vibrate (its oscillatory frequency or resonant frequency) and frequencies at which it don't want to vibrate.

 

[Wolfshoes]

Rather than a spoke or plastic, perhaps music wire would work better. It is available in a selection of diameters; even Walmart sells it. If a 11 ga spoke is about .070 of an inch in diameter, music wire appears to start at .018 (21018 as listed in link).

http://www.walmart.com/ip/PRECISION...00646369&wl4=&wl5=pla&wl6=97202968809&veh=sem

 

[Deleted member 12676]

plastic should be better because of its light weight. But that would probably require that it be longer.
But music wire would work also. Just experiment with different lengths.