How to balance a Crankshaft???

I have found they will vibrate the fillings out of your teeth unless you DO use rubber padding on your mounts. Anyone who says different has a HIGH tolerance for vibration or a motor that is better balanced than any I have seen from China. When bolts back out of your engine and frames and mounts fail because of vibration, vibration IS a problem.
 
I'd suggest reading some of the following links thoroughly, also noting there are alternate mount solutions that while a bit more complex, don't compromise mount integrity;

http://motorbicycling.com/f6/rubber-not-27364.html
http://motorbicycling.com/f6/rubber-mounts-19205.html
http://motorbicycling.com/f6/engine-mounts-31953.html
http://motorbicycling.com/f3/cheap-rubber-motor-mounts-18244.html
http://motorbicycling.com/f6/motor-mount-issue-solved-30191.html

Interestingly, from my experience the primary source of undue felt vibration isn't actually the engine itself (unless it's very poorly tuned), but from the kit supplied hard plastic chain idler/tensioner wheel as jack shaft rides haven't a fraction of the felt vibration although equipped with an identical engine.
 
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My grubee 66 vibrates badly so I probably got a bad one. Here's my experience with different mountings for what it's worth. I tried solid mounting with the mounts lapped to the frame and it didn't help so I tried leather and inner tube dampers and they failed miserably. I converted to a SBP shift kit this past spring and made my own copy of their front motor mount and it's now very smooth.
I have a couple of ideas why it's smooth,The first being that while it's still mounted solidly the front mount and bearing mount are acting as a damper. The second theory is the motor now has two rear mounting points which are much more secure than the one stock mount plus giving the shift kits 3 total mounting points the motor is now triangulated making it naturally stronger and more rigid. Any thoughts??
 
Oh as far as balancing goes I asked the motor buider that built my snowmobile race motor about balancing one of these motors. Basically he said it would be a waste of time and money because you can only balance a single to a specific rpm range so it's going to vibrate somewhere in the rpm range. The cost to have this done is way more than the cost of the motor so it's not cost effective.
 
One thing I've noticed so far,

of the 4 cranks I've look at, when you put them on a truing stand, 3 of them roll around so the crank pin is up. One of them rolled aound so the crank pin was down.

This leads me to believe the crank pin side is supposed to be lighter.

This was without a piston on the rod.
 
The 49cc motor are better balanced than the 66cc motors due to the fact all the Chinese did was to enlarge the bore and hang a heavier larger piston off the 49cc crank. Severly lightening the piston helps. jaguar once posted a photo of a piston he had drilled out in places where it wouldn't interfere with the function of the piston. Reducing the weight smooths some of the out-of-balance vibration but a good cycle shop can also balance a crank. It's up to you on weither it's worth it or not.
 
I just took another motor apart to check the crank. This one has the bolt on fly weights, is heavy on the crank pin side.

Of the 4 cranks I've checked, 2 were heavy on the crank pin side, and 2 were heavy on the counter weight side. This is without the piston, or wrist pin.

Can anyone confirm which side should be heavier?
 
biknut said:
I just took another motor apart to check the crank. This one has the bolt on fly weights, is heavy on the crank pin side.

Of the 4 cranks I've checked, 2 were heavy on the crank pin side, and 2 were heavy on the counter weight side. This is without the piston, or wrist pin.

Can anyone confirm which side should be heavier?
Click to expand...

I think you should weigh the piston, rings, pin and clips and then attach a similar weight to the rod and then see how it balances. Try that weight on your different crankshafts and see what balances and that should give you an idea what is right.... That would be my guess at least...
 
nightcruiser said:
I think you should weigh the piston, rings, pin and clips and then attach a similar weight to the rod and then see how it balances. Try that weight on your different crankshafts and see what balances and that should give you an idea what is right.... That would be my guess at least...
Click to expand...

I 2 that go one way, and 2 that go the other way. Which is right?
 
biknut said:
I 2 that go one way, and 2 that go the other way. Which is right?
Click to expand...

I don't know which is right, but if you rest the crankshaft on a parallel surface and attach a weight equal to the piston, rings, pin and clips to the rod, the one that is right should balance out, the other should want to rotate always to one position (where it is over weight).
 
nightcruiser said:
I don't know which is right, but if you rest the crankshaft on a parallel surface and attach a weight equal to the piston, rings, pin and clips to the rod, the one that is right should balance out, the other should want to rotate always to one position (where it is over weight).
Click to expand...

With the piston attached all of them go counterweight up, crank pin down.
 
biknut said:
With the piston attached all of them go counterweight up, crank pin down.
Click to expand...

I have heard it said that the whole deal is designed for the 48cc piston and never redone for the 66cc. So the piston is heavy on every 66cc motor if that is true, which seems to follow what you are reporting....
 
It seems like if the crank pin side is already heavy without the piston, adding the piston weight will make the crank pin side very heavy.

Seems like these cranks balance varies wildly.
 
This is what I think. Without the piston, the crank pin side should be up. With the piston the crank pin should be down. So if it's down without the piston it should be pretty far out of balance.
 
biknut said:
It seems like if the crank pin side is already heavy without the piston, adding the piston weight will make the crank pin side very heavy.

Seems like these cranks balance varies wildly.
Click to expand...

I thought you said it was heavy WITH the piston attached??? I think that is where the 60% balance comes in???
I don't doubt they very wildly, the last one I ripped open was way out of whack, recently I opened the motor I am running now and this one seems really smooth compared to the last one... The last one was not only out of balance but had a ton of wobble....
 
Biknut,

In answer to your question "what does a 60% balance factor mean?", a 60% balance factor refers to taking 60% of the reciprocating weight (piston, rings, pin, keepers, and the small end of the rod) and adding this to the rotating weight (large end of rod) to come up with a bob-weight to affix to the crank pin. With the bob-weight in place on the crank pin, the crank assembly should balance statically when the crank journals are supported on knife edges or bearings.

A bare crank without piston or rod in place should always orient with the crank pin up and counterweights down. With the rod and piston (pin, rings, keepers, etc) in place the crank should always orient with the crankpin down and the counterweights up. Only when some fraction of the reciprocating weight (usually 50-60%) is attached to the crankpin will it balance statically.

Below is a link to an instruction sheet for balancing a hog crank where they are balancing each half individually. You should be able to follow along through the example.

http://www.rbracing-rsr.com/downloads/sscycle.pdf
 
All this and there are those who like the sound of a two cylinder four stroke Harley that has the firing boom boom.........................boom boom........................ ect.

The cylinder firing is not 180 degrees apart between the two cylinders, but they probably have counter weights at the crank shaft to balance.

Just Saying.


Measure Twice
 
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