Balancing a crank/con-rod/ piston???
 

Out of curiosity, I put a crank from a single cylinder on a balancer. The weight side is much heavier than the con-rod pin side. I put the con-rod and piston on and now the piston side is heavier regardless of the position. Shouldn't the assembly balance in some orientation of piston/ con-rod?

RE: Balancing a crank/con-rod/ piston???
 

NO, that why you can't balance a single.
Basic is 100%l of the big end of the rod and 40-60% of the pistion and small end of the rod.
The 40-60% allows you to chosen where do you want the engine to vibrate, at full throttle or 1/2 throttle, cause its going to vibrate some where in the rpm band.
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RE: Balancing a crank/con-rod/ piston???
 

What Terry said. (tkg)

RE: Balancing a crank/con-rod/ piston???
 

Hi tkg,

First post here. Okay, so if I had an engine that currently shook at full throttle but instead I wanted it smooth at full throttle and to force the shaking to say half throttle; what would I do with the 40%-60%? Would I take more and use 60% or less and use 40%?

Thanks,
Dan

RE: Balancing a crank/con-rod/ piston???
 

Another question...how about redoing the piston to get some weight off it. Will that help and if so what RPM will it help. I have noticed some pistons are made ulta lite. Thanks Capt,n

RE: Balancing a crank/con-rod/ piston???
 

There is no % vs rpm formula, You just have to try and see what happens.
2 thoughts MOST engines have a heavy piston and need more counter weight. A cheap trick is to clamp on 2 hose clamps around the prophub with the tighten screws inline with the counter weight. You run the engine and see what happens. If you equally twist the clamps in opposite directions a bit at a time you will reduce the effective amount of the weight.
This is an easy way to tell if your engine is heavy on the piston or counter weight.

RE: Balancing a crank/con-rod/ piston???
 

This is very interesting, I know Homelite has a formula they use on there engines ,but it favored the piston side as being the heavy one if memory serves me right. Seems like it was 60/40.
When I made my engines I tried to get the rotating mass equal as possible. I have a engine that runs quite well and idles smooth, well smooth for a single cyl gas engine that is and the top end is vibration free as far as I can tell. I know you can't fully balance a single but I thought that was due to the mid stroke or side force harmonics the engine produces.
Here is avery short vid of it running, you can judge for yourself on the balance.
http://www.rcuvideos.com/video/RDs-55-cc

RE: Balancing a crank/con-rod/ piston???
 

I have an Alpina 330 that was marketed by Bill Bennett a few years ago. It has on the shaft just behing the prop driver an adjustable weight that is clamped on the shaft. I have not tried to adjust it, but it must serve the same function as the hose clamps you refer to.

I am thinking to convert this engine to ECDI and remove the flywheel and the spring starter to reduce weight. Will tthis affect the position of the adjustable weight? Tnanks.

Sincerely, Richard

RE: Balancing a crank/con-rod/ piston???
 

On higher rpm engines I've always leaned toward a slight overbalance. Usually somewhere around 52 to 54% "bobweight". As Terry said, it's more trial and error than science though. One reason for that is, how much fuel/oil is hanging on the piston whilst running? Answer, it's never the same and at different rpm's it's constantly changing affecting the balance just a tad. Just one of the many reasons why experimentation is necessary to find a balance that works best for your particular setup. Another is the harmonic frequency of the airframe/mount. It goes on and on and on.