The Act of Crank Balancing

[Fabian]

I like your weight reduction method Purple_Haze.

It's a relatively simple concept; it's relatively easy to do and it's a relatively low cost method of reducing weight, yet doesn't suffer any of the negative aspects of stress risers created in the wrist pin by the twist drilling process.

 

[Purple Haze]

I like your weight reduction method Purple_Haze.

It's relatively simple concept; it's relatively easy to do and it's a relatively low cost method of reducing weight, yet doesn't suffer any of the negative aspects of stress risers created in the wrist pin by the twist drilling process.

Thanks, Fabian. The method I described actually eliminates stress risers, hence making the part stronger. I believe the wrist pin could be lightened by a similar method, perhaps honing? Don't know what kind of tooling would be required, but I'm gonna look into it, as it usually requires lightening multiple parts to get the weight down.

 

[StrontiumEthics]

I think I have an idea of where you are talking about purple haze, but i'm not sure. Is the parting line on the crank arm in the middle? Thanks So far that sounds like an excellent idea!

 

[StrontiumEthics]

Ok So I have my crank and engine disassembled. I devised my crank balancing device and I have some numbers to work with. THe problem Is that im not sure wether to add wight to the crankshaft or remove weight. I read 2 posts one from Fabian and the other from Foureasy. Fabians method is adding wieghts, and foureasy's method is removing wieght.

 

[Fabian]

depending on the rpm sweet spot where your engine is desired to operate, you will either need to add or remove weight; and depending on what rpm the engine was previously intended to operate.
If using a 66cc engine, this changes the dynamics again.

There is no magic set figure and there is no magic set balance percentage either.
At the end of the day, you'll need to do the maths and then have these factors determine how much weight will need to be added or subtracted.
Even the frame and handlebar material can have an influence on your engine balance, as sympathetic resonance can occur at specific rpm; amplifying through the frame and handlebars.

No single bullet theory exists in this situation.

 

[StrontiumEthics]

I had some more time to tinker with this bike. It seems that the RPM sweetspot is about 5000-5500 rpms.
I would like to balance the engine at around 7000-75000 rpms. I created another static jig and weighed all the parts is the balancing factor what influences what rpms the engine will equalize itself (vibrations) at?

I read some posts where people added bob weights to there crank and added and removed weight based on that. Is this approaching the right direction?

The main thing for me is understanding what variables you have to manipulate to adjust the balance for a specific rpm.