BBC engine Balance

[robert1]

So your saying I can just hack the counter weights off because they do nothing. I have 2 cranks, 1 is a billet pc with large counter weights the other is a forged Lunati with small counter weights. It's not going to be perfect but I bought a wheel type fixture and I'm going to make a bob weight of the correct weight and do it like a motorcycle engine. I'm going to use 60% unless someone knows better. It's a kart by the way.


Add: The fomulas I'm finding say to use 1/2 of the reciprocating weight. Any input here?

[SchmidtMotorWorks]

1/2 reciprocating will have roughly the lowest bearing load peaks and will vibrate roughly the same vertical and horizontal.
Like Bill said, it all depends on the chassis and which direction the vibrations are best tolerated.

If acceleration is important and the crank has good oiling I would lighten the CWs until I found new problems.

[Carroll]

1623 seams kinda light?
Joe C

[Wolfplace]

1623 seams kinda light?
Joe C

I would guess it is the weight of the parts themselves, not the bobweight number

[mmmitch588]

1623 seams kinda light?
Joe C

I would guess it is the weight of the parts themselves, not the bobweight number

I agree. I think he needs to add another rotating weight into the total for his bob weight.

[panic]

Not to divert the subject, but I have yet to read a justification for the assignment of reciprocating vs. rotating weight of rods by the center of gravity.
I have also read "the rod length is not relevant" frequently, but how is the thrust component (~ with rod angle) not a factor?

[SchmidtMotorWorks]

Not to divert the subject, but I have yet to read a justification for the assignment of reciprocating vs. rotating weight of rods by the center of gravity.
I have also read "the rod length is not relevant" frequently, but how is the thrust component (~ with rod angle) not a factor?

They rotating and reciprocating masses must be treated differently because they create different types of vibration.

The rotating mass applies continuous force in a direction that rotates with the crank.
The reciprocating mass applies a cyclical force that changes with piston acceleration.

In regards to thrust component, there is nothing in balancing to compensate for anything beyond the masses of the mechanism.

It is possible to make a crank that has some intentional imbalance to reduce bearing loads from cylinder pressure but that is far beyond the capability of just balancing to a different bob weight. It requires analyzing the crank as a flexible object under loads and optimizing the CW layout to minimize bearing loads. To do this, starting from nothing with a very good CAD and CAE system is about 3 months work to set up for the first time then after that it would take a few hours to set up and a few days to run the analysis and optimization unattended.

[55chevygasser]

I had the combo balanced Friday. I had them do it 52% for an overbalance. I will post the actual bob weights and balance numbers when I get to the shop tomorrow. But they added 48 grams to one end of the crank and 45 to the other. I thought that was a lot because the rod and piston combo weighed the same as the old, so I was glad I had it done. The machine shop owner and I were talking about it and then he said that Eagle would have balanced the assembly at 50% so it would have been pretty close. He went to the balance sheet and showed me that at 52% it added 93 grams to the crank. So I guess the answer to my original question would be Eagle can do a pretty good job on the crank balance but I know they did nothing to the pistons and rods so who knows how close they were. My new Weisco pistons were 4grams different from the lightest to the heavyst. And the new eagle H beam rods were about the same. The shop did a great job getting them all within 2 tenths. And he did weight match both ends of the rods. Thanks to Shawn at Sweeten performance.

[SchmidtMotorWorks]

I had them do it 52% for an overbalance.

Ask him to tell you what the techical purpose of overbalance is at the physics level.

[justahoby]

I thought over balance was an advantage at lower RPMS, under-balance for upper rpms ?

I.e. usually a smoother idle with 52% and like 49% possible with like high rpm use. Maybe I am asking a confused question

[SchmidtMotorWorks]

I thought over balance was an advantage at lower RPMS, under-balance for upper rpms ?

Never heard that one before, any idea why imbalance in any direction would reduce vibration?

[justahoby]

I thought over balance was an advantage at lower RPMS, under-balance for upper rpms ?

Never heard that one before, any idea why imbalance in any direction would reduce vibration?

No I dont have the answer, had to do with some theory that at higher rpms other things show up with vibrations in upper rpms. Had last two people that balanced for me ask me specifically the rpm range I will be planning to run. On the street if I tell them 6000 rpm instead of 5 or 5500 there was always a weird shake at idle. Overbalance and underbalance is a school of thought out there.
People do as much as 52% for lower RPM smoothness at the expense of vibrations ect, as at lower RPMs is different than 7000 rpm, where the inertias and forces change.

Not a machine shop or engineer, was just wondering if you had any Idea why they do it. Maybe next time will tell them I want it at 50% period.

[SchmidtMotorWorks]

Not a machine shop or engineer, was just wondering if you had any Idea why they do it. Maybe next time will tell them I want it at 50% period.

Until someone comes up with physics based explanation, that is what I would suggest.

I have asked this question hundreds of times at all levels of racing never never heard a well reasoned answer.

Considering that it originated in an era and field where proper engineering was uncommon I wouldn't be surprised to learn that there is no rational for it.

[Kevin Johnson]

Not a machine shop or engineer, was just wondering if you had any Idea why they do it. Maybe next time will tell them I want it at 50% period.

Until someone comes up with physics based explanation, that is what I would suggest.

I have asked this question hundreds of times at all levels of racing never never heard a well reasoned answer.

Considering that it originated in an era and field where proper engineering was uncommon I wouldn't be surprised to learn that there is no rational for it.

Sigh. http://www.eatonbalancing.com/blog/2007 ... ng-part-5/ was published by Mr. Eaton 5 years ago and was certainly common automotive engineering knowledge prior to this. Sometimes getting an answer to a question involves actual research work on your own part: I know you took the time to do this with camshaft theory and design.

I would certainly term engineering design practice that evolved from empirically evaluated physical effects from over- and under-balancing to be "well reasoned" and "physics based".

Perhaps you are objecting that over- and under-balancing are sometimes blindly done without any knowledge of the empirical results of others. This should be the basis for a different sort of objection.

[55chevygasser]

The shop did not decide on the over balance I did. But I won't get into an argument I don't do that very well...I know from 30 years of machine shop experience and fabrication work that physics come into play, I understand that if you balance your teeter totter you would want 50% but if one end of the teeter totter had a cup mounted on it and it started raining 50% won't work very well. Then if one end has a fan blowing at it but sometimes it suck instead of blows it might make a difference. Not to mention that one end is upder a tree and bird are dropping crap on that end, might make a difference over time. I know this is over simplified but people can get goo goo eyed when somebody starts using words and terms they cant understand and they might form an unjustified opinion. Reminds me of an old saying "if you can't dazzle em with brilliance, baffle em with B.S." my point is it is just an opinion and everybody has one. LOL and I can prove that.... Just look at our country today.... I do know that a given amount of weight in an unbalanced situation will increase as rpm increases. ( spin an unbalanced crank by hand, then spin that sucker at 5000 rpms and see what happens) so to say that balance based on math is the only way to go is like wearing blinders. There are way too many factors that we can't prove on paper but if over simplified they make sense. As far as over balance vs under balance goes, it makes sense to me that as rpm increases you would want to be on the minus side ( or have the crank balanced for a heavier rod piston combo) than the other way around. Your error won't get lighter as rpm goes up it will get heavier, more load on the pistons, more oil etc... I am not looking to start a forum pissing match like some want to do but there are some people out there that think they know it all and then there are those of us that don't know anything but just take in as much info as we can get, dig through it all, toss the obvious crap out and use the stuff that that has some logic to back it up not just numbers. Some of the best discoveries were by accident and Some people said that can't be, the numbers don't support it. we all know the earth is round not flat. The following is from Eaton Engine Balancing



If a carbon build-up on the piston top was anticipated over the long haul, then this could be also added to the oil value at this point. If you have a preference for a different oil value to be used on your rotating assembly upon getting it balanced, then talk this over with your shop and get their input on this. Most shops will be agreeable to sutle changes in the bobweight values if you have specific preferences.

There are a variety of other conditions which would require “overbalancing” as part of the balancing process. A change in rod lengths or crankshaft stroke can benefit from a given amount of overbalance depending upon the amount of change in rod/stroke ratio. The use of nitrous oxide, superchargers, or turbo chargers typically also requires a certain amount of overbalance. Using nitro methane in conjunction with a blower is likely the worse case scenario as cylinder pressures are extremely high under detonation which artificially increases the piston weight by a more than a normal amount. Any form of blown engine will benefit from a given amount of overbalance simply due to the weight of the piston averaging artificially heavier not only from the increase in cylinder pressure at ignition, but the increase in cylinder pressure taking place while the cylinder is also filling during the intake stroke. In this instance, the piston is averaging an overall heavier weight when running at speed. A normally aspirated engine has a given amount of pressure counterbalance in that the piston is subjected to negative pressure when the cylinder is filling but is under increased pressure during compression and ignition. If an aspirated engine is working with an extremely well designed induction system and is benefiting from a ramming effect to fill the cylinders at the upper rpm ranges, then overbalancing also helps here. And then there’s the rpm factor. Balancing is linear up to a point throughout the rpm range but depending upon the masses at work within your particular assembly, there is a point in which the crankshaft rpm starts to out run the dynamics of the existing state of balance. Overbalance allows these dynamics to stay in tune or “caught up” to the rpm’s of the crankshaft. There are proprietary formulas that calculate these amounts of overbalance for all the different variables and will vary somewhat from shop to shop. Again, talk with your balance shop regarding overbalancing and determine if this would be best applied to your application.