Balanced Engine vs Balanced Crank

[Jaybr]

QUOTE

Originally posted by Swamp'dale
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The Crank must be balanced and I might add "dynamically balanced" with considerations in multiple planes.

This is exactly one of the things I was asking about. You say it must be done, but who is doing it?

QUOTE

If Cannondale added additional mass to a certain era of stock cranks, then they must have been made aware of a "crank balance" issue.

Nope, the new crank weighed less than the old crank and the other components (counter balance, piston, etc) where never adjusted. Thier solution was to simply add weight to the crank and they did it in a single spot.

QUOTE

The new aftermarket rods and pistons have been desiged with specific masses in mind to aide in minimizing the vibrations induced by the entire rotational and reciprocating assembly (crank, rod, pistion, rist pin, rings, rist pin bearing, rist pin clips, etc.)Starting off with an unbalanced crank only further compounds inherent engine vibrations.

I know the weights are being matched, but are we starting off with an unbalanced crank? and is anyone addressing that problem?

[Swampdale]

Nope, the new crank weighed less than the old crank and the other components (counter balance, piston, etc) where never adjusted. Thier solution was to simply add weight to the crank and they did it in a single spot.

Not sure what your getting at. What's wrong with adding weight to the crank? It may have been needed..........




I know the weights are being matched, but are we starting off with an unbalanced crank? and is anyone addressing that problem? [/B][/QUOTE]

I'm not sure what's actually being done. I only have an opinion of what should be done. If the crank geometry and mass was repoduced consistenlty, than it is conceivable to only "balance" the prototype or spot check production lots. I would very dissappointed if this were the case at Cannondale, and pissed if this were currently the case with certain engine builders. Each crank should be checked and worked (balanced) on these machines. Automotive cranks are a different story. They come with harmonic balancers and dampers to automatically balance the system. They might get away with mass production without actually balancing the crank and engine.

All in all, if the engine is reliable, powerfull and does'nt cause my kidneys to bleed while sitting on it at idle, I'm happy, but you're right, what is "actually" being done. Maybe Big Winky can explain....

[Jaybr]

QUOTE

Originally posted by Swamp'dale
Nope, the new crank weighed less than the old crank and the other components (counter balance, piston, etc) where never adjusted.  Thier solution was to simply add weight to the crank and they did it in a single spot.  

Not sure what your getting at. What's wrong with adding weight to the crank? It may have been needed..........

All I was getting at is that the weight was added to the crank to make the new crank weight the same as the old, thus balancing the overall system. The crank itself was not balanced after the weight was added.

I believe the term used when I was told all this was "Static Balance"

And yes, one could make the assumption that the crank came off the assembly line balanced and then add weight elsewhere to get the whole system to balance out. Exactly what I did when I replaced my 03 crank with an 02.

[Swampdale]

I have two comments:

1. I don't think statically balancing the crank would be satisfactory. Although some may prove otherwised based on it's simplicity. Measuring the geometry against a known "balanced" crank would not be accurate enough to reproduce a balanced second. We're talking fractions of an ounce. It's best to spin it and see what you have.

2. Ultimately a balanced or optimized engine is achieved by trial an error. There are formulas for developing a starting point on paper initially, but you don't know what you have until you start her up. If the consistency of the components are controlled to continually re-produce them exactly, maybe nothing is checked???Dunno what Cannondale did. Sounds like they had some serious widespread QC issues though, probably a result of sourcing components from multiple vendors, maybe simultaneously with out qualifying them.

The procedure is simple though. Dynamically balance the crank, then weigh and adjust the mass of the reciprocating components to match what you know works based on actual trials.

[Swampdale]

Jay,
I've got several books on this stuff from a previous career. I can discuss with you on the phone if you like. I doubt you'll get some of these engine builders to disclose what they actually do. They would be giving up trade secrets so to speak. Maybe I'm wrong. Call Jim, he may tell you. I think my engine vibrates less. It's fast and loud. Only time will tell though if it holds up. If it make it through round 14, I'll be happy. Putting the engine back in tomorrow night and can't wait to ride it all weekend.

[Jaybr]

I'll give you a call sometime Swamp

I'm not looking for anyones trade secrets here, I believe wether or not a shop balances the engine and crank, and what method they use are simple enough questions to answer without revealing any trade secrets. Dustin told us what they do and the results they've seen without revealing any secrets :clap:

I have talked to Wnky on the phone numerous times, and I'm waiting for him to respond here.


The dynamic vs static balance sort of reminds me of the old days when they used a bubble to balance tires vs today when everyone spin balances. How many of us would be satisfied with the bubble technique when purchasing a new set of tires?

[Big Winky]

the crank can not be dynamicly balanced perfectly due to the throws being 2 diffferent weights (sizes), I have no idea why this was done. the cranks can be staticly balanced for a certain rpm range, which is what we did (11,000 rpm), which reduces the vibration at racing rpms (although I have noticed they seem to be smoother at lower rpms also). But as I have told many people which have contacted me.... the crank has more important issues than balance to deal with, which is why some engines never have a crank problem. Crap inside your crank will do more damage quicker than any other problem, which is why we disassemble every crank that crosses our doorstep and clean it inside and out.

[cannondale27]

I get you now.Here is my take.Our cranks may not be truly dynamicly balanced.On a tire you assume it is riding on the center of the tread so if there were side to side movement do to all the weight being put on one side(static balance) you would feel it in steering.
In order to dynamicly balance ours the flywheel and timing gear would have to be on it.I suspect that flywheels were weighed and found to be the same in all.So I would bet that if you were to install the flywheel and timing gear and place the assembly on a knife edge right in the center of the rod it would balance or come close.
In reality both the tension of the timing chain and the magnetic pull of the stator flywheel will have a effect on this.We did have a fully floating crank with the stock bearings and I am sure that the crank wanted to move side to side but even with a plastic cage the bearing held(never saw cracked cages).Now we have Ball bearings which have some side load capability along with better RPM rating.
The crank is also trued before it is balanced.Measurements are taken on both webs while turning the crank on both the front and sides.That is the really hard part to get right.
Think of the forces up and down that is what is really the most important force to control and I am sure that it is being controlled by matching the weights.

[cannondale27]

Geez first Pdavis beat me to a post and now Winky!oops.

[Big Winky]

lol @ steve, Ya snooze, ya lose.