Building a new motor as a few of you know and am wondering if I need to balance my crank. On a V8 it is a no brainer for me and if I was using a new crank or changed the weight of the rotating assembly I would. I ask because I see a lot of people add billet rods and change pistons with out re balancing there cranks but these are on motors that have been running and not built from scratch. I have a new cone crank with a billet rod and predator dished piston going in a bored clone block. If I was putting stock parts in I would probably not worry about it but I have changed the weight of things and am thinking the crank should be balanced now. If I do get the crank balanced should it be done with the flywheel attached?
https://etd.lib.msu.edu/islandora/o..._reciprocating_engine_with_optical_access.pdf
This is a good paper very technical.
But phil1958 is correct the point of un balance is the counterweights 90° to the cylinder bore.
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This is a good paper very technical.
But phil1958 is correct the point of un balance is the counterweights 90° to the cylinder bore.
Sent from my iPhone using Tapatalk
I have been involved with trying to reduce vibration on british vertical twins years ago which are just like a big single. You cannot balance them but you can change the balance factor to change the planes of vibration. It is a complicated subject but basically you take 100% of the big end weight of the rod and then take the weight of the small end of the rod plus the piston, rings, clips and wristpin. and multiply that by the balance factor. what will give you the weight to place on the crankpin and it should be equidistant from the center of the crankpin not hanging off one side. A 50 percent balance factor basically spreads the vibration evenly in the horizontal and vertical planes where the vibration is the strongest. You will read a lot about balancing for a certain RPM but when you understand that the out of balance forces increase with the square of rpm. So the faster you turn the engine the stronger the forces. There are two types of out of balance on our little engines reciprocating, and rotational. (also a small amount of rocking couple. The rotational and rocking couple can be balanced out just fine reciprocating cannot. This is where the balance factor comes in. changing the balance factors higher or lower than 50 percent increases vibrations in one plane and reduces it in the other. British bikes use factors between 60 and 80 percent usually opting for more vibration in the horizontal plane. The reason people thing you can balance for a particular rpm has to do with resonant frequency. When our motor is mounted in a frame the whole thing will have a resonant frequency that when vibrated at that frequency can multiply the feeling of vibration by a large factor which is perceived as a bad vibration but by changing the balance factor and moving the vector of the major vibration force may change or eliminate reaching the resonant frequency of the vehicle. When you bring your predator to a shop and say balance it to reduce vibration at x rpm they can't really do that not knowing what the resonant frequency of you project is. Manufacturers of motorcycles have the facilities to test that and balance around that. That is why when Norton went from the roadholder frame which mounted the motor rigidly had a balance factor from 70 to 84 percent to the commando which had a rubber bushing to allow the motor to vibrate freely they went to a 52% balance factor.
So what I found out during all my british (BSA) flattrack days was to minimize vibration by selecting the lightest reciprocating parts I could find, which basically was the lightest piston and wristpin.
So when guys change the rod and piston it may change the way it vibrates if the parts are heavier or lighter but I doubt that you would notice much unless it was a LOT heavier.
So what I found out during all my british (BSA) flattrack days was to minimize vibration by selecting the lightest reciprocating parts I could find, which basically was the lightest piston and wristpin.
So when guys change the rod and piston it may change the way it vibrates if the parts are heavier or lighter but I doubt that you would notice much unless it was a LOT heavier.
Nice Ole4. Very easy to understand your explanation. And aren't flywheels balanced at least statically for some of the machines? I'm pretty sure I see holes in them.
I had a sail boat with a Yanmar two cylinder diesel. Gosh. Those motor mounts were $100 each, replaced every few years, and worth every penny.
I had a sail boat with a Yanmar two cylinder diesel. Gosh. Those motor mounts were $100 each, replaced every few years, and worth every penny.
Nice... Thanks for the response guys, good information ole4. Makes sense that even if it was balanced end to end that because the counter balance is spinning it would only be in balance for short periods at the bottom of the rotation. I was a little worried that I should be spending money to balance a small motor that is getting expensive as it is. trinik7592 recommended the flat top predator piston a few days ago over the dished and said it was lighter. I thought o-well I will just balance the motor anyways and will not notice the performance of a slightly lighter piston and got the dished. Looks like it effects more than the compression, good to know for next time.
Just as an example of balance factor, for the engine to be in balance at the bottom and top of the stroke it would need a 100% balance factor. That is all the reciprocating and rotating weight of rod piston pins rings and clips was equal to the counterweights. Then at 90 degrees before and after TDC you would be really out of balance and the motor would shake in the horizontal plane as the counterweights would be too heavy. A 50% balance factor gives you equal out of balance in the top and bottom tdc as well as 90 degrees before and after. it averages the out of balance forces all around and the peaks are at their lowest. It is a lot more involved but basically the factory balances them to some factor so the motor does not shake excessively and if we put a rod or piston or both we may change the balance factor but it wont be by much. If you get a stroker crank, and maybe a steel rod then you would probably need to rebalance if it vibrated too much. Usually bad vibration is because of bad engine mounting, loose mounting, no head steady if one is needed or a cracked frame. I have never had any issues with a out of balance clutch/TC but that could also cause issues.
Ideally building a 208cc you would run a wiseco piston , short 490 wrist pin and a 3.595 rod with a lightened polished crank .. as well as a light valve train with almost perfect geometry .. as ole has stated you will never get a perfectly balanced rotating assembly but by reducing weight you can make it run with virtually no vibration @ any given rpm
I would think an after market flywheel would tend to be better balanced than a mass produced stock one. I think on a small single the effect is so small you would not notice? Now add some of these balance beads to the oil and it should be smooth as glass! (j/k) https://www.youtube.com/watch?v=eq263AYgyYg
i went through the math of an ancient description of balance in a single cylinder reciprocating
engine. there is one train of thought where if you make the rotating mass exceeding large
in compared to reciprocating mass balance factor become less of a contributing term. this goes
against all common sense engineering but it does indicate that a heavier flywheel (Rotating Mass)
will lessen any orthogonal variations in balance factor.
there is a practical and fundamental limitation on reciprocating mass, but less of a practical
limitation on rotating mass as flywheels can easily vary in mass density and diameter.
now, there are also performance issues to deal with. Everyone that has built an HS-50
knows that when you let the throttle off that massive iron flywheel wants to keeps on spinning
for quite some time. so if you want to stop fast the brake system absorbs all the kinetic
energy of the flywheel.
the people who want the engine to respond rapidly to changes in the throttle will need
a lighter flywheel and will have to put up with more vibration.
if you want to cruise the neighborhood at a fairly constant speed that titanic flywheel in
the HS-50 will suit you just fine.
check out low speed single cylinder diesel lister engines used for irrigation in god forsaken
third world banana republics, they run at 600 rpm and have huge flywheels. huge rotating
masses and seemingly second order reciprocating masses.
please correct me if i am off base.
engine. there is one train of thought where if you make the rotating mass exceeding large
in compared to reciprocating mass balance factor become less of a contributing term. this goes
against all common sense engineering but it does indicate that a heavier flywheel (Rotating Mass)
will lessen any orthogonal variations in balance factor.
there is a practical and fundamental limitation on reciprocating mass, but less of a practical
limitation on rotating mass as flywheels can easily vary in mass density and diameter.
now, there are also performance issues to deal with. Everyone that has built an HS-50
knows that when you let the throttle off that massive iron flywheel wants to keeps on spinning
for quite some time. so if you want to stop fast the brake system absorbs all the kinetic
energy of the flywheel.
the people who want the engine to respond rapidly to changes in the throttle will need
a lighter flywheel and will have to put up with more vibration.
if you want to cruise the neighborhood at a fairly constant speed that titanic flywheel in
the HS-50 will suit you just fine.
check out low speed single cylinder diesel lister engines used for irrigation in god forsaken
third world banana republics, they run at 600 rpm and have huge flywheels. huge rotating
masses and seemingly second order reciprocating masses.
please correct me if i am off base.