Balancing a crank

Discussion in 'Performance Mods' started by CedarHillbiker, Aug 19, 2015.

  1. CedarHillbiker

    CedarHillbiker New Member

    I don't mind paying someone to balance a crank. Any idea who could do it in the Dallas/Ft Worth area? Also, I hear that there are one piece cranks available that are already balanced. Is that a fact?

  2. HeadSmess

    HeadSmess Well-Known Member

    save your money. "balancing" is a misnomer.

    a single cylinder engine cannot be balanced.

    the subject has been raised many times, and the point still remains... a single cylinder engine cannot be balanced.

    the "best" you can do is minimize vibration at one certain rpm.

    basically, the issues are these...

    the piston travels at different speeds during the top half of a stroke to the bottom half of the stroke, due to the angle of the conrod (basic geometry) so it would need a similar weight moving in the exact opposite direction with the same velocity. draw it out and it will make sense. ie, 90` before and after TDC is NOT halfway up the cylinder.

    a rotating weight on the shaft itself travels in a circular motion. not only does it try to go up and down, in opposition to the piston, as planned... it also goes side to side... from the 1 oclock to 7 oclock.... from 10 to 4 oclock, etc, ad nauseum....get it? it rotates.

    and that rotating weight follows a sinusoidal path...whereas, as mentioned, the piston does NOT... so even in the best scenario, the crank weight only partially cancels out the piston weight. AT ONE SPEED ONLY.

    mcculloch made a twin cylinder saw for that exact reason. except one cylinder is just a "dummy", simply a guided weight to oppose the piston itself... ran so fast it needed a governor(find ANY other chainsaw with a governor and ill buy you a beer-except maybe the dolmar rotary) and a redux gearbox....

    most cars planes etc are fours as they are the best to balance in all areas... theres other factors, like "rocking" from end to end, that happens in 2 and 3 cylinders, etc etc etc...

    even radials, that look perfectly balanced, are not, as one conrod is a master, following the crankpin, while the rest are slaved off that rod...all pivoting at slightly different angles to each other... therefore each piston travels in its own distinctive motion... they do NOT try and run, say, seven conrods on one crankpin. each would need its own bearing and thats much harder than using a master and slaves.

    the only truly balanced engines are those cam-drive or swashplate/wobble drive types.

    here endeth my rant :)

    no harm in fiddling. the aim is to make EVERYTHING lighter... whilst retaining enough "flywheel" for torque and low speed running... these engines have nothing much but the crankweights as a flywheel.
  3. jaguar

    jaguar Well-Known Member