Balancing the 8/1 engine.
I feel like I am coming a bit late to the party. While researching this I came across a lot of posts, older posts so maybe I missed the big surge when these engines started arriving but I will post my experience anyway. I first started trying to learn about balancing. I read posts here, post on single cyl motorcycle engines, big block Chevy’s, racing lawnmowers with single cyl kohlers. I learned how you balance 50% of the reciprocating mass and 100% of the rotating mass. Any more than 50 will turn a hopper into a slider. But in real life some v twins and motorcycles are going up to 70 or 80 and really smoothing out. I learned that a single cyl engine will always be a bit of a thumper. Even fitted with balancing shafts. But I felt like something was missing, most of what I read was talking about counterweighted crank shafts. Weight right on the crank. Not big flywheels. I remember learning in school about how if you taped a penny to a 48” steam turbine blade the high rpm and the distance from center are like astronomical. The weight of a penny is over 2 tons of force on the turbine blades. Then I came across this equation in a model steam engine forum.
“I found a formula in an old public domain book, as follows for calculating the counterbalance weight(s) for steam engines, as follows:
Equation for minimizing engine vibration:
W1 = [K*(W2+W3)*r] / X
where:
W1 = weight of the counterweight (lbs.)
W2 = weight of the crank webs outside of the main shaft and crank pin (lbs.)
W3 = weight of reciprocating parts (piston, piston rod, crosshead, one half the weight of the connecting rod) (lbs.)
X = distance of center of mass of counterweight from center of the crankshaft (inches)
K = constant (use 0.67 for minimum vibration at right angles to the engine centerline, use 0.75 for minimum vibration at crank dead center)
r = distance from center of crankshaft to center of crank pin (inches)
Generally, vertical engines should tend toward using the 0.67 value for "k", and horizontal engines should tend towards using the 0.75 value for "k".
So I took some measurements on my des 8/1:
Aluminum Piston, rings, piston pin and cir clips= 4pounds 7.30z
Entire rod with bushing and bearing 8pounds 12.2 oz.
Bushing end 3pounds 20z
Bearing end 6pounds 3oz
Estimated weight of crankpin based on 2.5” diameter and 5.5” length 7.65 pounds
Bore 4.49” stroke 5.49”
Then remembering to divide the bore by 2 and converting all the oz. into decimals I crunched the numbers in the equation and came up with the following recommended counterweight on 24” flywheel.
W1= .67(7.65+8.837)2.745/12”
W1=2.455 pounds.
So feeling like I had a starting point I moved from what my coworkers call “theory and bullshit” and started on the flywheels. Using the forklift forks with a strip of ¼” key stock on the and an old 2” pin from a backhoe bucket I proceeded to balance as can be found here and on you tube. Not the most accurate rig but with a bucket and string and kitchen scale I came up with the following initial numbers
Front flywheel: total weight 137#, key on bottom when on rack but 1 degree to left of center, counterweight 2.35pounds
Back flywheel: total weight 136.5#, key on bottom when on rack 1 degree to right. Counterweight 2.4437 pounds.
It was neat to see those numbers match up so close to my calculated value. I assume that some engineer in India calculated out the counterweight and designed it into the casting or it was there by luck when they copied the original.
From there I started with the lighter back wheel and instead of drilling holes I used a grinder. Given the shitty look of the cutouts it was no hardship. I cleaned up the insides of triangular holes until it hung straight. Then I took a measurement with the bucket and found the new counter weight was 2.506#. I then proceeded to make the heavy front wheel match the back.
Granted I have spent many hours welding and fabricating but I found it surprisingly easy to remove a pound with a grinder once you start. As soon as you touch it and the paint and putty blows away the natural instinct to contour it, smooth it and grind away the casting sand takes over. It worked out real nice in lowering the weight of the heavier front wheel. I only removed material from the side I needed to get it to hang straight. This both lowered the overall weight of the wheel, corrected the tilt as well as giving the counterweight more of an effect on the wheel. Essentially making it heavier to match the rear wheel.