How to balance a crank
- Home / Crank Balancing for Single Cylinder 2 Stroke Engines
One way of inc reasing both the reliability and lifetime of your single cylinder 2 stroke engine is to properly balance your crankshaft. Balancing a crankshaft can yield higher RPM’s because it equalizes the forces involved during operation. An unbalanced crankshaft can lose something like 2000 rpms off the top. Generally speaking, multi-cylinder engines have multiple crankshafts and/or flywheels that allow them to balance themselves out from precise computer calculation (dynamic crank balancing). Since our motorized bicycle engines is only single cylinder we can balance the engine to within a certain RPM range. There are two difference kinds of balancing: static and or dynamic crank balancing. Here we will be focusing on static crank balancing because dynamic requires the use of complicated moving machinery and computers which most of us dont have access to. In our experiments, we have managed to get a 66/80cc motorized bicycle engine up to about 8800-9000 RPMs under load, and it balanced itself between 6800-8500. This does not mean that my engine will not vibrate at all; it simply means that at around 6800-8500 rpm’s the engines internal crankshaft forces will balance themselves out and there will be little to no vibrations within that range. Now that’s impressive seeing that with a rear sprocket of 41t
gives about 37-40 mph without the vibrations to tear itself apart!
Here we will be showing you how to statically balance a crankshaft on a single cylinder engine. Statically balancing your crankshaft involves removing or adding material to the flywheel of the crankshaft while it is stationary. This will be particularly useful if you know your kind of riding style (eg. long distance rider vs city rider vs racing). Typically, a stock 66cc comes balanced within 3500-5000 rpms and any much after that it turns into a cheap massage chair like most of you know. It’s those internal vibrations that cause the engine to tear itself apart and bearings to fail prematurely. If you are one to ride your motorized bicycle for longer distances at higher cruising speeds outside its balanced RPM ranges then you might want to consider crank balancing to ensure the maximum life of your engine.
Balancing a crankshaft assembly for any reciprocating piston engine presents a variety of challenges and compromises. Unfortunately, there are virtually no designs that completely cancel all the competing primary and secondary forces that produce shaft vibration. What this means is that in reference to our small 2 stroke engines we can only balance the crankshaft to within a certain RPM range.
Understand that a single cylinder engine produces vibration primarily from two sources:Source: juicemotoparts.com