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         Crank Balancing for Single Cylinder 2 Stroke Engines

    One way of increasing 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.

 


                                                                                                                              The Concept

      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:

                  1. Rotating imbalance

                  2. Vibration induced by the reversion of the reciprocating mass at TDC and BDC


                                       

 

                                                                Preliminaries

READ through Some science of balance by Tony Foale Designs to get some idea of what balancing is and how it works. 

DOWNLOAD this spreadsheet Crank Balancing Calculator. It could be useful later on for some of the calculations. 


Access to the Crankshaft

Step 1- Refer to Changing Bearings to get access the crankshaft

Step 2- Remove the piston and keep all the parts in a safe place.

Step 3- Removing the crankshaft from the bearings could be a little tricky if it is seated in there tightly. You can use a little heat and a rubber mallet to remove it from the case halfs. What you have to do here is fully remove the crankshaft (with connect rod) from the engine motor. See picture below.You can choose to leave the main bearings on if you like as this can help with the balancing process.


                                                                                         


                                              Instructions

 

Step 1- Get a precision electronic digital scale. The more accurate the better your balancing will be.  Usually the lower the maximum weight a scale can measure is the more precise it is.

Step 2- You will now have to weigh the top end of your engine. This includes the piston & rings, wrist pin, wrist pin bearing, e clips, and small end of the crankshaft. Weighing the small end of crankshaft could be a little tricky so placing the crankshaft on an elevated platform and having the top end rest on the scale is probably your best bet. See picture below. To save you some time, generally the small end of the crankshaft is around 31g +/- 5g. 

 (adapted from motorbicycling.com)


Step 3- Calculate the sum of weight of the top end and multiply this number by the balancing factor. (eg 140g x 59%). Choosing a balancing factor can be a little tricky to decide because it is based on the RPM range in which you plan to run the engine in. According to Tony Foale, “In practice, balance factors are usually between 50% and 85%.” I personally have tried a balancing factor of 58-59% with engine RPMs equalizing between 6800-8200 RPMS. You will have to play with balancing factors to find out what’s best for you 60% is a good place to start.


Step 4- Measure the weight of the bottom end of the crank shaft and bottom end wrist pin bearing. To do this you’ll have to break open the halves of the crankshaft to have access to the lower wrist pin bearing and bigger end of the crankshaft. If you do not have the tools to this this, the value is usually around 47.5g +- 1 gram.


Step 5- Calculate the sum of step 3 and step 4. This is your “bob weight” or the weight that you will attach to your crankshaft to judge whether you need to add or subtract weight from your crankshaft.


Step 6- Carefully measure out your “bob weight” using nuts/bolts/weights and attach it to the crankshaft pin. You can use a threaded rod and bolt the weights to the side. See example picture below

                                                                                  

 

Step 7- Mount the crankshaft on its shaft on a parallel surface while holding the crank arm parallel to the surface you can affix the upper crank arm to a peice of string.  Affix the bob weight to the crank pin and examine the position of the crank pin relative to the crank. Ultimately you want to be able to freely rotate the crank on its axis without the crankpin (with affixed weights) falling to the bottom. You have 2 options: Removing material from the top of the crank or Adding material to the bottom of the crank. In the picture below this crank was balanced and I choose to remove material from the upper part of the crank flywheels. See Picture below


 

 

Each method has its benefits and drawbacks but in the end your engine will still be balanced at a certain RPM range. At this point it’s a matter of preference and what tools you have access to. Also if your crankcase has a cover you should remove it before proceeding to these methods.

                                                              

Method 1- (Increase overall crankshaft weight)- Involves drilling holes in the crankshaft and filling the voids with a heavier material like tungsten or lead. This method is good if you want to maintain or increase the weight of your crankshaft. This could beneficial for those of you who are planning on running your engine constantly for longer distances. The extra weight will contribute to higher rotational inertia allowing for stronger pulls offs and initial acceleration. You will need to drill holes and machine weights into the bottom of each half of the crankshaft flywheel and precisely pressed in.Refer to picture below to know where to remove and add weight.

Method 2(Reduce overall crankshaft weight)- Involves removing material from the top of the crankshaft and constantly testing the crank with the bob weight attached. This method is recommended for those of you who constantly stop and run the engine at a multitude of RPMS (eg city riding). With a lighter crank there will be less inertia, which results in faster RPM changes, but also means that the engine will be doing slight more work to keep the engine at a constant speed. FYI, great for racing engines and motorized bicycles who frequently stop and go.

The process will be ALOT easier if you have a crankshaft with covers on it. Remove the 3x m6 bolts holding the crank covers to the flywheel then you can just mill or grind away the inside cavities. See picture below.

Another way of doing this if you do not have a crank with covers you will have to remove material from the top using an angle grinder and remove material symmetrically or drill holes in this area and test for balancing. If you have access to a mill even better! The more precise and symetrical you can remove material the better you balance will be.



Step 8- Since you are removing material from the crankshaft flywheel you will need to replace this lost material or else crankcase pressure will decrease. You can fill the space that you grinded/drilled away with oil resistance silicon sealant (AKA RTV sealant). If the sealant doesnt have a cavity to be placed in the centrifugal force from the crank will remove it. In that case, you will have to stuff the crankcase with something like JB weld to compensate for the material loss from grinding the crankshaft. 

 

Step 9- Reassemble the engine. Finished!

 

Resources

http://www.enginebuildermag.com/Article/1097/crankshaftsstock_and_performance.aspx

http://motorbicycling.com/showthread.php?t=40568&highlight=balance%2Cbalancing%2Ccrank&page=2