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High Performance Bicycle Engine Tips and Motorized Race Bicycles We all want our bicycle engines to push our bikes faster. Get exclusive engine modification tips and suggestions from us. Also check out some of our racing members builds

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  #1  
Old 07-05-2012, 05:38 PM
multipaul multipaul is offline
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Default Crankshaft balancing and truing (part...)

Balancing a single cylinder engine can be done at home. This is done in two-stroke engines statically. The crankshafts are short. We have only forces to balance and no moments. No expensive machinery is needed.

1. Everything is fine, as long as every moving part is turning around.
Such a system of rotating masses can be balanced easily and with100%. Here all masses are concentric. So no counterweight is needed.


2. Now the connecting rod appears. The bottom part of it with the crank pin and its bearing is 100% rotating. So it should be balanced 100%, like a concentric flywheel. The further we move away from the crank pin toward the piston the less the connecting rod turns and the more it oscillates.
Trick: the lower portion of the rod is added to the rotating part and is balanced 100%.


3. Now let's look at the piston, the rings, the piston pin and it's bearing. They go up and down 100% without any rotation. So does the upper end of the connecting rod. But the more it approaches to the crankshaft the more its parts make an elliptical way. Nevertheless the weight of the upper connecting rod is added to the reciprocating masses.
If we don't balance these masses the engine will vibrate up and down.
If we balance these masses with 100% everything is fine in TDC and BDC, but the engine will strongly vibrate at 90 degr and 270 degr. horizontally.

Against the reciprocating weights I added the green masses. The green arrows show in which direction the forces point.


That's why in praxis the engine manufacturer chose a compromise. Most single cylinder engines use a balance factor between 0% and 60% of the reciprocating masses. The masses to balance can be added opposite the crank pin or mass can be removed near by the crank pin. Often its a combination of both.
The rotating masses are always balanced 100%. That is why we don't mention them!

Big single 2strokes (for example the Ossas) with their standing cylinder had balance factors of 58% (57 to 60). These engines were used in motorcross, trail and also as very successfull grand prix racer in the 1970th.

Part 2 is about the math.
Part 3 is about the dynamic forces and internal speeds.
Part 4 is about how I trued my crankshaft. With system but without a lathe.

(Part 2 and 3 will appear perhaps tonight)

Multipaul
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  #2  
Old 07-05-2012, 08:16 PM
ivan H ivan H is offline
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Default Re: Crankshaft balancing and truing (part...)

Cool, looking 4ward 2 rest of
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  #3  
Old 07-05-2012, 10:35 PM
multipaul multipaul is offline
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Default Re: Crankshaft balancing and truing (part...)

Part 2a: The math of STATIC BALANCING and doing it in practice.

The math is different from dynamic balancing. But the result is the same.
So it often happens that the persons using one and the persons who uses the other method, don't understand each other.

We now want to calculate a replacement weight, the bob weight. This weight is acting on the crank pin, such as pistons, rings, etc. , but only with the intended percentage.
When we add this bob weight to the down hanging conrod and the crankshaft stops in each position, the crankshaft is balanced statically.

Preliminary: Determine the weight of the
Piston
Piston pin
2 piston rings
2 clips
Small end bearing
Reciprocating part of the connecting rod

The first 5 positions are easy to determine.
The 6th makes some difficulties.
Position the connecting rod in that way that it lies as level as possible. The upper conrod end will now be weighed. (The weight of the lower connecting rod does not interest at all.)



...done. When we add our six positions above we get 110 grams.

Now we think. We want to compensate the 6 weights together, let us say with 40% of their true mass. 110g x 0,4 = 44g for the bob weight?
BUT THE CONNECTIONG ROD is hanging there with 100% of its weight.
It should hang with 28g X 0,4 = 11,2g. But it doesn't. From the rod there are hanging 16,8 grams too much. Problem...16,8g too much. So the 44g bob weight is wrong.

Don't laugh at me. You know the solution: Reduce the first calculated bob weight by 16,8 grams. 44g - 16,8g = 27,2 grams is the correct bob weight in this example.

Formula:
Sum all 6 positions => result1;
result1 x percentage /100 => result2;
upper conrod part x percentage /100 => result3
upper conrod part - result3 => result4
result2 - result4 => bob weight

Now take some nuts, washers or whatever you have laying around and fasten it somehow at the conrod. Ribbons, wire, a bag - no matter. But its weight becomes part of the bob weight and must be considered.
It should only hang freely and must not disturb the rotation of the crankshaft.

The appropriate base for our work:
You can position the crankshaft with the main bearing journals on two flat and exactly horizontal surfaces.
The easier and probably exacter way is to use absolutely new not oiled or greased 6202 bearings and some brake pads. You will be astonished how difficult it is to prevent this unit from rolling off. Even if you thought the base was horizontal.

The HT engines with 3 part crankshafts have kidney-shaped notches on the inside. This is the balancing from the factory. And this is the area, where we should take away some more material or add some.

Here is the state of the flywheels from factory



Miniature cutters in action



Here, the state afterwords. 38g less. I can fill the holes again. Either to add some weight for tests or to keep the original crancase volume.



(There will be a part 2b and a youtube video and an excel sheet for you to download. And the parts 3 and 4)

Kind regards to Biknut. He discribed the procedure correctly in another thread.

Multipaul
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  #4  
Old 07-06-2012, 11:11 AM
Thud Thud is offline
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Default Re: Crankshaft balancing and truing (part...)

hehe, Looks familure
I run a 49cc engine & just machined the entire web out of the exterior crank parts.


I carefully measured the bob weights off the lathe, they snap into the crank pin:


& then checked for ballance on a set of level & parralell beams(smoothed angle iron on magnets shimed level/par. with paper)

yes all the screws & running parts were in place....
I used 55% or recipricating mass as a starting point.
motor is smooth @ 9500 rpms now.
hope you don't mind me adding this here....your doing a much better job than i would have...excellent documentaion! Thanx
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  #5  
Old 07-06-2012, 11:21 AM
multipaul multipaul is offline
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Default Re: Crankshaft balancing and truing (part...)

Part 2b: How to determine the actual balance factor

When you put the crankshaft onto the apparatus for balancing 3 situations are possible:

1. The conrod stops in the top position.
(This is unlikely to HT motors. In other two-strokes this is rule)



Now, find the bob weight by try and error until the crankshaft stops in each position. When this is done, put the complete bob weight on a balance.



Math:
Sum the 6 positions (piston + +...+ + upper conrod part) => result1
bob weight + upper conrod part => result2
result2 / result1 x 100 => actual balance factor [%]

2. The conrod stops in the lowest position.



Look at the picture below. You have to determine the down pulling force.
Crankshaft pin and main bearing journal must be exactly horizontally. The conrod itself should press vertically to a scale. You must align and underpin a lot until everything fits. Read the value from the scale.




Math: now line 2 is different
Sum the 6 positions (piston + +...+ + upper conrod part) => result1
upper conrod part - value from the scale => result2
result2 / result1 x 100 => actual balance factor [%]

If the result is negative that means, that the reciprocating masses aren't balanced at all. Even the rotating masses aren't balanced completely.

3. Theoretical possible: The crankshaft stops in each position. In this case use the formula from case 1.
Set bob weight = 0

Now to the primised Excel sheet. I made it in Google-Excel.
You should download it from there. I can only see the German version.
In the pull down menu of "Datei" (=file) there is the option "herunterladen als" (=download as?) and then click "Microsoft Excel 97-2003 (.xls)"

https://docs.google.com/spreadsheet/...CMU5EQkE#gid=4

In the last line of the Excel sheet there is a suggestion how much mass to add or to remove. Remember, the area where material is removed or added, has a larger radius than the crank pin. That is why the values seem to be rather low.

Multipaul
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  #6  
Old 07-06-2012, 03:42 PM
multipaul multipaul is offline
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Default Re: Crankshaft balancing and truing (part...)

Quote:
Originally Posted by Thud View Post
hehe, Looks familure...

I used 55% or recipricating mass as a starting point.
motor is smooth @ 9500 rpms now.
hope you don't mind me adding this here....
Hallo Thud

Never mind.
My text is for those people who have no lathe. And a little bit for those who are not sure about how to calculate that all. There are different opinions about it from time to time.

Now I want to talk about some inner forces and vibrations.
This may know a lot. Then it's just a reminder.

Multipaul
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  #7  
Old 07-06-2012, 05:27 PM
multipaul multipaul is offline
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Default Re: Crankshaft balancing and truing (part...)

One addition. Thud's bill is different. As a result of the horizontal conrod with the wire, the bob weight in that process must be heavier (+ conrod reciprocating part). The rest is the same. More important: the result of balancing is the same.

Multipaul
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  #8  
Old 07-07-2012, 09:49 PM
multipaul multipaul is offline
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Default Re: Crankshaft balancing and truing (part...)

Part 3 Dynamic forces and internal speeds

In the www I found an interesting link. Had I known sooner, I would have probably saved a lot of work. Here you see the forces that make the engine vibrate. At least the quality and direction. It is in English.

How to use it.
- The balance factor is choosable (0% to 100%)
- rod to stroke ratio
47/90= 1.9 for 66cc
and 2.1 for 48cc
- click "With secondaries" (yes, the length of our conrod isn't endless)
- choose "Toggle radar plot"
- choose "Against global angle"

Then you can play very fine with different values.
It is the program "Engine balance analysis". I downloaded it and my computer is still okay. Or you can only execute it, if you want to.

http://www.tonyfoale.com/

Days before I had started the same calculaten by hand.
The equation of motion of the crankshaft and the piston gives answers about the piston position, its speed and the acting forces depending on the crankshaft angle. Then I programmed these formula in Excel, every 10 degrees one value. Although now I have the same curves, but the real ways, real speeds and forces of our HT-engines that appear on my charts.

I realize that for many this is not interesting. It is rarely written about.
It might interest perhaps one or the other.

1. The way of the piston (HT 48cc and aproximated 66cc)
Can be interesting for people who want to rise the ports.
Can be helpful to make a port mapping.
At 90 and 270 degr crank angle the piston is not on its half way, but below. This means that the piston is faster from the center of the path over TDC than BDC. That is not important, but one can have heard it once.

Someone might read about an exhaust timing of 180 degrees. That doesn't mean, that the exhaust is open the half stroke. It is less.



2. Piston speed (HT 48cc and approximate 66cc)
The average piston speed is uncritical in our small engines. At 7000rpm, which is often called the red line, it is just 9 m/s. The diagram below shows the true speed.



3. Forces at the piston pin bearing
This is the first time, I watch the 48cc and 66cc separatly. The difference is too big. Notice that both charts show the forces with NOT.



4. Vibrating forces
That's what it's about a bit in this thread.
After the ignition, the gas expands in all directions and has therefore no direct influence on the vibrations of the engine.
I made all diagrams for 7000rpm. Any other speed is possible. The forces increase or decrease the square of the speed.





...
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  #9  
Old 07-07-2012, 09:51 PM
multipaul multipaul is offline
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Default Re: Crankshaft balancing and truing (part...)

...








Puh...Multipaul
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  #10  
Old 07-10-2012, 05:50 PM
multipaul multipaul is offline
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Default Re: Crankshaft balancing and truing (part...)

Part 4a: Aligning and truing the crankshaft.

The last part is the try of truing the crankshaft.
Up to today I hoped but I was not sure whether I would be sucessfull.This is the first time in my life that I had to true a crankshaft.

I have no lathe, and I know that there are thousands of people where it's the same.
When I opened my engine, I suspected immediately that the shaft would be quite crooked. It seemed like the flywheels were splayed apart a bit. This could be caused during assembly into the motor or by pressing the components together. Unlikely that this had happened already in the lathe. The hole for the crank pin could be drilled not perpendicular and so on.

First, I built a simple device. That is what I needed. The bottom is 19mm plywood.


Initial measurements were 0.4 mm eccentricity of the journal:


I wanted to work as systemically as possible.
So I identified three possible errors which are described in the following picture:


Fortunately, in my engine mistake #1 did not occur. The threaded holes were all in alignment.

Mistake # 2 was clearly present. I measured the inner distances between the disks. Then I marked the positions where to pull or to press. I've read about broken HT crank pins. So I warmed it up electrically to about 150 C (300F).
Upper part shows how i pulled, below you see how I pressed. There was only few force required.


With these measures, the error on the journal decreased to 0.2 mm. Trying to make both disks exactly parallel, the error enlarged again.
So now I had to fight error No. 3.
Some attempts with little heat were hardly sucessfull. The main bearing journals are not hardened. I was very careful with them and treated them with hardwood and a 500g hammer. Note that only the upper disc is clamped.
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