Balancing Singles?

**TygerTung** · 14th November 2006, 18:59

I have a bucket racer, engine is a bit of a bitsa, it's a XL125S bottom end with 6 speed gearbox, crankshaft is a GL145 lightweight item (same stroke as 125) polished conrod, and it has a shortened XL185 barralel and piston which makes it 150 cc, ported head and hot cam.

Anyway I put it together last week for the racing on the weekend, except it vibrates pretty out of control, It was almost impossible to ride on the practice 'cos you couldn't hold onto it properly thats how much it was vibrating, I had to put as much weight as I could at the ends of the bars.

It was rideable after that, but I can't leave it like this as it's gonna shake the thing to bits, probably crack a crankcase or somthing.

How should I go about balancing the crankshaft? (the obvious source of vibration).

Would it be alright to do it on knife edges? I was just gonna split the crank, take off the piston and conrod bearing etc and weigh that then get the same weight using stick on tire weights and put that on and balance it like that, or will I need to spin it up on a balancing machine? I just had another idea of just leaving everything assembled and just supporting the bearings and letting it hang down and then just balancing it like that, or would there be too much drag on the bearings? I could clean them out and just spray in a light oil such as CRC or WD40 etc.

Let me know your thoughts, ideas, and advice

**Motu** · 15th November 2006, 09:45

A single cylinder crank isn't balanced like that,it's done on a percentage of reciprocating mass....like 65%,or 80%.Your piston is most likely heavier than the original for the crank.I don't know how you would get it right - bolt on weights,blobs of weld,something like that.Check the weight of your piston with the GL145 piston,then you will have an idea of how much weight is involved.

**Expert** · 15th November 2006, 16:41

Balancing the crankshaft on a single cylinder is very difficult, the bob weight on the crankshaft is designed to counteract the weight of the piston and gudgeon pin etc at either end of its stroke, so it tries to balance out up and down vibration at 12 o'clock and 6 o'clock, but the problem is it then creates an out of balance force at 9 o'clock and 3 o'cklock because it has nothing to work against. That is why you have to compromise and take a percentage of the weight of the piston, gudgeon pin etc some where between around 55-85% according to where abouts in the rev range you want the least vibration. This ammount varies from bike to bike and is not something you can look up or calculate, more a seat of your pants thing, or experience. Just to really confuse the issue model aircraft engines running at 25,000 rpm use a balance factor of 55%, Harly Davidson use 52% on their engines, my old man used 85% on his old bsa and that was good. I just did 60% on a xt crank coz i binned the counter rotating balance shaft and that vibrated like a twat and i had to add more weight to calm it down.
Also four strokes have a cam running at half engine speed which is a lumpy peice of metal spinning around biffing all sorts of heavy metal things around and causing vibration, two strokes don't have this problem.
Any jig that allows the crank to spin freely is good, i made mine from rollerblade wheel bearings for the crank to sit on and that seems ok.

**pete376403** · 15th November 2006, 16:51

nothing silly like loose engine mounts, is it? My speedway bikes would shake like a dog shitting a chain if the bolts were even slightly less-than-tight. In one case I had to ream out the holes in the cases and engine plates to make them a snug fit for the next size up bolts.

**TygerTung** · 15th November 2006, 17:48

I was under the impression that you had to weigh the big end half of the conrod, add that to the weight of the piston and rings gudgeon circlips etc and take 60% of that weight then add that number to the weight of the big end of the crankshaft again?

Then if you're gonna dynamic balance it balance it to 6.3 mm/sec?

I probably have access to a balancing machine so I can give it a dynamic.

**Motu** · 15th November 2006, 19:19

So if you know all that and have access to that kind of gear - why did you ask the question in the first place?

**TygerTung** · 15th November 2006, 21:19

I only just found out before and I'm not sure.

**Brian d marge** · 16th November 2006, 01:06

Originally Posted by Expert

Balancing the crankshaft on a single cylinder is very difficult,....... i made mine from rollerblade wheel bearings for the crank to sit on and that seems ok.

I used the sme set up ..I used Dry unsealed bearing to give a very Very free rotating knife edge, then balanced it at 64 % ( Enfield 500 single ) ,, BUT the rod has an Geometrical centre and a Recipricatng center, and they are not one and the same thing ,,

you need to find that reciprocating centre ,cause the wieght above it is the wieght u use for balancing ...

best thing is to take it to some one who has experience and get it done right ..you may find you will need a heavier crank ....OR the They could move it up /down the rev range ... ( but before that ,,check the mounting points ,,of engine ..are the holes elongated ??? any mounting rubbers that are hard /worn ????)

IF You dont mind ripping the engine aprt and can google the info IS out there ... ONce you have experience , then futre experiments are easier on the pocket !!!

Stephen

**oyster** · 16th November 2006, 21:08

Yep, hi TT.
I had this same problem once. So figured out if the original setup ran smooth with the lighter piston, then all thats needed is a correction of exactly the same mass removed from the crank on the piston side.
So weigh the two piston assemblies (don't forget gudgeon pins)
Now steel weighs about 8000kg per cubic metre. So using this big number, brush up on you maths and calculate a sensible diameter and depth for a total of four holes to be drilled either side of the crank pin, both sides. So you might need 4 holes of diameter 6mm, by 6mm deep (for example)
If you're a lazy bum like I was, just take off the barrel, mask off the crank with duct tape very securely and drill straight down. This saves having to pull it all to bits. Use a magnet and careful cleaning to ensure no remaining swarf, then lift the tape. Voila, all done. For me, it worked perfectly.
Have fun
Oyster

**Shaun** · 16th November 2006, 21:16

Originally Posted by oyster

Yep, hi TT.
I had this same problem once. So figured out if the original setup ran smooth with the lighter piston, then all thats needed is a correction of exactly the same mass removed from the crank on the piston side.
So weigh the two piston assemblies (don't forget gudgeon pins)
Now steel weighs about 8000kg per cubic metre. So using this big number, brush up on you maths and calculate a sensible diameter and depth for a total of four holes to be drilled either side of the crank pin, both sides. So you might need 4 holes of diameter 6mm, by 6mm deep (for example)
If you're a lazy bum like I was, just take off the barrel, mask off the crank with duct tape very securely and drill straight down. This saves having to pull it all to bits. Use a magnet and careful cleaning to ensure no remaining swarf, then lift the tape. Voila, all done. For me, it worked perfectly.
Have fun
Oyster

Man you even admit in public to being so crude

**TygerTung** · 16th November 2006, 22:02

It's fully stripped now, I think I have the right idea, I might google it a bit to see what the story is with these reciprocating centres and what have you.

**Brian d marge** · 16th November 2006, 22:55

Reciprocating centre of mass ,,or what ever they call it , all it is is a point on the conrod that seems to stay in the same place ,,,,ie the small end goes up and down ,the big end goes round and round ,,, the point which is the most confused it the recip centre of mass ,,
Most likely will be different from the point if you tried to balance the rod on a knife edge ....

Stephen

But please dont try any thing like this on a multi ,,,, it WILL CONFUSE u

**TygerTung** · 17th November 2006, 08:50

Yeah I found out some great infomation on there so I worked out what do do and I'll be balancing soon!

**Brian d marge** · 17th November 2006, 13:15

The good thing about buckets and race bikes is that u can try these things out ,, and if it all goes pear shaped ,, who cares ,, I have have a cr250 for that .. right now I have picked up an HRC front rim 19 inch and will lace it up ... It should turn in quicker ,,,if it dont ,,, well Ive only lost a bit of my time....

Stephen