Homemade crank balancing

[robin]

Egor --your dax engine is it a 2 cycle? i bought one from dax and it has a two piece crank which is out of the engine i got pullers and broke it apart (new engine)--should i drill 2 holes in it of 3/8 inches --doesnt seem to be any small holes the screws are very hard to open so am wondering if i can drill it without a dismantel ? thank you so much

 

[Egor]

Robin - If you are able to get the weights off you will notice that there are hollow spots near the crank pin. That is the reason that it is made that way, it acts like a stuffed crank to have the holes internal.
If you use the formula that has been postulated you may find that the lightening that has been done is not correct. On one of mine the wheels are out of round and so spoils the intended affect. I would hope that all are not so bad. I have one Dax engine that is very smooth, it is the best engine I have. I think if I were you I would run the engine and see how it does. Also you could check how true it is with a Dial Indicator, if out have it turned true. Also check all the bearings as you put the engine back together, that will justify tearing it down. I also run the engine in for a few miles and pull the barrel and check the piston for high spots, with a fine file I remove them with a few strokes. Keep us posted to what you find. Have fun, Dave

PS : If you look closely at the pic you can see how poor the casting was and the hollow spots that are not supposed to be there, all this unfortunately adds to the vibration.

Egor --your dax engine is it a 2 cycle? i bought one from dax and it has a two piece crank which is out of the engine i got pullers and broke it apart (new engine)--should i drill 2 holes in it of 3/8 inches --doesnt seem to be any small holes the screws are very hard to open so am wondering if i can drill it without a dismantel ? thank you so much

 

[robin]

Thanx Egor wish you were here!! my friend i showed him to drill 2 holes in the crank--because i needed a drill press --i could have killed him --it came back with 6 holes!--i give up--Dax said reassemble it --it will probably run--couldnt get rings closed so my other friend broke both rings--later we saw the small positioning pins!! iam going to bed!! will order new rings in a.m.

 

[robin]

The main positive thing it is fun learning and these engines are simple enough

 

[Egor]

Robin Sorry to hear that you are having a bit of a go there. I agree with Dax, and you never know it just might be in balance. If not you will have to keep the RPM's down a little. Yea the pins are to keep the ring ends out of the ports. You will get it just keep trying. Have fun, Dave

 

[emdude]

Hello:

I found your post from some time ago about the method you used for balancing your crankshaft very interesting. I’d like to possibly try this myself but have some questions as to your method and procedure:

Based on this:

>Piston+pin+rings+clips = 70 gr

That’s easily measurable of course.

>Light rod end = 35 gr

I’m beginning the think you measured this with the con rod attached to the crankshaft? See below

>Total reciprocating weight = 105 gr

Makes sense.

I gather that you did NOT remove the con rod at all. I’d like to get away this as well if I could.
Now, I’ve never balanced a crankshaft like this myself so I can’t speak from any experience. But as I understand it the “normal” method of crankshaft balancing takes into account not only the reciprocating masses but also takes into account the rotating mass of the big end of the con rod. This requires removing said to measure its weight at the big end.

From what I was able to understand you accounted only for the reciprocating masses in the calculation. Is this correct or did I miss something?

>I fixed a 55 gr weight to the rod journal

Is that the bolt like thing I see in the picture or was that some part of the crank?

And checked the crank balance using two metal rods (horizontal bubble level checked) and suspending the light rod end with a string

I don’t understand the reason for this but it looks interesting, could you expand on this? Maybe you have an easier than normal method for crank balancing and if so I’d like to know about it.

>Too heavy
>I started drilling the crank added side pieces, and found it had holes in it
>Was not enough
>I had to drill 1/4 holes through the center crank plates

Are those holes visible in the photo?

> And balance was perfect.

In other words, with the con rod suspended horizontally with the string and the crankshaft on the metal tubes the crankshaft will remain in any position it is placed in?

Am I correct in assuming that you balanced ONLY for the reciprocating masses as indicated earlier? And if so was this enough to get rid of the vibrations you were encountering. That could be easy!!!

Is it maybe that by using your method of attaching the calculated weight to the con rod journal and leaving the con rod in place and then balancing as indicated in the photo with the con rod suspended from the the string automatically accounts for the rotating mass? Wow that could be cool and easy. Do please expand on your method. I’m here to learn.

regards

emdude

 

[DIYMark]

2 crankshafts later and finally I have been successful in my efforts. I drilled two holes 3/8" in diameter through the existing holes already in the side plates. I only feel vibration at severely high rpms which I achieve from my boost ports so if anyone is attempting this it will be balanced for the rpms normally used in these engines (non boost ported) since you will not achieve the same insane rpms as i do... Good Luck! -Justin

Would these 2 3/8" holes still balance the crankshaft that has the 2 bolt on "balance" rings? Or would I need to re calculate the balance?

 

[DIYMark]

Ha Success!

After giving this some thought you can infact work out all the balance masses etc according to the correct method (( reciprocating mass + small end mass * % balance [ low 50's for low RPM, high 50's for high RPM]) + big end mass.

However, I worked out how to do it without needing to split the crank!!!

This is based of the info in the first post - little end mass is 35 grams.

To save everyone all the maths (might post it up later - much easier if I had a scanner) Ill just tell you how to do it. Now my method assumes that the con rod on all the engines is the same (which it is).

Okay. To balance the engine, dismantle it and remove the crankshaft. Set it up on two metal rods which are level and parallel (like a "railway" for the crankshaft to freely move on). Now you can do this 2 ways.

Either let the conrod hang freely and attach a 25 gram mass to the centre of the crank pin (or on the end on the con rod) and then drill the counter weights till they balance OR do what valerdejose did and hold the conrod horizontally and attach a 60 gram (he used 55 grams, not quite right thats why he had vibrations in the pedals - but anyway its was still a huge improvement over the stock balance). Once like this you drill until its balanced (no matter angle you turn the crankshaft, its stays put and wont roll to a more favorable side).

This is why I prefer my method (use the conrod as a mass and only add 25 grams) because when it comes to test for balance you can freely turn it with out worrying how level the conrod is like valerdejose did because he used the big end mass as a part of the balance mass.

So, anyone with their motor stripped down, or who wants it to be smoother - give this ago if you want to have your motor balanced.

PS - I you have the tools, true up the crank in the lathe and finally take a light cut over the counter weights (no doubt they are out of round). Careful when assembling, you may (highly likely) tweak the crank when assembling the 2 case half's so you would then have to re true the crank again ("slight" tap with a hammer over the big end/crank pin part of the counter weights) as the crank would probably get squashed in when you torque up the case bolts. Also, for gods sake don't beat the crankshaft and bearings into the case, support the case with a pipe around the back of the case where the bearing is and use the crankshafts momentum to drive home the bearing (use a wooden temporary spacer to in between the crank halves) to minimize distortion. I probally sound excessive, but a 3k RPM gain over stock is the best upgrade you can do - its like putting gears on your bike! At 10k RPM your stock geared HT will be screaming at 70 Km/h (43 MPH) and more if you port/polish, new carb, expansion chamber, etc.

If your smart about it, modifiy the engine so that it breathes easier AND does not lose torque (hard but possible). That way, you can go fast AND still climb hills. Using a 36T sprocket is a pathetic cover up for lack of revs (Tell me, what engine under 100cc's cant rev over 7K?...besides a HT)

 

[ken murphy]

good Job I'm right behind you on that balancing the diagram will be cool--------Did you go to the spooky tooth death race?were planing a rally need sugestions for the ride?
Let me know please I'm also In Phx rightin the center 16 th & Ind.

 

[impression]

Ha Success!

After giving this some thought you can infact work out all the balance masses etc according to the correct method (( reciprocating mass + small end mass * % balance [ low 50's for low RPM, high 50's for high RPM]) + big end mass.

However, I worked out how to do it without needing to split the crank!!!

This is based of the info in the first post - little end mass is 35 grams.

To save everyone all the maths (might post it up later - much easier if I had a scanner) Ill just tell you how to do it. Now my method assumes that the con rod on all the engines is the same (which it is).

Okay. To balance the engine, dismantle it and remove the crankshaft. Set it up on two metal rods which are level and parallel (like a "railway" for the crankshaft to freely move on). Now you can do this 2 ways.

Either let the conrod hang freely and attach a 25 gram mass to the centre of the crank pin (or on the end on the con rod) and then drill the counter weights till they balance OR do what valerdejose did and hold the conrod horizontally and attach a 60 gram (he used 55 grams, not quite right thats why he had vibrations in the pedals - but anyway its was still a huge improvement over the stock balance). Once like this you drill until its balanced (no matter angle you turn the crankshaft, its stays put and wont roll to a more favorable side).

This is why I prefer my method (use the conrod as a mass and only add 25 grams) because when it comes to test for balance you can freely turn it with out worrying how level the conrod is like valerdejose did because he used the big end mass as a part of the balance mass.

So, anyone with their motor stripped down, or who wants it to be smoother - give this ago if you want to have your motor balanced.

PS - I you have the tools, true up the crank in the lathe and finally take a light cut over the counter weights (no doubt they are out of round). Careful when assembling, you may (highly likely) tweak the crank when assembling the 2 case half's so you would then have to re true the crank again ("slight" tap with a hammer over the big end/crank pin part of the counter weights) as the crank would probably get squashed in when you torque up the case bolts. Also, for gods sake don't beat the crankshaft and bearings into the case, support the case with a pipe around the back of the case where the bearing is and use the crankshafts momentum to drive home the bearing (use a wooden temporary spacer to in between the crank halves) to minimize distortion. I probally sound excessive, but a 3k RPM gain over stock is the best upgrade you can do - its like putting gears on your bike! At 10k RPM your stock geared HT will be screaming at 70 Km/h (43 MPH) and more if you port/polish, new carb, expansion chamber, etc.

If your smart about it, modifiy the engine so that it breathes easier AND does not lose torque (hard but possible). That way, you can go fast AND still climb hills. Using a 36T sprocket is a pathetic cover up for lack of revs (Tell me, what engine under 100cc's cant rev over 7K?...besides a HT)

if you could post up pictures step by step that would be awesome

 

[DIYMark]

I plan too once i get my engine off ebay - boost porting, balance/true, reeds, exhaust, head work and a few others are what I plan on doing - ill try to grab pictures of the working being done.

 

[Egor]

I marked this post a long time ago. I wanted to use weights to balance the crank on at least one of my bikes. A few weeks ago I had one apart for a crankcase gasket, (bad from factory) would not run. So I tried to balance the crank. The only problem was I have no access to a scale, so I guessed and just made the crank start to move back from the big end. It is far better but no cigar. Then I got the idea, I need to find something that I can depend on for weight. Penny weight, no, what about coins, they are minted they must have an accurate weight. Sure enough a Nickel weight is 5 gr. even. a Quarter is a little more at 5.670 gr, Dime is 2.268 gr. so now we have some objects that have accurate weight to use to counter the crank. Have fun, Dave

The United States Mint Coin Specifications

 

[mountain80]

So after reading this very informative set of posts I see that there are 2 balance factor percentages mentioned by my calculations 52% and 57%. If I use 55% would that be a "good" all around balance and if not throw me some percantage as I have a brand new spare crank that I am itching to true and balance and also what kind of rpms can a guy safely run in you're estimations?

 

[Egor]

Mountain, There is no perfect balance numbers for a single cyl engine. I run most of mine under the rpm that they start to vibrate at. but there are times when I want to spin it up a bit and can't, it would tear the bike up. I am going to go for the 57% with a new engine I have waiting for a tear down. The last one I did with the guess is OK far better than it was from factory. I have one engine out of 10 that is smooth running. Is your new crank a one piece or the two piece wheels. I am looking for one of the one piece cranks. at least you can true them up if out of round. The only thing that keeps the rpm's limited on these engines is the wrist pin, 10mm "why". Should be 14mm for an engine this size. I have to say I have never heard of a failure. I push some of mine pretty hard. Have fun, Dave

 

[mountain80]

Egor, my spare crank has the one piece wheels that is to say they don't have the cheesy plates with screws bolted on it and I noticed that the outide of each half has been surface ground which is kind of impressive for what it is. I just measured up the parts for weight but I am too lazy to press the crank apart and measure the rod weight so ill hang it horizontally and measure the small end weight that way. I would like to get a balance factor % so that I can run her up to 7-8k rpm max. I run the stihl bearing and can make a better wristpin than stock at work. One thought i had is to machine the bobweight till everything balances to find out what the balance factor from factory is on this crank just for kicks. Any thoughts??

Cheers, Jody.

 

[foureasy]

remove some material near the crankpin and install. the vibration will move up in the rpm range. the higher you want the sweet spot, the more material you will have to remove. it's too much work to put it on a knife edge, (since you're probably checking for the "balance factor" on a couple of 2x4's anyway).

 

[mountain80]

I will be using balancing stands at the machine shop where I work meant for checking shaft runout and balance, not 2x4's.

 

[corgi1]

ok,,,To balance a crank,I will be balancing the weight of the "piston ,rings ,wrist pin and conecting rod" against the counter weight of the crank,is this right?

 

[Egor]

Corgi - Yes that is correct. It is true the % best is 57, and the method that is the easiest is the hanging rod with the 25 gr. attached to the end of the rod. All the professional shops use the suspended rod, so there must be some reason. It seems for our use the hanging method works good enough. You could use 2X4's with slots cut in them and steel edges inserted, you will need to keep the edges as plumb as possible at any rate. Most of my cranks have the added weights and they are so poor machined that they have a lot of run out, it can't be fixed as it would alter the weight distribution. I did get one crank with no counter weight holes at all. It is an old Kings with a bushing Gudgeon pin. I have one engine that has no vibration an early Dax also with a bushing, I have threatened to take it apart and see whats up. I would like to get a GruBee as I think there is a chance his outfit is trying to do a better job, he is the father of the import of these engines. Have fun, Dave

 

[corgi1]

I've set up the level straight rods ,set the crank on them,rod and piston hanging down(small piece of tape accross the piston to hold it straight w/the rod)...Is the piston too heavy?,,,do I need less weight where the piston is?,,Is this where I need only 57% of the weight of the piston,pin,rings,& clips?(or 25 grams),,,I fix cars but I never did this just heard about it,,, I think these answers will get me started, and I know too remove weight in small bits os its not too much,,,I'm looking for no roll