Balanced Crank?

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Feb 6, 2018
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#1
Know of anyone who Balances Cranks, or sells them? Seems the balanced cranks I read about are old ones, or from companies that seem to be out of business. Seems like a worth while mod to me, especially since it would help protect the engine and other parts as well, at least. Doubt it's something I could do with minimal equipment. Thanks for any ideas, etc. I'm kind of new to this.
 


FurryOnTheInside

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#2
My interpretation is when we say "balanced crank" this just means the best balance of piston, con rod and crank counterweight that we can get for our own peak RPM.

You can get a better balanced (than a stock generic) engine using very minimal tools. :)
It is definitely a worthwhile mod. The early posts on this forum are full of stories about bolts falling out, frames cracking from metal fatigue, and people needing foam grips and springs under the saddle to stay comfortable.


Personally, I started by drilling a couple of holes, one in each crank cheek. Radially drilled, so no need to split the case. Starting with a 3/16" tungsten carbide bit and then used a 3/8" tungsten carbide bit. Drilled about 15mm deep measured nearer the side of the hole than the middle so it is the average depth (you can drill shallower then go back and drill deeper later on if you wanted to).
I then lightened the upper half of the connecting rod somewhat just by grinding a bit of material from around the edges using the grit stone whatsit on the Dremel. I haven't seen any posts about bent con rods so I assumed there is some excess material, and removing steel makes sense for lowering weight.
I did remove quite a bit of aluminium from inside the piston skirt using a tungsten carbide burr in the Dremel, but it's a less dense metal. I haven't seen any bent piston skirts so I assumed that there's excess material there too (and also the fact that there are thinner and thicker bit of the skirt so I think it can all be a thin as the thinnest part).

I have VERY little vibration up to 7000 RPM and not excessive vibration up to 8000 RPM (which is the highest RPM I've tried).

 
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#3
YES! What a great answer, right off the bat! So, looks like I can do a little after all. Was wondering the other day about lightening the piston too, though I hadn't heard of it having been done. Think I'll do a little of those mods on my new 49cc kit, which should be delivered Monday. Not going to go overboard on anything. Not trying to make a racer or anything like that. Just hoping to make a more efficient, longer lasting engine, perhaps with even a little better mileage. Thanks so much Furry...! I did attempt searching posts, just didn't seem to see what I was looking for. Oh, photos were Very Helpful too!
 

FurryOnTheInside

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#4
YES! What a great answer, right off the bat! So, looks like I can do a little after all. Was wondering the other day about lightening the piston too, though I hadn't heard of it having been done. Think I'll do a little of those mods on my new 49cc kit, which should be delivered Monday. Not going to go overboard on anything. Not trying to make a racer or anything like that. Just hoping to make a more efficient, longer lasting engine, perhaps with even a little better mileage. Thanks so much Furry...! I did attempt searching posts, just didn't seem to see what I was looking for. Oh, photos were Very Helpful too!
Well, I have to thank Jaguar for his posts here on the forum and his wonderful dragonfly75.com site which enabled me to only do the balance jobs once and be close enough to ideal balance that the vibes are not an issue at all, up to about 7000rpm, because his figures worked so well for me. :)
http://www.dragonfly75.com/motorbike/
I might get a lightened wrist pin sometime, but it will be a luxury and not a necessity.

There is no need as far as I can see, to entirely disassemble the engine, remove the crank, weigh the parts, use any kind of calculator, drill the balance holes sideways and reassemble the whole thing again.
The method I used is quick, effective and easy to do using minimal tools that most people have.
 
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#5
Great again! Checked out a site just a little so far. Can see I'll be spending some time there in the future too. Thanks again!
 

Street Ryderz

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#6
Know of anyone who Balances Cranks, or sells them? Seems the balanced cranks I read about are old ones, or from companies that seem to be out of business. Seems like a worth while mod to me, especially since it would help protect the engine and other parts as well, at least. Doubt it's something I could do with minimal equipment. Thanks for any ideas, etc. I'm kind of new to this.
Balenced as Furry said is just a lose figure of speach here for our engines since they are now where near being balenced but rather balenced to within a factor meaning that if you weigh all the reciprocating part's,piston with rings,wrist pin,bearing,and retainer clips along with the hung weight of the rod that's the total mass.Let's say the total mass is 130 grams and with the radial cut's Furry with the crank on a balence stand you can hang around 25 g of the rod and it will staticly balence that's a balence factor of 20% just slightly above but you can see what I mean here.I have belenced out a crank to 100% perfectly staticly balenced and It was supper smooth but didn't work well at all for bottom end or lower tooth count gear ratio's,It lost to much rotational mass trying to achive the balence factor most of my cranks currently are between 40-50% balence factor and still retain enough rotational mass as to not affect the bottom end much and rev out to 11 grand though I try to stay under that!Most people who are doing there own are using a balence factor of 30% with very good results.First pic is weight reduction with the crank removed,Radialy works aswell Oh yeah if you remove the crank for balencing do your self a solid and true it before reinstalation,I put the crank between live centers on a lathe then preload a dial indicator on the shaft to see where its out and the straighten bit by bit then in the next pic I squared it up against the chuck and double check it,this one managed to get to within a thou this also makes a big difference in running smoothly!I know many folks dont have a lathe or other means to do this but if your going to make the balence better it only makes sence to true it aswell and many good bike shops provide this service it cost a bit but will pay you back in the long run.
2018-04-06 17.55.06.jpg
2018-04-18 11.21.11.jpg
 

FurryOnTheInside

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#7
Balenced as Furry said is just a lose figure of speach here for our engines since they are now where near being balenced but rather balenced to within a factor meaning that if you weigh all the reciprocating part's,piston with rings,wrist pin,bearing,and retainer clips along with the hung weight of the rod that's the total mass.Let's say the total mass is 130 grams and with the radial cut's Furry with the crank on a balence stand you can hang around 25 g of the rod and it will staticly balence that's a balence factor of 20% just slightly above but you can see what I mean here.I have belenced out a crank to 100% perfectly staticly balenced and It was supper smooth but didn't work well at all for bottom end or lower tooth count gear ratio's,It lost to much rotational mass trying to achive the balence factor most of my cranks currently are between 40-50% balence factor and still retain enough rotational mass as to not affect the bottom end much and rev out to 11 grand though I try to stay under that!Most people who are doing there own are using a balence factor of 30% with very good results.First pic is weight reduction with the crank removed,Radialy works aswell Oh yeah if you remove the crank for balencing do your self a solid and true it before reinstalation,I put the crank between live centers on a lathe then preload a dial indicator on the shaft to see where its out and the straighten bit by bit then in the next pic I squared it up against the chuck and double check it,this one managed to get to within a thou this also makes a big difference in running smoothly!I know many folks dont have a lathe or other means to do this but if your going to make the balence better it only makes sence to true it aswell and many good bike shops provide this service it cost a bit but will pay you back in the long run. View attachment 81679 View attachment 81680
I would definitely still like to try some further reduction of the reciprocating mass. My "no workshop" tools limit me but I'm hoping to acquire a drilled out wrist pin (4.5g lighter) that has been done on a lathe, and perhaps even try removing a little more material from the upper half of the connecting rod myself.
I plan to fit a very slightly heavier wrist pin bearing (KT101415) anyway so it should be more than enough to make up for that.

Reducing the reciprocating mass is always going to be better than removing rotational mass, so I would like to do as much as possible of the former and just enough of the latter for good smoothness at my self-imposed RPM limit of 8000 or maybe 8500. :)
 
Joined
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#8
Balenced as Furry said is just a lose figure of speach here for our engines since they are now where near being balenced but rather balenced to within a factor meaning that if you weigh all the reciprocating part's,piston with rings,wrist pin,bearing,and retainer clips along with the hung weight of the rod that's the total mass.Let's say the total mass is 130 grams and with the radial cut's Furry with the crank on a balence stand you can hang around 25 g of the rod and it will staticly balence that's a balence factor of 20% just slightly above but you can see what I mean here.I have belenced out a crank to 100% perfectly staticly balenced and It was supper smooth but didn't work well at all for bottom end or lower tooth count gear ratio's,It lost to much rotational mass trying to achive the balence factor most of my cranks currently are between 40-50% balence factor and still retain enough rotational mass as to not affect the bottom end much and rev out to 11 grand though I try to stay under that!Most people who are doing there own are using a balence factor of 30% with very good results.First pic is weight reduction with the crank removed,Radialy works aswell Oh yeah if you remove the crank for balencing do your self a solid and true it before reinstalation,I put the crank between live centers on a lathe then preload a dial indicator on the shaft to see where its out and the straighten bit by bit then in the next pic I squared it up against the chuck and double check it,this one managed to get to within a thou this also makes a big difference in running smoothly!I know many folks dont have a lathe or other means to do this but if your going to make the balence better it only makes sence to true it aswell and many good bike shops provide this service it cost a bit but will pay you back in the long run. View attachment 81679 View attachment 81680
There's apparently a lot more to this "Balancing" than I expected. Thanks to much for your in depth reply!
 

Street Ryderz

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#9
I would definitely still like to try some further reduction of the reciprocating mass. My "no workshop" tools limit me but I'm hoping to acquire a drilled out wrist pin (4.5g lighter) that has been done on a lathe, and perhaps even try removing a little more material from the upper half of the connecting rod myself.
I plan to fit a very slightly heavier wrist pin bearing (KT101415) anyway so it should be more than enough to make up for that.

Reducing the reciprocating mass is always going to be better than removing rotational mass, so I would like to do as much as possible of the former and just enough of the latter for good smoothness at my self-imposed RPM limit of 8000 or maybe 8500. :)
I didnt do the one pictured above but some times I'll remove all the casting seams from the sides of the rod and even round out or even knife edge the sides between the big and little ends,This can shave up to 12g of the rods weight (knife edged) but anything helps.
 
J

Jonathan hemsworth

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#10
Know of anyone who Balances Cranks, or sells them? Seems the balanced cranks I read about are old ones, or from companies that seem to be out of business. Seems like a worth while mod to me, especially since it would help protect the engine and other parts as well, at least. Doubt it's something I could do with minimal equipment. Thanks for any ideas, etc. I'm kind of new to this.
Arrow sells balanced cranks Now!
 
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