Crank balance

[Street Ryderz]

Ok I want to run this by the pros to make sure my mind is working right. I have a total weight of 117.5grams, for piston, rings, circlips, wrist pin & bearing & small end of rod. So for a 45% balance factor you need 52.8grams, for 50% balance factor you need 58.75grams & for a 55% balance factor you need 64.625grams. Are these figures correct?

Yes that's right.

 

[Street Ryderz]

Sounds good to me, now you need to know the crank weight/cc to know how much material to take off.

Big math problem, I feel like I'm back in trade school.

No need for the cranks total weight or cc, The hung weight just has to be able to statically balance to find the factor. The amount of weight that statically balances while hung is how much of the reciprocating mass it accounts for.

 

[DieselTech]

Thanks @Street Ryderz that's the answer I'm looking for, if my weight figures were right or not, I will adjust size & depth of hole needed. Thanks again Sir!!! All help greatly appreciated.

 

[Karl Snarl]

I'm not sure about the weight per hole and size, Jag and I argued about this because he uses a program (charges for it's use) had incorrect figures and said he could hang 63 g with a single hole each side so I showed every one that was impossible. The crank material's density can differ from one to another and I'd rather know exactly, so I do it physically by hanging the weight off it and seeing for myself, I start with pilot holes then enlarge them and check with the hung weight, I think my holes on the last one I showed are only 12mm dia and 14-15mm deep but I'm using a lighter piston with that one so again it will vary.

We are out of 13 and 14mm bit here. Good thing ya posted this, I was going to wait for them. Lol. I'll use a 12mm one instead. An just check as I go. Now I just need to remember where I put my scale.

 

[Bizal]

This thread really simplified the concept. I'm sorry I missed it. In your guys experience whats the best balance factor to have on these engines? I know people say 55% on a vertical cylinder, but ours aren't 100% vertical. Ours sits at about 30° to the front. I think they say a horizontal cylinder works best with a 45% balance. So does the 30° make a difference?
I've had a lot of success with balancing my cranks, but I don't know if I'm squeezing the last drop of performance out it. I've always done 50% balance because that's the easiest to figure out.
Would it be worth going past 50% or lower the 50% a little because of the cylinder pitch?

 

[DieselTech]

I'm not for sure, I'm new to 2strokes & trying to learn them. That's a question for the professionals. I'm sure someone will chime in.

 

[Karl Snarl]

I don't understand it clear enough to teach it yet. Still learning it all with these single piston engines. I'd like to know if my understanding is right too. Lol. So I won't saying anything in case I'm wrong, don't want to put wrong info in someone elses head. I'll wait for one of the racers to chime in, they gone make or break my understanding of it.

 

[DieselTech]

I got a good understanding of crank balance, but not enough where I feel safe of trying to teach it.

 

[Karl Snarl]

I got a good understanding of crank balance, but not enough where I feel safe of trying to teach it.

I think I do, but would rather not mess someone else up if I'm wrong. Rather wait an have one of the racer guys tell me Im wrong or right.

 

[Street Ryderz]

This thread really simplified the concept. I'm sorry I missed it. In your guys experience whats the best balance factor to have on these engines? I know people say 55% on a vertical cylinder, but ours aren't 100% vertical. Ours sits at about 30° to the front. I think they say a horizontal cylinder works best with a 45% balance. So does the 30° make a difference?
I've had a lot of success with balancing my cranks, but I don't know if I'm squeezing the last drop of performance out it. I've always done 50% balance because that's the easiest to figure out.
Would it be worth going past 50% or lower the 50% a little because of the cylinder pitch?

Like all things when it comes to building up a 2 stroke finding the happy medium and building it to suit your needs is what usually works best! Rotating mass and it's inertia is needed not just for balance but also to help over come the dynamic compression which is higher than that of the static compression. After or above 50% balance factor this becomes very apparent as it will struggle with trying to reach higher rpm, also the forces that cause vibration start to shift out of the vertical plane to the horizontal plane with nothing to help correct these can cause a new set of issues.
As with many things testing can be very informative and I tried higher factors only to find that it would rev up faster no load but then struggle badly loaded, and this lead me to stick with better all around balance factors of 50% or less (I like 40-45% with higher compression builds). There are programs like Tony Foale designs that are very helpful in giving a visual comparison of different factors effects, like these showing how a 20% factor is still very bad (which is basically the stock cranks factor) then how at 50% it's all most perfect in both planes.

then you can see that just 2% over the 50% factor it starts to shift into the horizontal plane and just gets worse the higher you go.