Spin Balancing welded crank

[platinumjason]

When welding the crank, does it need any balancing after that? Talked to my building asking how my motor was coming along and he was wanting to find a shop around here to spin balance the crank after its welded. Ive never heard of anyone doing this before, is just welding it going to cause that much difference in weight to where it needs to be rebalanced again? If not Id like to just say to weld it and put it back together being I dropped it off 3 weeks ago and he just started it. Any input? Thanks.

[Big Blue]

I dont think they are banlanced in the first place, I could be wrong thought but I thought a banshee motor balanced itself.

[platinumjason]

I dont think they are banlanced in the first place, I could be wrong thought but I thought a banshee motor balanced itself.

 

Yeah I didnt think so either, I thought it just got welded and thrown in. Anyone else have a guess?

[sheefreak]

I have never heard of spin balancing, maybe he is refering to checking to see if it is true. If it is not true you will get very unwanted vibration leading to more problems later.

[BdBanshee]

Have it TIG welded without adding any metal.

[RATBIKE0130]

There is no way to spin balance a banshee or any two stroke crank for that matter. The crank wheels are counter weighted to account for the rod length and pistion assembly at a certain rpm. There is a ratio to follow but it's not a strict design number.

[FireHead]

There is no way to spin balance a banshee or any two stroke crank for that matter. The crank wheels are counter weighted to account for the rod length and pistion assembly at a certain rpm. There is a ratio to follow but it's not a strict design number.

 

You are very wrong. Where did you ever hear that from?

 

Any two stroke crankshaft can be spin balanced, you will just need to have an accurate fixture to hold the rods whil balancing. This would be in place and sort of in addition to the bob weights used on a normal crankshaft. What it really depends on is how many planes of pins you have on a given crankshaft, as to what you attach to the pins during balancing.

 

How do you think that the mid-80's Moto GP bikes were able to turn 20k rpm's? It would certainly not be possible for an unbalanced crankshaft.

 

[Cotton eyed Joe]

I have never heard of spin balancing, maybe he is refering to checking to see if it is true. If it is not true you will get very unwanted vibration leading to more problems later.

 

There are 2 ways to balance something that spins. Either dynamically or static.

 

My soon to be ex father in law has a dynamic crank balancer in his engine shop, and it just spins it, detects where its out of balance and then tells you where to remove matierial, or drill a hole and fill it with lead to add weight.

 

They will put the crank between centers, and instead of your pistons and connecting rods, they will hang what is called "bob weights" to simulate your pistons and connecting rods. The weight used will be the lightest piston in your engine.

 

Then after that is done they will match the rest of the pistons to the lightest pistons weight. Usually to less than .5g.

 

I'm a huge believer in not only having a crank balanced along the axis, but also around the axis of rotation...although I've never been able to afford it.

 

I was quoted the "thanks for marrying our daughter and taking her off of our hands" price, and it was about $400.00 or so.

[RATBIKE0130]

You can static balance a crankshaft which will do you little good. I was talking about dynamic balance. There is a balance ratio usually between 50 and 60% to limit vibrations but it is impossible to balance a 2 stroke crankshaft completely. It's impossible! The problem is the crank rotates and the rod is always changing position in relation of the crank wheels. You can use a machine spinning the crank at a certain rpm with the rod pinned in a certain location but what good is that in real life? Do you account for the weight of the oil that ends up everywhere in a running motor? What happens when carbon builds up on the piston? What happens if you spin balance the crank at 8k rpms and then drop it to 2k rpms. Go find a tilt a whirl and tell me what happens when the thing starts to spin faster.

 

Why do you think honda's very own trx250r still needs an engine counter balancer!

[FireHead]

You can static balance a crankshaft which will do you little good. I was talking about dynamic balance. There is a balance ratio usually between 50 and 60% to limit vibrations but it is impossible to balance a 2 stroke crankshaft completely. It's impossible! The problem is the crank rotates and the rod is always changing position in relation of the crank wheels. You can use a machine spinning the crank at a certain rpm with the rod pinned in a certain location but what good is that in real life? Do you account for the weight of the oil that ends up everywhere in a running motor? What happens when carbon builds up on the piston? What happens if you spin balance the crank at 8k rpms and then drop it to 2k rpms. Go find a tilt a whirl and tell me what happens when the thing starts to spin faster.

 

Why do you think honda's very own trx250r still needs an engine counter balancer!

 

That is a good explanation of rotating versus statis balance.

 

:baseball_bat: