I am in need of a heavier counterweight for my tone arm. From the checking around I have done, best I can tell my current weight is 75g and my cart. is 8.5. My new cartridge will be 13.5g. A guy at needledoctor says I should get the 80 gram weight for my arm. That just does not seem right in my mind.

The front part of the arm is way long with respect to the stub where the counterweight is, right? so won't aprox 5g more on the long part have a lot more affect on the balance then an additional 5g on the back??? It is a lever afterall, no?

Currently my arm is in balance with a quarter and penny taped to it and the weight all the way back...almost falling off. I quick google search tells me a penny and quarter weighs about 7g+-

BTW the current cart. is a denon 103, the new one will be a zu 103r and my arm is a stock Pro-ject 9. If anyone has the time I greatly appreciate you checking my weights:

Stock Pro-ject counterweight 75g (I think)
Denon 103 8.5g
Zu 103r 13.5g
Penny aprox 2.5g
Quarter 5.2g

My options for a counterweight are 80g 95g and 120g.

So is my thinking all screwed up and the NDr. guys is correct, or am I correct in thinking a gram increase from 75 to 80 won't be enough.

 Well, you could get all accurate and calculate the moments. But, I think the 95g weight is what is needed. I had the Zu on my RB300 and I had to add a giant 1.25" nut securely taped on to the standard weight. It was a HECK of a lot more than 5g for sure. And even then, I had it all slid way back. 

Let's see if we can thumbnail it here --

On my arm the pivot to cart center is 9" and the pivot to weight center is 2". So, adding 1g to the front means you need 4.5g on the back. Your new cart is 5g heavier, so you need to add 4.5 x 5 = 22.5g.  So, the 95 will be close, I think.

Thanks for the reality check Mark...it just did not sit right with me that 80 would do it. Now another question since my current weight plus Larry/Marty's change is all the way back, and I mean way back would it stand to reason that the 95 will still have to be pretty far back? Am I correct in thinking that in a perfect world you want you counterweight close to the pivot. Something about moment of inetertia? Or am I worrying about things too much? Point being that might the 95 weight have to be way back to equal the current one + coins AND sitting way back?

Surely there must be a formula that takes in desired tracking force, length of arm, weight of cartridge and spits out counterweight needed? Or at the very least a math geek in the forum that has a bit of physics in his back pocket. Where is MaxG when you need him? I bet he knows this stuff in his sleep.

Rough numbers:
Arm=9 inches, Cartridge/screws=14g, Desired tracking force = 2.5-3.5g all of that would be in front of the pivot. Counterweight stub is about 2 inche long like you stated, so say the weight could vary from 1/2 inch to 1.5 inches from pivot.

mdeneen:

 Well, you could get all accurate and calculate the moments. But, I think the 95g weight is what is needed. I had the Zu on my RB300 and I had to add a giant 1.25" nut securely taped on to the standard weight. It was a HECK of a lot more than 5g for sure. And even then, I had it all slid way back. 

Let's see if we can thumbnail it here --

On my arm the pivot to cart center is 9" and the pivot to weight center is 2". So, adding 1g to the front means you need 4.5g on the back. Your new cart is 5g heavier, so you need to add 4.5 x 5 = 22.5g.  So, the 95 will be close, I think.

It looks like 95 will be close but still 2.5 short and he is already all the way back.  But teh next jump is to 120 or 22.5 over (if the weight is all of the way out).  But if he has 2" of travel, he can considerabely cut the moment of the counterweight by moving it in so the 120 may work. 

I'd just call Zu.  They make this heavy cart and shurely have dealt with this issue.