Piston Weight and Balance Factor?

APRRSV said:
Hi Dan,
Do you have a JS part # for your pistons.

Ed
Click to expand...
You can order them from JS Motorsport but be forewarned, there is a pretty significant lead time.

Norton Motorcycle Parts

jsmotorsport.com jsmotorsport.com

Piston Weight and Balance Factor?


Piston Weight and Balance Factor?
 
Dan1950 said:
No, these are designed for stock rods. Static CR should be about 10.1:1, dynamic CR with the Web 312a cam and flat tappets should be about 7.5:1 compared to 7.1:1 with stock cam/pistons.
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That's nice that JS offers a half-JS option if you don't want to get too deep into a rebuild. I'm interested to hear how much the change affected vibration. IMO, the isolastics allow the entire assembly to act as a counterweight since the assembly will be moving in the opposite direction of the pistons, but it's complicated by the fore and aft movement when it will be moving opposite to the counterweights.
 
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When you lighten the pistons (reciprocating weight) and do nothing else you reduce the vertical shaking of the motor but the horizontal shaking stays the same. This holds true until your balance factor reaches 100% but you are never going to be able to lighten the pistons enough to reach a 100% balance factor. So your bike and motor will feel smoother and stress on the crank and cases will be reduced. Your balance factor will increase and if you want you can rebalance the crank back to stock balance factor specs to accomodate the lighter pistons. This is all very fine. But either way the lighter pistons will give you a smoother ride whether or not you rebalance the crank.

The lead time on the stock rod pistons is better now as long as I've ordered them before.

An 850 with the lighter JS pistons for stock rods and no other changes will increase the stock balance factor of 52% to approx 57%

A bare stock medium 850 piston weighs approx 317 grams
A JS 850 high compression piston for stock rods weighs approx 278 grams (a JS 850 medium compression piston would weigh less).
 
jseng1 said:
When you lighten the pistons (reciprocating weight) and do nothing else you reduce the vertical shaking of the motor but the horizontal shaking stays the same. This holds true until your balance factor reaches 100% but you are never going to be able to lighten the pistons enough to reach a 100% balance factor. So your bike and motor will feel smoother and stress on the crank and cases will be reduced. Your balance factor will increase and if you want you can rebalance the crank back to stock balance factor specs to accommodate the lighter pistons. This is all very fine. But either way the lighter pistons will give you a smoother ride whether or not you rebalance the crank.

The lead time on the stock rod pistons is better now as long as I've ordered them before.

An 850 with the lighter JS pistons for stock rods and no other changes will increase the stock balance factor of 52% to approx 57%
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Thanks for the details on that. I have a 920 which I assume has the RGM kit in it. I'm mounting it in a Wideline although it came out of a Commando. The whole exercise is temporary to test out whether I like the chassis and it's straight etc., and will come apart for painting anyway. Would you have an estimate of the balance factor change going from RGM 920 pistons to yours? Thanks!
 
hhh said:
Thanks for the details on that. I have a 920 which I assume has the RGM kit in it. I'm mounting it in a Wideline although it came out of a Commando. The whole exercise is temporary to test out whether I like the chassis and it's straight etc., and will come apart for painting anyway. Would you have an estimate of the balance factor change going from RGM 920 pistons to yours? Thanks!
Click to expand...
The bare weight of the medium compression JS 920cc piston for stock rods is 290 grams. I don't know the weight of the RGM 920 piston.
 
hhh said:
Thanks for the details on that. I have a 920 which I assume has the RGM kit in it. I'm mounting it in a Wideline although it came out of a Commando. The whole exercise is temporary to test out whether I like the chassis and it's straight etc., and will come apart for painting anyway. Would you have an estimate of the balance factor change going from RGM 920 pistons to yours? Thanks!
Click to expand...
I've recently done this. The RGM 920 pistons weigh the same as standard size stock 850 pistons, so changing to the RGM 920 leaves the BF at stock. That is something Roger made a deliberate aim at.
Seems to work so far.

Glen
 
worntorn said:
I've recently done this. The RGM 920 pistons weigh the same as standard size stock 850 pistons, so changing to the RGM 920 leaves the BF at stock. That is something Roger made a deliberate aim at.
Seems to work so far.

Glen
Click to expand...
So I can expect my teeth to rattle out at high RPM in my Wideline at 52% then.
 
So what I take from the information here is that the reduction of piston weight will raise the RPM range of minimal vibration. Given that the Web 312a cam I will be running will have an elevated RPM range over stock, that seems like a benefit.
 
worntorn said:
Yes, if left as is I suppose it will be a shaker alright.

Glen
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Now the question is, can I pull the barrels, tap the drilling(s) and insert weights. Anybody have a slug of depleted uranium, about half inch diameter, lying around unused?
 
As mentioned in post #23, lighter pistons (equals increasing the balance factor) reduces the vertical vibration but the horizontal vibration stays the same. Reducing the vertical vibration is beneficial because your ass and your feet feel less of a buzz because of gravity pressing you down on the seat and pegs. This may be the reason that higher balance factors are recommended for solid frames. But you have to specify wet or dry BF. A wet balance factor of 65% equals a dry balance factor of 72% and so on.
 
jseng1 said:
As mentioned in post #23, lighter pistons (equals increasing the balance factor) reduces the vertical vibration but the horizontal vibration stays the same. Reducing the vertical vibration is beneficial because your ass and your feet feel less of a buzz because of gravity pressing you down on the seat and pegs. This may be the reason that higher balance factors are recommended for solid frames. But you have to specify wet or dry BF. A wet balance factor of 65% equals a dry balance factor of 72% and so on.
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I'm thinking dry balance factors are what are assumed if it's not specified, like in factory docs? I can't see somebody filling the crank with oil while balancing it.
 
When you take all the parts to be dynamically balanced, have him balance the rods and pistons as well as the crank. I use 63% dry balance factor for a Commando. Works out to be about 55% or so when wet. Perfect for a rubber mounter Norton.
 
robs ss said:
Actually, I don't use the ignore list.
I believe it is incumbent on all of us, as users, to call out crap when we see it.
Not to do so potentially leaves a trap for some poor unsuspecting reader to trust unchallenged bullshit.
If, in the next few posts, the rubbish is exposed - we have done our job.
My view anyway.
Cheers

Edit: Not that I am always correct either - I hope I get called out as necessary
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When I was about five, I came in the house crying because a bully hit me. My mother wasn't having it, so she told me I was bigger and didn't need to take crap from a bully and sent me back out. When I came back in crying, she explained that she was about to give me something to cry about and to out and handle it.

So out I went and learned two things. A straight left to the nose sends bullies home crying and that I should have done the same thing but not as hard - broke his nose which wasn't needed to get my point across.

Being in a military family that moved a LOT and being overweight and having a last name that bullies liked to turn into other things, every time we moved, at least one bloody nosed bully got the message.

When I'm attacked in a personal attack here or when I see them against others, I ignore the attacker since I can't bloody the nose - then I can go back to enjoying my time here.

It is fine to show people where they are wrong or have misspoken but attacking is simply not acceptable to me. It is so easy to say "I disagree because bla bla bla" rather than your modus operandi. Consider this being called out as necessary :) Hopefully, you'll take this as constructive, but if not...
 
marshg246 said:
When I was about five, I came in the house crying because a bully hit me. My mother wasn't having it, so she told me I was bigger and didn't need to take crap from a bully and sent me back out. When I came back in crying, she explained that she was about to give me something to cry about and to out and handle it.

So out I went and learned two things. A straight left to the nose sends bullies home crying and that I should have done the same thing but not as hard - broke his nose which wasn't needed to get my point across.

Being in a military family that moved a LOT and being overweight and having a last name that bullies liked to turn into other things, every time we moved, at least one bloody nosed bully got the message.

When I'm attacked in a personal attack here or when I see them against others, I ignore the attacker since I can't bloody the nose - then I can go back to enjoying my time here.

It is fine to show people where they are wrong or have misspoken but attacking is simply not acceptable to me. It is so easy to say "I disagree because bla bla bla" rather than your modus operandi. Consider this being called out as necessary :) Hopefully, you'll take this as constructive, but if not...
Click to expand...
...and you're calling me out on my post #14 because?
You believe that we should ignore or pass over factually incorrect posts?
We'll have to agree to disagree.
 
robs ss said:
...and you're calling me out on my post #14 because?
You believe that we should ignore or pass over factually incorrect posts?
We'll have to agree to disagree.
Click to expand...
Always happy with "agree to disagree" but try reading again. I'm fine with whether he was right or wrong and you disagreeing with him. Instead, you simply attacked him personally and nothing more and you should notice that I didn't respond to the post where you attacked him (post #12). In post #14 you expressed an opinion on a basic premise that I agree with. It's the manner in which you call someone out.

In this particular case I don't personally know enough to say who is right or wrong. It is an interesting subject just because of that. Your attitude that you know and others are stupid/wrong/etc does not give me any knowledge because you give no reason why you are correct. Is it training, experience, because you studied it, a God-given bit of knowledge, a simple opinion, or what?

If you read through the thread you'll see posts making a list to take to the "machinist" or "balancer" or "dynamic balancer". In my opinion some are simply regurgitating what they've been told because:

1) Any machinist is a joke in my opinion. Must be someone knowledgeable and setup to balance old British twin cranks.
2) They must be told the balance factor you want. You specified your opinion in one post and another guy did in another.
3) Then as most have said, he must have all the rotating parts to weigh, or you must provide that weight.
4) If you actually want it "right" for your application or desired outcome, you forget 3 & 4 and tell them what weight bob weights to use what they are allowed to do to the crank to achieve balance.

How critical the individual component weights are to each other is not clear to me or how much weight differences for the different components left and right affect the final results. For instance, weighing the bottom of a rod verses the top of the rod on both sides. Or is it correct to weight each side's components and average the two, or what? Is dynamic balancing actually better than the original static balancing that was done, if so, why?
 

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