Theoretical crank balance question

Fast Eddie · Sep 18, 2023

When balancing a wheel, we don’t apply a factor, we just try and get it fully ‘balanced’.

I think I’m right in saying that if we did that to a crank assembly it’d be the equivalent of a 100% balance factor ?

So… with a long stroke motor like a Commando, what would and engine run like if balanced like this ?

baz · Sep 18, 2023

Fast Eddie said:
When balancing a wheel, we don’t apply a factor, we just try and get it fully ‘balanced’.

I think I’m right in saying that if we did that to a crank assembly it’d be the equivalent of a 100% balance factor ?

So… with a long stroke motor like a Commando, what would and engine run like if balanced like this ?

I think I read it transfers the vibes to for and aft ?

Fast Eddie · Sep 18, 2023

baz said:
I think I read it transfers the vibes to for and aft ?

Yes I recall reading something like that.

But what does it mean? What will it fell like ?

baz · Sep 18, 2023

Fast Eddie said:
Yes I recall reading something like that.

But what does it mean? What will it fell like ?

Someone on here will know
And hopefully chime in

Burgs · Sep 18, 2023

Hi fast Eddie, its not that simple, get a copy of "Tuning for Speed' (Phill Irving) if you do not already have one.
When I first put my Manx on the road it vibrated something terrible, when I rebuilt it in around 1986, I balanced it to suit the rev range I was going to race/ride it at according to Phils calculations, it is still the smoothest single I have every ridden.
Balancing at 100% will just about break every thing on the bike including the riders arms and wrists.
PS wheels don't have piston/s and conrods going up and down, just around and around and can be balanced to 100%.
The factor accounts for out of balance forces at various revs, strokes weights etc.
A 360 degree twin is not that much different to a single ie they both go up and down at the same time, 180 degree twin introduce another variable.
This al from my experience and old memory.

Hope this helps.
Burgs

robs ss · Sep 18, 2023

Fast Eddie said:
When balancing a wheel, we don’t apply a factor, we just try and get it fully ‘balanced’.

I think I’m right in saying that if we did that to a crank assembly it’d be the equivalent of a 100% balance factor ?

So… with a long stroke motor like a Commando, what would and engine run like if balanced like this ?

I don't think the wheel analogy is applicable.
The inherent imbalance of a 360 twin is caused by the mass of the reciprocating parts (pistons & top half(ish)of rods)
The term "balance factor" is simply a way of distributing part of the vertical vibration into "some" horizontal vibration. This is achieved by removing weight (drilling) the cheeks and/or flywheel at the appropriate place. The amount of this distribution is the balance factor number.
A wheel has no reciprocating parts so is not an appropriate analogy

Fast Eddie · Sep 18, 2023

robs ss said:
I don't think the wheel analogy is applicable.
The inherent imbalance of a 360 twin is caused by the mass of the reciprocating parts (pistons & top half(ish)of rods)
The term "balance factor" is simply a way of distributing part of the vertical vibration into "some" horizontal vibration. The amount of this distribution is the balance factor number.
A wheel has no reciprocating parts so is not an appropriate analogy

I agree.

But I’d still like to know what the theoretical outcome would be of balancing a Norton engine thus…

texasSlick · Sep 18, 2023

Fast Eddie said:
I agree.

But I’d still like to know what the theoretical outcome would be of balancing a Norton engine thus…

Utter destruction....... vibration is a killer

Fast Eddie · Sep 18, 2023

Burgs said:
Hi fast Eddie, its not that simple, get a copy of "Tuning for Speed' (Phill Irving) if you do not already have one.
When I first put my Manx on the road it vibrated something terrible, when I rebuilt it in around 1986, I balanced it to suit the rev range I was going to race/ride it at according to Phils calculations, it is still the smoothest single I have every ridden.
Balancing at 100% will just about break every thing on the bike including the riders arms and wrists.
PS wheels don't have piston/s and conrods going up and down, just around and around and can be balanced to 100%.
The factor accounts for out of balance forces at various revs, strokes weights etc.
A 360 degree twin is not that much different to a single ie they both go up and down at the same time, 180 degree twin introduce another variable.
This al from my experience and old memory.
Hope this helps.
Burgs

What kind of vibes would balancing to 100% cause, high end, low end, or ?

Can you recall the factor % you ended up with on the Manx… or is it a secret ?!

robs ss · Sep 18, 2023

Fast Eddie said:
When balancing a wheel, we don’t apply a factor, we just try and get it fully ‘balanced’.

I think I’m right in saying that if we did that to a crank assembly it’d be the equivalent of a 100% balance factor ?

So… with a long stroke motor like a Commando, what would and engine run like if balanced like this ?

...or, you could throw away the piston and conrod and balance it as you say - but you may have a bit of a power reduction.

Fast Eddie · Sep 18, 2023

texasSlick said:
Utter destruction....... vibration is a killer

Where in the rev range would you expect the worst vibes to occur ?

concours · Sep 18, 2023

Fast Eddie said:
Where in the rev range would you expect the worst vibes to occur ?

There is more to it.
Balance to offset the piston rod at 0/180, then at 90/270 there is only a slight amount of the mass, just the big end of the rod.
Reference 90 degree v-twin.

Engine balance - Wikipedia

en.m.wikipedia.org

Burgs · Sep 18, 2023

Hi Eddie
Tuning For Speed has it all, you can balance, a single or twin for the rev range that you want to ride at, my experience tells me Phill new a lot more then the average jockey yesterday and today.
You can calculate the rev range where you require the least vibration, piston/pin weights, conrod, flywheel, weights etc, rev range you want to have least vibration, about as good as it gets.
You just need to set the rev range you need least vibration, and maximum vibration and aim for somewhere in between in my experience.

Burgs

Richard Tool · Sep 18, 2023

And I know you have a copy of “Tuning for speed “ - we had a thread going on the subject a while back IIRC …

Fast Eddie · Sep 18, 2023

Richard Tool said:
And I know you have a copy of “Tuning for speed “ - we had a thread going on the subject a while back IIRC …

Of course I do.

Doesn’t everyone ??

marshg246 · Sep 18, 2023

Fast Eddie said:
When balancing a wheel, we don’t apply a factor, we just try and get it fully ‘balanced’.

I think I’m right in saying that if we did that to a crank assembly it’d be the equivalent of a 100% balance factor ?

So… with a long stroke motor like a Commando, what would and engine run like if balanced like this ?

Not an expert but think about this. You're not balancing just a wheel, you're balancing a wheel with an appendage that is changing "weight" all the time. When the big end is at 0 or 180 degrees its "weight" is very different than when at 90 or 270 degrees. Plus, when the piston is going up it has more effect than going down and vice versa depending on cycle the piston is in. So, it's a matter of trying to even all that out. That's not possible to do across the RPM range so a factor is used that makes the balance the best at a certain RPM.

So, to answer your actual question - there is no way to balance to 100% other than statically and it would not be 100% when running.

marinatlas · Sep 18, 2023

Hi FE , not the answer you need but it may help a bit to both of us

JS Motorsport Balance Specs

Installation of the JS Motorsport lightweight pistons and rods will automatically change a 750/850 stock Commando wet balance factor from 52% to around 65% with no modifications to the crank.

jsmotorsport.com

kommando · Sep 18, 2023

Fast Eddie said:
I think I’m right in saying that if we did that to a crank assembly it’d be the equivalent of a 100% balance factor ?

So… with a long stroke motor like a Commando, what would and engine run like if balanced like this ?

1. No as you cannot fully balance a 360 crank unless you add a balancer shaft. 100% just means the out of balance weight it pointed either fully up or forward (can't remember which way). When balancing a 360 crank the % just changes the direction of the out of balance weight but not the weight.

2. Likely the iso's would not work as well as normal, Norton used 50% balance ie 45 degrees as then that left the iso's to do their work.

Onder · Sep 18, 2023

I'd love to do a bunch of motors at different balance factors and then ride them
and see what the difference really is. Sure, I realize the Commando is a different animal
as the rubber doughnuts reaction are part of the equation but still would be interesting.

MikeG · Sep 18, 2023

When I had my A10 balanced the term "factor" never came into it. I was asked the RPM range where I wanted it the smoothest and that's what was done. I do know that he had to remove very little weight to do it so BSA must have had some idea when it was designed, and built to the best that mass production of the day allowed.

Theoretical crank balance question

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