Engine Balance

So just how does one calculate engine balance. I know the Commando 850 is supposed to be 53 % of the reciprocating weight. (something like that) I asked the dynamic engine balancer company how do you deal with the volume of oil mass in the sludge trap in the crankshaft and got a response "we compensate" Is it not imperative to know the weight of the oil in the crankshaft when doing these calculations? And what about the Commando engines going into a featherbed frame what balance factor is to be used in this situation and how is it calculated? Please enlighten.

Here's a little tech on balance;

A 52% balance factor means you take 52% of the reciprocating weight [ ie the weight of the pistons,pins,rings, clips and the small end of the rod ] and add it to 100% of the rotating weight { ie the big end of the rod with bearing shells and bolts ] and this figure is the weight of the bob weight that will be clamped to the rod journal when it is placed in the balancing machine. Remember that the 52% figure is correct when the crankshaft is full of oil. If the crank is balanced dry then you should use a factor closer to 56%. This is where things start getting a little harder to understand.
The least amount of imbalance possible in a 360 degree twin is obtained at 50% balance factor. That is when the uncompensated weight of the piston is attempting to shake the motor up and down vertically and the uncompensated weight of the crankshaft is attempting to shake the motor forward and rearward horizontally by the same amount.
To understand what is happening imagine increasing the balance factor. Let's go all the way to 100%. So you add the few hundred grams to the crankshaft weight to completely offset the weight of the piston and rod at top dead center and bottom dead center. Now the engine does not vibrate up and down vertically at all. But when the crankshaft is 90 degrees away from TDC or BDC there is no horizontal piston and rod assembly to counteract the weight of the heavy flywheel weight so 100% of that weight will be trying to shake the engine forward and rear [horizontally]. It will shake horizontally by the same amount that the engine would shake up and down [vertically] if the balance factor were 0%.
A 90 degree v-twin [Ducati and others] is so smooth because there is a horizontal piston to counteract the weight of the crank flyweight at 90 degrees so the engine is balanced to a factor of 100%. Called perfect primary balance.
A Norton Commando motor is balanced to a figure a little over 50% because the speed of change happens a little faster near TDC than it does near BDC. so the moment weight of the piston is different at BDC than it is at TDC. The moment weight of the piston is the weight assigned to it when momentum is figured in so rpm and rod lengths are part of the equation. 52% is used because that gives an average of 50% when you figure that 52% is going to overcompensate for the reciprocating weight at BDC but undercompensate for the reciprocating moment weight at TDC because of the differences in piston speed change.
A 50% balance factor means the engine is going to shake round and round in a small circle. Perfect for a little round rubber isolastic. It also applies the least amount of load to the engine cases and crank bearings.
Raising the balance factor to a higher percentage causes the engine to shake less vertically but more horizontally. It will apply more shaking forces to the engine mounts and cases but depending on the frame design may be felt less by the rider. This gets really complicated as frame resonances then come into play. On a solid mounted engine the best balance factor is normally found by the cut and try method to see what feels smoothest to the person holding the handlebars. It may come out different on two similar bikes because very minor things [even the weight of the rider] will influence how the bike vibrates.
The balance factor only changes the plane of the vibration. Up and down or forward and rear. An engine will always vibrate more at higher speeds than at lower speeds. If it is felt by the rider or not depends on the whether the frequency and plane of the vibration is transmitted to the rider by the frame and whether or not it happens to excite a natural resonce in that frame
Now comes secondary shaking forces which are caused by the different moment weights of moving parts and where I am going to stop. Jim Comstock

That was a great explanation. Thank you.

swooshdave wrote;

That was a great explanation. Thank you.

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I'll second that. Even I can understand that.......more or less :lol:

Wonder how a Ariel Square Four does for balance?

So this has me asking, (Jim) What are your thoughts on an offset cranks and their drawbacks and benefits? Thanks in advance.

That was a great explanation......I think if I could just magnaflux the gimbelhead I would have nearly as good of luck.....

So this has me asking, (Jim) What are your thoughts on an offset cranks and their drawbacks and benefits? Thanks in advance


Offsetting the crank throws turns one big jerk as the pistons reach top and bottom center into two smaller jerks. It also crates a rocking couple which makes the engine try to rotate on an axis placed horizontally from front to rear. It is not what you want on an engine in isolastic mounts as they can't flex in that direction. In some rigid mount frames they may transmit less vibration to the rider. They also create more shakes per revolution which may or may not excite frame harmonics. Predicting whether this is going to make an engine feel smoother or rougher is about impossible. I tried a couple combinations a few years back and ended up back at 360 degrees. The engines seemed a little smoother at lower speeds but buzzier at higher speeds. It also messed up the tuning with 2 into 1 exhaust pipes. I also tried a 180 degree crank and found it was nearly un-ridable at higher engine speed. It also destroyed the engine cases in nothing flat. Jim

That was a great explanation......I think if I could just magnaflux the gimbelhead I would have nearly as good of luck.....

I've been looking for a gimblehead magnafluxer for years. If you find one let me know. Jim

Wonder how a Ariel Square Four does for balance?

No clue. I have never researched one. But probably similar to an inline four with perfect primary balance but serious secondary vibes.

comnoz said:

Now comes secondary shaking forces which are caused by the different moment weights of moving parts and where I am going to stop. Jim Comstock

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Jim, Please continue if you have the time and inclination. That was great.
I remember seeing some specs on balance for the Commando where they reversed the balance numbers for wet and dry, so that the wet figure was higher than dry. Can't recall where it was. I checked Kim White's CD that has a shop manual for the 750/850 dated 1973. It calls for 52% dry. The manual for the 850 MkIII dated 1975 calls for 52% wet 63% dry, which makes more sense. It wouldn't be impossible for them to have run a typo, seeing as how they published that diagram with the oil feed line going to the scavenge pipe.
I know of someone that did get an engine balanced at 52% dry and it seemed to run ok for pretty mild street duty.

I know of someone that did get an engine balanced at 52% dry and it seemed to run ok for pretty mild street duty


I don't think the balance factor used with isolastic mounts is very critical. I have balanced my street bike several times from 52% to 65% and really couldn't feel much difference. The things that made differences I could feel were piston weight [that's why 750s are smoother than 850s] and isolastic mount softness. The frame mounts also need to be checked to make sure the isolastics are relaxed. [IE the bolt holes line up without preloading the rubber]. I moved the shocks to a verticle position for a year on my bike so the shocks didn't load the isolastics and found the bike was smoother but it sure didn't look as cool. Jim

Thanks for the reply Jim, Lots of things to consider when planning a motor build, Chuck.

Now comes secondary shaking forces which are caused by the different moment weights of moving parts and where I am going to stop.

Bob, I would love to be able to write up an article on secondary shaking forces. Unfortunately my understanding of them is pretty basic and I don't think I could do any justic to the subject. Fortunately they don't apply to the Norton motor in any way that would be of any benefit when building an engine unless of course you were designing counterbalancers into your Norton. Jim

comnoz said:

I don't think the balance factor used with isolastic mounts is very critical. I have balanced my street bike several times from 52% to 65% and really couldn't feel much difference. The things that made differences I could feel were piston weight [that's why 750s are smoother than 850s] and isolastic mount softness. The frame mounts also need to be checked to make sure the isolastics are relaxed. [IE the bolt holes line up without preloading the rubber]. I moved the shocks to a verticle position for a year on my bike so the shocks didn't load the isolastics and found the bike was smoother but it sure didn't look as cool. Jim

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I tried that too for the same reasons. It wasn't completely vertical. I chickened out because it didn't seem too strong, the way I did it. It looked like it needed a strut in tension going down to the main frame to the shock pickup point.
The possibilities for the reduction in reciprocating mass is phenomenal with modern pistons and pins. Not measured in grams, it's fractions of a pound.

Bob, I looks like my shocks were a little straighter than yours. The top mount is where I have the luggage rack mounted. I used shorter shocks. The frame reinforcement is around .100 thick and it actually worked very well. If I had a good set of short shocks I would move them back. Jim

Wonder how a Ariel Square Four does for balance?
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Back in the early seventies I put an Ariel Square Four engine (4G mk.2) into a Slimline Featherbed frame and from memory there was virtually no vibration at all, really wish i'd kept it!
Dave.

Gday Dave, that sounds like a sweet bike, the Squariel in the featherbed, any photos to post? FOXY