Crankshaft balance

I just got the invoice from MARINE CRANKSHAFT for balancing an 850 crank for a featherbed frame. I chose them because of Jim Comstock's recommendation.

$212...inspect, magnaflux, micro polish journals and balance.

$45...return shipping with $1000 insurance

Total $257. Not bad

How much to ship it there?
Just saying

$40

What balance factor you going for, 72% was perfect for my set up, done way back in 1981, be good to see another 850 in a Featherbed frame, lots of torque in a great handling frame if set you right.

When I got my crank balanced for fhe Seeley recently (so also rigid mounted) I gathered as many knowledgable opinions as I could and the vast majority were in the 72-78% range.

So I opted for 75% (bang in the middle) and it is wonderful!

I believe that ensuring a good, well executed balancing job (ie research the person doing it), is actually more important than the exact % choice so long as you’re in the range of 72-78 ish.

ashman said:

What balance factor you going for, 72% was perfect for my set up, done way back in 1981, be good to see another 850 in a Featherbed frame, lots of torque in a great handling frame if set you right.

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When I built mine it was a 750 commando engine with power max 10.25-1 pistons, triple S cam with a Mick hemmings head and a severely lightened/polished crank
It was balanced to 72% with the motor leaning forward in a wide line frame
The vibration was appalling,I had to wire everything up
It'd shake the float bowls off and the rocker spindle plate bolts etc just for fun
It split the petrol tank and the lugs broke off the battery box
You could barely keep your feet on the pegs
But I think I made a mistake making the engine plates from stainless steel that I had mirror polished
The bolts would never stay tight

baz said:

When I built mine it was a 750 commando engine with power max 10.25-1 pistons, triple S cam with a Mick hemmings head and a severely lightened/polished crank
It was balanced to 72% with the motor leaning forward in a wide line frame
The vibration was appalling,I had to wire everything up
It'd shake the float bowls off and the rocker spindle plate bolts etc just for fun
It split the petrol tank and the lugs broke off the battery box
You could barely keep your feet on the pegs
But I think I made a mistake making the engine plates from stainless steel that I had mirror polished
The bolts would never stay tight

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Dynamically balanced or static?

Balancers do make mistakes. Owen Greenwood balanced a Triumph crank for me yonks ago (and he knew his Triumphs) and that vibrated as you describe plus it broke things… like the frame… it was freakin’ horrendous !! Later I had it re-done by Bassett Down and it was night and day different.

FWIW… Dave Degens was quite strong on the topic that alloy engine plates reduced vibration a lot, also that 1/4” was better than the 3/8” plate that many seem to prefer these days.

Is balance factor related to engine speed? Go fast you want to balance for high rpm smoothness if you are more
the road going type then you may want to have it best at your normal cruise speed what ever that may be?

From around 1981 or so
I think the stainless engine plates were not being gripped by the bolts?
I was only young when I built it

Onder said:

Is balance factor related to engine speed? Go fast you want to balance for high rpm smoothness if you are more
the road going type then you may want to have it best at your normal cruise speed what ever that may be?

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Mine was smooth at low revs but as they increased the vibes got worse and worse
I never managed to get it above 6000rpm it just wouldn't rev any higher , maybe the carbs were frothing

Fast Eddie said:

Dynamically balanced or static?

Balancers do make mistakes. Owen Greenwood balanced a Triumph crank for me yonks ago (and he knew his Triumphs) and that vibrated as you describe plus it broke things… like the frame… it was freakin’ horrendous !! Later I had it re-done by Bassett Down and it was night and day different.

FWIW… Dave Degens was quite strong on the topic that alloy engine plates reduced vibration a lot, also that 1/4” was better than the 3/8” plate that many seem to prefer these days.

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It was done by Owen greenwood
No Idea if static or dynamic

baz said:

It was done by Owen greenwood
No Idea if static or dynamic

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Seriously ?!?

Maybe he got both our piston and rod weight details mixed up !!

I’m pretty sure he only did static balancing IIRC.

baz said:

Mine was smooth at low revs but as they increased the vibes got worse and worse
I never managed to get it above 6000rpm it just wouldn't rev any higher , maybe the carbs were frothing

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I know not everyone agrees… but I believe that a crank that’s bad enough out of balance puts a cap on power as the forces involved consume power as well as limit revs. Things like fuel frothing are also factors of course, but I just feel that the forces involved also consume power.

My 1007 Seeley was out of balance when first built. The Dyno graph just cut off at under 6k and stayed flat and at 6k it simply would not rev any more.

After dynamic balancing to 75% (and using JS pistons & rods and a lighter flywheel) the graph maintained its upward trajectory to produce more power at 6k than before AND the revs went comfortably past 6k (self imposed limit of 7k now) with the power still climbing. So the before and after effect was considerable.

Obviously this was a combined effect of balancing and reducing internal rotating and reciprocating weight, but the rest of the engine spec was unchanged, so the effect was down to reducing these internal forces, and the performance change was huge.

Fast Eddie said:

Seriously ?!?

Maybe he got both our piston and rod weight details mixed up !!

I’m pretty sure he only did static balancing IIRC.

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It's a long time ago now
I bought the engine in bits from a mate of a mate he'd had the crank balanced to run the engine rigidly mounted
He said it was done at greenwoods, I've no doubt that he did as everything he told me about the engine was correct
I remember going back the next day to collect the crank from him ,it was wrapped in some oily rags in his kitchen cupboard
Those power max pistons were quite heavy
I'm also wondering now if the crank had been polished after being balanced!!! Nobody would do that surely??

Fast Eddie said:

I know not everyone agrees… but I believe that a crank that’s bad enough out of balance puts a cap on power as the forces involved consume power as well as limit revs. Things like fuel frothing are also factors of course, but I just feel that the forces involved also consume power.

My 1007 Seeley was out of balance when first built. The Dyno graph just cut off at under 6k and stayed flat and at 6k it simply would not rev any more.

After dynamic balancing to 75% (and using JS pistons & rods and a lighter flywheel) the graph maintained its upward trajectory to produce more power at 6k than before AND the revs went comfortably past 6k (self imposed limit of 7k now) with the power still climbing. So the before and after effect was considerable.

Obviously this was a combined effect of balancing and reducing internal rotating and reciprocating weight, but the rest of the engine spec was unchanged, so the effect was down to reducing these internal forces, and the performance change was huge.

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Sounds like you've hit the sweet spot for your engine
I have always assumed that any vibration must be wasting power

seattle##gs said:

I just got the invoice from MARINE CRANKSHAFT for balancing an 850 crank for a featherbed frame. I chose them because of Jim Comstock's recommendation.

$212...inspect, magnaflux, micro polish journals and balance.

$45...return shipping with $1000 insurance

Total $257. Not bad

Click to expand...

Nice one..

My 850 crank balanced at 72%, my cam was built up to a 2S profile/grind, running 40thu Hepilite flat top pistons, the head been shaved and still running original valve but the ports have been ported back in 1981, I run with 5mm steel engine plates and is the same set up today, my motor is a very smooth running motor, has never shaken anything off in the 44+ years of running with this set up, my motor will run very freely right through the revs, just got to watch it don't over rev.
In all the time I built this motor for the Featherbed I have only lost one muffler mount nut and the top gear box mount nut, I am still running all my original engine bolts and the bolts that mount the motor to the engine plates, I also have hollow round spacers in-between the engine mounts to frame lugs as well one big through bolt with spacers at the mid bottom rail lugs and engine plates.
I had some vibration in the ends of the steel handle bars at some revs but fix that by a set of renthal alloy bars with 6mm wall thickness, steel bars are only 1.6mm wall thickness I think, I can ride my Norton all day without any problems at all, its a great fun bike to ride.
When I first built this bike back in the early 80s there wasn't stuff like what Jim sells for smooth running engine bits, mine was built on the cheap when I had the money took me 2 years to build as money was tight back then (in between jobs) but it has never let me down motor wise and is very reliable, was an everyday ride right up to 2013 (when I retired early at 55) and has clocked up a lot of miles, it still pulls strong after all these years with a few newer upgrade added in the last 15+ years.

Is the 75% you used wet or dry? I never hear anyone say which way they do it. If wet, how much was allowed for oil in the crank?

When I bought the Atlas in 1987 it was low miles and commando pistons. It had vibration like you wouldn't believe. I drove it from Lake Tahoe to Sacramento one day...I could feel the vibration then pain creeping up my arms as the ride continued. Unfortunately I was with four other people or I would have turned around.

The crank in my Seeley 650 is balanced at 72%. I filled the hole which had been drilled into the cast iron counterweight with the threaded steel plug. I was about to drill the other side of the flywheel to get it up to about 75%, but thought about what I was doing and stopped. My motor would rev to 8000 RPM easily, but I am superstitious - it never goes over 7,300 RPM. I am surprised that the 850 motor can be so good with so little modification. You cannot get the most out of the motor, if you do not have the right gearbox.
Some people make the mistake of getting car guys to balance their cranks. Car cranks do not usually have a weight bias, but you cannot tell a car expert anything. A car camshaft and exhaust system does not often make the bottom 4000 RPM of the rev range disappear.
My bike is really lovely to ride - the way it feels at low revs gives me the required adrenalin rush. At high revs, it is perfect everywhere. Something so bad, should not be so good !
In writing this, I am getting the urge to race again, I am remembering how it feels.
If your motor will not rev over 6000 RPM, the most likely reason is a restrictive exhaust system. Most 2 into 1 systems cut 1000 RPM off the top of the usable rev range, but give more in the middle where it is needed - the tail pipe diameter and length is important.