Crank balance factor

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baz

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I've read a fair bit on here about crank balance factor when a commando engine is rigidly mounted
The more I've read the more I've not really understood!
I used to have a rigidly mounted commando 750 motor leaning forward in a wide line it was balanced to 72% and was terrible at all revs
This was probably compounded by using polished stainless steel engine plates
The vibration just got worse as the revs rose
I'm planning to get a crank balanced for a project at some point in the future, probably a 750 motor
I think an atlas was balanced to 84%?
What would be a good dry crank figure to aim for maybe 82%?
I'd like it as smooth as possible between say 3500 and 5500 rpm ish
 
The prefect balance factor will not cure the vibration of a Commando engine in a solid frame. The only cure is lighter pistons (less reciprocating weight. I tried many balance factors from 49% up to 85% in a solid frame. They all shook too much and things would crack and fall off. All that changed with the lighter pistons and longer rods.

The higher the balance factor (up to 100%) the less vertical shaking but it increases horizontal shaking. Using lower balance factors gives less horizontal shaking but then it increases vertical shaking. That's what changing your balance factor does. You have to find a compromise of about 66% (WET) for a solid frame. The Norton twin is like two side by side singles. It is inherently out of balance and you have to fix the problem at its source - the reciprocationg weight (piston, top end of rod and going to longer rod length).
 
My 850 motor has standard compression 850 pistons. I rebalanced it by fillingthe hole in the counterweight with a threaded steel plug. It's balance factor is about 72 %. The motor is rigidly mounted in a Seeley frame and stressed backwards fom the top of cylinder head by two rods with rose joints at each end. Theoretically, the top of the motor can vibrate slightly sideways. In it's usuable rev range from 5,500 to 7000 RPM, there is very little vibration. My frame is a Mk3 Seeley with piece of gusseted chrome-moly tube in front of the motor where the ladder is normally installed. The Seeley frame is extremely rigid and all of my mounts are very tight. I suggest a balance factor of about 70% would be normal for a road-going Dominator 88.
A good racing 650cc Triumph would have a balance factor of about 80%. My mate does not rev his over 6500 RPM. Most go to 8000 RPM. It is a very pleasant ride. It is a neutral handling featherbed Triton, and fast in a straight line.
I suggest the usual Commando crank is an Atlas crank which has had a hole bored into the counterweight to stop the bike frpm rocking forwards and backwards while idling. The comparison between an Atlas 750 and a Honda CB750, when they are standing and idling would have been very off-putting for Norton customers who were not experienced.
If the frame does not have isolastics and the mounts of the motor become loose, it is easy to crack the frame with vibration.
 
My hard mount 850 Commando motor in the Featherbed frame was balanced by an old English gentleman back in 1981 Ivan Tighe my cam builder here in Brisbane at the time put me on to this gentleman as soon as I told him what I was building he started to rattle off numbers and balance factors and what was needs for balancing, was too much for my young brain to absorb and I handed my crank, pistons, conrods, bearings, pushrods and all too him to do, he said around 72% balance factor.
Well whatever he did my hard mount Norton runs pretty smooth down low and mid-range but there are 2 rev ranges that the vibs in the handle bars that get a bit bad in the higher rev range but it smooths out over those two points in the rev range, other than that its quiet smooth around the suburbs, on the highway and where I use it the most up in the ranges so all the way up and over 100MPH its quiet good.
My stock Norton use to only rev to 6500 before losing it but after the balancing my crank, it revs very freely but have to be careful not to over rev it as it just keeps spinning up, but of course the crank is not the only thing that I have done too my motor but wasn't overdone.
My hoon days are no more but my motor works so well in my rev range, 2nd and 3rd gets a good workout throughout the rev range, but I no longer over rev my engine, but it gets up and over the ton very fast and surprises a lot of folks when out and about in the twisties.
I now run renthal 6.5 thick wall alloy handle bars and no longer get any vibs through the handle bars at those to rev ranges and in 44 years built this way I have only lost the top gearbox top mount nut and one nut off the left hand rear muffler nut, not bad for a hard mount motor that suppose to vibrate a lot and it was an everyday rider most of its life with me now its semi retired just like me.
Whether I been lucky or the old English gentleman knew what he was doing or I just have a freak Norton, but my 850 Commando/Featherbed has been the same way I built it in 1980/82 with only a few mods upgrades in the last 15 years, but the motor is very much the same when I first built it, it's a fun bike to ride, its light, plenty of torque and get up and go and handles like it's on rails, it's not as smooth as my 2016 1200 Thruxton but it's pretty good but it's not bad for a heavy 360 swing crank motor.
I love to build an 850 Commando motor with Jim's bits and pieces, but I don't have that much money in my kitty to play with, I built my motor when things were cheap to do and so far my motor has proved it worth for being reliable and not have things fall off since I brought it new, although its semi-retired now it's still my favorite bike to ride I have so much fun on it.
4 years as a Commando 44 years as Commando/Featherbed, would I do it again, sure would and would I change anything, no, as I have a 1960 Norton project bike build waiting to be restarted.
So yes, you can get them to run smooth with crank balancing if done right and in the right hands of the balancer.

Ashley
 
baz said:
I've read a fair bit on here about crank balance factor when a commando engine is rigidly mounted
The more I've read the more I've not really understood!
I used to have a rigidly mounted commando 750 motor leaning forward in a wide line it was balanced to 72% and was terrible at all revs
This was probably compounded by using polished stainless steel engine plates
The vibration just got worse as the revs rose
I'm planning to get a crank balanced for a project at some point in the future, probably a 750 motor
I think an atlas was balanced to 84%?
What would be a good dry crank figure to aim for maybe 82%?
I'd like it as smooth as possible between say 3500 and 5500 rpm ish
Click to expand...
I compiled a list of recommended figures from lots of knowledgable Norton folk (Inc JS, Comnoz, LCR Ken, Steve Maney, etc).

The list ranges from 72-78%. So anywhere within that range should be good.

I had a Norton cranked Triumph engine balanced to 80% some years ago, at the suggestion of Basset Down, and that worked well too.

I have had two of my Norton cranks dynamically balanced, most recently a Maney crank. Steve says dynamic balancing is not needed and indeed I have one statically balanced by him that’s very good. But on this recent occasion my balancer said it definitely needed work that only showed up dynamically. Comnoz posted a video about this once whereby a correctly (statically) balanced crank was nearly jumping out of his rig when spun up, and very smooth after dynamically balancing it. My balancing guy basically said mine was the same.

So… IMHO and FWIW, anywhere between 72-80% should work… with the rule of thumb to err on the higher % for higher rpm and vice versa….and I would definitely say to get it dynamically balanced… and use alloy engine plates.
 
Last edited:
ashman said:
My hard mount 850 Commando motor in the Featherbed frame was balanced by an old English gentleman back in 1981 Ivan Tighe my cam builder here in Brisbane at the time put me on to this gentleman as soon as I told him what I was building he started to rattle off numbers and balance factors and what was needs for balancing, was too much for my young brain to absorb and I handed my crank, pistons, conrods, bearings, pushrods and all too him to do, he said around 72% balance factor.
Well whatever he did my hard mount Norton runs pretty smooth down low and mid-range but there are 2 rev ranges that the vibs in the handle bars that get a bit bad in the higher rev range but it smooths out over those two points in the rev range, other than that its quiet smooth around the suburbs, on the highway and where I use it the most up in the ranges so all the way up and over 100MPH its quiet good.
My stock Norton use to only rev to 6500 before losing it but after the balancing my crank, it revs very freely but have to be careful not to over rev it as it just keeps spinning up, but of course the crank is not the only thing that I have done too my motor but wasn't overdone.
My hoon days are no more but my motor works so well in my rev range, 2nd and 3rd gets a good workout throughout the rev range, but I no longer over rev my engine, but it gets up and over the ton very fast and surprises a lot of folks when out and about in the twisties.
I now run renthal 6.5 thick wall alloy handle bars and no longer get any vibs through the handle bars at those to rev ranges and in 44 years built this way I have only lost the top gearbox top mount nut and one nut off the left hand rear muffler nut, not bad for a hard mount motor that suppose to vibrate a lot and it was an everyday rider most of its life with me now its semi retired just like me.
Whether I been lucky or the old English gentleman knew what he was doing or I just have a freak Norton, but my 850 Commando/Featherbed has been the same way I built it in 1980/82 with only a few mods upgrades in the last 15 years, but the motor is very much the same when I first built it, it's a fun bike to ride, its light, plenty of torque and get up and go and handles like it's on rails, it's not as smooth as my 2016 1200 Thruxton but it's pretty good but it's not bad for a heavy 360 swing crank motor.
I love to build an 850 Commando motor with Jim's bits and pieces, but I don't have that much money in my kitty to play with, I built my motor when things were cheap to do and so far my motor has proved it worth for being reliable and not have things fall off since I brought it new, although its semi-retired now it's still my favorite bike to ride I have so much fun on it.
4 years as a Commando 44 years as Commando/Featherbed, would I do it again, sure would and would I change anything, no, as I have a 1960 Norton project bike build waiting to be restarted.
So yes, you can get them to run smooth with crank balancing if done right and in the right hands of the balancer.

Ashley
Click to expand...
What you have said about the 72% balance factor, is pretty much what I have also found. When I first got my 850 motor, I recognised it was a strange. It seems to be an alternative concept. I did not really believe in it until I got the inlets, exhaust system, and cam timing to where they all worked with the gearing and handling. I now believe the design is very correct, but different. Usually when the overall gearing is lowered on a race bike, the bike will accelerate faster. When I tuned the 850 motor it did not seem to accelerate faster until I raised the overall gearing. It did not do what I expected from my experience with other bikes.
 
What I have also found with the Commando motor, is the power needs to be used in a different way. For me on a racetrack, the next straight begins at the start of it's preceding corner. I brake into the corner and immdeiately I am in the corner, it is full blast all the way around it with less lean. With any other bike I have ridden, I would crash if I did that. It has worried me a bit. I have never used the 6 speed box to do that, and the thought makes me a bit apprehensive. The acceleration rate through corners was already a bit quick, when passing other guys at the ends of the corners. I was watching a vid about2027 MotoGP rule changes. They mentioned the seconds saved in corners add-up. What they don't seem to realised that seconds saved in corners, mean you come out of the corner faster. It is acceleration in corners which is important. If I don't accelerate, the bike does not steer itself in the correct direction. The rear squats and that has effect through the trail, when the rake increases - addition of vectors.
I seems that historic racing is coming back to Winton, so there should be practice days.
 
My Molnar crank is balanced somewhere between 72%-75%. I'm not sure if it was done dynamically or statically, or if that is a wet or dry number. It was done by TGA. My crank is 3lbs lighter than a stock 750 crank and has long rods and short skirt pistons attached to it. Engine is in alloy plates and solid mounted.

Only significant difference I notice between the Molnar crank and the stock crank with long rods and short skirt pistons balanced to 62% is the engine spins up a lot quicker in neutral with the Molnar crank. Engine still shakes its booty at idle and lower RPM on the road. Very smooth at HWY speeds, but the shake turns into a high frequency vibration in the bars. Not a problem riding on winding roads with plenty of turns, but kind of annoying when droning home on the HWY at 70 mph after a 6-hour day in the saddle.

To me the long rods and short skirt pistons are an improvement particularly if you want to wring the engines neck, but if getting old, slowing down a lot, and riding like a gentleman of leisure, the stock parts work well enough if the crank is balanced correctly for the mount. As JS mentioned, you can't stop that Norton lump from shaking with a standard crank configuration where the pistons are going up and down in the bores side by side with alternating combustion.
 
Fast Eddie said:
I compiled a list of recommended figures from lots of knowledgable Norton folk (Inc JS, Comnoz, LCR Ken, Steve Maney, etc).

The list ranges from 72-78%. So anywhere within that range should be good.

I had a Norton cranked Triumph engine balanced to 80% some years ago, at the suggestion of Basset Down, and that worked well too.

I have had two of my Norton cranks dynamically balanced, most recently a Maney crank. Steve says dynamic balancing is not needed and indeed I have one statically balanced by him that’s very good. But on this recent occasion my balancer said it definitely needed work that only showed up dynamically. Comnoz posted a video about this once whereby a correctly (statically) balanced crank was nearly jumping out of his rig when spun up, and very smooth after dynamically balancing it. My balancing guy basically said mine was the same.

So… IMHO and FWIW, anywhere between 72-80% should work… with the rule of thumb to err on the higher % for higher rpm and vice versa….and I would definitely say to get it dynamically balanced… and use alloy engine plates.
Click to expand...
Why alloy for the engine plates recommendation ?
 
Fast Eddie said:
Absorbs vibration.
Click to expand...
Really?
I didn't know that
My 750 motor balanced to 72% may very well have been ok if hadn't made polished stainless engine plates ?
It was so long ago I can't remember all the details but I more than likely had stainless washers,bolts etc that were probably causing most of the issues but I was only 21 or 22 so I was more clueless than I am now!!!
The vibration was by far the worst I have ever had on a motorcycle
It sheared my aluminium battery box
Various bits used to shake off
It broke the silencer mounts several times
It used to shake the float bowls off
And the rocker spindle cap bolts
The aluminium tank split down the middle on the underside
It was difficult keeping your feet on the foot pegs and you physically could not rev it past 6000rpm , maybe the carbs were frothing?
It had a lightened /polished re balanced crank
Power max 10.25-1 pistons
Hemmings big valve head
Triple s cam
The crank was so light it would never idle
I wish I owned it now!!
 
From over 40 years ago
 

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baz said:
Really?
I didn't know that
My 750 motor balanced to 72% may very well have been ok if hadn't made polished stainless engine plates ?
It was so long ago I can't remember all the details but I more than likely had stainless washers,bolts etc that were probably causing most of the issues but I was only 21 or 22 so I was more clueless than I am now!!!
The vibration was by far the worst I have ever had on a motorcycle
It sheared my aluminium battery box
Various bits used to shake off
It broke the silencer mounts several times
It used to shake the float bowls off
And the rocker spindle cap bolts
The aluminium tank split down the middle on the underside
It was difficult keeping your feet on the foot pegs and you physically could not rev it past 6000rpm , maybe the carbs were frothing?
It had a lightened /polished re balanced crank
Power max 10.25-1 pistons
Hemmings big valve head
Triple s cam
The crank was so light it would never idle
I wish I owned it now!!
Click to expand...
I’ve never done back to back tests to verify it Baz, but plenty of people from Degens to Basset Down say so. I’ve always thought it made sense.
 
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baz said:
From over 40 years ago
Click to expand...
Looking at this photo now
It's no wonder the silencer brackets kept breaking
And that battery box looks way too close to bell mouths
And the cabling could use a tidy up and and and 🤣🤣🤣🤣
Hindsight is a wonderful thing
 
Are the alloy engine plates usually made in the same thickness as the standard steel domi plates?

Glen
 
Schwany said:
My Molnar crank is balanced somewhere between 72%-75%. I'm not sure if it was done dynamically or statically, or if that is a wet or dry number. It was done by TGA. My crank is 3lbs lighter than a stock 750 crank and has long rods and short skirt pistons attached to it. Engine is in alloy plates and solid mounted.

Only significant difference I notice between the Molnar crank and the stock crank with long rods and short skirt pistons balanced to 62% is the engine spins up a lot quicker in neutral with the Molnar crank. Engine still shakes its booty at idle and lower RPM on the road. Very smooth at HWY speeds, but the shake turns into a high frequency vibration in the bars. Not a problem riding on winding roads with plenty of turns, but kind of annoying when droning home on the HWY at 70 mph after a 6-hour day in the saddle.

To me the long rods and short skirt pistons are an improvement particularly if you want to wring the engines neck, but if getting old, slowing down a lot, and riding like a gentleman of leisure, the stock parts work well enough if the crank is balanced correctly for the mount. As JS mentioned, you can't stop that Norton lump from shaking with a standard crank configuration where the pistons are going up and down in the bores side by side with alternating combustion.
Click to expand...
The high speed freeway riding is about the only time I'm aware of the 650ss vibes. If I ride within the Province then an hour or more of this type of riding is required to get to the interesting roads.
I've thought about tearing the bike down to make the change to lighter pistons, but hate to tear an almost fresh good running engine apart for an expensive change that should help, but might not make a huge difference.
One day perhaps.
It would also be nice to do a dynamic crank balance . It would be ideal to make each change then try it out in order to see the effect.
As you know, that is a lot of engine work!

At 70 mph on the 650SS the bars are quite smooth, mirrors still useable.
There is a fair bit of vibration coming thru the footpegs.
There is some vibration coming thru the seat.
The seat vibration was awful until I purchased the correct Domi rubber bits that are like little isos for the seat. When the bike was rebuilt in 1999 the rebuilder made his own rubber mounts as none were available. He made a good try, but the proper parts are now available and they reduced vibration greatly vs the homemade parts.
I've noticed that the rider part of the seat still transmits far more vibration than the passenger part. I think this is because the seat foam is old and compressed in the rider area from years of use. The previous owner was an upholsterer. He made a nice leather seat cover for the bike but used the old worn out seat squab.
This is an easy area to fix. Leighton has top quality squabs.
The footpegs are a little tougher to isolate, but it can be done.

I recently purchased some of the old fashioned Amal style hand grips. This type of grip is already on the bars and they are excellent for reducing felt vibration to almost nothing.
The plan is to fill a pair of the grips with squishy Silicone then fit them to the footrests. It might work!

Glen
 
worntorn said:
The high speed freeway riding is about the only time I'm aware of the 650ss vibes. If I ride within the Province then an hour or more of this type of riding is required to get to the interesting roads.
I've thought about tearing the bike down to make the change to lighter pistons, but hate to tear an almost fresh good running engine apart for an expensive change that should help, but might not make a huge difference.
One day perhaps.
It would also be nice to do a dynamic crank balance . It would be ideal to make each change then try it out in order to see the effect.
As you know, that is a lot of engine work!

At 70 mph on the 650SS the bars are quite smooth, mirrors still useable.
There is a fair bit of vibration coming thru the footpegs.
There is some vibration coming thru the seat.
The seat vibration was awful until I purchased the correct Domi rubber bits that are like little isos for the seat. When the bike was rebuilt in 1999 the rebuilder made his own rubber mounts as none were available. He made a good try, but the proper parts are now available and they reduced vibration greatly vs the homemade parts.
I've noticed that the rider part of the seat still transmits far more vibration than the passenger part. I think this is because the seat foam is old and compressed in the rider area from years of use. The previous owner was an upholsterer. He made a nice leather seat cover for the bike but used the old worn out seat squab.
This is an easy area to fix. Leighton has top quality squabs.
The footpegs are a little tougher to isolate, but it can be done.

I recently purchased some of the old fashioned Amal style hand grips. This type of grip is already on the bars and they are excellent for reducing felt vibration to almost nothing.
The plan is to fill a pair of the grips with squishy Silicone then fit them to the footrests. It might work!

Glen
Click to expand...
Given what I've done for no good reason other than that old favorite... because I felt like it, I say leave the motor as it is. It sounds about as good as it gets. My bike was nearly as much fun to ride with the stock parts in it. It does sound different when the RPMs are near the limit, but riding there on the street is foolish, and does tend to blow head gaskets. I am desperately trying to wean myself off of doing stupid stuff. However, stupid is as stupid does and it might take a while.

Alloy plates for the engine and gearbox on a '67 P11 are 1/4" thick. I think Nigel uses 5/16" or thicker on the Seeley. Well, it looks like it in the pics anyway.
 
worntorn said:
Are the alloy engine plates usually made in the same thickness as the standard steel domi plates?

Glen
Click to expand...
Good question. The Atlas plates are 3/16" while the Commando is 1/4". Is there any reason that 1/4" alu plates would not fit in the Atlas? I would imagine alu plates would tend to wear out. Any reports from those that have alu plates?
 
worntorn said:
Are the alloy engine plates usually made in the same thickness as the standard steel domi plates?

Glen
Click to expand...
All the ones I have seen have been thicker than the standard steel plates, typically 1/4", 5/16", 6mm, and 8 mm. I think the stock steel plates were more like 3/32" (I don't have any to measure), and modern replacements in stainless steel are typically 4 mm. On my featherbed with Commando engine (with Commando tilt angle), the alloy plates are .250" thick, and they have been problem free to decades for road racing and a good bit of landspeed racing. I've used engines dynamically balanced to 62% and 72% for both road racing and landspeed racing, and never noticed any problems with excess vibration, but that's with them running mostly above 5000 rpm. I think they would have been fine as street bikes too, but never used them for that. In my ex-Jim Schmidt monoshock road racer (also with the engine tilted forward), I've used engines dynamically balanced at 70%, 72%, and 80%, and also had no vibration issues with them. My old notes do mention that the 80% was particularly good, but it's been so long ago that I don't have any specific memories left.

Ken
 
20 years ago Ken Armann balanced my Atlas crank to 65%, do not know if that was wet or dry. It runs very well in town or at freeway speeds. I rarely take it over 5000 rpm and it is very good to ride. Single 34 Mikuni and a Boyer.
It was balanced for 10-1 forged pistons which were very heavy lumps. I could never stop it from pinging so I went back to stock Commando pistons. To keep the balance It was necessary to have heavier wrist pins made. This arrangement worked very well. However, during the 20 years the heavy wrist pins got lost and I had to use the stock Commando pins. I noticed a slight difference for the worse though it is still very rideable. JS claims a key factor is reducing reciprocating weight and I discovered RGM has tapered wrist pins which are lighter than the stock ones. Also very inexpensive. My plan this winter when the weather gets bad is to have the crank rebalanced using the RGM pins. I REALLY WISH there were lightweight pistons ( just like the JS pistons ) I could use in a stock motor. Comstock has recommended MARINE CRANKSHAFT in southern California.
To sum up, before balancing, the Atlas was a vicious shaker. Balancing really tamed it.
 
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