Crank balance factor

My 920 has the RGM kit. For those kits Roger sourced pistons that weigh almost the same as stock standard size 850 pistons. Surpringly, the steel skirt 920 pistons are a few grams lighter than stock 850.
Some earlier 920 kit pistons were quite a bit heavier than the stock 850 pistons.
Roger thought that it was important to keep the weights the same as this would keep the balance factor at the factory ideal number.

I had the crank dynamically balanced to 62% wet.
The bike is very smooth above 2800 rpm.
On first go round I totally forgot to assemble the isos with grease. I noticed the mistake at some point but carried on thinking that perhaps the grease wasn't really needed. On the first ride around the block the bike shook like a big bore Atlas.
I took the isos apart and coated things in the special rubber grease.
It's been a very smooth runner since then.
So, although I'm a believer in dynamic balance, it obviously wasn't enough to quell vibes for a solid mounting of my 920 engine, which is almost what I had without the isos greased up. I suspect full solid mounting of that engine would be even more vibratory. The isos should work to a degree without the grease, but it sure didn't feel like they were doing much.


Glen

lcrken said:

Actually, only one of the data entries, the first one stating the stock Commando BF as 52% dry, is based on possibly wrong factory info. I got that one from a factory service manual. All the other entries are from other sources, not based on factory info, with many from my own measurements. It is not a recommendation, simply a compilation of other peoples recommendations, as well as measured data from various crankshafts I have used.

Ken

Click to expand...

Apologies Ken, I was back handed. My comment was in response to Commando/Isolastic road bike stock crank b.f. figures. I have altered my posting.

- Knut

From the original post by Baz it was all about ridged mounted Commando motor in a WL Featherbed frame, we all know that the isoelastic made a big improvement to the later Commandos, but getting a hard mount Commando motor set up right in a Featherbed frame is completely different to a Commando frame set up, the Featherbed frame is designed to be all part of solid mounting, its making the motor, GB and everything part of the frame, you can't just throw in a Commando motor into a Featherbed without trying to make the motor and everything connected to the motor to run smoother.
A 650 Domie motor seems to run a lot smoother than a 750 Atlas motor and the earlier smaller motors ran smoother in the WL Featherbed frames, seems the bigger the motor the more vibrations, whether the cranks have gotten heavier or everything else as well.
I have ridden many Featherbed framed bikes from 650 Domies to 750 Atlas with stock motors but found the 750 Atlas to be the worst for vibrations, but I can honestly say with the work my 850 motor has had done and built for the Featherbed frame it's so much smoother than a stock 750 Atlas by far, I have done a lot of all day rides in the saddle and still feel good after as well a few straight 600 mile runs without any problems at all.
Whether I have just been plan lucky or I found someone who got the BF right with my crank when I built it all them years ago, reading back looks like so many haven't got it right including Jim who has a lot of knowledge and understanding, but Jim's bike is not a Commando motor in a Featherbed frame and of course each bike/build is different to the next one.
I have no answer to why mine is so good, yes its not perfect but the little vibs it does get is all in the right places like any old British motorcycle and I don't get things falling off and my head not shaking from the BF being way out and I don't get sore hands when riding, I never wear gloves except when its real cold of course, I like to feel my bikes when riding, I don't get any vibration through my foot pegs at all and my motor is tilted as far forward as it can, would I have put up with bad vibrations for 44 years, no way and my Commando/Featherbed was an everyday rider for most of those 44 years on the road since I built it and even my wife enjoyed being on the back, but it does have a comfortable duel interstate seat when she use to come out on it, but now the Norton is solo rego and the Thruxton is duel rego for her outings now.
As I have said many times over I haven't had the problem like most have said they have with hard mounted Commando motors in a Featherbed frame and I wish I had the answers, all I know is mine works so well for me, lucky or freak of a bike who knows, but it's not a stock motor with a full open exhaust.
You can tell in the pics it's a well ridden bike and not much has changed in the 44 years I built it except for a few upgrades.

Ashley

I went through a long thought process about using a featherbed frame, Seeley, etc for a Commando build.

I concluded Seeley, which is obviously rigid mounted, BUT it is only for track use, so prolonged riding at high vibration inducing cruise speeds is a none issue.

Having looked closely at a very well built featherlastic, my own personal conclusion was that there is no point. It seems to me that you lose the advantages of the stiffer featherbed design, and the lightness of, well, both (a featherbed mounted with ISO’s and cradle will definitely be heavier than a Commando frame, and offer no benefit other than style).

So, I concluded that the best approach is a solid mounted featherbed or Seeley (etc) with care taken to balance the motor and protected against unavoidable vibes, OR use isolastics… and the best frame to use to house an isolastic mounted Commando engine is… a Commando frame !

But that’s just me and my own brand of perverse logic !

Fast Eddie said:

I went through a long thought process about using a featherbed frame, Seeley, etc for a Commando build.

I concluded Seeley, which is obviously rigid mounted, BUT it is only for track use, so prolonged riding at high vibration inducing cruise speeds is a none issue.

Having looked closely at a very well built featherlastic, my own personal conclusion was that there is no point. It seems to me that you lose the advantages of the stiffer featherbed design, and the lightness of, well, both (a featherbed mounted with ISO’s and cradle will definitely be heavier than a Commando frame, and offer no benefit other than style).

So, I concluded that the best approach is a solid mounted featherbed or Seeley (etc) with care taken to balance the motor and protected against unavoidable vibes, OR use isolastics… and the best frame to use to house an isolastic mounted Commando engine is… a Commando frame !

But that’s just me and my own brand of perverse logic !

Click to expand...

I agree
I don't really understand the featherlastic apart the classic cafe racer aesthetics
If you don't use a solid mount engine in a featherbed then you lose the engines rigidity in stopping the headstock floating around
I've seen a few featherbeds with an additional bracing tube from the headstock to the top cross brace tube
But the rest of the frame is left wide open
And we know the importance of connecting the headstock to the swinging arm pivot

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...

So we have alloy plates at back and rusty steel ones at the front
And an oversized chain that's skimming the tyre

Fast Eddie said:

I went through a long thought process about using a featherbed frame, Seeley, etc for a Commando build.

I concluded Seeley, which is obviously rigid mounted, BUT it is only for track use, so prolonged riding at high vibration inducing cruise speeds is a none issue.

Having looked closely at a very well built featherlastic, my own personal conclusion was that there is no point. It seems to me that you lose the advantages of the stiffer featherbed design, and the lightness of, well, both (a featherbed mounted with ISO’s and cradle will definitely be heavier than a Commando frame, and offer no benefit other than style).

So, I concluded that the best approach is a solid mounted featherbed or Seeley (etc) with care taken to balance the motor and protected against unavoidable vibes, OR use isolastics… and the best frame to use to house an isolastic mounted Commando engine is… a Commando frame !

But that’s just me and my own brand of perverse logic !

Click to expand...

Fast Eddie said:

I went through a long thought process about using a featherbed frame, Seeley, etc for a Commando build.

I concluded Seeley, which is obviously rigid mounted, BUT it is only for track use, so prolonged riding at high vibration inducing cruise speeds is a none issue.

Having looked closely at a very well built featherlastic, my own personal conclusion was that there is no point. It seems to me that you lose the advantages of the stiffer featherbed design, and the lightness of, well, both (a featherbed mounted with ISO’s and cradle will definitely be heavier than a Commando frame, and offer no benefit other than style).

So, I concluded that the best approach is a solid mounted featherbed or Seeley (etc) with care taken to balance the motor and protected against unavoidable vibes, OR use isolastics… and the best frame to use to house an isolastic mounted Commando engine is… a Commando frame !

But that’s just me and my own brand of perverse logic !

Click to expand...

The Commando frame is a simple but clever solution to a problem that plagued the entire motorcycle industry right up to that point. Some bikes were worse than others but they pretty much all vibrated more than a Commando.
The Honda and Yamaha twins that I rode back then were awful for vibrations, but we were young and tolerated it.
Harley Davidsons vibrated so much that Harley just proclaimed that their buyers preferred motorcycles which vibrated.

Glen

worntorn said:

The Commando frame is a simple but clever solution to a problem that plagued the entire motorcycle industry right up to that point. Some bikes were worse than others but they pretty much all vibrated more than a Commando.
The Honda and Yamaha twins that I rode back then were awful for vibrations, but we were young and tolerated it.
Harley Davidsons vibrated so much that Harley just proclaimed that their buyers preferred motorcycles which vibrated.

Glen

Click to expand...

I would love to have been a fly on the wall during those early discussions…

“Why don’t we rubber mount that engine?”

“Because it will cause havoc with the chain tension and alignment you wanker”

“Well why don’t we rubber mount that as well then you muppet? You know, rubber mount the entire power train as one unit?”

“We can’t do that you dick because… errr… well… yeah actually, why don’t we do that”?!

I suggest it is impossible to balance a Commando crank so that is good for both race and road use. If you want to do both, you probably end-up with a compromise. The reason I do not ride a motorcycle on public roads is a motorcycle is not worth riding unless you fang it, and I like staying alive. Riding on a race track might look the same, however it is completely different from riding on public roads. It is actually much safer, except for the first time you do it. I am never afraid of crashing in a race, but on public roads on a motorcycle, I am always extremely nervous.
The other day I watched a video about beginners getting into bends too hot, on public roads and running wide. They suggested counter-steering. Everytime I raced, I always managed to get into a corner too hot. I would not counter-steer - that is not the way out of the predicament. You need to trail-brake until you lose enough speed, then accelerate when you think the tyres will stick. All bikes self-steer when you accelerate and most will go in the correct direction. You usually know the speed at which the tyres will slip and you need to be patient and wait until you get below it. Psychologically you always get time dilation. Your brain speeds up and everything happens slower. It usually takes me 5 laps of a race track before I am fast enough in my head.

acotrel said:

I suggest it is impossible to balance a Commando crank so that is good for both race and road use. If you want to do both, you probably end-up with a compromise. The reason I do not ride a motorcycle on public roads is a motorcycle is not worth riding unless you fang it, and I like staying alive.

Click to expand...

With drum brakes?

acotrel said:

I suggest it is impossible to balance a Commando crank so that is good for both race and road use. If you want to do both, you probably end-up with a compromise.

Click to expand...

Traditional balancing is a compromise in both cases. It can be performed for minimum vibration in the 3000-4000 rpm range, or in the 6000-7000 rpm range, for instance; in both cases, vibrations will be higher outside these ranges, and sometimes intolerable, so the rider has to sweep through it.
Complete balancing is possible by adding two counterbalance shafts only. Norton 961 has a single gear-driven counterbalance shaft, and vibrations aren't completely tamed still.

What do you mean by "fang it"?

- Knut

acotrel said:

I suggest it is impossible to balance a Commando crank so that is good for both race and road use. If you want to do both, you probably end-up with a compromise. The reason I do not ride a motorcycle on public roads is a motorcycle is not worth riding unless you fang it, and I like staying alive. Riding on a race track might look the same, however it is completely different from riding on public roads. It is actually much safer, except for the first time you do it. I am never afraid of crashing in a race, but on public roads on a motorcycle, I am always extremely nervous.
The other day I watched a video about beginners getting into bends too hot, on public roads and running wide. They suggested counter-steering. Everytime I raced, I always managed to get into a corner too hot. I would not counter-steer - that is not the way out of the predicament. You need to trail-brake until you lose enough speed, then accelerate when you think the tyres will stick. All bikes self-steer when you accelerate and most will go in the correct direction. You usually know the speed at which the tyres will slip and you need to be patient and wait until you get below it. Psychologically you always get time dilation. Your brain speeds up and everything happens slower. It usually takes me 5 laps of a race track before I am fast enough in my head.

Click to expand...

Who's talking about racing?

mdt-son said:

Norton 961 has a single gear-driven counterbalance shaft, and vibrations aren't completely tamed still.

Click to expand...

That is something of an understatement !

mdt-son said:

What do you mean by "fang it"?

Click to expand...

Cane it.

Thrash it.

Spank it.

Ride it like you stole it.

Etc !

mdt-son said:

Norton 961 has a single gear-driven counterbalance shaft, and vibrations aren't completely tamed still.

- Knut

Click to expand...

Tradition is important. Shake Ya Bootie

Was the Wankel 588 rotary engine as close as Norton ever got with a bike that didn't shake? That and Ash's bike.

Any member (not Guy Martin) on this site ever ride a Norton 588 rotary engine powered race bike?

Darn that's all irrelevant.

I've often heard riders of old vibratory bikes say " It vibrates quite a bit at such and such rpm then smoothens right out above that as it's balanced for high rpm" or words to that effect.
I've never noticed that effect on any of the bikes I've ridden, other than the Commando which needs a bit of rpm to make the rubber isos work.
With all of the solidly mounted bikes I've ridden, the vibrations just got worse and worse with rpm.
Jim Comstock explained that this is how a crank vibrates- there is no sweet spot for some specific rpm. The vibrations just increase with rpm.
So when someone tells you that their solidly mounted engine is a little shakey at 4000 but smooth as glass at 7000- this isn't possible.
It could be that the vibes are so bad at 7000 that numbness has set it.

Glen

worntorn said:

I've often heard riders of old vibratory bikes say " It vibrates quite a bit at such and such rpm then smoothens right out above that as it's balanced for high rpm" or words to that effect.
I've never noticed that effect on any of the bikes I've ridden, other than the Commando which needs a bit of rpm to make the rubber isos work.
With all of the solidly mounted bikes I've ridden, the vibrations just got worse and worse with rpm.
Jim Comstock explained that this is how a crank vibrates- there is no sweet spot for some specific rpm. The vibrations just increase with rpm.
So when someone tells you that their solidly mounted engine is a little shakey at 4000 but smooth as glass at 7000- this isn't possible.
It could be that the vibes are so bad at 7000 that numbness has set it.

Glen

Click to expand...

You need to ride the bike. Mine is dead smooth at 7000 RPM. If it wasn't, I would be worried. Most of the old British race bikes were like that - not much power under 4000 RPM and rough, but smooth over 4000 RPM - particularly if they are long-stroke. My 500cc short stroke Triton used to vibrate, but it revved to 10,000 RPM. The Seeley 850 is a joke. It smoothes-out at 4000 RPM and is dead smooth and tractable above that It does not vibrate below 4000 RPM - what you get is very strong pulses. For a long time now I have wanted to make an on-board video. Sooner or later Iwill get around to doing that.
I would suggest one thing, I do not believe many guys would do what I did to balance the crank. There is a hole in the counterweight whichis there to lower the balance factor. I have a threaded steel plug in there which has been glued with blue Loctite. If it ever comes loose, the motor is finished. Effectively,the crank is now an Atlas crank. - BF=72%. An Atlas mighthave vibrated to much for road use - always below 4000 RPM.

I sometimes wonder how much racing some people have actually done. The theory is the best motor in the best frame creatres the best bike. However, it does not actually work like that. A 1000cc VIncent motor in a featherbed frame is probably the worst bike, even when it has the best rider. A genuine Manx with someone such as Tom Phillis riding is extremely difficult to beat. And I am not talking about two-strokes - that is a different game. I would not race on the IOM and particularly with a two-stroke. I sold mine to a kid who won 28 races and 4 championships with it. I like knowing in advance when I am going to crash.

acotrel said:

I sometimes wonder how much racing some people have actually done. The theory is the best motor in the best frame creatres the best bike. However, it does not actually work like that. A 1000cc VIncent motor in a featherbed frame is probably the worst bike, even when it has the best rider. A genuine Manx with someone such as Tom Phillis riding is extremely difficult to beat. And I am not talking about two-strokes - that is a different game. I would not race on the IOM and particularly with a two-stroke. I sold mine to a kid who won 28 races and 4 championships with it. I like knowing in advance when I am going to crash.

Click to expand...

You are the only one taking about racing

acotrel said:

You need to ride the bike. Mine is dead smooth at 7000 RPM. If it wasn't, I would be worried. Most of the old British race bikes were like that - not much power under 4000 RPM and rough, but smooth over 4000 RPM - particularly if they are long-stroke. My 500cc short stroke Triton used to vibrate, but it revved to 10,000 RPM. The Seeley 850 is a joke. It smoothes-out at 4000 RPM and is dead smooth and tractable above that It does not vibrate below 4000 RPM - what you get is very strong pulses. For a long time now I have wanted to make an on-board video. Sooner or later Iwill get around to doing that.
I would suggest one thing, I do not believe many guys would do what I did to balance the crank. There is a hole in the counterweight whichis there to lower the balance factor. I have a threaded steel plug in there which has been glued with blue Loctite. If it ever comes loose, the motor is finished. Effectively,the crank is now an Atlas crank. - BF=72%. An Atlas mighthave vibrated to much for road use - always below 4000 RPM.

Click to expand...

On this subject I will go with the info from the guy who has a Dynamic crank balancer in his shop- and my own experience.