Balance factors

[jseng1]

Norton balance factors can be confusing and range from 52 to 85%. Part of this wild variance problem is due to not specifying "wet" or "dry" - including or not including the weight of oil in the crank.

The best way to think about crank balance is to understand that with a 100% BF the Norton has perfect balance at TDC when the counterweight is opposite the rod/piston. But then the Norton is way out of balance when the piston is at the 1/2 way point of its stroke with the heavy counterweight sticking out to the side. So you have the choice of balancing the Norton when the rod is aligned with the bore center or at half stroke. Perfect balance at TDC eliminates the vertical shaking force. Perfect balance at 1/2 stroke eliminates horizontal shaking forces. But you can't have it both ways.

Calculating the perfect BF at the 1/2 way point of stroke is complicated. Figure it would include the bottom end of the rod as well as part of the rod that is out of line (angular) with the cylinder bore centerline. For simplicity lets assume that the rod big end weight along with 1/2 of the angular rod weight is less than the reciprocating weight (piston, pin, rod small end etc). It stands to reason that a compromise BF weight would be roughly mid point between the calculations. And thats why you hear of successfull BF factors generally in the 60 to 70% range (rubber mounts throwing in a wild card).

The norton is basically a side by side single and cannot be balanced to run smooth. The only way to smooth out the vibes is with rubber mounts or by reducing reciprocating weight (pistons and rod small ends etc) The 90 deg V twin Ducati on the other hand runs smooth because the counterbalance weights contribute to counteract the shaking forces throughout the stroke - eliminating shaking forces at TDC as well as at 1/2 stroke - 90 deg is the only twin corfiguration that provides perfect balance. But we cann't just take a bandsaw to our Nortons and rearrange the cylinders anyway we want.

So we keep messing with BFs, piston weights and rubber mounts and put up with the vibes when they are tolerable.

My own pet project (besides reducing reciprocating weight) is measuring vertical and horizontal vibration and finding a compromise between the two. I've measured it with a solid frame but not with isolastics and recommend BFs in the mid 60s percent range for solid frames (wet BF). I'd be interested if anyone would make a "scratch test" at 2000 4000 and 6000 RPM on a Commando frame with isolastics - But where on the frame as it might shake in different directions at different locations. The scratch test involves mounting a needle in a tiny tube like 1/32" ID K&S brass tubing (strapped to a weight) and tapping the needle momentarily to a polished piece of metal and then looking at the orbital shaped scratch under magnification (see scratch tool in photo). Its a difficult test but it can be achieved with a little development and enough attempts.

Also see this page:

Balance factor scratch test tool

I decided to post this as a new topic. First of all - when people suggest a BF how do they qualify it? By the sensitivity of their butt and how well they remember how a Nort vibrated one day and they how it vibrated a few days later after they changed the balance factor? The first clue is that...

www.accessnorton.com

 

[mean gene]

Because I always have questions, where would you mount this ? On "Z" plates being an area that you feel the vibration? Or somewhere flat on engine/trans mounts that is the "real" vibration? Or both?

 

[Mart UK]

I had assumed this was done for me, by the factory, in the good old British way... Roughly work it out, then get it right by testing it with different BFs, until you get the best compromise.

Higher spec cams and racing spec bikes etc. I have no experience. But, my bike (std engine 850 mk3, except +20 bore/pistons) was restored 7 years and 3,700 miles before i bought it. When I first got it, having never previously ridden a big parallel twin, nor a bike of this era, I had nothing to compare it with. It was OK, but I definitely felt my teeth chatter until somewhere between 2,500 and 3,000rpm (pulling away and around town). I read about how smooth these bikes are and scratched my head a bit. I gave it a full service and found most of the nuts and bolts you'd check after run-in were a bit below torque spec. As soon as I re-torqued the engine mounts, I was in heaven. Smooth from 2,000rpm up. I was going to check the iso vernier settings, but it handles fine, so I've left alone.

So, at risk of stating the obvious to those much more experienced than me, if you are starting to experience some vibration low down, check the engine mounts first.

 

[jseng1]

Mean Green - I think you'd want to check it somewhere on the frame. Z plates would be a good place to start. Also up near the steering head. If they're both the same then it wouldn't matter which.

 

[johnm]

I have a question.

My 850 has the JS longer rods and pistons. It was dynamically balanced to the BF recomended on your website. And also balanced left and right. The bike has CNW isolastics front and back and a Dave Tayler headsteady. I did have a low speed shimmy in the bars slowing down but that turned out to be an out of balance and slightly out of spoke alignment front wheel. The bike has had the full world's straightest commando alignment check.

The bike now has mild vibration mostly through the bars up to 2300 rpm. After that totally smooth to 6000 and above. I mostly ride in the 3 to 4 k range. So it all works fine.

But going back to the dynamic crank balance and this left and right balance.

What's that about? And even if the crank is balance end to end when it's in the bike one end has the chain or belt sprocket plus a rotor and nut while the other only the timing sprocket and oil pump drive. Very different in weight.

So what's the point of the left right balancing? Or does only the weight between the main bearings count. I realise the rotor sprockets etc are symmetrical about the crank rotation to they don't impact the vertical horizontal balance but left right I don't get.

 

[ntst8]

Hazy memory but if correct circular trace should be the outcome on iso's.
50% balance factor, for an infinitely long con rod, would share the vibes equally between horizontal and vertical.
Con rods aren't infinitely long, as a result the piston speed/acceleration is higher around TDC than around BDC. Then 52% balance factor is a real world factor which shares vibes equally horiz and vert. Hence the circular outcome.
Suspension and tyres aren't rigid so who knows what that would do to the trace.

 

[gortnipper]

I have a question.

My 850 has the JS longer rods and pistons. It was dynamically balanced to the BF recomended on your website. And also balanced left and right. The bike has CNW isolastics front and back and a Dave Tayler headsteady. I did have a low speed shimmy in the bars slowing down but that turned out to be an out of balance and slightly out of spoke alignment front wheel. The bike has had the full world's straightest commando alignment check.

The bike now has mild vibration mostly through the bars up to 2300 rpm. After that totally smooth to 6000 and above. I mostly ride in the 3 to 4 k range. So it all works fine.

But going back to the dynamic crank balance and this left and right balance.

What's that about? And even if the crank is balance end to end when it's in the bike one end has the chain or belt sprocket plus a rotor and nut while the other only the timing sprocket and oil pump drive. Very different in weight.

So what's the point of the left right balancing? Or does only the weight between the main bearings count. I realise the rotor sprockets etc are symmetrical about the crank rotation to they don't impact the vertical horizontal balance but left right I don't get.

I found this explained it to me:

Some balancers also compare the results of the left and right sides to see how the forces interact (3-plane balancing) before displaying the index location and amount of imbalance that needs to be corrected. Weight is then removed by drilling or machining the counterweights, or added by installing heavy metal tungsten (“mallory”) plugs into the counterweights to offset the indicated imbalance. The crank is then spun again to check the corrections that were made. This procedure is repeated as many times as it takes to achieve the desired degree of balance.

 

[johnm]

So the left and right ends of the crank are matched. And then any balancing changes required are made exactly in the middle.

The weight of components attached at each end don't matter because they are centred and symmetrical about the crank axis?

 

[gortnipper]

So the left and right ends of the crank are matched. And then any balancing changes required are made exactly in the middle.

The weight of components attached at each end don't matter because they are centred and symmetrical about the crank axis?

I would do a bad job explaining. Start watching at 17:00

 

[jseng1]

Side to side out of balance or "dynamic balance" is not as critical as "static balance" but still makes a difference. "Static balance" can be checked and adjusted by using knife edges and symetrical weights attached to the journals of an assembled crank.

Adjusting "dynamic balance" is more difficult but not impossible. To adjust dynamic balance - remove the flywheel. Insert the 1/2" dowel pin in the center (or use a 1/2" bolt & nut). Then rotate one cheek so the crank journals are in a 180 deg configuration (must be reasonably accurate). Then put it on knife edges and balance it so each cheek has the same weight. This will get you close to correct dynamic balance.

Frames can have a "harmonic vibration". When racing I once developed a crack at a solid frame engine mount which allowed more flex. I was amazed to see the seat vibrating up and down about 1/2" when I revved the bike up in the pits. When I discovered the crack I realized what the problem was. In this case the crack allowed the frame to flex more and amplified the vibration.

 

[johnm]

I would do a bad job explaining. Start watching at 17:00

Got it now thanks.

 

[johnm]

Side to side out of balance or "dynamic balance" is not as critical as "static balance" but still makes a difference. "Static balance" can be checked and adjusted by using knife edges and symetrical weights attached to the journals of an assembled crank.

Adjusting "dynamic balance" is more difficult but not impossible. To adjust dynamic balance - remove the flywheel. Insert the 1/2" dowel pin in the center (or use a 1/2" bolt & nut). Then rotate one cheek so the crank journals are in a 180 deg configuration (must be reasonably accurate). Then put it on knife edges and balance it so each cheek has the same weight. This will get you close to correct dynamic balance.

Frames can have a "harmonic vibration". When racing I once developed a crack at a solid frame engine mount which allowed more flex. I was amazed to see the seat vibrating up and down about 1/2" when I revved the bike up in the pits. When I discovered the crack I realized what the problem was. In this case the crack allowed the frame to flex more and amplified the vibration.

I don't know what frame you were using but old feather bed frames do have issues when used for racing. Kenny at New York Norton posted photos of his broken frame back in the past. My Domi which wasn't a very bad vibrating bike cracked the frame down by the swinging arm area several times. I have also seen the bottom rail broken on a Velo engined featherbed race bike.

I think the causes are down to age and possibly much higher loads from modern tyres and suspension when used by good riders.

 

[ggryder]

.......Calculating the perfect BF at the 1/2 way point of stroke is complicated. Figure it would include the bottom end of the rod as well as part of the rod that is out of line (angular) with the cylinder bore centerline. For simplicity lets assume that the rod big end weight along with 1/2 of the angular rod weight is less than the reciprocating weight (piston, pin, rod small end etc). It stands to reason that a compromise BF weight would be roughly mid point between the calculations. And thats why you hear of successfull BF factors generally in the 60 to 70% range (rubber mounts throwing in a wild card)......

Jim, Isn't it the case that, conventionally, shaking force in the vertical axis (assuming a vertical cylinder) is = (Reciprocating mass) x (1 - balance factor).
And shaking force in the horizontal axis is = (Reciprocating mass) x (balance factor).

So a 70% balance factor results in vertical shaking force of 30% (of the unbalanced reciprocating mass) and horizontal shaking force of 70%. While a 50% balance factor results in vertical shaking force of 50% and horizontal shaking force of 50%.

So anything other than a 50% balance factor always results in larger shaking force in one axis or the other?

That being the case, if you know nothing about the way an engine is mounted a 50% balance factor is the "safe", logical choice.

Balance factors of other than 50% are only chosen to suit particular engine mounting arrangements.

 

[HopperXL]

Or do what Phil Irving told the Brit factories they should do in the 1950s sometime and change their crankshafts to 270 degree crankpin offset, which Triumph finally got around to doing with the new generation Bonnevilles and smoothed them right out.
Classic racing guys started doing it with Nortons many years ago. Relatively easily done on a Norton with its bolted together crankshaft. Just relocate the bolt holes. Then you have to cut the camshafts, rotate to suit and weld up again.
Problem solved. Plus a glorious loping V-twin-like exhaust note to boot!
I have been threatening to do it to my Atlas for years as it slowly shakes itself to bits and cracks every piece of tinware on the bike.

 

[johnm]

Or do what Phil Irving told the Brit factories they should do in the 1950s sometime and change their crankshafts to 270 degree crankpin offset, which Triumph finally got around to doing with the new generation Bonnevilles and smoothed them right out.
Classic racing guys started doing it with Nortons many years ago. Relatively easily done on a Norton with its bolted together crankshaft. Just relocate the bolt holes. Then you have to cut the camshafts, rotate to suit and weld up again.
Problem solved. Plus a glorious loping V-twin-like exhaust note to boot!
I have been threatening to do it to my Atlas for years as it slowly shakes itself to bits and cracks every piece of tinware on the bike.

My understanding is they didn't go this route at least in the 1950s and early 60s because they were using magnetos and single carbs.

I have ridden a street Norton 600 with a 270 crank modification and that was very nice with little vibration.

However I have heard Norton twin race bikes with 270 cranks have had cam chain harmonic issues. This is hearsay only. I have no personal experience and would be interested in hearing from anyone with actual experience.

 

[robs ss]

Or do what Phil Irving told the Brit factories they should do in the 1950s sometime and change their crankshafts to 270 degree crankpin offset, which Triumph finally got around to doing with the new generation Bonnevilles and smoothed them right out.
Classic racing guys started doing it with Nortons many years ago. Relatively easily done on a Norton with its bolted together crankshaft. Just relocate the bolt holes. Then you have to cut the camshafts, rotate to suit and weld up again.
Problem solved. Plus a glorious loping V-twin-like exhaust note to boot!
I have been threatening to do it to my Atlas for years as it slowly shakes itself to bits and cracks every piece of tinware on the bike.

I prefer the brit-bike sound and feel compared to my mate's "Triumphati"
A 360 twin's vibration can be quite well managed.

 

[Matt Spencer]

Too Right .

Of Course , Non Unit Powerplants are Superior in that aspect , also .

Weight & % factor relate to what r.p.m.'s she'll be smoothest at .
If we're gunna run high r.p.m.'s , it wants to be smoothest there ,
as the forces are accereateing and suchlike more quickly .

ONE could actually do a central counter rotating planetry geared flywheel .
A scatter vsheild might be a idea at the test stage . If triumph hasnt got the
brains to do it , these days , the drawings not their property .

Dunno how many H.P. a Tret 100 is .

But thats ar elephant . Im not sure that youd do it exactly like that , either .

 

[Matt Spencer]

After All , we were still in the stone age , here in Australia , 250 years ago .

Double Acting . That should smoot it out .

In fact , theres No reason NOT to do a 120 degree Twin . A Triple minus one . Re Laverada flat plane triple ( four minus one ) Or Triumphs 90 degree triple .

 

[acotrel]

A balance factor of 100% means you have balanced 100% of the reciprocating weight when the crank is stationary. When the crank is spinning the rods and pistons have inertia, and so does the counter weight in the flywheel. With a balance factor of 54% ,the rods and pistons, and the counterweight are in balance at about 4,000 RPM. If you want to spin the motor higher, you need a higher baance factor, or you start shagging crankcases The crank in my 850 is halanced at 72% - at 7000 RPM it is dead smooth running, but at 4000 RPM and lower the motor shakes. The piece of steel I have inserted into the hole in the counterweight, weighs about 500 gram.
When the Commando was made, Norton were freaked out by the smooth running CB750 Honda. So they did all the bullshit -it was all going in the wrong direction for performance. But an 850 Commando is still quick enough for the average commuter guy. Most of them would probably kill themselves with a 650 Triumph Bonneville, if they tried to use it in anger. The Commando is the next generation motorcycle. If it is set up right it can be quicker than a Z900 Kawasaki, but why would you do that with a road bike, when you do not rev them high ?
When I rebalance my crank, I was about to drill the flywheel on the opposite side to the counterweight. I thought about what I was doing and stopped. It is bad enough heving the bit of steel threaded into the hole with locktite with the ends of the thread punched. If it comes out during a race, I have lost a motor.
I think Norton were bloody idiots to do that to the crank to get smooth running. I don't think they knew whether they were Arthur or Martha. An Atlas is a very nice bike, if you like being turned on when you ride. The Commando was inteded for the duffle-coat brigade.
However, that being said - I watched an A grade rider win an Allpowers Race with an unmodified 850 Commando in about 1974. So what they were was not all bad.

 

[robs ss]

A balance factor of 100% means you have balanced 100% of the reciprocating weight when the crank is stationary. When the crank is spinning the rods and pistons have inertia, and so does the counter weight in the flywheel. With a balance factor of 54% ,the rods and pistons, and the counterweight are in balance at about 4,000 RPM. If you want to spin the motor higher, you need a higher baance factor, or you start shagging crankcases The crank in my 850 is halanced at 72% - at 7000 RPM it is dead smooth running, but at 4000 RPM and lower the motor shakes. The piece of steel I have inserted into the hole in the counterweight, weighs about 500 gram.
When the Commando was made, Norton were freaked out by the smooth running CB750 Honda. So they did all the bullshit -it was all going in the wrong direction for performance. But an 850 Commando is still quick enough for the average commuter guy. Most of them would probably kill themselves with a 650 Triumph Bonneville, if they tried to use it in anger. The Commando is the next generation motorcycle. If it is set up right it can be quicker than a Z900 Kawasaki, but why would you do that with a road bike, when you do not rev them high ?
When I rebalance my crank, I was about to drill the flywheel on the opposite side to the counterweight. I thought about what I was doing and stopped. It is bad enough heving the bit of steel threaded into the hole with locktite with the ends of the thread punched. If it comes out during a race, I have lost a motor.
I think Norton were bloody idiots to do that to the crank to get smooth running. I don't think they knew whether they were Arthur or Martha. An Atlas is a very nice bike, if you like being turned on when you ride. The Commando was inteded for the duffle-coat brigade.
However, that being said - I watched an A grade rider win an Allpowers Race with an unmodified 850 Commando in about 1974. So what they were was not all bad.

...and your point is?