Crank balance factor

'Some things are so bad that they are good'. I know I have an illness. Reading the posts on this topic, I know I must get the Seeley 850 going again. I have never started it at home, only ever at a race circuit, and there is a very good reason for that. When the motor is running, the imbalance moves the bike backwards and forwards when the motor is idling and you can see it. I always remember what the bike feels like when it lurches away and the motor smoothes-out and really gets going. If I start the bike, I am mentally compelled to ride it. In my situation - that will be up the street. It will probably take about 30 minutes for the cops to arrive - by then I should be back inside.

If I was going to use my Seeley 850 in the same way as a Triton 650, I would balance the crank to 80 %. The crank in my mate's Triton is balanced to that and does not ever get revved above 6,300 RPM. It is extremely fast in a straight line. It has 18 inch wheels and the motor back a bit, so it handles like a Japanese bike - lovely and neutral - OK around corners and very fast down the straights. My Seeley 850 is faster around corners and almost fast enough down the straights. In historic races, the two bikes race in different period classes. He will probably never race against me again, but he does not seem to know that. His head is still back in 1973.

I am still surprised that the Commando 850 engine is so good without much modification. It took a bit of work, to find out how to use it. The Seeley is very different from a Triton.

I use the same thickness steel plates as the Commando (5mm thick), my first set I made for the Featherbed/Commando motor was cut up Commando rear plates with welded extensions to fit the frame, a few years later I made new alloy plates out of 5mm alloy but found the motor had too much vibrations I should have made them out of 8mm alloy (I used what I had at the time), but I ended up going back too 5mm steel, really the 5mm steel weren't that much heavier than if I used 8mm alloy and besides the engine plates are down low so not make any real difference, but of course I always use a solid top steel mount.
I found that using steel handle bars the wall thickness is so thin under 2mm and when I got to those vibs area of my revs range the handle bars where bad but it smoothed out when past, but since going to Renthal alloy bars I no longer get any vibs though the handle bar throughout the rev range they are smooth as and not bad on open highway riding, my Norton loves open roads at 65mph to 90mph and will sit on those speeds all day but of course we got to sit on the speed limits, our open road speed limits are 100km and up to 110kmph, get caught doing over 40km over the speed limit here and they take your bike or car for 30 days, get caught 3x doing that and you lose your bike or car.
Here are pics of my bottom plates from the first set to what I made out of 5mm alloy, I now have the 5mm steel I made the same shape as the alloy ones I made, and a strong top engine mount.
My fuel tank is not bolted down it sit on top of the Featherbed frame rubbers, if my Norton had bad vibrations the tank would shake all over the place but it doesn't, I have a reason why my tank is not bolted or clamped down, the great fire of 83 I saved my bike by being able to remove the tank when it went up, don't run carbs without air filters, lesson learned.
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seattle##gs said:

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?

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I don't know about 1/4" plate thickness fit on an Atlas. My '67 P11 has had the same 1/4" alloy plates since 1967. They still function although they don't look brand new. Moving the gearbox back and forth has left a few marks on them. The bike did not have clocks on it when I bought it in the early '70's and may not have more than 25K total miles on it, so might not be the best wear example. If the fasteners are kept tight, the alloy plates should last a long time.

I've seen two BSAs that, at idle and on the center stand, would wag back and forth like a fish swimming in a stream. I assume that was crank imbalance.

WhenI was trying to get my 500cc Triton to handle, in corners the front was always feeling too light and indefinite. And the exhaust system had separate pipes with megaphones. It had been impossible to get anywhere near a decent lap time. I was able to move the motor forward about an inch, So I made new engine plates out of 1/4 inch steel. They were too heavy but the combination of the new plates and 2 into 1 exhaust system made the bike competitive against bikes of much larger engine capacity. The frame was a mild steel copy of a Manx, and I cracked it a few times. It is important to remove stress raisers with gussets.
When you are halfway through a corner on a race bike, you must be able to give it a big handful of throttle and know where it is going to go. The first time you ride any race bike, enter a corner slow and do that when you are in the middle - leave yourself room to recover. You will find you will be quicker everywhere. Do not wait until you are in trouble to find out what your bike is going to do, They have minds of their own.

acotrel said:

WhenI was trying to get my 500cc Triton to handle, in corners the front was always feeling too light and indefinite. And the exhaust system had separate pipes with megaphones. It had been impossible to get anywhere near a decent lap time. I was able to move the motor forward about an inch, So I made new engine plates out of 1/4 inch steel. They were too heavy but the combination of the new plates and 2 into 1 exhaust system made the bike competitive against bikes of much larger engine capacity. The frame was a mild steel copy of a Manx, and I cracked it a few times. It is important to remove stress raisers with gussets.
When you are halfway through a corner on a race bike, you must be able to give it a big handful of throttle and know where it is going to go. The first time you ride any race bike, enter a corner slow and do that when you are in the middle - leave yourself room to recover. You will find you will be quicker everywhere. Do not wait until you are in trouble to find out what your bike is going to do, They have minds of their own.

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...and the relevance to the thread topic (Balance Factor) is?

seattle##gs said:

I've seen two BSAs that, at idle and on the center stand, would wag back and forth like a fish swimming in a stream. I assume that was crank imbalance.

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A big heavy crank and a 360 swing will always make a bike wag around in your own words when on a centre stand, off the stand the front wheel will shake a little, but they put out a better note from the exhaust pipes, my 1200 Thruxton is so much smoother with the 270 swing of the crank, but the exhaust note is also a lot quieter even with a decat and open exhaust, on the Thruxton 1200 the crank is also lighter (lighter than the 1200 Bonnie's) so it spins up a lot quicker.
A Norton with a well balanced crank will run smoother (but not as smooth as a 270 swing) as well spin up a bit quicker but will still have that thump to the exhaust and sound better.
Its not only the crank that get balanced, but everything bolted to it has to weight the same, pistons, conrods etc., even my push rods got weighted and slipper bearings.

Ashley

I'm not sure it's the 270 crank that makes the Triumphs smooth. It could be the twin balancer shafts helping.
How was your 360 degree Thruxton 900?
I believe those have a single balancer shaft.
The Norton 961 has a 270 crank and a balancer shaft but still seems to be a vibratory engine, according to most testers.

Glen

Hi Glen, the old 900 Thruxton was a bit smoother and had that thump to the exhaust note but get up in the higher revs/speed you did notice the vibs a bit more but the 1200 are so much smoother no matter what revs you pull even up to the rev limiter, and of course the older British bikes never had balance shafts, well my old Norton or my old 81 650 Triumph Thunderbird didn't have them, but the Thunderbird was a smooth bike with the shorter stroke pistons and rods.
But the old 74 850 Norton's are so much different to modern bikes of today and I can't compare a modern Norton as they were way out of my price range.
I only go by what I feel between my legs when riding lol.

Ashley

When I was young, I used to ride hotted-up road bikes, mainly 650cc Triumphs. I did not race until I completed my first diploma at age 29. When I started racing, I found the amount of restraint I was forced to use, very frustrating. It stayed like that until I got the Seeley 850 right. The amount of throttle I can use and where I can use it , is really stupid. It is not me, it is the bike - flick it into corner and go - is a very convincing argument. When I do it, it feels so wrong. You need a sense of humour.

robs ss said:

...and the relevance to the thread topic (Balance Factor) is?

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What do you think cracks frames ? If you did not have isolastics on a Commando, that flimsy frame would crack everywhere. Then the whole thing comes down to weight. A featherbed frame with steel engine plates, probably weighs more than a Commando with isolastics, and steel engine plates. But a Commando would be more comfortable.

acotrel said:

When I was young, I used to ride hotted-up road bikes, mainly 650cc Triumphs. I did not race until I completed my first diploma at age 29. When I started racing, I found the amount of restraint I was forced to use, very frustrating. It stayed like that until I got the Seeley 850 right. The amount of throttle I can use and where I can use it , is really stupid. It is not me, it is the bike - flick it into corner and go - is a very convincing argument. When I do it, it feels so wrong. You need a sense of humour.

What do you think cracks frames ? If you did not have isolastics on a Commando, that flimsy frame would crack everywhere. Then the whole thing comes down to weight. A featherbed frame with steel engine plates, probably weighs more than a Commando with isolastics, and steel engine plates. But a Commando would be more comfortable.

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Balance factor?

Fast Eddie said:

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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It doesn't make sense to me. Approx. 2/3 of the vertical forces of a parallel twin transfers through the front mounts. The shear area at disposal is minimal, so a larger shear angle doesn't really come to fruition.
Alloy plates have other drawbacks - poor bearing pressure capacity (bolt holes are considered to be "bearings" for the bolts), thus plate thickness have to be substantially increased over steel plates. Thereby some or all of the weight advantage is lost. AMC increased alloy plates for the P11, but not enough. Used engine plates usually exhibit ovalized holes. Another drawback is compressibility due to the low Young's modulus. Engine/frame bolts will come loose much sooner than when using steel plates. Loose bolts amplify bearing wear of course.

Alloy plates are good for racing bikes, where the objective is weight reduction. Racing mechanics are used to keeping all bolts tensioned at all times, and they will inspect and replace plates when worn. For a road bike, I'd stay with steel plates. They will provide a much longer life, and there is less fuss about them.

- Knut

Years ago my brother worked up the math to add a balance shaft to a Commando, situated where the starter is, run off the timing side like a magneto. Don't know what happened to it, but I thought it would have worked to help tame the vibes.

milfordite said:

Years ago my brother worked up the math to add a balance shaft to a Commando, situated where the starter is, run off the timing side like a magneto. Don't know what happened to it, but I thought it would have worked to help tame the vibes.

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That's interesting

Alloy plates work fine on a street motorcycle that is maintained by a meticulous experienced mechanic. No problems with my 1/4" P11 alloy plates. Holes in my engine and gearbox plates are all still round except for that elongated hole for gearbox adjustment.

I personally don't care what anyone uses for plates. I like the alloy plates because that's what came on the P11, they work, and don't rust or need to be painted. Also much easier to drill holes in and thread if one has a reason for doing so. I would never change them to any type of steel. Not sure why anyone else would either.

What about titanium? Too brittle?

milfordite said:

Years ago my brother worked up the math to add a balance shaft to a Commando, situated where the starter is, run off the timing side like a magneto. Don't know what happened to it, but I thought it would have worked to help tame the vibes.

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Dough Hele designed a Commando with a balance mechanism. Dubbed "Jake", it resides with AN or Joe Seifert (ZFD). There was an article in "The Source" a few years ago.

A chain-driven balance shaft won't last long. The alternating torque loads will kill the drive. One or two balance shafts need a substantial gear drive, and this adds weight and complexity to the engine.

- Knut

That is interesting, when did Doug Hele do that? I know he left Norton for Triumph many years before the Commando came into existence. Did he return to Norton later on?
The other question is why would anyone bother to go to that effort when we already have (for road use) the wonderful isolastic Commando frame that sorts the vibration problem out so nicely?

Perhaps they were looking at solidly mounted race applications of the Norton parallel twin.

Glen

What we need is something to actually measure the amount of vibration and its direction at various RPMs. The closest I could come was with a polished plate attached to the motor and a sharp point to scribe the movment. I got an approx .020" oblong circle with the lightweight pistons and longer rods similar to the hand drawn image below.