Theoretical crank balance question

jseng1 said:
Please - if you're going to quote balance factors then state if its wet or dry balance. Otherwize the numbers are misleading. I worked in a BSA shop long ago and the prefered balance factor was 66% wet. That turns out to about 72% dry. After all these years that's still correct. But the BSA A65 buzzes badly simply because of its heavy high domed pistons. I traveled on one once and after an 11 ride the skin on my right thumb (throttle hand) actually split open from the constant shaking. I had to sit on a foam rubber pad to mellow out the seat vibes.
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When you fill the hole in the 858 Commando crank with steel, the balance factor comes out at about 72% dry. When I was balaxcing my crank and discoverd that, I did not go and further. Between 5,500 RPM to 7000 RPM, it is beautiful. When I ride the bike, the revs are always between those two numbers. With the close box, it is magic.
Commandos are designed for normal people on public roads. I am amazed at how good the 850 motor is for road racing when the crank is rebalanced. I think the Atlas must have been very good. But it could never compete with a CB750 Honda for smooth running. Is that what an experienced rider would want from an Atlas ? I do not think so.
If I never had to slow down, my Seeley 850 would make a lovely road bike. But the reality is that if I tried to ride it on public roads, it would probably jump into the back of a car.
I kid you not - on a race circuit, it is a really lovely ride. When my old mate rode it, as he came back into the pits he had a great big smile on his face.
 
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If somebody buys a 961 and tries to develop it for road racing, when they are changing cams, they might have a problem due to the unusual crankshaft. The lobes on the cam for the two cylinders cannot be at 180 degrees. With a normal cam in a twin cylinder motor, if there is an error in the cam timings when you conpare both cylinders, you usually split the difference.
I like freedom of choice.
 
I quoted balance factor numbers from the text description on the TGA website for the cranks he has manufactured and sells. I'd have to ask him if those are wet or dry numbers. Gut feeling is they are the resulting wet numbers. That said I'm fairly certain everything I say has the potential to be misleading. :)

jseng1 said:
Please - if you're going to quote balance factors then state if its wet or dry balance. Otherwize the numbers are misleading.
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Aside from the Commando vs Atlas frame discussion, I have not noticed any comments on the balance factor needs of other frames. I built 2 Norton 750s; one went into an Atlas and the other into a Matchless frame. Same balance factor. The Matchless was quite plesant to ride at all speeds and the Atlas was harsh, at all speeds. When the Atlas blew a piston I rebalanced it for 65MPH and that is a nice sweet spot, though a bit brisk for me now. It's still harsh at around 3,000 RPM. I wrote down the balance factors in the build sheets but I'm away from them just now. Sold the G-15. Wish I hadn't.
 
For anyone interested, this is a list I've made up over the years, showing different recommendations for balance factors for Commando twins. It also includes weights and some info for crankshafts I've used. I've posted it before, but this time I've updated it to show whether the numbers are wet or dry balancing values, in cases where I have that info. For the rest of them, I don't have that info. The balancers I've used always add a bit for the weight of the oil in the big ends when calculating BFs, so I would assume that most of the other values in the table are also wet, but I can't be sure of that. I've been collecting this info for just over 50 years, and by now I don't remember much of the history of this data, other than what I've already included.

Theoretical crank balance question


Ken
 
If you had asked me what my Steve Maney 80.4mm short stroke crank was balanced to, I would have said 78% using piston and rod data from JSM for +0.020 pistons.

I wouldn't know if that was wet or dry!

Ken's chart bears that out, pretty much, but still doesn't confirm wet or dry.
 
SteveA said:
If you had asked me what my Steve Maney 80.4mm short stroke crank was balanced to, I would have said 78% using piston and rod data from JSM for +0.020 pistons.

I wouldn't know if that was wet or dry!

Ken's chart bears that out, pretty much, but still doesn't confirm wet or dry.
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I know it’s a bit subjective Steve but how does that feel to you ?
 
Fast Eddie said:
I know it’s a bit subjective Steve but how does that feel to you ?
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Subjective.....do add JSM long, long rods and pistons so reduced reciprocal weight.

When I first built the bike in '76 Owen Greenwood balanced it. He said if it vibrates, note the rpm and bring it back. I never did. But it did break the odd bracket and exhaust. And I preferred thick Tomaselli natural rubber grips

I rode a Seeley with a Maney engine, nothing dramatic, a little bit of vibration but nothing to worry about, thick rubber grips.

I rode a Rickman with a Commando engine straight out of an isolastic frame. My wrists swelled, my vision was affected, it was difficult to hold it flat out, honestly I couldn't believe how it vibrated. Oh, and bits fell off.

I rode mine. No apparent vibration whatsoever! Smoothest 360 degree twin I've ridden in a rigid mounted frame! Really not far from a Commando! I use thin Renthall grips. Very little by the way of broken parts due to vibration, cracked belt cover, cracked head steady lug. I moved to a little rubber in the head steady.

No way would I mess with it.
 
Regardless of frame type the balance factor probably depends on how high you intend to rev the motor. Reciprocating weight, the cam profile and the valve train affect that. A low balance factor might be good if you never need to rev the motor above 5000 RPM.
When I use my 850, it is usually revving between 5000 RPM and 7000 RPM. A 650 Triumph can rev safely to 8000 RPM.
If you use bigger valves, you are more likely to have valve float. If you have bigger pistons, they can weigh more.
I did three things to my 850 motor - I rebalanced the crank by screwing a steel plug into the counterweight. I gave the cam a mild racing grind. I reshaped the entrances of the 30mm inlet ports to match up to 34mm Amal carbs.
I use methanol fuel which hides up my tuning errors, and stoprs the heat problem.
My motor is fast enough to be competitve, but the Mk3 Seeley frame makes a big difference. It took me a long time to figure out how to use it to best advantage.
'Mick Hemmings recommendation for 750 with isolastics - 74' ! - It probably revs high, but has lighter pistons ?
 
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