Author Topic: engine balance  (Read 716 times)

Online RDfella

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engine balance
« on: 19.02. 2019 11:00 »
Having just posted an update on the frame registration concerns I had, I was reminded about the engine I will be using and how the engine balance method I used could help those with A10 vibration issues. We hear about 54% and 58% balance factors, but every alteration requires a complete engine strip. Well, not necessarily. Here’s the trick: remove sump plate. Turn engine until pistons are at bottom. Now the flywheel bolt should be in the centre of the crankcase hole. Mark a spot about one inch infront of that bolt and another one inch behind. Mark two more, about one inch infront and behind the first two marks. Now drill those marks 9mm dia to a depth of 12mm. Now open those holes to 9.9mm (10mm will do) and tap to 7/16 UNF. Now make up plugs from 7/16 HT bolts to fit those holes, such that they do not protrude (cut a decent screwdriver slot across them before fitting). Now you have balance weights you can fit / remove until you reach the best balance compromise to suit the rev range you prefer – all without stripping the engine.
Below is an example from the crankshaft I made for my vee twin project. Crank is nitrided EN40B and the flywheels – drilled and tapped for balance weights – are mild steel. When you have an engine where the balance factor can at best only be a guess, it would be tiresome to have to strip the engine every time a different factor was tried.
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Online Greybeard

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Re: engine balance
« Reply #1 on: 20.02. 2019 17:15 »
Lovely work!  *good3*
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Online Triton Thrasher

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Re: engine balance
« Reply #2 on: 20.02. 2019 18:09 »
You need to be sure they’ll not come loose.

At least one of the old factories did tests in a similar way, with tungsten plugs.

I arbitrarily increased the balance factor on my 650 twin of another popular make, by enlarging a hole at the big ends’ side of the flywheel with a drill poked in at the sump plate.  It may have reduced vibration a bit.
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Online muskrat

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Re: engine balance
« Reply #3 on: 20.02. 2019 19:20 »
G'day RD.
Nice work mate.
I agree balancing acts are time consuming. I'll have to consider that method next time the Cafe is down. I have to lower her bf as she walks backwards out the door when on the center stand when the throttle is blipped.
Cheers
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Offline mikeb

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Re: engine balance
« Reply #4 on: 21.02. 2019 02:11 »
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I have to lower her bf as she walks backwards out the door when on the center stand when the throttle is blipped.
can you explain that comment Musky - mine walks backwards too
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Online WozzA

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Re: engine balance
« Reply #5 on: 21.02. 2019 04:45 »
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I have to lower her bf as she walks backwards out the door when on the center stand when the throttle is blipped.
can you explain that comment Musky - mine walks backwards too
I imagine Musky's the same as mine...   *doh*
When I give the throttle a quick squirt the bike moved backwards on the centre stand..
Why it does it is  *dunno *  *conf2*
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Offline mikeb

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Re: engine balance
« Reply #6 on: 21.02. 2019 07:38 »
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Why it does it is  *dunno *
yeah that's the question, and how does crankshaft balancing influence that?  *conf*
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Offline duTch

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Re: engine balance
« Reply #7 on: 21.02. 2019 08:46 »

  Probably better to go backwards than forwards *eek*... mine goes backwards too, but  generally don't put it on the centre stand for dynamic things
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Offline cyclobutch

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Re: engine balance
« Reply #8 on: 21.02. 2019 09:07 »
... but  generally don't put it on the centre stand for dynamic things

Generally I just don't put mine on the main stand at all.
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Online JulianS

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Re: engine balance
« Reply #9 on: 21.02. 2019 09:10 »
Balancing for the very very brave - bulletin from Hap Alzina in 1959.

Not a method I would try!!
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Online Greybeard

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Re: engine balance
« Reply #10 on: 21.02. 2019 09:33 »
Balancing for the very very brave - bulletin from Hap Alzina in 1959.

Not a method I would try!!
Flippin heck! We have to take into account that new crankshafts were available then, for sensible money.
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Online berger

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Re: engine balance
« Reply #11 on: 21.02. 2019 15:11 »
mine dances backwards too it always has on the centre stand when I rev it , but it flys a lot better going forward when I resume to nutter on a beezer race mode *grins* forgot to say front wheel and forks bounce on tick over also. I have seen many that don't have tick over wheel bounce but maybe those that are without tick over bounce have vibes at revs where mine doesn't --- I don't know --not ridden another one.
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Offline trevinoz

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Re: engine balance
« Reply #12 on: 21.02. 2019 20:15 »
It's all very well to talk about drilling holes in the flywheel but to do the job properly, i.e. dynamically, the crank has to be put onto a balancing machine.
Metal is not just taken from the centre line of the flywheel but also from the bob weights.
Static balancing is better than no balancing but dynamic is better again.
Just my opinion for what it's worth.
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Online RichardL

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Re: engine balance
« Reply #13 on: 21.02. 2019 21:15 »
Balancing for the very very brave - bulletin from Hap Alzina in 1959.

Not a method I would try!!

Julian's service document seems to take very lightly, "Pull the engine. Drill some holes. Thoroughly clean out a bunch of swarf (not, "Swarfy") that's been distributed in, and stuck to the sides of, the oily crankcase. Put the engine back in. Test ride and repeat a couple of times." If you were to use the 'drill from below method' described maybe the whole thing is worth while. I could see it being easier to set up a bike inversion stand than going the engine removal route. All this said, I'll just be leaving the A7 to the machine shop for dynamic balancing (if I ever get around to it).

I'm still amazed how Julian comes through with the obscure documents.

Richard L.

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Offline mikeb

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Re: engine balance
« Reply #14 on: 21.02. 2019 21:18 »
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I'm still amazed how Julian comes through with the obscure documents.
me too. i suspect dark powers. its awesome
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