E: Engine Crankshaft Balance Method

Pushrod Twin

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Terry's cranks are pretty much set at 60%........... The balance of a crank in a stock frame should be near identical to the Egli frame on account of how it's mountings are the same......... When the engine is in a partial or full loop frame, that is different....... well going off the run of series "A" twins I have done recently.......and building 2 Norvin's, a single (my racer) and a 1200 Twin as we speak.
Greg, I assume that is 60% using the pistons Terry offers? I have one of his "standard" cranks, standard stroke & 1" main spindle. I foolishly forgot to ask what piston weights he recommends & used the +.020 Kempaloids which I had used with my factory crank. I regret not spending some time doing some balance calcs. My bike is noticeably more vibey than with the Vincent crank & Alpha BE. I suspect my pistons are lighter than Terry might use.
 

greg brillus

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I think the piston weight variance can be quite large, most Vincent pistons should be around the 450-460 gram mark. I have checked the weight of stock 84 mm Omega's and similar against the 92 mm version used in Terry's top end kits and they are within a couple of grams, so i'm not sure how to advise on that one.
 

Little Honda

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Years ago, I rebuilt a DINO 246 engine. The works figures of Ferrari allowed differences up to 3,5gr per piston
as std. With Vincent pistons I found the greatest differences in weight with the gudeon pins. Lightest ones
were those with conical inner ends in combination with circular rings without ears. Those with ears had to be
used with the heavier pins with cylindrical inner bore. In one case I turned two heavier pins conical on the
lathe to match the weight of the original pistons with the new ones. By this way, I achieved the same weight
within 1 gr to std.
 

Little Honda

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I can only compare my first std. Rapide, which was smooth up to 4000rpm and had some vibs over 5000rpm,
using MKIII cams. It reached max. 5.800 rpm at 110mph. I had the crank done by Tony Maughan, who´s
balance factor I did not know. When racing Ernst´s BL, it was different: It was dead smooth up to 6500rpm
and pulled much stronger than the Rapide, no comparison. I think, Ernst told me once, he used 55% balance
factor, but I am not sure. His BL was the only twin, which did not move backwards on the paddock stand,
when revving up to 6500rpm. My Norvin, using a D-Shadow engine with an original crank, does vibrate more
than the BL, but is also smoother, than the Rapide, using MKII cams, 8:1cr, 32mm inlets, double ignition.
My impression: For speeds over 100mph, a higher balance factor is recommendable.
Also Erich Kruse´s Comet with Ernst´s short stroke engine revs up to 8500rpm is quite smooth, but I do not
know more abt its engine. It is stronger than Ernst´s 1948 racing Comet with normal crank and max revs
up to 6200rpm. Erich´s engine feels a bit less torquey under 4500rpm, but stronger, exceeding 5000rpm, than
the older Comet. Erich uses a 5 speed box, which compensates the lower torque, compared to the 4 speed
box of the 1948 Comet racer.
 

Cyborg

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A couple of years ago I asked Terry and he replied “ My cranks are balanced to 60% so they will accept a wide range of piston.”
 

BigEd

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I put new + 0.020" Omega 8:1 pistons in my Rapide last year. They weighed 450.38 grammes each complete with pin, circlips and rings. I had it bored with 2½ thou clearance. It ran fine after a short running-in period and I've two or three thousand miles since including a bit of "spirited" riding.:)

I put together an Excel spreadsheet to calculate what the balance figures would be. I can't upload it to the forum as the extension is not allowed. The link below is to a Google Drive account. If it works it may display the sheet and you may be able to download it and open it in MS Excel. If you have a Gmail account you should be able to open it online using Open With Google Sheets. If anyone has been able to access this could they confirm it? If you can use Excel please tell me if my formulas are wrong. I have left some sample figure in place to play with.
https://drive.google.com/file/d/1pZ9XRJmmoTOnahJpSb1z_VwWQ3_NXmU8/view?usp=sharing
 

vibrac

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Opens on sheets with Android . Will try on Linux with open office ( who can afford or want microsoft privately)
 

BigEd

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Opens on sheets with Android . Will try on Linux with open office ( who can afford or want microsoft privately)
Good thought Tim re Linux. I've just downloaded it and tried it using Linux Mint. It opened in Libre Office but wouldn't let me change anything until I saved it with a new file name. e.g. "crank balance calculator2".
(I can point out ways of making MS Office 365 more attractive as a viable option to purchase but that would be in a new thread. It will never be as good a bargain as the free Libre Office. :))
 
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