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E: Engine Crankshaft Balance Method


Brian Thompson

Well Known and Active Website User
VOC Member
I have the Twin running nicely in the original frame and was about to put it in a Mcintosh Egli frame. It has one of Terry's 40 mm big end and 30 mm main shaft cranks in it with 600 cc top end kits on it. Its nice and smooth in the Vin frame. I"m hoping it doesn't vibrate to much when it moves into its new home. ( Won"t happen for a while now) I didn't ask Terry what this cranks balance factor is. I"m sure if there was an issue he would have mentioned it. It has 350 gram pistons. The newer pistons I have are 300 grams. Does anyone else have this combination in an Egli style frame?
Cheers
Brian
 

Bill Thomas

Well Known and Active Website User
VOC Member
Can you go blow by blow with us as you do it Greg, I am still not sure of the maths.
There is a bloke on youtube that is different, And most people think he is right.
 

greg brillus

Well Known and Active Website User
VOC Member
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.
 

Vincent Brake

Well Known and Active Website User
VOC Member
I noticed
At 48% a more up n down vibration.
And over 52% a more fore and aft one.

Guess it also has to to with the secondary vibration.

Now i want a straight six
 

timetraveller

Well Known and Active Website User
VOC Member
I think that Vic is a hero building his own dynamic balancing rig. One suggestion which might already be incorporated is to mark the rim of the flywheels with numbers every centimetre or so. That way the strobe will show a number and the position. Good luck with it Vic.
 

oexing

Well Known and Active Website User
VOC Member
Norman,
I don´t have a strobe on the rig but get positions from the electronics for each side, for adding weights or drilling for reduction. Only questionable setup is my stickers or felt pen marks for getting a rough idea about circular positions. I can use the woodruff key slot on the SR cranks for zero position -and the rest is a bit of guesswork and eyesight. Certainly one could add a degree disc on the mainshaft end but then I do this only once in a few months for a friend with his hobby workshop.
In case somebody were interested in that kind of equipment - I got the professional tester with a lot of luck from Ebay € 500.- or so - I first tried a USB hobby kit from Australia but somehow this did not work as promised, maybe broken. So I could get help from an oscillograph app on the laptop hooked to the g-sensors and with many test runs I achieved great smoothness.
Here is a link and youtube clips about this USB Dynex Hobby kit for € 150.- or so which got about the same logic as my prof tester. But you have to do a lot of configuration on your computer to see useful results. So when I found my tester on Ebay I just had to have it.

Vic

Dynex Hobby kit:
P1050316.JPG

HRD crank with Dynex:
P1050258.JPG

laptop app Dynex:
P1050263.JPG

youtube Dynex

Dynex Hobby home
 
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Little Honda

Well Known and Active Website User
VOC Member
Norman,
I don´t have a strobe on the rig but get positions from the electronics for each side, for adding weights or drilling for reduction. Only questionable setup is my stickers or felt pen marks for getting a rough idea about circular positions. I can use the woodruff key slot on the SR cranks for zero position -and the rest is a bit of guesswork and eyesight. Certainly one could add a degree disc on the mainshaft end but then I do this only once in a few months for a friend with his hobby workshop.
In case somebody were interested in that kind of equipment - I got the professional tester with a lot of luck from Ebay € 500.- or so - I first tried a USB hobby kit from Australia but somehow this did not work as promised, maybe broken. So I could get help from an oscillograph app on the laptop hooked to the g-sensors and with many test runs I achieved great smoothness.
Here is a link and youtube clips about this USB Dynex Hobby kit for € 150.- or so which got about the same logic as my prof tester. But you have to do a lot of configuration on your computer to see useful results. So when I found my tester on Ebay I just had to have it.

Vic

Dynex Hobby kit:
View attachment 32127

HRD crank with Dynex:
View attachment 32128

laptop app Dynex:
View attachment 32129

youtube Dynex

Dynex Hobby home
Vic,
do I see correctly: The device operates with a swinging arm of sheeted metal left and right, carrying two
rolls to take the crank each side, taking the wired gravity sensors inside the sheeted metal plates, being
wired to the electronic unit on the left base plate which measures the varying loads in grams, when the
crank starts rotating, driven by the belt, tensioned by a tensioning roll?
An admittedly beautiful tool to work with. Congratulation to your buy at ebay!
 

oexing

Well Known and Active Website User
VOC Member
Salü,
yes, the two ball bearings at each side sit on their mini linear guide with rubber stops to limit lateral strokes. I did not want rubber elements there as a crank can have some more kilograms so better have a sturdy rig. The last photo above shows the Dynex kit that I have binned for the professional test equipment. The crank supports have g-sensors no. 1 and no. 2 at the ends which are hooked up to the electronics and send acceleration readings into the set. A proximity switch at the right mainshaft picks up on the woodruff key for taking revolutions and angular position - roughly. This night I got the feeling that the proximity switch is not quite OK when finishing the second SR crank. Have to check my stock . . . .
Below is the present setup with a SR crank - as shown in the youtube clip.

Vic

previous Dynex kit with HRD crank
P1050246.JPG

professional sensors and SR crank
P1070224.JPG
 

Little Honda

Well Known and Active Website User
VOC Member
Vic,
do I see correctly: The device operates with a swinging arm of sheeted metal left and right, carrying two
rolls to take the crank each side, taking the wired gravity sensors inside the sheeted metal plates, being
wired to the electronic unit on the left base plate which measures the varying loads in grams, when the
crank starts rotating, driven by the belt, tensioned by a tensioning roll?
An admittedly beautiful tool to work with. Congratulation to your buy at ebay!
Salü,
yes, the two ball bearings at each side sit on their mini linear guide with rubber stops to limit lateral strokes. I did not want rubber elements there as a crank can have some more kilograms so better have a sturdy rig. The last photo above shows the Dynex kit that I have binned for the professional test equipment. The crank supports have g-sensors no. 1 and no. 2 at the ends which are hooked up to the electronics and send acceleration readings into the set. A proximity switch at the right mainshaft picks up on the woodruff key for taking revolutions and angular position - roughly. This night I got the feeling that the proximity switch is not quite OK when finishing the second SR crank. Have to check my stock . . . .
Below is the present setup with a SR crank - as shown in the youtube clip.

Vic

previous Dynex kit with HRD crank
View attachment 32130

professional sensors and SR crank
View attachment 32131
why not fitting ignition pick-ups at any side of the crank, ie. Sachse Ignition, which allows precision to less
than 1 deg.? What about primary drive components re balancing? I bet, that´s a lot more than 1 gr.
 

Pushrod Twin

Well Known and Active Website User
VOC Member
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

Well Known and Active Website User
VOC Member
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

Well Known and Active Website User
VOC Member
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

Well Known and Active Website User
VOC Member
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

Well Known and Active Website User
VOC Member
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

Well Known and Active Website User
VOC Member
VOC Forum Moderator
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

Well Known and Active Website User
VOC Member
Opens on sheets with Android . Will try on Linux with open office ( who can afford or want microsoft privately)
 

BigEd

Well Known and Active Website User
VOC Member
VOC Forum Moderator
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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