Author Topic: Full Circle Or Counterweighted? (Read 5886 times)

Black-Back · « **on:** July 28, 2004, 07:05:59 PM »

not that I'm building an engine, but what is the difference/benifit, of a full circle crank over a standard counterweighted one.

aren't the counterweight designed to balance the weight of the piston during the stroke? and if so wouldn't a full circle crank be similar to stock?
or does it have something to do with the extra weight acting like a flywheel?

AHHHHHH!!!!

Mike

jim martin · « **Reply #1 on:** July 28, 2004, 08:29:44 PM »

i'm sure you know what a vw crank looks like so knowing this you can see that two rod journals are allways opposite each other at any point during a 360 deg revolution. there are 2 main killers in a crankshafts life,FLEX AND VIBRATION,so in order to fight flex a stronger crank is made by use of material and thickness at key areas ,as well as in order too offset the outward centrifical force of the spinning crank .the point opposite the rod journal a weight is added which pulls the other side of the crank outward which in turn keeps the whole assemble spinning true.and for vibration the crank is spun on a special machine the measure vibration and weight is usally removed via a drill into one of the counterweghts .just like balancing a tire on a high speed tire balancer.
now as far as a full circle crank the counterweight mass is built around the full 360 deg instead of at the oposing side only.benefits,stronger due too assembley style/less flex at any position while running /can run less deck height/smooth acceleration/higher rotating mass.nothing revs like a full circle crank,and on and on
here is a neat inside scoop ,when the us military was accepting aplication for engine parts for there assault vehicles ,the demello machine shop full circle crank was the only crank that did not fail ,thats why they have the contract for us military now ,and that was using a 90mm crank[secret].plus there is a huge amount of top racers that back them as well.

Black-Back · « **Reply #2 on:** July 28, 2004, 08:35:12 PM »

Hey Thanks Jim! its all a lttile clearer now!

Mike

Cameron · « **Reply #3 on:** July 28, 2004, 11:44:03 PM »

Demello does not make a true full circle cranks, it is only a name. They call it a full circle crank because the two webs between the rod journals are beefed up with extra metal. This stops the crank from flexing at higher RPMs. These webs are machined at the same time as the counterweights, and give the appearance of a full circle crank.

See the pictures here!

The centre crank is the \"full circle\" crank. But you can clearly see the counterweights on it. Only the two webs between the rod journals have been made round.

IMO Demello makes a nice crankshaft, and I'm willing to put my money on it.

Bruce · « **Reply #4 on:** July 29, 2004, 03:50:54 AM »

As Cam said the only difference is the extra material added to the two cheeks between adjacent rod jouranls. This stiffens the cheek to reduce flexing. If you look at a Vanagon waterboxer crank, instead of counterweighting it, VW increased the size of the cheek just like DMS's full circle crank.

silas · « **Reply #5 on:** July 29, 2004, 08:42:46 AM »

great info. thanks bruce, cam and jim.

Black-Back · « **Reply #6 on:** July 29, 2004, 10:43:09 AM »

Yeah, thanks again guys! great explaination.

Mike

jim martin · « **Reply #7 on:** July 29, 2004, 08:39:10 PM »

word of advise ,do not leave a full circle crank on a work bench unattended.
they will roll away on you.ouch,great too get some other input thanks guys.
here is something i noticed now that my motor is apart.with my old scatt crank the centre split bearing use too have heavy wear on the top and bottom sides of the bearing showing crank shaft flex was present,don't get me wrong the motor ran well ,so far with the bearings used on the demello crank no heavy wear at present so present bearingswill stay in,i also used 0.005 positive deck and a .040 copper head gasket with no problem.i wanted too see if it was posible too use copper gaskets that way without maching a groove in the piston head for clearance.it does work.

Black-Back · « **Reply #8 on:** July 29, 2004, 10:17:18 PM »

Hey, as I was surfing the curl they call the net, I came accross this on the Berg site. have a look here, and scroll down a little.

Gene berg website

Now I know that everyone has they're own opinion, but what it says make a little sense......doesn't it?

Just thought I'd throw it out there

Mike

jim martin · « **Reply #9 on:** July 29, 2004, 10:49:38 PM »

sorry just going off topic a bit,this is my 2 cents on the vw parts scene right now,first years ago with fewer suppliers around you could by anyones parts.toss an engine together and there was a damb good chance it would run with no trouble forever,i have a 76/90.5 in my garage that was put together with no measuring and you can't kill it,so that one was lucky . but,the point is now there is a lot more demand and a lot more product,of which some is good and some is bad.
which means that everone makes there parts too there own demensions , tollerances and quality,which in turn means you have to tripple check everthing as so many parts will not work together without modification.that was the case with my demello crank and scat rods,took alot of time but finnaly worked out.in a nutshell , at this point if you by quality parts from a well named supplier you will get high quality , but it is up too you too make them work for you.

Black-Back · « **Reply #10 on:** July 30, 2004, 02:16:51 AM »

True.....its like anything, you get what you pay for, you hope........

Thanks for the info

Mike

Bruce · « **Reply #11 on:** July 30, 2004, 03:37:01 AM »

Quote

Hey, as I was surfing the curl they call the net, I came accross this on the Berg site. have a look here, and scroll down a little.

Gene berg website

Now I know that everyone has they're own opinion, but what it says make a little sense......doesn't it?

Just thought I'd throw it out there

Mike

The first thing anyone needs to know is that was writtin a couple of decades ago by Gene Berg (who's been dead for 8 yrs).
GBE does not make welded counterweighted cranks anymore, haven't for many years. Why they keep that obsolete info up is beyond me.

The Berg reference to full circle cranks is different from what Cameron and I am describing. If you look at just a regular welded counterweighted crank, (see Cameron's link to Demello's site) the counterweight is added only to cheeks between main journals and rod journals. There are 4 of them. The counterweight ends exactly at the half way mark. That is, all the counterweight's mass is completely on the opposite side of the crank from the rod journal.

Now there was a time when some crank makers welded on a different counterweight in those 4 locations. This different counterweight not only added weight opposite to the rod, but also where other's counterweight ended, all the way to the extreme of the rod journal. Thus the cheeks between the rod journals and the mains was a full solid disc. This is totally the wrong way to do it because it adds material to the rod side of the crank. This type of crank was also called \"full circle\" back in the 60s and 70s. This is what Gene was refering to.

The cranks sold by DMS called full circle only add material to the cheek between the 2 adjacent rod journals. Since the 2 adjacent rod journals cancel each other's weight out, there's no need for a counterweight, nor is there any room for it. But if you add material to the crank 90

Black-Back · « **Reply #12 on:** July 30, 2004, 02:19:43 PM »

OIC, so what is now referred to as a full circle crank is a little different in design than in past years, and is considered to be better, but a good forged counterwieghted crank still revs well and keeps from flexing as much as a stocker does. Is there a reason one would chose either for a different application? or is it just that the full circle is stronger and thats it?

Sorry to pick your brains so much, its just really interesting to me

Mike

Tom H. · « **Reply #13 on:** July 30, 2004, 08:44:31 PM »

There is still somebody making full circle cranks, and they come complete with finger grip holes opposite the rod journal. These \"Finger Grip\" holes make it easier to put the crank into the case. Other than that, the holes have to be the WORST idea currently advertised in full colour in a magazine.

jim martin · « **Reply #14 on:** July 30, 2004, 08:59:52 PM »

no , no tom you have it wrong .those holes aren't for your fingers.
they are there too put nickles in so you can balance it at home !! :lol: :lol:

Bruce · « **Reply #15 on:** July 31, 2004, 02:54:13 AM »

Quote

OIC, so what is now referred to as a full circle crank is a little different in design than in past years, and is considered to be better, but a good forged counterwieghted crank still revs well and keeps from flexing as much as a stocker does. Is there a reason one would chose either for a different application? or is it just that the full circle is stronger and thats it?

Tom's right, the absolute worst crank is the ones with holes in them. If you really think that's a good idea, I have a couple of cranks with the absolute lightest counterweights possible. Problem is, they flex quite a bit.

A regular counterweighted crank will flex a lot less than a stock crank. If that cwt'd crank was given the DMS full circle treatment, it would flex even less. The only legitimate reason to not go for the full circle upgrade is cost. More work to weld it on = more money. Also there are a lot of ignorant people out there that think the DMS full circle is the same as what was done wrong in the old days. I think DMS would be able to sell more if they came up with a new name.

Cameron · « **Reply #16 on:** August 04, 2004, 12:29:42 AM »

Here is another view of a DMS \"full circle\" crank. Again, you can clearly see it has counterweights.
Only the two webs between the rod journals are round,
from extra steel welded on to strengthen those webs.
BTW That's dial test guage, but you can't make out the face, because the photo is slightly over exposed.