Crankshaft Balancing

superhornet59 · Post by **superhornet59** » Wed Jun 27, 2007 10:07 pm

Hi everyone, Matt here and I'm new

Okay, alot of V8's are at 90 degree angles, that means you have 2 throws opposite of each other verticaly, and 2 throws opposite of each other horizontaly.

so seeing as each throw is balanced by another one... why the counterweights?

I mean i realise it could be beacause there is significant distance between te throws because 1 and 3 and 2 and 4 would balance eachother out so that distance between them could do to tornional vibration cause issues for the bearing between them,

but what if instead you had throws 1 and 2 opposing eachother verticaly and the other 2 opposing horizontaly.

after re-adjusting firing order, your have a crankshaft with almost no vibrations but alot lighter due to the lack of counterweights... but im not sure how torsional stress would act on it.

it sounds like a flat plane with little second order vibrations

-Matt

Post by **katanaman** » Wed Jun 27, 2007 10:11 pm

counter weights balance the pistons and rods

superhornet59 · Post by **superhornet59** » Wed Jun 27, 2007 10:18 pm

so rather than using a balance shaft on a flat plane, why cant they also use counterweights?

Post by **katanaman** » Wed Jun 27, 2007 10:40 pm

loosing the extra weight of having counter weights is part of the whole idea of a flat plane crank. Faster revving more responsive which is why they are used in race engines or high end sports cars. The balance shaft is for secondary vibes anyway as a flat plane is naturally balanced. http://www.autozine.org/technical_schoo ... mooth4.htm have a read

Post by **katanaman** » Wed Jun 27, 2007 11:01 pm

Actually if you go to the start of the article http://www.autozine.org/technical_schoo ... mooth1.htm it explains a lot more about primary and secondary vibration and why counterweights and balancer shafts do what they do.

superhornet59 · Post by **superhornet59** » Wed Jun 27, 2007 11:11 pm

okay I see what they mean, but I still dont understant how having the weights on the crank or on a seperate shaft makes any difference, either way it takes engine power to overcome the rotational interia of all that mass, and because of the friction of the second shaft, id think its less efficient... but that doesnt seem to be the case. why?

-Matt

Post by **katanaman** » Wed Jun 27, 2007 11:22 pm

you need to read the rest of the article to understand how balance shafts work.
Basically twin plane crank = bad primary balance good secondary. Primary balance is much greater so needs big lumps of steel i.e counter weights.
Flat plane = good primary balance poor secondary. its also at a different point so if you added counter weights you would screw up the primary balance hence a separate shaft is used.

superhornet59 · Post by **superhornet59** » Thu Jun 28, 2007 12:42 am

Oh alright I understand what you mean, okay well thanks alot!

-Matt

ChrisJC · Post by **ChrisJC** » Thu Jun 28, 2007 7:18 am

That's a good website, some lunchtime reading beckons...

Chris.

Post by **katanaman** » Thu Jun 28, 2007 8:58 am

Yes its a good read, if you dig back to the start http://www.autozine.org/technical_schoo ... index.html there is some interesting stuff in there. Its worth it just to see a W12 crank, what an odd looking thing that is!

ChrisJC · Post by **ChrisJC** » Thu Jun 28, 2007 2:33 pm

I'm really dubious about those VAG cranks with split journals (or whatever you call them!). The Audi RS6 crank looked very thin in x-section through the big ends.

Guess it works though....

Chris.