Subjective crankshaft vibration

Homebuilt Aircraft & Kit Plane Forum

Help Support Homebuilt Aircraft & Kit Plane Forum:

rmeyers

Active Member
Joined
Dec 20, 2013
Messages
35
Location
Denver, CO
Here's a question for you guys with a lot of VW experience. How much difference is there in the apparent smoothness between a counterweighted and un-counterweighted crankshaft? What effect does displacement, bore and/or stroke, have?
 

Hot Wings

Grumpy Cynic
HBA Supporter
Log Member
Joined
Nov 14, 2009
Messages
7,717
Location
Rocky Mountains
How much difference is there in the apparent smoothness between a counterweighted and un-counterweighted crankshaft?
None - presuming that both are correctly balanced. The counterweights are used for structural reasons.
Saw a 4 cylinder boxer in half, like a half VW or the O-100, and they then can make a noticeable difference in engine shake.

Stroke is probably going to effect the 2 cylinder shake more than bore.
Displacement is related to the power pulse magnitude.
 

Pops

Well-Known Member
HBA Supporter
Log Member
Joined
Jan 1, 2013
Messages
9,136
Location
USA.
Haven't used a counterweighted crank with a prop. But, as Hot Wings said, if correctly balanced the un-counterweighted crank is very smooth. On the SSSC with a balance stock VW 69 mm crank, etc, and the same CR with each cylinder, its almost electric motor smooth with a balanced prop. That means more power and longer life.
 

rmeyers

Active Member
Joined
Dec 20, 2013
Messages
35
Location
Denver, CO
At our direct drive RPMs there should be no difference between counterweighted and un-counterweighted. Nice to see you guys real world experiences backs up the theory. Thanks.

Next question then; where does one find an un-counterweighted 84 mm stroke crank? All of the strokers that I have been able to find have counterweights. I have multi-axis CNC capability and could cut the weights off but it would be nice to find an affordable crank than didn't have counterweights.
 

Hot Wings

Grumpy Cynic
HBA Supporter
Log Member
Joined
Nov 14, 2009
Messages
7,717
Location
Rocky Mountains
Other than weight savings there is no reason not to use counterweights. They may not make a noticeable difference in engine vibration, but they DO serve a very useful purpose.
Without going into a lot of detail ......... they make the engine last longer. As RPM, stroke and reciprocating weight increase counterweights become more important.
 

proppastie

Well-Known Member
Log Member
Joined
Feb 19, 2012
Messages
5,107
Location
NJ
could cut the weights off
not very up on VW....so as long as we are here I can learn something.....when you say counter-weight....is that a pivoting weight like on the counterweight aircraft engines or is it just a larger lobe on the bottom of the crank opposite the rod?....In either case I would assume the idea is to balance the weight of the piston and rod and do not see how cutting it off would be a good thing.
 

proppastie

Well-Known Member
Log Member
Joined
Feb 19, 2012
Messages
5,107
Location
NJ
big half circle(ish) slab opposite the rod journal.
welded.....I guess there are no stock engines of that size?.....I can not imagine VW would do that....but I really do not know.
 

Hot Wings

Grumpy Cynic
HBA Supporter
Log Member
Joined
Nov 14, 2009
Messages
7,717
Location
Rocky Mountains
I picked a welded one because it is just one option for homebuilders - especially 1/2 VWs.

Normal aftermarket cranks have the counterweights forged as an integral part of the crank. Most aftermarket cranks seem to be kind of skimpy on the counterweights. This works out OK on the 4 cylinder versions.

The 1/2 VWs need all the weight they can get - even to the point of drilling the opposing rod journal to reduce the weight on that side of the crank. Same reason Pete had a custom crank made and used light weight rods and pistons for the O-100.
 

TFF

Well-Known Member
Joined
Apr 28, 2010
Messages
14,692
Location
Memphis, TN
It’s part of the casting/forging of most car engines unless you are talking MGs and Triumphs or some other tractor engines car. The moving weights on airplane engines are a different harmonic fix. They are not on every journal and act like a harmonic balancer not an overall balance.
 

Richard Roller

Well-Known Member
Joined
Feb 15, 2018
Messages
57
Location
Olathe, Ks.
No pendulum weights on a VW crank, just a big half circle(ish) slab opposite the rod journal.
View attachment 110313
Some people attach counterweights to the Ford model A crank used in the Pietenpols. In the case of N34KP the weights are attached with cap screws and welds on the ends.
 

Attachments

Pops

Well-Known Member
HBA Supporter
Log Member
Joined
Jan 1, 2013
Messages
9,136
Location
USA.
I built one 1/2 VW and welded counter weights on the crank that Hummel engines sells for the job. Very hard to do. Had to use a very small cup using Tig and a hard job, I would not build another. For all the machine work, Scott's prices at Hummel are a good buy for the crank or a complete engine.
 
Top