# Westbury balance

### Help Support HMEM:

#### methuselah1

##### Well-Known Member
I'm an ETW fan, and note that his little engines often specify the conrods as bronze or duralumin. On my engines I always go for the latter. Looking at my Kiwi last night, it struck me that there is nothing on the drawing about amending the crank counterweights, ditto for the Seagull or Wallaby.

Has anyone got any thoughts on this? My Seagull's conrods owe a lot to Graham Meek 's ideas, but he mentions no balance problems.

Andrew UK

#### Charles Lamont

##### Well-Known Member
Out of interest I did some calculations for the Seagull a while back. With alloy pistons and con-rods, and for the conventional balancing of rotating mass plus half the reciprocating mass, I found it would be necessary to to thicken the semicircular weights by 0.040" on each side of each web. I didn't
.

So far, I have only run the engine bolted down. It did not cause noticeable vibration of the lathe!

#### Vietti

##### Well-Known Member
Surprised it took more weight to balance an aluminum rod &piston. Maybe the original plan was too light??

#### stevehuckss396

##### Model Engineer
Project of the Month Winner
I designed my V8 with round weights. Doc built 2 engines. One he left round and the other he cut out the counterweights. Said there is no difference between the two so I wonder if the tiny amounts of stroke and miniscule weights we work with really mean much when it comes to balance. I'm talking 5/8 of an inch stroke.

#### Eccentric

##### Well-Known Member
I used aluminum conrods on my Wallaby and did some calculations to determine the proper weight of the counter weights.

Since the pistons do move in unison the engine can be modeled as a single cylinder engine with twice the mass of one of the con rods, pistons, rings and wrist pins. The formula I found is as follows: Weigh the top half of the connecting rod and add it to the weight of the piston, wrist pin and rings. Then take a percentage, 55%, and add it to the weight of the bottom half of the rod. Place this weight in the CAD model at the center of both connecting rod journals on the crankshaft. Then adjust the weights of the counter weights to balance the entire rotating assembly.

I discuss this here:
Wallaby Crankshaft