When it comes to balancing engines of a single throw nature which includes the FIAT 500 style unit, it is not straight forward. It is not possible to dynamically balance the engine so that it does not vibrate in both the vertical and horizontal attitudes. But it is possible to have a satisfactory result by finding the best compromise between vertical and horizontal shaking !
As the engine of the FIAT has the cylinders in the vertical axis and it is mounted on engine mounts that are relatively soft in the vertical direction, it will tolerate some vertical shaking throughout the rev range, but getting the tolerance right is all a matter of “Balance Factor”.
When the crankshaft rotates it has two big ends of the two con-rods always attached to the big end journals, so of course it will always see this corresponding weight. However, when the pistons are at TDC or BDC, the amount of weight transfer of the reciprocating parts to the crankshaft big ends is changing between a maximum and minimum amount. What proportion of this variable you choose to use is the “Balance Factor” !!
So the FIAT factory, worked this out and it is a closely held secret by them and by all balancing shops providing a service. Even if you know what the original factory value was for standard parts, when you change to after market parts of different weights and dimensions, it’s all change.
Vibration Free Ltd who are a balancing shop near Oxford however have a different method available as well as the original methods. That is to run the crankshaft in the crankcase while it is attached to the con rods and pistons while it is sat on a special balancing machine stand. In this way the crankshaft actually measures the weight transfer directly from the con rods and pistons, instead of it being a calculated weight transfer.
Vibration Free have being going for 20 years to date and don’t just deal with Motorsport (F1, Historic, Vintage, Indycar...) but aerospace / MOD / Marine / Oil & Gas in fact any sector.
Fiat 500 Engine
Fiat 500 Balance Factor