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The Expert Advices | A long or short connecting rod? Two different philosophies compared

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A LONG OR SHORT CONNECTING
ROD? TWO DIFFERENT
PHILOSOPHIES COMPARED

13 June 2016
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Of all the changes in the 2016 homologation, one in particular highlighted the different philosophies of the manufacturers: the length of the connecting rod (from the centre off the piston-pin opening to the centre of the big-end bore). Two makes, in particular, have made opposite choices: Vortex, with its new RKZ shifter engine, has changed tack with respect to the previous approval, lengthening the connecting rod from 110 mm to 115 mm., On the other hand, for its MKZ engine Modena Engines has chosen a 106 mm connecting rod, which is very short compared to the standard measurements, such as those used by IAME, which has chosen a “traditional” length of 110 mm.

Before going into the details of the choices made by Modena Engines and Vortex, let’s take a look at the general theory concerning the length of connecting rods. At an equal crankshaft height, an engine with long connecting rods can mount a shorter and lighter piston, reducing the alternating masses to the advantage of reliability and decreased component wear. Furthermore, long connecting rods also have reduced angles during movement, generating less lateral thrust and friction (less wear) on the cylinder walls. This is why road engines favour smaller pistons at the expense of longer connecting rods.
In contrast, in high-performance engines, in which pistons are already as small as possible (at high rpm, reducing masses is essential, since they generate forces of inertia that are proportionately high), the crank drive is taken to an extreme by also shortening the connecting rod. The part has an alternating motion for a fraction of time and a rotary motion for the remaining fraction. Therefore, reducing it means reducing the masses that move in both one of the motions, as well as the other. The result is a net advantage in terms of performance, but a somewhat reduced duration and a need for expensive surface treatments on the cylinder walls to mitigate the effect of increased lateral thrust. In general, this is the case with two-stroke kart engines, on which long and short connecting rods, with the same stroke, determine changes to transfer and exhaust port opening and closing times and changes to the flows.
Giovanni Corona, a manager at Vortex, and Roland Holzner, the technical director at Modena Engines, explain the choices that led to the definition of the length of the connecting rod in their latest homologation KZ engines.

MECHANICS AND FLUID DYNAMICS: LONG AND SHORT CONNECTING RODS AFFECT THE MAIN CHARACTERISTICS OF AN ENGINE
1 WHAT IS THE MECHANICAL REASONING BEHIND LONG AND SHORT CONNECTING RODS?

Holzner: “Obviously, it wasn’t the mechanical aspect that led us to choosing such a short connecting rod, but a number of other factors involving fluid dynamics and resonance. A short connecting rod unquestionably increases the lateral thrust forces of the piston on the cylinder and, consequently, friction and wear. To preserve the advantages of a short connecting rod and reduce the disadvantages, we have developed a new piston with the piston rod seat moved up by about 4 to 5 mm, a solution that reduces the force acting on the piston wall resting on the cylinder”.

Corona:: “It wasn’t the primary reason for our choice, but there is no doubt that reduced lateral forces help a lot with the very high rpm of two-stroke engines. Piston and piston ring wear is significantly reduced. The two elements are exposed to less stress, to the advantage of reliability. In addition, the lower amount of friction results in a reduction of losses, which is particularly important at high rpm, or “red line” as we call them in the trade, at which friction has the greatest impact”.

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