CON-RODS

All our con-rods are made from the highest grades of steel billet to specification 817M40T/EN24T (or forgings where applicable), are shot peened to improve durability and supplied with the superior ARP 12-point headed bolts, which provide the optimum closing strength without stretching. Using FEA (Finite Element Analysis) simulation, our rods are optimised for reduced weight, in both 'H' and 'I' beam configurations. 

 

We have bespoke design capabilities, for both small and large order quantities - even if you only require a set of four rods.

Con-rod Design Process

For all Phoenix designed con-rods, FEA studies are conducted to stress test and prove designs, optimising for weight saving where possible. This process allows us to design con-rods suitable for race applications, where high strength and low weight are a must.

To accurately simulate the con-rod through the operational cycle, three separate FEA studies are conducted. One to simulate the con-rod when under compression during the expansion phase of the cycle just after TDC, forced downwards via the gas pressure acting on the piston. The second to simulate the con-rod again in compression, but this time on the upward stroke, pushed up via the inertia force from the crankshaft. And lastly to simulate the con-rod in tension when at TDC and pulled down by the inertia force of the crankshaft during the intake phase of the cycle.

To calculate the forces involved and which to apply to the con-rod for each study, several equations have to be used. For the first study, the con-rod is being simulated at the point where combustion takes place, the pressure of which acts downwards on the piston with the inertia force via the crankshaft also pulling the con-rod down from the big end. The resultant force that would need to be applied to the small end would therefore be the force experienced from the combustion pressure minus the inertia force. Using the ideal gas law of Equation 1 to obtain the combustion pressure, and then multiplying by the cross-sectional area of the bore, shown by Equation 2, the force acting downwards on the piston during the cycle can be determined.

As the volume required for Equation 1 will change relative to the position of the piston in the cylinder, Equations 3 and 4 below are used to determine this change in volume according to a given crank angle position.

The use of the three Equations 1, 3 & 4 together determine the pressure relative to the remaining volume of the cylinder above the piston at a given point during the cycle. It is shown from the calculations that as the piston moves upwards in the cylinder, the remaining volume decreases and the pressure increases, displayed by Figure 1. This is why the piston moves upwards during the compression stroke, increasing the pressure as the remaining cylinder volume decreases. After which the expansion stroke, where combustion occurs and peak pressure is experienced, forces the piston back down the cylinder and rotates the crankshaft.

Before calculating the inertia force, it must first be considered that the smaller the inertia force, the higher the resultant force. Therefore, to ensure suitability of the con-rod for all operating conditions, the inertia force for this load calculation should be for when the crankshaft is rotating at the lowest speed for combustion to take place, which will be during the start-up phase from the starter motor. The inertia force is based on the angular velocity of the crank, multiplied by the reciprocating mass at a distance from the centre point of the crank, as shown by Equation 5.

For the remaining studies, the force applied via the combustion pressure is not present, so it is just the inertia force that is acting on the con-rod that is to be simulated. Again, the use of Equation 5 is required, however instead of using the lowest possible rotational crank speed, the maximum will be used to ensure that the design can withstand the highest inertia forces during operation.

Included in the design process of the con-rod is the determination of the con-rod bolt size and material. For this, several calculations to verify the strength of the bolts when subject to the loads experienced during operation must be undertaken. Equation 6 below is the rearranged formula for stress, force over area, to make the root diameter of the bolt thread the subject.

The allowable stress value is the minimum typical tensile strength of the bolt material, the force is the reciprocating weight, which comprises of the piston and a third of the con-rod mass, multiplied by the stroke and by the max rpm squared, as shown by Equation 7 below.

Phoenix designed con-rods exclusively use ARP (Automotive Racing Products) rod bolts, due to their superior quality and design for the motorsport sector.

MicrosoftTeams-image (37).png

Disclaimer:

Customers are reminded that Phoenix Crankshafts Ltd manufactures automotive crankshafts and con-rods and does not supply assembled bottom-ends ‘ready for use’. With regard to our line-boring service, crankshafts are only fitted into the crankcase for delivery as proof that the crankshaft freely rotates in the main bearings and for ease of transport and product safety.

 

It is the responsibility of the customer and/or engine builder to ensure that the crankcase(s) are thoroughly cleaned and free from any swarf or debris. Furthermore, checks should be made to ensure all oilway plugs etc. are properly fitted to the crankshaft and that following these initial steps that everything else is fit for purpose, before re-assembly.

 

It must be noted that once fully assembled, the engine performance falls under the responsibility of the engine builder and not Phoenix Crankshafts Ltd.