In motorsport, the four-stroke engine is dominant in terms of numbers, and the vast majority of them use what we would describe as conventional shell bearings for both crankshaft main bearings and big ends. The most common method used to ensure proper location of these bearings and to prevent rotation is a combination of proper preload and tags (or tangs) which locate in precisely machined slots in the con rod big-end bore. This method is also that used by most production engines. There are alternative location features used in some race engines, but these can be costly to produce compared to tags.
There has been a trend in modern engines for production vehicles, in part because of the relentless drive to cut costs, to dispense with the tags/tangs on big-end bearings and, of course, with their corresponding slots. The elimination of a machining operation on both the rod and its cap reduce the cost of the con rod. The method of producing the tag in the bearing is a deformation process rather than machining, and there is a cost saving when eliminating this process.
Of course, a large part of motorsport is based on the use of production engines and parts, so some con rods with tagless bearings are already being used. Could the same cost reduction strategy for con rods work in racing?
In terms of preventing rotation of the bearings, this should be taken care of by the proper sizing of the con rod bore and the amount of preload applied to the bearings. The major risk in deleting the bearing tag and the slots in the con rod big-end bore is the loss of precise axial location of the bearing.
As we know, race engines are generally designed to be as small and compact as possible, and the stressed components are designed with component fatigue life at the forefront. The axial location of the big-end bearing can be of critical importance for a number of reasons. It is essential that the edge of the bearing shell does not encroach on the crankpin fillet radii, and any loss of precision in location means the design clearance between the bearing and the fillet must be increased.
In limited space, this means that either the crankpin fillet radii must be decreased or the width of the bearing must be decreased. Both of these have an impact on the life of the engine, either by possibly reducing the endurance limit of the crankshaft or by increasing the service pressures on the bearing.
Where the small end of the con rod is fed by pressurised oil from the big end, poor axial location could restrict the flow of oil, or possibly stop it entirely if the oil feed hole in the bearing and the rod are seriously out of alignment.
Of course, where the centres of the bearing shells are out of alignment relative to the applied load from the small end, and also to the neutral axis of the rod, bending stresses will be increased.
For these reasons, it is unlikely that we will see widespread use of tagless bearings in bespoke race engines in the near future.
Fig. 1 – Bearing tag slots, as seen here, are likely to remain a feature of racing con rods (Courtesy of Arrow Precision)
Written by Wayne Ward