Smaller and lighter is always better

Tuesday, February 16, 2010

Tags :  con-rods

con-rodsIn the case that you don't have this article delivered to your inbox, I hope that the title has drawn you here either in the hope that you will find how to make your con rods smaller and lighter (in which case you may be disappointed) or because you disagree with the fundamental statement of the title.

Previous articles on the subject of con rods have talked about some of the material choices for these parts, and in the recent magazine Focus article on the subject of con rods, the author discussed the merits of material choice with leading manufacturers of con rods from all over the world. There is a general feeling that reduction of mass and inertia is a good thing, and in very many cases this is true and the title of this article can be taken almost as a 'universal truth' within the limits of maintaining both satisfactory function and sufficient reliability.

For those chasing light weight, low inertia and reduced mechanical loads, there are indeed significant gains to be had from working on the con rod. Light con rods make less demand for counterweighting on the crankshaft and so the any mass saved on the con rod can be carried on through the design of the whole cranktrain. In many forms of motor-racing this is the philosophy that is followed with good results. Low inertia is felt to improve the responsiveness of the engine, making it 'pick-up' quickly when transferring from a closed or neutral throttle to an open one. In a similar vein, low inertia turbine and compressor wheels in turbocharged applications prevent excessive lag and improve responsiveness.

So, are there any racing applications where low inertia and low mass are not primary goals? The answer to this question in a surprising number of motorsport situations is 'yes'. Whilst we can usually be certain that riders / drivers / pilots of racing motorcycles, cars and boats will want more power than their rivals, there is an aspect to performance which is gaining increasing understanding, but is little understood by many (probably most) involved in racing engine design.

When we talk of performance, those involved in engine design alone will generally judge peak power to be the metric by which performance is measured. Those who are perhaps more enlightened may have other methods by which they judge performance. Ideally we ought to judge performance by the stopwatch, because I suspect that there are very few of us who compete on dynamometers alone for any sort of peak power prize (although I have seen an extremely cheap in-house 'Power Cup' awarded to the builder of the most powerful engine) and so our efforts are judged on the track.

To be of greatest use on track, even the most power-hungry engine supplier realises that his product needs to have a certain useful operating range, and this is a step in the correct direction and this represents a loose grasp of the subject of 'driveability' or 'rideability', which goes beyond providing a high output.

There are a number of applications where low inertia is actually a barrier to performance as measured by the stop-watch. In some of these applications we have, by constantly producing parts of lower inertia, not provided what is required. In some instances, we have actually seen titanium con rods being replaced with steel rods, and furthermore the steel rods are specially designed for increased inertia, in order to control engine response. These have proven repeatedly to offer improved performance when measured in terms of lap time and race times. Such con rods appear ungainly to the eye when judged against 'normal' racing con rod designs. However, they do represent a measurable improvement in performance, and so should be judged on their considerable technical and economic merit. Providing a similar gain in laptime by increasing power alone would cost a great deal more in spent treasure.

Fig. 1 - Steel rods may offer real performance gains compared to lighter titanium rods in some applications.

Written by Wayne Ward

Photo courtesy of Auto-Verdi.

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>surprising number of motorsport situations
>a number of applications where low inertia is actually a barrier to performance
>In some of these applications
>In some instances

The article caught my attention with the tongue in cheek title, but disappointed with no specific, IE. why choose reciprocating vs. prime mass if strength isn't the problem or how increased mass on the rod equates to which attributes a driver can benefit from, etc.
Please finish your thought in part 2, I am interested!
I think Wayne Ward is plainly wrong, in asserting that a heavier rod may make for better performance because of a more drive-able throttle response - a result of greater or lesser inertia. Such rotational inertia, or damping, of throttle response is least appropriately sought in the reciprocating bits, as these are being accelerated and decelerated each stroke. Such drive-ability might be sought in the rotating bits, like the clutch and flywheel - but never the reciprocating parts. Before ANY power is transmitted to the crankshaft, the inertia of the rod must be overcome on every power stroke. At the end of each stroke there is no stored energy at the big end to assist the next stroke, but must begin again, unlike the rotating bits.
In reply to Jim Wolf's post, I can't mention specific applications as I've been shown these parts in confidence. However, we can say that it is sensible to design the rod such that the extra mass is around the big end rather than the small end.

This brings me to answer Basil van Rooyen's post. The mass around the big end of the rod is classed as being part of the rotating inertia of the engine and is treated as such in most calculations. I've not mentioned specific applications, but there are instances where it isn't possible to just fit a bigger flywheel etc, hence the reason why these parts have been designed and run, in race series up to international level. The teams are happy with the parts and because the lap time and, more importantly, consistency of lap-time is improved, they see this as a performance increase when judging vehicle performance.

I agree that, given the choice, we would certainly look to add mass to the flywheel etc but, where this isn't possible, it seems that an approach such as the one described seems worthwhile.

Best regards,

Wayne Ward
Hi Wayne

Would it not be better to match then a tungsten big end to a titanium rod? Alternative is to have more material on the crank, if possible.
Hi Wayne,
                 I have been thinking about this subject lately as the type of engine I am building at the moment has tradionally been improved by fitting titianium rods.
      These are physically larger and do not allow the tight squish possible with the latest steel rods. If a coated titianium pin is then substituted instead of a coated steel one, it seems a win, win.  
      I can no longer see any advantage in fitting titainium rods, but for the above reasons. I would not consider an increase in reciprocating mass as an advantage period, as I have spent large sums decreasing it to obtain smoother faster engines.
      If the rider could not handle the decreased mass, quicker spinning engine, then an increase in the dynamically balanced flywheel is where I would put it. Hope I am not missing something, pls correct me if I am.

Best regards Ian

The point you make is correct; the best place to put the extra inertia is probably on the flywheel or the crankshaft. However, where this isn't possible, people have successfully turned to steel rods to increase inertia in an attempt to make power delivery more docile.