Heavy conrods are better sometimes

[GregLocock]

http://www.ret-monitor.com/articles/842/smaller-and-lighter-is-always-better/

Can anybody come up with a sensible explanation? Flywheels seem to me to be a cheaper lighter better solution if inertia is the game.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[evelrod]

Well boys and girls, it would be a genuine difficult ride on an observed trials course with an "ultra light inertia" motocross bike!!! Not enough room for a sufficiently sized flywheel? Good guess. My Bultaco had a heavy rod and a big flywheel along with a smallish Amal carb and some really close ratio gears in the tranny.

Rod

 

[ivymike]

in the comments portion, the author does go on to say

"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.
"

 

[MikeHalloran]

Bultacos are a special case. They ALL use the same bottom end, regardless of bore. Hence, all but the largest have 'oversized' rods and flywheel.

As for other engines, and oversized rods vs. oversized flywheels, there used to be a market among dirt bikers for 'twingle' cams for Triumphs, the operating theory being that smooth power was not as effective for traction as intermittent power. ... or something like that.



Mike Halloran
Pembroke Pines, FL, USA

 

[SomptingGuy]

Given that the application is motor racing, "cheaper" doesn't come into it. Redistributing your rotating inertia will have effects on the dynamics of the crank/block, which may or may not be beneficial to mechanical reliability. It may give beneficial mass distribution.

Unfortunately for us, the prime measure used is lap time, which includes vehicle, driver/rider and track. Nothing objective we engineers could measure.

When the "big bang" bike engines started to appear in the 90's, there was much hand-waving about why the riders could get faster lap times on them and much unscientific experimentation.

- Steve

 

[tbuelna]

GregLocock,

All other things being equal, a heavier rod would increase the inertia of the reciprocating components that must be accelerated and decelerated twice per rev. This increases friction losses at the rotating joints due to the higher inertia forces, which in turn hurts efficiency. So I would disagree with the author's basic premise.

However, there are some instances where additional rod mass may be beneficial. For example, in large slow speed, high pressure, two-stroke diesel engines the additional rod mass (as well as piston mass) can help to counter the wrist pin and rod bearing loads due to high combustion pressures around TDC. Strategically placed material around the rod structure that supports the bearing shells is also a good use of metal, since a rigid support of the journal bearing is critical for good bearing fatigue life.

As for rod mass being an effective method for increased crank polar inertia, you are correct that a flywheel mod would be better. In general, only about half (the big end) of the rod assembly is considered as rotating mass.

Of course, considering the cost of a set of Ti rods, the benefits may not be worth the cost for most racers.

Regards,
Terry

 

[LadaTrouble]

If we are talking motorcycles,extra flywheel weight is usually added to an external flywheel.This causes problems in turning the bike hard (MX or flattrack).I guess adding weight to the rod and crankshaft itself would keep it more central and at a smaller radius.

Also as mentioned - a 2 stroke trials bike does the things it does with flywheel weight and lack of engine braking.A skilled rider also helps.

 

[db600]

Surely the connecting rod only needs sufficient mass to achieve its goal of acting as a strut and a tie as extra mass needs energy to speed it up and to slow it down. Inertia and lateral loading whether at high or low speeds is still an important consideration. As a light assembly running at high speed is similar to a heavy assembly running at low speed all other things remaining equal. All internal forces need to be resolved and contained within the structure.

 

[evelrod]

db600, If you read the OP, I think you have missed the point.

Rod

 

[Compositepro]

I finally read the article in Greg's link. It was interesting in its assertion but said nothing about why a heavier con-rod might be a benefit. The author responded to comment and that, I think, clarified the mystery (and tbuelna mentioned it). Mass added to the big end of the rod (at the crankshaft) does add to flywheel inertia.

What I'm not sure about is, in what cases is this better than adding mass to the flywheel? The rotational inertia of the "big end" is centered at the axis of the crank arm, and not at max radius of the crankshaft. Also, it seems, that balancing this added mass requires removing mass that could be placed in a fully balanced flywheel. So, I think this would only be a solution given certain other constraints.

 

[kevindurette]

The force required to accelerate a heavier con rod will balance the pressure load of the piston and help to eliminate failure in compression (but it will make the tension problem worse while the cylinder is being filled/emptied). Whether this is good or bad all depends on where your torque is developed in the rpm range. (For naturally aspirated high-rpm engines, this really won't help at all.)

Durette

 

[MastDesgin]

One big problem with heavy rods is stretching especially if the rod is heavy on the wrist pin end.

Like Tbuelna said the more weight on the rods the more inertia that needs to be stopped. In my experience you lighten the internal mass till you cant anymore then what you need to run smoothly you add to the flywheel not only is the flywheel much easier to change then the entire rotating assembly but its easy to reduce the weight on, be it drilling or machining the surfaces, so you can start high and go down. Finding the balance of rotating mass is hard and generally never perfected (I cant claim I have ever built the perfect engine).

One of the biggest questions you have to ask though before you go into balancing and lightening your rotating assembly is how much torque do I need and how much HP do I need. If your going to pull something you want all the inertia the engine can handle, basically a diesel. If you want to road race and your RPMs are never going to be below 5000 RPM make everything feather weight so you can rev fast and high and stay there (F1 is the best example of HP over torque). Look at RX-7's they have probably the lightest rotating mass of any street car engine in the world, they are fast because they can be made to rev 15k and not think twice.