Polished Rods

[RDfella]

Combustion should not affect balance, as no mass is altered. It will, however, introduce torsional vibration of the crankshaft as combustion attempts to accelerate the flywheel. Good luck with calculating that one!

re crankcase 'window' - it's actually quite easy. Who would want to remove and strip an engine several times to adjust engine balance? Usually access is possible by removing the sump plate. Wasn't possible on my Weslake (which had outrageous vibration, like sitting on a jackhammer) but there was a 'window' on one side of the crankcase - a small alloy plate fastened with two screws (purpose unknown). So when I had the engine stripped to make the necessary alterations to the flywheels. I replicated that plate on the other side. That way engine balance can be altered in five minutes and am pleased to say the Weslake now has an acceptable vibration compromise.

[RichardL]

Combustion should not affect balance, as no mass is altered. It will, however, introduce torsional vibration of the crankshaft as combustion attempts to accelerate the flywheel. Good luck with calculating that one.

My theory is that there is too much force on one side at time of combustion for a slight rod weight imbalance to make much, if any, difference in vibration. Apologies if I am being accidentally thick, as is like to happen.

Richard L.

[Radlan2]

Hi Richard
        Something I thing I found dissapointing when I invested in a new set of high strength alloy forged rods, sold as balanced, for my Trident was that although they all weighed the same 453.6 grammes.
         One was from probably a different batch and the holes were offset differently in the forging which showed up when compairing the weights of the individual end caps. I now always do a comparison of caps which can help pinpoint the odd gramme. I have read the Stan Shenton weighing each end proceedure but never got reliable and repeatable results.
         With the billet rods now widely availiable this should not be an issue.
               Best of luck
                      Chris

[RDfella]

Richard - I was referring to torsional vibration - that which destroys cranks through fatigue. ie the twisting back and forth along the length of the crank as combustion forces try to accelerate the flywheel, only for it to be slowed down as a piston reaches the compression stroke. A nightmare trying to calculate all the harmonics and what revs are critical (at least I find it is). For example, on 6cyl in-line engines a vibration damper is virtually mandatory if you want the engine to last more than 100hrs before torsional vibration causes the crank to break.

[RichardL]

RD,

I see what you mean. In my thinking the tortional stress you describe would not be measurably influenced by slight rod weight imbalance, which goes to my original question. (The key phrase being, "In my thinking.")

Richard L.

[Greybeard]

My current car, a BMW 1 series has a 3 cylinder engine with vibration damping of some sort; it's very smooth.

I used to have a twin-cam 16 valve 4 cylinder Volvo. The engine had an engine balancing lobed shaft that made it as smooth as silk.

[Jules]

Quite honestly Richard, all this discussion is irrelevant for our engines, they are not highly tuned, tightly clearance controlled machines that require such heavy investment in sophisticated balancing. I would suggest that the "matching" you have achieved to date is good and the polishing equally great, it maybe the the polishing that creates a smoother engine too as it whirls through the air/oil vapour, but either way, unless you invest in some sophisticated process you have what you have, so to speak - maybe worth assembling the pistons now and then doing a recheck on the comparative difference end to end, but I'm sure you could keep going with this forever to try and achieve perfection, but for what purpose?
I note that Muskrat hasn't commented here nor RR and maybe they are perhaps the most experienced with tuning and racing these engines, to comment on the effects of balancing and smoothing all the parts.....

[trevinoz]

I have my engines dynamically balanced and believe that the cost is money well spent.

[RichardL]

Thanks all, again.

Jules, I definitely am not seeking perfection and not racing. Started out mentioning one gram difference between my rods and the discussion went to balance factor, etc. While all that is fun to talk about (me knowing little, others much more), I will not be going for dynamic balancing. I will be confirming for myself that the imbalance in total weights and end weights doesn't go beyond this, then call it an experiment. I didn't dynamic balance my A10 and I think it runs great and pretty smooth. Also, I don't want to spend the money. (This said, total respect for Trev. On my fantasy bucket list is shipping my A10 to Oz or the UK and riding with my friends there. Next year's Orabandan? Sadly, quite unlikely.) The polish on the rods started out as removing imperfections and the forging part lines to reduce stress points. Then, I got carried away because a friend loaned me his awesome two-ended buffing machine.

Richard L.

[trevinoz]

Richard, at least static balance your engine.
A friend of mine has done several at around 60% and is happy with the results.

[RichardL]

Trev,

I think I need to report here the balance factor, as-is, so we can discuss whether it will be worth adding or removing crank counterweight in trying to reach a specific number. Not moving at light speed, but when I get to it, I'll be looking forward to the comments. (Comments now OK, as well, of course.)

Richard L.

[Black Sheep]

I guess I am just lucky not to mention unspeakably idle. The A10 has unpolished rods and has never been balanced in any way and yet is a pretty smooth machine. Was out on it yesterday despite near zero temperatures, rain and muddy roads just for the fun of it. Similarly the A7 Star Twin. And, you may want to be sitting down for this, my 745cc Altas engined Dominator is remarakably smooth, to the extent that a Commando owning friend reckoned it was just as smooth as his machine.

You can't really know how a bike is going to vibrate until after you have ridden it. It's at that point you would have to strip it down and play with balance factors in the hope that you are altering the balance in a way that will improve things. Balancing a vertical twin is all about compromise so it's not ever going to be perfect throughout the rpm range.

If you polish rods, should you shot peen them to restore the skin tension?

[orabanda]

Hey Richard L,
Just bring yourself to next year's Orabandon; I'll provide the 2 wheeled chariot!
Richard The Benevolent

[RichardL]

What to do with these nuts: send them back and get the right ones, or put washers under them so socket can fit?

[berger]

they should be fitted with thickish washers, if not the ally crimps into the threads and causes trouble trying to part the cap