I've stripped my WD/CO engine for two reasons. The first is to try and reduce the vibration, the second is to rebuild it into the correct crankcase for my RAF contract.

Admittedly there is very little rubber on this bike to absorb the vibes, but at 40mph it makes my fingers tingle. I'm taking the crank to my engineer tomorrow and wondered what would be the optimum balance factor to reduce the vibration at say 30-45mph? I've heard 60% spoken about.....Ian?

Cheers Ron

email (option): ronpier@talk21.com

I think this is a very sensible move Ron. Nine years ago I had the crank dynamically balanced in my AJS trials bike. the chosen balance factor was 52% in that engine as it is used mostly at low revs. What was interesting is that very different amounts of metal were removed from different places on each flywheel. The result is brilliant,ultra smooth steady power,after nine years of hard work it still runs beautifully.Just done the Arbuthnot trial again on it,an all day thrash.
My mate Steve finished building his M20 about a month before riding it to Normandy,it runs very smoothly having had its crank dynamically balanced and piston skirt coated it gets better every time its ridden.
The actual balance factor you use is very much up to what you expect from it,I seem to remember a figure of 60% for road engines.When I asked on a trials forum about the best figure for a trials engine I got no positive advice at all,just rudeness... So I was on my own really,but my main point is that the engine is bound to be better with dynamic balancing if differing amounts of material are removed,(Or added) from different places.A smooth rotating assembly must mean less vibration and wear.

I had my BSA B 25 dynamically balanced at Andrews Precision, Worc. it was 58% but they usually ask what rev range you want it smooth for and the balance factor is irrelevant as it will vary between rev ranges. They've done a few engines for me now, very knowledgeable blokes.

https://andrewsprecision.co.uk/

email (option): horror@blueyonder.co.uk

Dynamic or static balancing of the crank should bring an improvement...
I have had standard flywheel assemblies rebalanced and there is an improvement over the factory job...I would think there was a degree of tolerance during production and I sometimes wonder about the enthusiasm (and accuracy) of the chap that was balancing multiple units daily..

Different engines can have different balance factors depending on use but for most road bikes the objective is to move the 'sympathetic vibration point' away from the area of performance most used...On a road going single that's pretty much the 45-55mph area...Most weren't driven much flat out so vibration in the upper rev range wasn't really an issue...Likewise, the lower end of the rev range was something you just passed through to get to the 'cruising speed'...

Sympathetic vibration is the point where the out of balance forces that cause vibration of the engine unit reach the same frequency that the frame assembly will resonate at, thus making the entire machine vibrate...

That is why modifications to the frame matter and say, the omission of a head steady or loose engine plate studs and worn holes can make a difference...

Anyone who has ridden an M20 without its head steady fitted will know what I mean...The resultant lack of triangulation of the engine/frame assembly causes a lot more vibration at usual operating speeds as the sympathetic vibration point is altered...

So, in the case of the Enfield the balance factor for that bike would be the one to go for...If you don't have, or can't find that information 60% is probably a workable compromise that will produce an improvement...Many road bikes fall in the 55-60% range...

I don't think the figure is absolutely critical...Through rigid, plunger and s.arm versions BSA M and B Series engines always seem to have the 58% figure applied, despite quite different frame types...As the calculation is easier I've always had mine done to 60% with good results....Ian

email (option): ian@wright52.plus.com

Thanks for the input chaps. My engineer will be looking at checking the flywheels for true and static balancing. I've worked out that the dome top +40 piston that's fitted is approximately 94 grams (3.3 ounces) heavier that a standard flat top piston. This is all interesting stuff to me and i'll be using a guy who is not adverse to letting me in on the process. I'll report back.

Strangely These Royal Enfield's don't have a head steady? Maybe it's because of the large oil filled crankcase? But then from memory, nor do the Norton's? Ron

email (option): ronpier@talk21.com

The 'rule of thumb' is that a piston weight difference from standard of more than 1 1/2ozs. should mean rebalancing...

Obviously the crank should be set up to recommended tolerances regarding 'truth'...An M20 for example should run true to a tolerance of .002" max. on the main shafts and .005" max on the flywheel faces...

Regarding sympathetic vibration and the altering of it, that is the thinking behind handle bar end weights. Increasing the mass here alters the vibration frequency...Maybe you should experiment with internal bar end weights later on if there is still a problem?....Ian

email (option): ian@wright52.plus.com

Cheers Ian. That's worth considering and easy to do. Ron

email (option): ronpier@talk21.com

email (option): sam-cormier@hotmail.com

Ron, is that +40 piston an old original or a modern replacement ? Some of the modern ones seem to be very heavy,with very thick crowns.They also often have very heavy gudgeon pins.The Australian JP piston in my AJS had quite a bit of material milled out from inside the crown to get the weight down,and the pin I think we taper bored as it was so thick in the wall.

Yes Jon it's a modern Royal Enfield Bullet piston and comparing it with one of Jan's standard bore flat top originals, it's at least 3 ounces heavier. It's with 'Ainsley' now for him to check and weigh and and let me know what he intends to do. I have every faith in Ainsley, He's well known here and apart from the mundane stuff, he builds racing and speedway engines. Ron

email (option): ronpier@talk21.com

Ron, we are the lucky ones having someone we can trust... I have a good friend who builds allsorts of race engines,including "Mountain Motors".(14 litre v8 drag racing engines)
The point is he looks at every component for all the right reasons,no matter how basic the engine is he is working on.That piston you are using is probably very much a compromise,without weakening it too much it can likely have a fair amount of material removed from inside the crown - Anything your chap can do here will help.
Me and my riding mates often chat about the opportunities we now have to build engines to run sweeter,cooler,more power and last longer etc,than when they had to be almost thrown together to do a job.

I collected my crankshaft from Ainsley today. The outcome is that when he first checked it, he calculated the balance factor at 43%. After drilling the flywheels at the big end area, he's achieved a balance factor of 58%.

My 'RAF' crankcases arrive from the Netherlands yesterday and I've fitted the new main bearings and ordered new gaskets and small parts from Hitchcock's, so I can proceed with the engine build tomorrow. Ron

email (option): ronpier@talk21.com

I've rebuilt my engine into the RAF crankcase and done 2 x 7 mile test runs on Monday and the vibration is vastly reduced. I'm planning another approx 30 mile run in the morning to watch Gedd try and straighten Tim's Indian Chief forks in his frame straighten jig after he ran into a car a few week ago. Ron

email (option): ronpier@talk21.com