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vibration

derek

Well Known and Active Forum Website User
VOC Member
Do Vincents vibrate? Twins and Comets, what is considered acceptable vibration. Does any one know the aimed for frequency and amplitude. Is there an easy way of testing for the desired figures, or is there anyone that an do this?
 

david bowen

Well Known and Active Forum Website User
Non-VOC Member
If your Vincent vibrates first check that the correct long shaft ET154 are fitted to your head bracket FT3 to UFM.
 

mercurycrest

Well Known and Active Forum Website User
VOC Member
They're pretty smooth. My '82 BMW R100CS is "Buzzy" compared to my D Shadow, which kinda just rumbles along. Comets are about as smooth as a 500cc single can get. If you have an unexplained vibration don't forget to check things that aren't so obvious, like the clutch for example.
Cheers, John
 

Bazlerker

Well Known and Active Forum Website User
VOC Member
Vibration is all relative,...Vincents do not "tingle" the hands when they are properly setup, but at 55mph there is a gentle, soothing, thumping beneath the rider..
 

davidd

Well Known and Active Forum Website User
VOC Member
I think that David and John are giving you the good advice. Check things to see that things are done right. The traditional way of building engines requires a certain "balance factor" in which the flywheels are lightened to minimize vibration. The balance factors can be different for twins and singles and these factors also take different frames into consideration. Each engine builder has his balance factors which are based on experience.

Most owners do not have to worry about balance factors unless they modify their engines. The usual culprit is installing a new piston that is heavier or lighter than the existing one. If the difference is great, then the flywheels must be balanced to the new piston. The balance of the stock flywheels would be suitable for a good selection of stock pistons even with higher compression.

If the fit of the big end pin in the flywheels is too loose, the flywheels can twist. They can also be knocked out of alignment. This would be more likely on a twin than a single because the single flyweel is narrow.

Based on the above, most of the engine vibration is built in to the engine. Measuring it after the fact is not something that is done. Excessive vibration can be a symptom of a problem. To enlist more aid, it is best to describe the rpms or the speed where the vibration is the worst. Does the vibration cause your feet to vibrate off the pegs? or go numb? The same for your hands? These would be serious signs.

David
 

Monkeypants

Well Known and Active Forum Website User
VOC Member
Check the wheels to see if they are causing the problem. Its easy to do, just run the bike up to the speed at which the vibes are bad, then pull the clutch in and coast along with the engine idling. If the vibration stays about the same then the engine is not the source and the likely culprit is one or both wheels.
I had a terrible shake in my first Vincent, it was all in the rear wheel which was 1/4" Out of round and none too straight side to side.

Same thing with the Norton 650ss, when I first rode it the vibration at 80 mph was awful. The wheels were new and professionally built, but off by almost as much as the Vincent rear. Now that the wheels on both bikes are trued to .010" in both directions, the bikes are smooth runners. The wheels should also be balanced.


Glen
 

jim burgess

Well Known and Active Forum Website User
Non-VOC Member
I have ridden two prpoerly set up Vincents and they both had as much vibration as a modern Jap 4. My Black Shadow virates horribly. So I set about having it dynamically balanced. Turns out the 'rods were 55 grams different in weight. There are apparently 4 different forgings for our 'rods. Back in the good old days when you sent your crank off for a new big end, the builder pulled out the next set of rods with little or no care as to the weights. Add in different types of big end, variations in piston types, bad assembly, poorly balanced carburation, out of balance/buckled wheels.... I would strongly advise that if you follow the route of crankshaft and engineering you talk to Maughans... Normal balance fator is 46%, it will vary for other types of frame. Bon Chance!
Jim Burgess
 

ogrilp400

Well Known and Active Forum Website User
VOC Member
Glen,
At 60mph, a mile a minute, the rear wheel rotates at approximately 780 revs per minute. This is a very low frequency compared to the 4.5 times that, very high frequency associated with engine vibration.
I have a Rapide that vibrates quite badly to the extent it has split the rear of the tank even with the tie bolt in. The bike had been involved in an accident and a thought was that the flywheels had shifted. Dissassembly of the engine showed .004 round out, hardly huge but room for improvements. The engine has 8 : 1 specialloid pistons in it and reverse calculations show the balance factor to be with in 6 grams of 35%. I was loath to deviate from this figure as the engine is in very original condition and quite good inside. Dynamic balancing only needed two small holes drilled partially into the flywheels on the crankpin side, one 3/16" on one side1/4" on the other flywheel. Truing the crank to .002 and reassembly, I had I hopes that it would be very smooth. Sadly I was dissappointed as it still vibrates quite badly. Up to 60mph it is smooth, over that and the vibrations creep in. By 70 mph the bars are "expanding in diameter" and the tank is "doing an orgital sander" I have tried different tensions on the cylinder head bracket bolts but with no improvements. My Norvin, norvins known to be a bit more vibratory than a standard bVincent, is markedly smoother than my HRD. I'm at a loss to explain it so just keep riding it.
 

Monkeypants

Well Known and Active Forum Website User
VOC Member
In trueing these wheels I took out mutiple lumps, big and small, not just a single high spot. Add in additional vibration caused by the lateral runout and you have potential for several thousand shakes per minute at 70 or 80 miles per hour. The result feels a lot like engine vibration from a slow revver such as a Vincent.
The overall ratio for a twin with standard gearing is 3.5 to one, not 4.5. Engine RPM at 60 mph is only 2760.
If you havent already done so, it would be a good idea to check the wheels for out of round, out of plane and out of balance (with tires on).
A couple of hours spent with a spoke wrench and dial indicators can transform the bike, with zero cost for parts or specialist help.
Glen
 
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Glyn Baxter

Forum Website User
VOC Member
I suspect that your vibration problem could well be due to the balance factor you are using of 35%. The correct value, according to the book is 47%. My Shadow engine is balanced to this factor, with the drive side shaft showing zero run out and the timing shaft 0.002" run out, and there is merely a slight tremor in the bars.

Glyn Baxter
 
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