Dynamic Balance

worntorn

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My 1360 always vibrated a lot. There wasn't a smooth spot anywhere, the higher the revs the higher the vibration level.
The fellow who supplied the crank and a lot of other motor parts suggested it would smoothen out after break-in .
It didn't.

I messed with carb synch and valve timing to no avail.
Finally two weeks ago I disassembled the engine to have a dynamic crank balance done.
This meant pressing the flywheels apart here in my shop (12 tons!) , removing the rods and rod bearings, then reassembling and trueing without rods. I
He insisted that I true the crank to the same spec as it would get on final assembly, which makes sense, but what a job for a temporary thing! I was at least 5 hrs getting it to run to spec. You get close then get far away again, start all over. It doesn't help that this is a " Picador" type crank, considered unserviceable when Vincent built them. That was because of the 12 ton interference fit which makes the crank tend to hang together but very hard to adjust in tiny increments.

The balancer then makes up a temporary bobweight which fits on the crank pin for the dynamic balance.

The balancer couldn't drill the flywheels with his little Mill/drill as the flywheels are made out very hard steel.
I brought the crank home and drilled it to his specs in the K&T mill. It was a lot , approximately 330 grams or 3/4 of a pound to get the 60 percent BF we were after. There was also a 22 gram side to side difference , so we corrected that as well.
When it came back from the Balancer for the final time, I pressed it apart and refitted the conrods. After reassembly and trueing I checked the balance factor with the crank in the lathe on oiled centres.

With a made up weight hung on the connecting rods( made to 60% BF calculations) the crank was perfectly balanced. It would sit whever it was left.
I added one flat washer to the weight and it wouldn't balance, it would rotate so that the conrods hung straight down.
So his balance job was spot on at 60 %.

I finished reassembling the bike today and went for a ride.
What a difference. Its now as smooth as a 998 Vincent at 70 mph and that is pretty smooth. From 80 mph on up it is smoother than the 998 s, .
For the size of motor and the tune level, this is a good outcome.
After many hours of struggle, I managed to get the mainshafts running to a total indicated runout of .001", that probably helps a little. Runout was only .0015" before, so this is only a slight improvement.
I think the dynamic crank balance is the main factor in the improvement.

The only way to go!


Glen
 
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Glen That's really interesting. I am glad you got that bike to run smoothly. As Grandpaul said that shows real persistence. I was beginning to wonder if maybe the engine was just to big to run smooth.

From what you are saying though if the engine had been static balanced properly to a 60% balance factor. the balance would have only been out by 22 grams side to side. Before you took off the 330 grams what would the balance factor have been? Did that weight come off the pin side of the crank or the side opposite (was the balance factor to high or to low)

I am assuming that you took the weight off the side further from the pin because usually if you have to take more of on the pin side it is not possible and you have to add weight (by adding tungsten) to the other side. If that was the case the balance factor must have been much to high. If that is the case and assuming the pistons weigh around 500 grams each, and also assuming that the small ends of the rods are about 250 grams each. (the rods might be heavier than that) Assuming all of that to balance the crank at 60% the weight you would need would be 900 grams. But if that weight was 330 grams heavier than that (1230 grams) then the balance factor would have been 82%. It was out by 22%! Maybe since the balance factor was the same as a Triumph Bonneville it was vibrating like a Triumph!

I have a Terry Prince crank standard stroke. I think for sure I will check the static balance before I use it. I am lazier than you so I may not bother taking the crank apart to dynamic balance the whole thing. My Rapides crank was dynamic balanced though. Neville Higgins seemed to believe in accurate static balancing, maybe because he wanted to do everything himself. I think he sometimes balanced the flywheels individually and in that way he would have corrected most of the side to side imbalance as well. Maybe he was right though. I wonder if within 22 grams is close enough?

I think sometimes there is confusion about the balance factor. The way you checked the balance after assembly causes some of the confusion. The thing is sometimes people do this by just applying the balance factor to the piston weight only and they ignore the rod weight. I often wonder about the people who state that some Vincents were balanced to 35% and were very smooth whether they had done the calculations the same way you did.
 
From what you are saying though if the engine had been static balanced properly to a 60% balance factor. the balance would have only been out by 22 grams side to side. Before you took off the 330 grams what would the balance factor have been? Did that weight come off the pin side of the crank or the side opposite (was the balance factor to high or to low)

I am assuming that you took the weight off the side further from the pin because usually if you have to take more of on the pin side it is not possible and you have to add weight (by adding tungsten) to the other side. If that was the case the balance factor must have been much to high. If that is the case and assuming the pistons weigh around 500 grams each, and also assuming that the small ends of the rods are about 250 grams each. (the rods might be heavier than that) Assuming all of that to balance the crank at 60% the weight you would need would be 900 grams. But if that weight was 330 grams heavier than that (1230 grams) then the balance factor would have been 82%. It was out by 22%! Maybe since the balance factor was the same as a Triumph Bonneville it was vibrating like a Triumph!

I have a Terry Prince crank standard stroke. I think for sure I will check the static balance before I use it. I am lazier than you so I may not bother taking the crank apart to dynamic balance the whole thing. My Rapides crank was dynamic balanced though. Neville Higgins seemed to believe in accurate static balancing, maybe because he wanted to do everything himself. I think he sometimes balanced the flywheels individually and in that way he would have corrected most of the side to side imbalance as well. Maybe he was right though. I wonder if within 22 grams is close enough?

I think sometimes there is confusion about the balance factor. The way you checked the balance after assembly causes some of the confusion. The thing is sometimes people do this by just applying the balance factor to the piston weight only and they ignore the rod weight. I often wonder about the people who state that some Vincents were balanced to 35% and were very smooth whether they had done the calculations the same way you did.

Hi Nigel

In educating myself on balance factor I encountered three different versions for the temp. bob weight calculation.
The calculation used by Hines, the balancer manufacturer, is simple and seems to be the most widely used.

Its the same calculation that Jim Comstock described in his excellent post on balance factor. The temporary bobweight should be made equal to( 100 % of the rod big end weights complete with bearings and collars)plus the desired balance factor x (the rod small ends + complete piston weights).

From my calculations the original bf for this crank was close to 90 %.
I'm not sure if the 22 gram side to side made much of a difference, but every correction helps.
You are correct, the weight all came off opposite the crankpin.

I too wonder if some of the unusual bf numbers come from use of a different version of balance factor.
Maybe that's why this crank was so heavy on the counterweight side.

Do you find your Rapide to be a smooth runner?

Mine are both quite decent and similarly smooth to about 75 mph, then vibes creep in.
The OZ Rapide has a statically balanced crank ( by Terry Prince), the McDougall bike has a dynamically balanced Maughans crank.

Glen
 
It would be interesting to know which made the bigger difference, the change to BF or the side to side. Balance factors move the sweet spot around and side to side will have an effect that just increases with revs. As you had vibration at all revs then one scenario could be the old BF was good at high revs but you never experienced that due to the rocking couple kicking in when the BF was hitting its sweet spot. Considering the efforts you had to go through it would have made no sense for you to add to that by doing it twice over so it will stay a mystery and of course the BF of 90% could have been so far off there was no sweet spot at all.
 
Glen

My Rapide is pretty smooth even at 80 or 90 MPH. Not as smooth as my Ducati. The vibration it does have is less intrusive than the vibration of my Triumph 955 Sprint. The Sprint is almost perfectly smooth but there is a high frequency tingle that can even cause my right hand to tingle after many miles. The Vincent has no vibration like that. (except for Allyson's feet)

Allyson my passenger said she never noticed any vibration at all on the Rapide until I installed the luggage rack and panniers, then her feet started vibrating rather badly.

I think that when some people calculate the balance factor they are only figuring it out to do a static balance with the rods assembled on the crank. When they say a 50% balance factor they mean that they balance the assembly with only 50% of the weight of the pistons. I would think that by the more normal way of calculating it that would put those engines at around 35%. It would have to be less than 50% anyway.

The thing that always bugs me is to do with the Vincents that apparently were balanced to only 35%. Apparently some of those were the smoothest of any of them.
 
One of us will have to take apart our perfect running motorcycle and re balance it to 35% to find out. I don't think that will be me!
 
Won't be me either!
However if one of mine ever needs a bottom end refurbish, the crank will definitely make a short trip to the local Hines dynamic Balancer.

Strange that your Sprint 955i has the tingleys. They are generally known to be smooth runners.
I have the Daytona version and really can't feel any vibration anywhere from idle to 12,000 rpm. I think Fast Eddie Nigel would say it is too smooth! For me there is no such thing.

It does have all different engine internals than the Sprint.
Or maybe there is still a little variation even in modern production bikes.

Glen
 
The Sprint does seem almost dead smooth. The engine is almost as smooth as a Goldwing. But after a whole day of riding you do start to notice the vibration of the handle bars. I think I am going to change the handlebars. It isn't as comfortable as the Rapide for a very long day of riding. It has a Corbin seat that is a bit too hard. It is amazing that it really isn't as good as the Rapide in some ways. It is a lot faster and I guess it uses less gas you can't really compare the power the braking or the suspension, they are in a completely different category.
 
I had a 1000 Twin with a Maughans crank, dynamically balanced by Basett down. It was smoother than my Commando.
I also had a 1330 Godet Egli, Patrick also has all his cranks balanced by Basset Down, it shook a bit at tickover etc but on the open road it was amazingly smooth.
 
Vibration can be due to pistons being too heavy. Balance factor is usually higher if the motor is intended to run at higher revs. it is a matter of setting it to suit the bike's operating conditions. It doesn't matter what you do a four stroke single or twin will be out of balance somewhere in the rev range and vibrate. Even rotating balance counterweights geared to the crank, will not stop what is really happening.
 
Is there someone in the states that offers a dynamic balance service ?
 
Is there someone in the states that offers a dynamic balance service ?

Jimbo, I'll bet there are hundreds of places that offer standard dynamic balance service.
I only found one that could do a rods on dynamic balance and as it turned out, that machine was for Harley only.
For split conrod type engines such as Norton twins, a standard Dynamic balancer is ideal. Jim Comstock has one and he knows the best numbers to use.

Glen
 
Jimbo, I'll bet there are hundreds of places that offer standard dynamic balance service.
I only found one that could do a rods on dynamic balance and as it turned out, that machine was for Harley only.
For split conrod type engines such as Norton twins, a standard Dynamic balancer is ideal. Jim Comstock has one and he knows the best numbers to use.

Glen
I'll try to convince JIM C to reconsider working with my Royal Enfield crankshaft, last time I asked he didn't want to.:(



Dynamic Balance
Dynamic Balance
 
Thanks , but I would rather find a shop that is familiar with 360* vertical twin motorcycle crankshafts and the balance factors needed. . I don't need to educate when I don't know enough myself! Why reinvent the wheel. I can send the crankshaft anywhere, but preferably the USA
 
Jimbo ,I don't think that matters, a dynamic balance by a trained tech should get the result needed.
The shop I used had no knowledge of the balance factor for my crank, I gave them that info. Not surprisingly, they had never seen a crank like it. There are just 3 of these cranks in the world at the moment.
None of that mattered, It came out great.
I guess a dynamic balance is a dynamic balance, as long the tech is properly trained and knows how to utilize the dynamic balancer.( in this case trained by Hines)

I'm sure that you can find the original factory spec BF for the Enfield crank and provide that number to the balancer.
If you are only looking for a balance shop already familiar with RE cranks, it could be a tough search.

Glen
 
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