smooth 65% balance factor and best sealer

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The oil mass is added to the rotating weight and opposes the flywheel counterweight. Jim
Click to expand...

Ok that's the geometry fact I was missing up on.

Btw is it possible for the crank speeding oil out the shells to out run the oil supply and essentially run dry? At least to the practical red line limits. We know zero pressure showing at hwy speed is not much an issue as so common in past but might be approaching oil starvation. What's your sense of this, w/o some obvious oil supply fault?
 
hobot said:
The oil mass is added to the rotating weight and opposes the flywheel counterweight. Jim
Click to expand...

Ok that's the geometry fact I was missing up on.

Btw is it possible for the crank speeding oil out the shells to out run the oil supply and essentially run dry? At least to the practical red line limits. We know zero pressure showing at hwy speed is not much an issue as so common in past but might be approaching oil starvation. What's your sense of this, w/o some obvious oil supply fault?
Click to expand...

I am sure there is plenty of oil being pumped to the crank to keep the sludge trap full. Jim
 
As Irving has been mentioned it is also not well known that Irving also designed a 4 valve head for his Vincent in the event that more power might be required or that the change might be a powerful sales stimulant. There is a brief mention of this head in Vic Willoughby’s book Classic Motorcycle Engines.
The November issue of Old Bike magazine (Australia) has an article on a search for the plans which were kept in his former Australian home. English Vincent enthusiast Andy Thompson obtained copies of the originals and had the heads cast for use in his Vincent Comet outfit .According to the article, the power of the Comet went from 34 to 44 horses after fitting the 4 valve head.
ando
 
Vincents may well be the best steam punk looking and functional craft ever fielded as well as first in many modern trends.
IF only had developed a rubber isolation...

Prior here it was stated sludge trap only filled halfway with oil while Jim C states its kept full. Which is it or does it change with rpm, ie: sometimes full something half empty running?

As the engine is the source of vibration, what ever allows it best smoothness all by itself, implies it'd also be smoothest in what ever mounting installed. In solid mounts there may be resonate ranges that feel better at higher BF, but Schmidt's report implies best to aim for similar BF as rubbery mounts. BSA research showed least internal 360' crank engine stress gotten in the low 50's wet BF.
 
hobot said:
Vincents may well be the best steam punk looking and functional craft ever fielded as well as first in many modern trends.
IF only had developed a rubber isolation...

Prior here it was stated sludge trap only filled halfway with oil while Jim C states its kept full. Which is it or does it change with rpm, ie: sometimes full something half empty running?

As the engine is the source of vibration, what ever allows it best smoothness all by itself, implies it'd also be smoothest in what ever mounting installed. In solid mounts there may be resonate ranges that feel better at higher BF, but Schmidt's report implies best to aim for similar BF as rubbery mounts. BSA research showed least internal 360' crank engine stress gotten in the low 50's wet BF.
Click to expand...

Any oil that was "slung" out of the crankshaft by centrifugal force would have to be replaced by something. Even without an oil pump, oil would be drawn into the crankshaft -as long as there was oil there in the first place.

The only way to decrease the amount of vibration in a 360 degree twin is to reduce the reciprocating mass. Changing the balance factor only changes the plane of the forces- not the amount of force. IE as the vertical shaking forces are decreased by raising the balance factor then the horizontal shaking forces are increased by exactly the same amount. Jim
 
Was out on the Commando yesterday evening, paying close attention to vibes. It gets totally smooth above 3200 rpm and stays that way right to 6500. Below 3200 thereare just enough vibes to remind you that you are on a motorcycle that is running. At 3,000, it is barely noticleable, at 2500 it reminds you to either shift down or twist the throttle a little (or a lot). If this is typical of a stock Commando, then vibration is a non-problem with these bikes. That was the original plan by Hooper and Bauer wasn't it?

If I ever need to rebuild the engine I will have a good look at the JS pistons mainly because they look to be high quality items. If the sub 3200rpm vibration is lessened, that would be a side benefit, but not a big one because I don't ride at that RPM for long.

Glen
 
Dances with Shrapnel said:
You really cannot go wrong with the lighter (and more durable) components that JS offers. Lighter is almost always better.
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That is true as long as what you are looking at is the amount of vibration. Maybe not so much when you take into consideration the longevity of the lighweight parts. They are racing pistons. Jim
 
Fair enough but do you really think there's that much of a difference in durability.

Granted, about the most obvious difference is less skirt area and thinner rings of race pistons but then there is the improved location of the wrist pin, probably improved alloy and lower skirt thrust due to lower mass of the small end of the rod.

To me, the thinner rings would probably be the biggest difference in rate of wear.
 
When the isolastic system was being developed, every imaginable type of rubber was tested. It was found that in order to remove the more gentle low rpm vibrations a fairly soft rubber was needed. This soft rubber was overwhelmed by the high amplitude Atlas type vibrations that showed up at higher Rpm. With the soft rubber in place, things were smooth up to a certain speed (fairly low I think) then the rider really got nailed.
The eventual solution was what we use now- harder rubber that lets some (not all) of the low speed vibes thru but really soaks up the powerful vibrations from 3,000 rpm on up. It makes for a very rideable machine, one that in terms of vibration is the opposite of every other bike I have owned or ridden in the last 46 years, (the newish Triumph triple excluded no vibes at any rpm), as rpm increased the vibration also increased. The Commando is the only one that you rev up to make vibration go away.

So this has all been tested and designed as a unit way back when. Often when you change one thing, it changes everything, not always for the better.
If the vibration amplitude of the engine at cruising speed of 4000 RPM is, thru use of lighter pistons, rebalancing, or some other change reduced to what is once was at 2500 rpm, why won't the isolastics respond just the way they do at 2500 rpm? At 2500 rpm they let a good deal of vibration come thru, we know that.
I realize that the frequency of the vibration is higher at 4,000 rpm, but I believe the isolastics, as they are, require sufficient amplitude to function properly. I don't see frequency as the important factor.
This is a delicately tuned part of the bike. I think it was recently mentioned that the 750 isolastics are a bit softer than the 850 isolastics. Presumably Norton saw the need to tune the elastics differently for the larger shaking forces present in the larger motor.

Glen
 
It is the short piston which causes rocking in the bore which makes a race pistons life shorter. When the piston rocks the rings do not seal as well against the bore.
The reduced contact area between the skirt and bore will mean higher spot loading and does increase the wear- both to the piston and bore.
Moving the pin closer to the top of the piston also places the side thrust nearer the ring lands which are not able to cope with the thrust as well as a full skirt.
The pistons available today all take advantage of new alloys whether they are cast full skirt pistons or slipper skirt race pistons.
The forged pistons advantage is in its ability to remain strong when hot. There is no advantage in wear due to the fact that is forged.
The reduced life of a race piston has been well documented over the years. Good things are not without their tradeoff Jim
 
Jim Comstocks says that with isolastics - "The least vibration felt with a standard iron barrel motor was at about 58% dry which would have been in the lower 50's wet. When I went to an aluminum barrel the vibration in the bars increased so I raised the balance factor to around 62% dry which made it better." (mid/upper 50s wet).

Dave Watsons winning 1007cc in a Seeley frame ran smoothest with 65% wet BF and that was with the stock rods and heavy pistons. Then he went to lightweight JS pistons and it "ran much smoother". I took this as encouragement to reduce the BF in my featherbed to 65% wet and yes it is smoother than 72% wet. I don't know for sure at what point it vibes worse when the BF gets too low. But I remember trying 50% wet in a featherbed and had to bring it back up because it shook pretty bad. And I haven't heard anything good about putting a commando BF crank into a solid frame. My Freind Mic Olfield tried that once at a National race in a emergancy when his motor blew. It got him through the race but he swaped it out later.

As for longevity. After 4 hard years on the lightweight pistons I have just pulled them out when rebalancing to 65%. Everything looked good and I didn't even change the rings (I usually always do that). Its up & running again.

My pistons are ultra light at 170 grams and this Atlas is now smooth enough to ride around all day without being bothered.
 
Jim. Air is a great replacment for oil! sucked in via the cover seal.
comnoz said:
hobot said:
Vincents may well be the best steam punk looking and functional craft ever fielded as well as first in many modern trends.
IF only had developed a rubber isolation...

Prior here it was stated sludge trap only filled halfway with oil while Jim C states its kept full. Which is it or does it change with rpm, ie: sometimes full something half empty running?

As the engine is the source of vibration, what ever allows it best smoothness all by itself, implies it'd also be smoothest in what ever mounting installed. In solid mounts there may be resonate ranges that feel better at higher BF, but Schmidt's report implies best to aim for similar BF as rubbery mounts. BSA research showed least internal 360' crank engine stress gotten in the low 50's wet BF.
Click to expand...

Any oil that was "slung" out of the crankshaft by centrifugal force would have to be replaced by something. Even without an oil pump, oil would be drawn into the crankshaft -as long as there was oil there in the first place.

The only way to decrease the amount of vibration in a 360 degree twin is to reduce the reciprocating mass. Changing the balance factor only changes the plane of the forces- not the amount of force. IE as the vertical shaking forces are decreased by raising the balance factor then the horizontal shaking forces are increased by exactly the same amount. Jim
Click to expand...
 
comnoz said:
The reduced life of a race piston has been well documented over the years. Good things are not without their tradeoff Jim
Click to expand...

Care to give your opinion on life of a stock piston compared to that of a race piston as it pertains to a Norton Commando for street use? Is it fair to say that people generally install race pistons to race, thus a different application to that which I had in mind.

The reason I ask is that most everything comes as a compromise. A gain in performance and reduced vibration is the very desireable trade off I am seeing here.

Just my personal preference here but knowing the limited miles I ride my street Norton (an I suspect a majority of Norton owners), I don't think relative piston/ring durability is a factor - you will need a head job before then.

Now if I were looking at doing a couple of cross county trips I would want to see some history on the lighter pistons.

On the other hand, take a look at some late model Ducati pistons. Yes, I know, different bore and stroke but...
 
john robert bould said:
Jim. Air is a great replacment for oil! sucked in via the cover seal.
comnoz said:
Very true if there were no oil pump. With an oil pump hopefully there would be at least a slight amount of positive pressure to keep the air from being drawn in. Jim
Click to expand...
Click to expand...
 
Dances with Shrapnel said:
comnoz said:
The reduced life of a race piston has been well documented over the years. Good things are not without their tradeoff Jim
Click to expand...

Care to give your opinion on life of a stock piston compared to that of a race piston as it pertains to a Norton Commando for street use? Is it fair to say that people generally install race pistons to race, thus a different application to that which I had in mind.

The reason I ask is that most everything comes as a compromise. A gain in performance and reduced vibration is the very desireable trade off I am seeing here.

Just my personal preference here but knowing the limited miles I ride my street Norton (an I suspect a majority of Norton owners), I don't think relative piston/ring durability is a factor - you will need a head job before then.

Now if I were looking at doing a couple of cross county trips I would want to see some history on the lighter pistons.

On the other hand, take a look at some late model Ducati pistons. Yes, I know, different bore and stroke but...
Click to expand...

Lower piston speeds and better cooling make big improvements in piston life with modern stuff. Even then I have heard of problems with oil consumption on a few of the Ducks.
I am all for using lightweight stuff in a bike that is a limited use hot rod or a race bike. I suspect the life will be plenty long enough for that.
If I were building a bike that is going to be a rider for years to come I might look at something designed more for longevity.
As for as how long can a set of full skirt pistons last- I have seen oem Norton pistons over 100,000 miles.
The full skirt pistons in my bike have over 80,000 miles on them now. And yes it has had head work but I didn't need to spend
money for pistons, boring or new rods. Jim
 
The problem with full skirt & all pistons is that the ring lands still get pounded out from use - they get sloppy and they start to smoke. The bore still wears and the rings still wear and you lose compression. There is no point in making your piston skirts last longer than your rings/ring lands and when the ring lands get loose - the piston is junk. In the mean time - all that extra vibration from having long skirts can crack your crank and cases and other parts as well and that gets very expensive. The goal is to design a piston that is light without losing longevity.

So far there is only one customer with lightweight JS pistons who put so many miles on a Norton that the top end needed a rebuild and it was because of excessive wear on the bore. Bore wear is measured near the top because of the rings - as shown by the difference at the ring ridge.

The top photo is a lightweight JS pistons with a shortened but reasonable skirt (and tang) - designed to reduce vibes and save your motor and last as long as the rings.

smooth 65% balance factor and best sealer




The photo below is of an extremely short Ducatti racing piston with minimal skirt (no tang). They are not the same.

smooth 65% balance factor and best sealer
 
I once fitted modified BSA Gold Star 350 pistons to a 650cc Triumph, they were a bout 30gm lighter than the standard ones. The motor spinned up quicker. I'd machined the edges of them to give a bit of squish band. I don't understand this stuff about sealing the crank. I use a PCV valve to suck the seals and gaskets and reduce oil leaks, there is always blow-by past the rings.
 
The problem with full skirt & all pistons is that the ring lands still get pounded out from use - they get sloppy and they start to smoke. The bore still wears and the rings still wear and you lose compression. There is no point in making your piston skirts last longer than your rings/ring lands and when the ring lands get loose - the piston is junk. In the mean time - all that extra vibration from having long skirts can crack your crank and cases and other parts as well and that gets very expensive. The goal is to design a piston that is light without losing longevity.
Click to expand...

Hm, good alert to me. I must check Trixie Combat lands someday as even with 2nd set of new rings, .20 over bore on the tight side of normal she still pushes oil out if i use much throttle over 70. MIght just be grit form air boot leak though. Smokes on alternate sides on first cold start ups but then clears up in a min. rest of the days stop and goes. Wes says he thinks they didn't hone quite deep enough so some low spots blow by.
 
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