Piston Weight and Balance Factor?

ludwig said:

That amount of imbalance could be detected by bolting the 2 crank cheeks ( minus flywheel) together at 180° and balance it.
Not perfect, but close enough for most..

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Had further thought on this.
Ludwigs idea (above) would, I think, work and is actually a good approximate way to dynamically balance.
Great for those not near a capable shop or are short of cash.

robs ss said:

Had further thought on this.
Ludwigs idea (above) would, I think, work and is actually a good approximate way to dynamically balance.
Great for those not near a capable shop or are short of cash.

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I now see what you mean. It would interesting to find out if there is any rocking couple with a standard crank. But I think that, considering the total mass involved witn a Commando crank, you would not feel it, if it was there - , unless you lightened the flywheel.

it is likely to be a small rocking couple. However, if one is going for FINE tuning, then may be worth addressing: depends on priorities. The reversing method is a good tip!

acotrel said:

This is all stupid discussion. A Commando crank is probably an Atlas crank with a hole drilled in rhe counterweight to stop the bike from shaking at low revs. When you fill the hole with steel, the balance factor becomes about 70% - which is correct for a road-going Atlas. The guys who raced the Atlas, used to rev them to 8,000 RPM, and used an even higher balance factor. But if you drill holes in a cast irom flywheel, you are asking for trouble. The cheeks on the crank are probably forged steel. Better to replace the flywheel with a steel one, than fill the hole in the cast iron flywheel.But if you do noy intend to race your Commando - forget it.
Commando cranks kill crankcases if revved high - probably due to the low balance factor. Motors which rev high, run snooth if the balance factor is high. Atv 7000 RPM, my 850 motor would rev eassily to 8000 RPM. I stop it from doing that, because I am superstitious, and I don't want to die with a crank in my chest. With my Seeley, there is not much obove the motor.

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Stupid?
revving an Atlas to 8000?
"probably"
If you ever ride your bike again, and the crank breaks, the flywheel will come out the bottom. There is as much above the motor as there is in an Atlas
Your experience of Commando cranks? you keep on about mythical holes to screw a counterweight into etc. When was the last time you actually saw one?
Please do a little bit of research on the subject

Do not get your crank rebalanced by car guys, Phil Irving's book Tuning For Speed tells you how to do it.

Seeley920 said:

Stupid?
revving an Atlas to 8000?
"probably"
If you ever ride your bike again, and the crank breaks, the flywheel will come out the bottom. There is as much above the motor as there is in an Atlas
Your experience of Commando cranks? you keep on about mythical holes to screw a counterweight into etc. When was the last time you actually saw one?
Please do a little bit of research on the subject

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The hole is the inch diameter one which has been bored into the counterweight to turn an Atlas crank into a Commando crank.
The 850 motor in my Seeley would rev easily to 8000 RPM and possibly reliably. I stop it at 7,300 RPM. It gets there extremely quickly.
I am too lazy to photograph my spare crank.

Fast Eddie said:

Triumph cranks were not machined all over until the TSS I believe.

I think Triumphs problem (as with most of the others) was consistancy. A good Triumph was / is surprisingly smooth, a bad one is basically self destructive.

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Triple cranks are machined all over of course. Just one reason as to why they are better than the twins (ducks and runs for the door).

SteveBorland said:

Triple cranks are machined all over of course. Just one reason as to why they are better than the twins (ducks and runs for the door).

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I think the 120 degree throw kinda helps too…

Lighter pistons are better. Every time a piston reaches top or bottom it reverses direction, so inertia is a consideration. Commandos use a low balance factor to reduce vibration at low revs. If the balance factor becomes higher because of lighter pistons, the motor would be more suited for slightly higher revs. So it does not matter.
I would never give anything motorcycle to a car guy to be fixed.

Fast Eddie said:

I think the 120 degree throw kinda helps too…

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Triples have their own problems. The one I rode stood up very easily, and it was not engine power which did it. When you accelerate on a motorcycle, it rotates fore and aft around it's centre of gravity. Accelerating a triple while in a corner might not be good. There is an on-board video of a triple on the IOM, the rider always gets it very straight, before giving it the berries.

Dan1950 said:

I picked up a postage scale at Harbor Freight. I realize it's probably not accurate enough for precise balancing, it still revealed that my .020 over JS pistons are more than 1oz. lighter than the std. bore OEM specimens that were in my engine.

How will this affect the balance?

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Have you removed the gudeon pin and weighed them without , it could be the pin , you can swap the pins over or put piston in lathe and turn certain areas or drill press and use a shallow ground drill on the underside of crown , shallow though .

My friend reads all about tuning on a Triumph forum. He sent a one piece Triumph crank to a car guy to get dynamically balanced. It came back with a piece ground off one of the cheeks - STUFFED ! Balancing a Triumph or Norton crank IS NOT rocket science, and Phil Irving was not a dill. When I balanced my 850 crank, I made parallel straight edges welded together - grabbed one end in a vise, and supported the other - using an engineers level. Then used a cheap electronic balance and a calculator. I did not drill the flywheel. Filling the idiot hole in the counterweight with steel was enough. If the balance factor is between 70 and 80 percent of the reciprocating weight, it will be OK.. The standard balance factor is car guy bullshit.
Isolastics are external - they do not stop the crank from trashing the crankcases, when you rev a crank which has a 54% balance factor.

There is some misunderstanding about balancing.

If you just lighten the pistons without rebalancing then it will shake less vertically and the same horizontally. If you rebalance it for the lighter pistons then it will shake a little less in both directions than originally.

If you add heavier pistons without rebalancing then it will shake more vertically and the same horizontally. If you rebalance it for the heavier pistons then it will shake a little more in both directions than originally.

jseng1 said:

There is some misunderstanding about balancing.

If you just lighten the pistons without rebalancing then it will shake less vertically and the same horizontally. If you rebalance it for the lighter pistons then it will shake a little less in both directions than originally.

If you add heavier pistons without rebalancing then it will shake more vertically and the same horizontally. If you rebalance it for the heavier pistons then it will shake a little more in both directions than originally.

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well said. One of the problems in perception is that people talk about static balancing of piston/rod etc, whch is like balancing things on scales, where it is all nice and stable. BUT the 360 degree crank is a fast SPINNING lump, and can never be perfectly balanced ( unless there are no rods and pistons involved!)

Agreed - "balance factor" is simply how much vertical shake you want to reduce at the penalty of horizontal shake.
I suspect the rev/frequency issue is more to do with the natural frequency of the frame/fittings that the engine, which is merely the exciter.
Much of the uncomfortable vibration can be substantially reduced by paying attention to mounting of fittings.
Cheers

robs ss said:

Agreed - "balance factor" is simply how much vertical shake you want to reduce at the penalty of horizontal shake.
I suspect the rev/frequency issue is more to do with the natural frequency of the frame/fittings that the engine, which is merely the exciter.
Much of the uncomfortable vibration can be substantially reduced by paying attention to mounting of fittings.
Cheers

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Congratulations, you are quite correct (Several years of my life were spent doing vibration surveys, modal analysis and problem solving in the aviation business, including on the Norton rotary engines/installations for manned and unmanned aerial use).

Dynamic loads at the main bearings due to engine out-of-balance forces increase in proportion to the square of the engine speed e.g. the load at 8000 rpm is 64 times the load at 1000 rpm regardless of the value of the balance factor. The isolastic frame was introduced as a clever solution to the twin cylinder engine vibration (and much cheaper, lighter and more effective on a road bike than any other method). Norton had no money for a 3 cylinder engine.
The shaking at low speed of correctly adjusted isolastics shows that they are working as intended and is no problem at all.

The beauty of the isolastic system is that even though it was designed for an 8.9 to one cr 750 cc engine it still works perfectly with an 850cc engine or a 920cc engine at 10 to one cr, and that is with standard weight reciprocating items. I would guess that a 1007cc would also be smooth in an isolastic frame.
The isolastics do add a few pounds to the bike so I was tempted to go with some type of solid frame when attempting to build a very lightweight bike using the Commando engine.
In the end I realized that the weight of the isos wasn't much compared with the benefit of smooth running above 3000 rpm. The compromise was to make the iso tubes and mounts in al.

If I was building a solidly mounted parallel twin engined bike for the road then it would be best to do everything possible to reduce the reciprocating weight. This would be even more necessary if the engine was overbored and high compression. Both of those factors increase the shaking forces a lot.

Even so, I doubt that any version of a solidly mounted Commando powered bike would approach the smooth running of the stock isolastic bike.
You really start to appreciate those isos at about mile 250 on a 400 mile day!

Glen

There is an option besides isolastics - adding weight to the flywheel helps smooth out the vibes. Nourish cranks are the heaviest so far but you can make probably them heavier. An alternative to adding weight with isos. Combine that with Extreme lightweight pistons with drilled skirts (170 grams for 750, 180 grams or so for 850) along with taper bored pins and ultralight rods. I've been running pistons with drilled skirts since the 1980s with no problem. The Vance & Heins dragster employed drilled skirts to avoid scuffing.

early experiments racing with 750 lightened Hepolites around 1984





The first lightened Wiseco 850 pistons around 1985




Drilled skirt and tapered pin bore



Checked for wear on early tear down. This piston has been in my test bike over 50,000 miles now.


3D Undercrown milling for consistant .150" thickness around valve pockets. Some Triumph race bikes have run as thin as .120"



This technique of adding weight (500cc racer) could be applied to a stock Commando crank.

That could work too as long as you have lots of money in the hot rodding budget.
On the other hand, Steve Maney feels his lightened cranks greatly reduce nasty forces on the cases. I know that he broke quite a few stock cases and crank before developing his strengthened components.

I can't say that I have a preference for crank weight for road riding. The Thruxton R has a very light 17 lb crank which Triumph says gives it very fast throttle response. I guess it does but the stock 850 Commando is 22+-? and they seem to respond to the throttle plenty fast when in tune.
My 1360 Vin has a 28 pound crank and has much faster throttle response than the Triumph or Commando. I think that might be due to the accelerator pumps on the Dellorotos.
All of that might matter in drag racing but really isn't important on the road.

In my case with the 920 the low miles ebay stock MK3 crank was $140 , the Dynamic balance was $170 and the home made iso mounts cost about ten bucks for material, so that all solved the vibration issue very nicely at a very low cost.

I would guess that a custom crank and all lightweight reciprocating parts would cost 6k us or so, about my total budget for parts and material on the lightweight 920.
For racing though I think those expensive parts are a safety necessity, along with strengthened cases and a TTI gearbox. In short, get rid of pretty much everything Norton for a race engine!

Glen

Having the pistons as light as can be achieved without compromising reliability is the ultimate aim but, that is for racing. Norton were never going to do that for a road bike where the costs and development times were imposed on the engineers. I also recall adverse comments about the pistons used in the Dunstal 810 kit.
The increased flywheel inertia, reduces the rotational speed fluctuation due to the mass of both pistons stopping together twice per revolution. Lighter pistons further reduce the rotational speed fluctuation. The increased flywheel inertia has a similar effect on the engine power delivery. The resulting smoother power delivery reduces transmission shock loads and losses, resulting in a higher top speed (as discovered by Doug Hele when testing a Manx with different flywheel weights).
The engine is a fairly complicated spring/mass system and any change to a mass in this system (e.g. adding weight to the flywheel)) is going to change natural frequencies, but I don't see that increasing the flywheel weight a relatively small amount is going to have a noticeable effect on the out-of-balance forces being transmitted to the frame.