Piston Weight and Balance Factor?

worntorn said:
Jim Comstock made a great video on this but I cannot find it. The statically balanced crank bounced around a lot and upset some folks. He might have removed the video for that reason.
I see that his Dynamic Balance machine now has safety loops on it.
I did find this post by Jim.
It might explain why some Commandos move around at idle on the centre stand while others do not.
I'm firmly in the Dynamic Balance camp when reasonably possible (already apart) especially if the engine is going to be solidly mounted.

Here's Jim's post on the subject-

There is an advantage to dynamic balancing a Commando crank. It eliminates the possibility of a rocking couple which the isolastic mounts can not isolate from the chassis. A rocking couple makes the motor shake sideways which you do feel. Of course some cranks are pretty good from the factory and some are not. It was all just chance as none were ever dynamically balanced from the factory.

Have you ever noticed that some Commandos are really smooth and some are not....

There is nothing to be gained by changing the balance factor in a Commando. Isolastic mounts work best between 50 and 60% regardless of the rpm.
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Was it this one?
 
jan nelder said:
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!)
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The balance factor determines the revs at which the motor will run smoothly. With a factor of 72% , my motor runs dead smooth at 7000 RPM, but you could never use it in a road bike for slower riding, you might start breaking frames and brackets. When my motor idles, the whole bike moves backwards and forwards. A higher balance factor might be OK at low speeds, with isolastics.
It would have been impossible to sell Commandos which moved backwards and forwards when idling. - I wonder if the Atlas used to do that ?
 
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Norton probably became paranoid about the Honda CB750. I rode one when they first arrived. I didn't like it - it felt very stiff, and heavy. These days the CB750s which race are 1100cc and methanol fuelled. My Seeley 850 is faster.
 
cliffa said:
Was it this one?
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No that must be a later video.
It makes me want to pull some statically balanced engines apart for Dynamic balancing.
" Much better balanced than necessary" sounds just about right!

Glen
 
When you pull a Commando motor apart, it is obvious what has been done to the crank. All I did was restore the crank to what it was before the modification. It is not rocket science - the 650SS and Atlas were good motorcycles, but not comfortable.
My mate still has an Atlas 750 which he used to race successfully in Allpowers A grade against the 500cc Manx Nortons. Even with the capacity difference, that was an achievement. The guys who had Manx Nortons back then were not idiots., some were international riders.
 
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hybridracer said:
... 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.
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Maybe not. I'm just quoting what Comstock said about heavier flywheels smoothing them out.
 
jseng1 said:
Maybe not. I'm just quoting what Comstock said about heavier flywheels smoothing them out.
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I once had weight taken off the flywheel in a Triumph 650. I could not feel the difference. My 500 was stroked from 82 mm to 63mm and the crank was very light. That was very different.
I cannot see how lightening a Commando crank would change much. I did not think I would like the heavy 850 crank, but it is as good as any. My Seeley 850 is an excellent ride. It's major advantage is in it's handling, but the power characteristics are beautiful. With it's cornering, it does not need to be so fast elsewhere. And because of my old bike, I am used to being scared- it is burned into my memory. So I can still ride pretty quick, if I am permitted. The Seeley is tame.
 
jseng1 said:
The bare weight of the medium compression JS 920cc piston for stock rods is 290 grams. I don't know the weight of the RGM 920 piston.
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My balancing notes show 306 grams for the RGM 920 Piston
 
If you are increasing capacity, you obviously want more power without increasing increasing the rev limit. The stock balance factor is designed to make the motor run smooth at low and medium revs. If you don't raise the balance factor - due to heavier pistons, you might get more vibration in the low and medium rev range, but raising the balance factor without drilling the journal side of the cast iron flywheel is not easy. You need to add weight to the opposite side. I tapped the balance hole which is there in the 850 crank, and fitted a steel plug. But it is not foolproof or failsafe.
You might be able to mill a scallop into both sides of the rim of the flywheel, above the journals. You would need to be careful to avoid creating stress-raisers.
 
I just had the crank dynamically balanced to 60% wet and the 920 is smooth as glass once above 3000 rpm, very similar to the stock MK3.
I'm not sure that the dynamic balance is at all needed or even helpful with isos, my guess is probably not. It seemed foolish not to do it given the low cost and close proximity of the Dynamic Balancing shop.

Glen
 
worntorn said:
I just had the crank dynamically balanced to 60% wet and the 920 is smooth as glass once above 3000 rpm, very similar to the stock MK3.
I'm not sure that the dynamic balance is at all needed or even helpful with isos, my guess is probably not. It seemed foolish not to do it given the low cost and close proximity of the Dynamic Balancing shop.

Glen
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You probably won't feel the effects of dynamic balance - it will extend the life of main bearings though.
 
I think what Dan (who started this thread) did with his build is a good compromise.

I'm not sure how much difference it would make, cuz I have not done it, but a lighter Molnar crank balanced for whatever one thinks is right, the stock rods and a set of JSM medium compression stock rod pistons would make for a lot less rotating and reciprocating mass. The stock rods are lighter weight than the popular steel Carrillo rods and the JSM pistons are lighter weight than the stock pistons. All that and a belt clutch. I think the lighter parts might make for a spunky little 850.

BTW after watching that Comstock video on dynamic crank balancing, I definitely won't be pushing toward 8000 RPM anymore with my 750. That crank spinning at 1200 RPM looked scary enough. lol
 
I've been reading some of the older threads about racing with Commandos.
There is quite a bit of interesting discussion about using hopped up Commandos with standard bottom end for racing in the late 70s and 80s.
If the engines and riders are viewed as disposable, then it worked OK.

There is one comment that Dynamic Balancing of the crank " made the cases last longer"
There is also discussion of racing with an all new engine, a Maney 1007 and having that disintegrate. I believe there was a 920 Maney blow up in there too.

My takeaway is that you need to be very brave to run an aftermarket strengthened Norton style engine at the sustained high rpms seen in racing. You might need to be a little crazy to try that with stock cases and crank.

Glen
 
worntorn said:
I've been reading some of the older threads about racing with Commandos.
There is quite a bit of interesting discussion about using hopped up Commandos with standard bottom end for racing in the late 70s and 80s.
If the engines and riders are viewed as disposable, then it worked OK.

There is one comment that Dynamic Balancing of the crank " made the cases last longer"
There is also discussion of racing with an all new engine, a Maney 1007 and having that disintegrate. I believe there was a 920 Maney blow up in there too.

My takeaway is that you need to be very brave to run an aftermarket strengthened Norton style engine at the sustained high rpms seen in racing. You might need to be a little crazy to try that with stock cases and crank.

Glen
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I raced in the early 80's (when not using my domi 500 motor) with what was basically a Combat motor with 4s cam, racing 2 into 1 exh bolted in my wideline. I assumed the crank would be ok without checking the balance because it came from an alcohol burning grass track outfit. That level of tune was fine for UK short circuits as very few commando motors were any more powerful and it was even good enough for B.O.T. It was all good fun because we kept away from the classes for 1000cc 4cyl stuff and I didn't see engine bits flying everywhere!
 
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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The head steady is also very important especially hard mounted motors.
 
The head steady on my Seeley is two short tubes with rose joints at each end. The motor can rock sideways, but not backwards and forwards. Commandos are not a new invention. The effects of balance factor have been known for a long time by people who road race. The problem is, most of the guys who raced in the 1950s are now dead. What I know about Norton engines comes from way back in the past. Joe Craig and Francis Beart and Phil Irving were not stupid. However what we were never told might be important.
In Australia, there has been two guys who never talked much - in Victoria- it was Harold Carter, and in NSW- it is Alec Henderson.
Alec Henderson is still alive.
When the Japanese went racing, they built upon European knowledge, and improved. In the 1950s the really fast guys had 6 speed gearboxes, and knew about jetting. I think the Gilera had 7 gears.
I suggest Commandos are worth racing, if only for fun alone. The 961 motors might become available. In the old days, a Manx motor could not be bought without buying a whole bike.
 
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worntorn said:
I just had the crank dynamically balanced to 60% wet and the 920 is smooth as glass once above 3000 rpm, very similar to the stock MK3.
I'm not sure that the dynamic balance is at all needed or even helpful with isos, my guess is probably not. It seemed foolish not to do it given the low cost and close proximity of the Dynamic Balancing shop.

Glen
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A lot depends on which part of the usable rev range you use the most. If I race, the revs never drop below 5,500 RPM, even in the slowest corners. Road bikes don't usually get used in that way.
 
Its all about reciprocating weight and rod length to stroke ratio. That's where the crank and case breaking stress and vibration is.
 
yes, cos it’s not in the symmetrical rotating weight part of the assembly, which spins very nicely on those bearings.
 
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