JS lightweight pistons for stock rods in 850 balance factor change

hhh

hhh

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If JS lightweight pistons are installed in a stock 850 with stock rods, what is the resultant balance factor?
 
I just checked the site again and found the "missing link" with the answer: it will increase the stock balance factor of 52% to approx 58%. I'm actually considering installing them in a 920 that I believe to have the RGM 920 kit with "slightly lighter than stock" pistons, so I'm thinking maybe 59%?
 
59% might be good at 6000 RPM max. 73% is better at max. revs - 7000 RPM. You probably do not need that.
 
Isolastics affect engine performance ? The engine must vibrate somewhere in the rev range - the point where it vibrates can be moved - up slightly should be better. A Commando engine with 73% balance factor runs dead smooth at 7000 RPM. Most Commando motors probably remain below 4000 RPM. If the balance factor is slightly higher than normal, it would suit a fast rider better. Lighter pistons usually give better acceleration.
 
acotrel said:
A Commando engine with 73% balance factor runs dead smooth at 7000 RPM. Most Commando motors probably remain below 4000 RPM.
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You've outdone yourself this time. There is no RPM where a Commando motor runs dead smooth. Any discussion of balance factor of a Commando engine can assume to have at least a non-zero probability that it will be mounted in an Isolastic frame and must take the frame into account or it's worthless since the resultant balance factor is entirely dependent on the frame. I know and have known many Commando owners and not one has kept the RPM below 4000 RPM, not even warming up.
 
hhh said:
You've outdone yourself this time. There is no RPM where a Commando motor runs dead smooth. Any discussion of balance factor of a Commando engine can assume to have at least a non-zero probability that it will be mounted in an Isolastic frame and must take the frame into account or it's worthless since the resultant balance factor is entirely dependent on the frame. I know and have known many Commando owners and not one has kept the RPM below 4000 RPM, not even warming up.
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That's because Commando's were built to RUN....
 
Jim Scmidt's light pistons should barely make any difference to vibration in a normal Commando, but you might get a slight improvement in acceleration. If the motor does vibrate, the vibration would occur slightly lower in the rev-range than usual. My motor with the high balance factor is obscene at low revs - the whole bike moves backwards and forwards when the motor idles. As soon as it is revved, the motor smooths-out. At 5,500 to 7000 RPM, it gives a very pleasant ride. Which would you rather ride, a Honda CB750 or a Commando 750 ?
Most of you guys were probably not even alive in the 1960s.
 
acotrel said:
Most of you guys were probably not even alive in the 1960s.
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Nobody's right if everybody's wrong. Buffalo Springfield 1966

I was around in the 50's, but real life experience doesn't mean a lot on the internet. No test required to be an expert. Don't even have to use a product to know everything about it and hand out advice.

hhh said:
If JS lightweight pistons are installed in a stock 850 with stock rods, what is the resultant balance factor?
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You get what you get if you don't have the crank rebalanced to a specific percentage. Just do it if you've got the desire to do it without rebalancing so you know what the balance factor is. Not likely the factory did it to the same exact percentage to begin with knowing the engine was going into an ISO cradle. Oops that sort of sounds like instant expert BS. lol
 
Schwany said:
Nobody's right if everybody's wrong. Buffalo Springfield 1966

I was around in the 50's, but real life experience doesn't mean a lot on the internet. No test required to be an expert. Don't even have to use a product to know everything about it and hand out advice.



You get what you get if you don't have the crank rebalanced to a specific percentage. Just do it if you've got the desire to do it without rebalancing so you know what the balance factor is. Not likely the factory did it to the same exact percentage to begin with knowing the engine was going into an ISO cradle. Oops that sort of sounds like instant expert BS. lol
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I suggest the factory just did what it needed to do to reduce vibration at lower speeds. A low balance factor does not improve performance, but a Commando is still fast enough to keep-up with an H2 Kawasaki. That surprised me - my mate used to do pre-delivery on both, and used to road test them. My own Commando 850 motor surprised me - I did not believe it had so much potential. It is seriously fast enough to keep-up with most other bikes in a road race, and with the Seeley frame is better. I do not really know why I built it - it has some similarity with a Gus Kuhn Commando - but not much. In some races, the JPN Nortons with Peter Williams were competitive with TZ750 Yamahas - I think that is incredible. Not many motorcycles make the ends of the straights look extremely narrow.
 
acotrel said:
I suggest the factory just did what it needed to do to reduce vibration at lower speeds. A low balance factor does not improve performance, but a Commando is still fast enough to keep-up with an H2 Kawasaki. That surprised me - my mate used to do pre-delivery on both, and used to road test them. My own Commando 850 motor surprised me - I did not believe it had so much potential. It is seriously fast enough to keep-up with most other bikes in a road race, and with the Seeley frame is better. I do not really know why I built it - it has some similarity with a Gus Kuhn Commando - but not much. In some races, the JPN Nortons with Peter Williams were competitive with TZ750 Yamahas - I think that is incredible. Not many motorcycles make the ends of the straights look extremely narrow.
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No, Al, just NO.
 
I do not believe the Norton Atlas 750 ever sold as well as the Commando 750, so the Isolastic frame and low crank balance factor made the bike better for road use. Motorcycling changed in the 1960s.
 
acotrel said:
Do you race a Commando with the Isolastics ? I would not even try to do that. A friend actually won a production race with a Commando, but he could outride most others with any bike.
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Of course.
Like you say: "slow bikes finish first"

You're disdain for screaming two strokes aside yeah OK you think the H2's not fit for golf cart . But
the TZ 750 Al?
 
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concours said:
Of course.
Like you say: "slow bikes finish first"

You're disdain for screaming two strokes aside yeah OK you think the H2's not fit for golf cart . But
the TZ 750 Al?
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My mate was sponsored to ride Jack Walters' TZ750 at Phillip Island. On one lap it rattled, but because he was in the lead - he kept going. On the next lap as he crossed the finish line , the motor exploded. My mate bounced off the guard rail. The bike somersaulted past the end of the guard rail standing on its rear wheel and went vertical between two trees, missing a guy who was standing there, then bounced a few times in a dam full of water. My mate came out and rode in the next race, on another bike. There was a missing carburetor, but the rest of the TZ750 was recovered. It had to be stretched back to its correct length. When a bike like that gets thrown away - big things happen. If I race, I do not worry about crashing because my bikes are usually only fast enough to be in the middle of the race.
 

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