Rod/stroke ratio effects explained

To consolidate the facts, @Gene, Long rods mean more torque. Long stroke and long rods equals lots of torque. Tractors have crank off center to bore and long rods and long stroke Huge torque low rpm's! 302 Chevy short rods short stroke low torque HIGH rpm's. 10,500 rpm's not unheard of but 6,500 -7,000 to get it off the line is also not unheard of. Norton's long rods and off center to bore equals a high torque and a engine that doesn't like high rpm's.

mean gene said:

To consolidate the facts, @Gene, Long rods mean more torque. Long stroke and long rods equals lots of torque. Tractors have crank off center to bore and long rods and long stroke Huge torque low rpm's! 302 Chevy short rods short stroke low torque HIGH rpm's. 10,500 rpm's not unheard of but 6,500 -7,000 to get it off the line is also not unheard of. Norton's long rods and off center to bore equals a high torque and a engine that doesn't like high rpm's.

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Actually, from the article, the low (short) rod ratios make a little more torque than the high rod ratios at low to medium rpm.
The difference in performance from low (1.5) to high (2.0) is said to be tiny.
Not to be confused with Long stroke vs short stroke, that is a different, but related, subject.

Glen

Matt Spencer said:

Oppsie !

( Read the THIRD column . )

Better put the other page on . One Wonders WHY it may notve been running quite right .

Obviously , the correct running in proceedure , is to take it out of the crate , and rev it as igh asitll go . off of the stop lights .

The bikes raced early on , were using 7500 & pulled down after each meeting . Dunstall , Kuhn maybe , etc . After CHAMPIONSHIPS , 69 & 70 .
The Short stroke pulls 8.000 so the valve gears not entirely the problem .

But PAST 7.000 is overstressing it , and leads to premature wear . If further abused demolition .
The ' 5800 continuous ' was the 850 rateing . Stock . for reliable intercontinental Touring . NVT Era .

Race Engines ordinarilly are asembled with the utmost care . Any flaws are liable to manifest in mechanical failure if not countered .

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What happens with a Manxman is different to what happens with a Commando when you rev it. The Manxman probably has a balance factor of 75 %,. The Commando balance factor is about 54%. The lower balance factor suits revs of about 4000, the higher factor suits rev of about 7000.. Vibes kill motors and isolastics do not change what happens inside the crankcases.

acotrel said:

What happens with a Manxman is different to what happens with a Commando when you rev it. The Manxman probably has a balance factor of 75 %,. The Commando balance factor is about 54%. The lower balance factor suits revs of about 4000, the higher factor suits rev of about 7000.. Vibes kill motors and isolastics do not change what happens inside the crankcases.

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I agree - but as I've posted before, balance factor is merely a trade off to reduce vertical vibration at the cost of increasing horizontal vibration.
Most "bad" vibration in bikes is to do with how well the rest of the bike is put together - the engine is merely the "excitational force".
Loose things have a natural frequency and will vibrate like hell at multiples of that frequency.

For a given engine configuration (eg: Commando) longer rods and lighter pistons will slightly reduce that excitational force though.

robs ss said:

... For a given engine configuration (eg: Commando) longer rods and lighter pistons will slightly reduce that excitational force though.

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It depends on how much you reduce the piston weight. If you reduce it a few percent then you only get a "slight" improvement. But when you reduce it by 33% you get a significant improvement.

robs ss said:

I agree - but as I've posted before, balance factor is merely a trade off to reduce vertical vibration at the cost of increasing horizontal vibration.
Most "bad" vibration in bikes is to do with how well the rest of the bike is put together - the engine is merely the "excitational force".
Loose things have a natural frequency and will vibrate like hell at multiples of that frequency.

For a given engine configuration (eg: Commando) longer rods and lighter pistons will slightly reduce that excitational force though.

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Regardless of which way the vibration goes, a motor with a high crank balance factor at high revs canl be as smooth as a motor with a low crank balance factor at low revs.
A motor with a high crank balance factor will shake at low revs, and a motor with a low crank balance factor will vibrate at high revs.
I prefer a low frequency shake at low revs to a high frequency vibration at high revs
I also prefer a heavy crank to a light crank. A Triumph Saint never performs as well as a pre-unit Tiger 110.
The problem with the heavy crank in the Commando , is it does not work well with a wide ratio gearbox.

acotrel said:

Regardless of which way the vibration goes, a motor with a high crank balance factor at high revs canl be as smooth as a motor with a low crank balance factor at low revs.
A motor with a high crank balance factor will shake at low revs, and a motor with a low crank balance factor will vibrate at high revs.

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I guess we'll just have to agree to disagree.

robs ss said:

Loose things have a natural frequency and will vibrate like hell at multiples of that frequency.

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I should have added that all physical barts have natural frequencies - but the stiffer & tighter things are, the higher the natural frequency. If done well, above the frequency of the excitational force (engine rpm).

Pre-unit Triumph 650s have long rods and heavy cranks. Unit construction Triumph 650s have short rods and light cranks. My rival from back then, had a 650 Triumph which was extremely fast and was set up to pull like a train. It runs extremely high gearing. It had the long rods and heavy crank. He is now building a 750 Triumph which has the long rods and heavy crank. I once rode his 650 at Calder Raceway and blew-off a 750 Kawasaki two stroke which was the full bit. We came around the corner before the straight side by side, and the bike I was on simply out-accelerated the Kawasaki. The Triumph did not even have a close-ratio gearbox, it now has 5 speeds as a 750.
It still won't be quicker than my Seeley 850. He ran into a vibration problem.

Matt Spencer said:

Oppsie !

( Read the THIRD column . )

Better put the other page on . One Wonders WHY it may notve been running quite right .

Obviously , the correct running in proceedure , is to take it out of the crate , and rev it as igh asitll go . off of the stop lights .

The bikes raced early on , were using 7500 & pulled down after each meeting . Dunstall , Kuhn maybe , etc . After CHAMPIONSHIPS , 69 & 70 .
The Short stroke pulls 8.000 so the valve gears not entirely the problem .

But PAST 7.000 is overstressing it , and leads to premature wear . If further abused demolition .
The ' 5800 continuous ' was the 850 rateing . Stock . for reliable intercontinental Touring . NVT Era .

Race Engines ordinarilly are asembled with the utmost care . Any flaws are liable to manifest in mechanical failure if not countered .

Click to expand...

One of my friends had a Manxman, the only other one I have ever seen. It was an absolute gem. When Norton started making them bigger, they lost the plot. It is probably possible to get a 650 Norton going quicker than an 850. I saw one race at Bathurst in a Senior Open Class in about 1968. It won convincingly. It was ridden by Jack Forrest who used to be a works BMW rider.

Would YOU lend your bike to THIS MAN ! .

Orses for courses . The main problem is if you start overstressing parts , you reduce their life . sometimes Substantially . The steel rods in the short strooke mustve been a lot harder on the crank shaft .

Lifing components , particularly alloy rods , and thourough inspections & crack testing , if your a throttle merchant , go hand in hand on a rebuild .
A thrashed to death bike will need much greater work at overhaul , at the same milage , than a well kept unabused one . Conversely it will run a much greater milage between overhauls .
Theres evidance of 100.000 miles on a 750 with decent filters , never taken past 6000 rpm .

Talk in NZ of Combats hitting 8.000 from Whites ( in Newmarket ) & cars. At 80 mph , at the lights to the west of the shop . Some people didnt realise a tachometer was supposed to be observed.
Not to mention a ' Red Line ' .
Phil hill even manadged to overrev , in top gear , Aston Martons manadging directors DBS V8 back in 74 ( or so ) The world Champion F1 driver watching the speedo & tacho needles lowly but surely
climb of the top of the readings , at 175 mph . " a Valve problem Occured " as they put it . In ' Road & Track ' mag .

So the ' 750 ' / ' 850 ' rod length is not the paramount limitation in powerplant output . Altering it will shift things , and other things ( forces ) along with it . At some point its no longer a ' Commando '.
Which dont go to bad as they were supplied . If carefully assembled .

The Other Atlas Test , showing they were starting to learn , but hadnt entirely caught the jist of ' Maximum ( safe ) rpm's .


For all that , they didnt go to bad ( at the time ) anyway .
Similar treatment handed out to the ' Atlas Scamblers ' there , also .

The first Norton Atlas 750 to arrive in Melbourne had it's barrel flange crack off. You will note the Commando 850 has four through bolts holding the barrel down, not just the flange. To get the best out of an 850, the revs need to be kept between 5,500 RPM and 7000 RPM so the top three gears need to be close together. That way the revs don't drop below 5,500 RPM on up-changes and you are not so dependent on throttle response. When you have a Seeley 850 going quick, it is difficult to watch the tacho. But I know what 7000 RPM sounds like.
My friend has an Atlas 750. It is very good. Back in the old days he rode it with some success in A grade races. It still had the license plates on it, which used to upset a lot of the fast guys. His motor has MAP rods and pistons.
The trouble was, very few people had Nortons in the 1960s. There were a few Dominator 88s and 77s, but the Manxman, 650 SS and Atlas were very rare. I now know they were better than Triumphs

I would not plan on much if any vibration reduction from a higher rod ratio. The BSA A65 has a 2.06 rod ratio and remains one the most vibratory machines I've ever ridden. From chatting with others, high vibration levels were just part of the A65, get used to it. Actually it comes with all the parallel twins of that size or larger.

Norton was the only manufacturer to get serious about dealing with it ( Commando) You can add all of the other vibration reduction measures together, including dynamic crank balance, and they won't equal the effect of the rubber ISOs.
It was brilliant really.

Glen

When you talk about Phil Hill over-revving in top gear - as you improve your motor, you need to raise your overall gearing or you don't get the benefit if improvements in torque. Because of the heavy crank, the motor always tends to spin-up at the same rate. It can take you to high revs fairly quickly but not be pulling as hard as it can, when you keep it spinning high and race-change up through the gears Often when you raise the gearing, the bike can accelerate faster. The trick is to raise the gearing just enough to take full advantage of the torque. If you go beyond that , the bike accelerates slower.

worntorn said:

I would not plan on much if any vibration reduction from a higher rod ratio. The BSA A65 has a 2.06 rod ratio and remains one the most vibratory machines I've ever ridden. From chatting with others, high vibration levels were just part of the A65, get used to it. Actually it comes with all the parallel twins of that size or larger.

Norton was the only manufacturer to get serious about dealing with it ( Commando) You can add all of the other vibration reduction measures together, including dynamic crank balance, and they won't equal the effect of the rubber ISOs.
It was brilliant really.

Glen

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The ISOs do not alter the fact that at revs above 4000 RPM the crank is probably still doing damage because of the low balance factor. The ISOs just keep your fingers from going numb. My motor is rifgidly mounted and my crank is balanced to 72 %. At 7000 RPM the motor is almost vibrationless, but at low revs the motor shakes. At low revs the destructive forces due to the imbalance are smaller. In short, you choose how you are going to use the bike before you balance the crank. When you alter a Commando, you are usually converting a commuter bike into a sports bike If I had a road-going Commando, I would not change it. If I was going to race one, I would raise the crank balance factor and live with the shake at low revs. I would not spin a crank with 54% balance factor to 7000 RPM.

How much damage due to incorrect balance ?
I'm all for dynamic balance at time of rebuild, however a friend just rebuilt his standard Commando owned since 1976 and bottom end never apart until now. It has spent a lot of time above the 4000 rpm damage point you mention but still covered128,000 miles.
That'll do for most of us!

I owned a BSA A65 and rode it from Calif to Indiana and back (didn't quite make it back). My 1st day was a 10hr marathon. It was cold and I was gritting my teeth. The vibration was so bad it split the skin on my throttle hand between the thumb and index finger and my teeth didn't fit together the same.

The BSA A65 is a tall high dome piston. Its heavy and that weight plus the 74mm short stroke high RPM is what produces all those vibes.

My point is that the 2.06 rod ratio doesn't make the bad vibes go away.

Glen