Advantages of longer rods in Nortons

Note that the Norton 500 twin had a rod stroke ratio of 2.1 to 1 - this is much more ideal.
The 600 twin ratio was 1.82 to 1
They kept increasing the stroke but not the rod length (to retain the same engine height) and the commando rod length to stroke ratio is 1.68 to 1

Some rod/stroke ratios:

Stock Commando - 1.68
Stock Short Stroke 750 - 1.96
JS lightweight kit for stock 89 mm stroke - 1.83
Short Stroke 750 with JS lightweight (6.400" c-c) rods - 2.02
Maney 1007 cc kit with stock rods - 1.60
Maney 1007 cc kit with JS lightweight (6.400" c-c) rods - 1.75
Maney Ultra Short Stoke 750 with stock rods - 1.99

worntorn said:

I think the Norton engine works quite well as designed, that is as a low rpm high torque engine. With the updated bearings it makes a great road engine. If the desire is to turn it into something it is not - a high rpm race type engine, then it needs a major revamp, or as Steve Maney and Kenny Deer both decided, a complete redesign.

It was mentioned that a long rod ratio is used in Formula one race engines. There does seem to be a consensus that the longer rod ratio works best with very high rpm engines of this type.
I don't see the Norton engines as having much in common with that type of engine, nor would I want to attempt to remake my engines into high rpm Screamers, invariably at loss of low and midrange punch.
Best to start again for that.
Or buy a litre Sport bike and watch the tach go effortlessly to 12 or 14 grand without vibration or risk to engine.

Glen

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When I built my 850, I knew that a top end motor was one place that I could not go. Built to pull harder with a better gearbox, the commando is excellent. I raced a lot when I was a kid, and I deeply regret that I did not find the Seeley commando - things would have been very different for me. It is the most versatile lovely bike that I have ever ridden. With that power characteristic and the way the handling/geometry is set up, you can do anything with it. I desperately need to race it again, however I've run myself very short of funds. It won't always be that situation.

Before I knew about Jim's short pistons and long rods, I grabbed a second hand 73mm Honda Fireblade piston with a view to making shorter barrels. Compared with an 850 piston, the fireblade one is more than 100 gram lighter, is teflon coated, and has very thin rings. I did not think of using longer rods to get the heights right. Jims items will be the first go-faster stuff I will buy for my motor when I get flush again. I believe they are an excellent improvement, especially as 12 to one comp. is available. I like his cam followers too, however I don't believe I will ever pull the revs to need them.

I notice a few comments about modern bikes in the posts on this and other topics. My feeling is that four cylinder bikes and two strokes with deltabox frames have no relevance to Norton commandos. What ARE relevant are 851cc and 900cc Ducatis which have air-cooled motors. They all have two valve motors with the same development problems as the old commando. There is a 1000cc Paul Smart replica - if you can beat that with a 1000cc Commando, you have achieved.
Where is the race class which facilitates that battle ?

bears?

There is an old saying 'horses for courses' and it could well apply to con rod lengths for a particular engine. From notes made over many years, covering but a sample of 33 different engines, I find rod/stroke ratio's vary from the shortest 1.552 for a Subaru, to the longest 2.85 of the 1.5 V8 Coventry Climax engine of the late 50's.

The original length rods in the Summerfield 500 Manx engines were 139.7 mm centres for a rod/stroke ratio of 1.863. Many users found these engines vibrated so badly that riders came in from a race with dead fingers. A lengthening of 1/4 inch made a huge difference, with no loss of performance.

The Cosworth P86 twin that Gary Flood eventually played a major role in sorted out had a rod/stroke ratio of 2.05.

The latest Japanese MX engines have rod/stroke ratio's way down, Yamaha YZ450R at 1.694, Honda CRF 450 at 1.643.

It would seem there is no 'perfect' rod/stroke ratio, all kinds of combinations can be made to work, and in the past convincing arguments have been made for both long and short versions.

The V8 fraternity would seem to have the greatest difficulties in fitting a rod much longer than the standard. Easiest has to be single cylinders, for accommodating a longer rod is often little more than a spacer under the cylinder.

I would say that any Commando owner would certainly do well to take note of Jim Schmidt's recommendations regarding rods suitable for different engine configurations. Over his years of being involved with the engine (s) he must surely have covered all the obvious possibilities, and probably a good many of the less obvious also.

oops

acotrel said:

Before I knew about Jim's short pistons and long rods, I grabbed a second hand 73mm Honda Fireblade piston with a view to making shorter barrels. Compared with an 850 piston, the fireblade one is more than 100 gram lighter, is teflon coated, and has very thin rings. I did not think of using longer rods to get the heights right. Jims items will be the first go-faster stuff I will buy for my motor when I get flush again. I believe they are an excellent improvement, especially as 12 to one comp. is available. I like his cam followers too, however I don't believe I will ever pull the revs to need them.

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I use honda shadow pistons 79.5 mm, with short stroke rods (norton steel) on a std crank.
Also needed to shorten the cylinder to achieve zero deck.
This was done long before Jim's kits were available. If I were to build a new engine today it would be much easier to use Jim's parts. I was in collage and money was tight at the time.

Snotzo said:

There is an old saying 'horses for courses' and it could well apply to con rod lengths for a particular engine. From notes made over many years, covering but a sample of 33 different engines, I find rod/stroke ratio's vary from the shortest 1.552 for a Subaru, to the longest 2.85 of the 1.5 V8 Coventry Climax engine of the late 50's.

The original length rods in the Summerfield 500 Manx engines were 139.7 mm centres for a rod/stroke ratio of 1.863. Many users found these engines vibrated so badly that riders came in from a race with dead fingers. A lengthening of 1/4 inch made a huge difference, with no loss of performance.

The Cosworth P86 twin that Gary Flood eventually played a major role in sorted out had a rod/stroke ratio of 2.05.

The latest Japanese MX engines have rod/stroke ratio's way down, Yamaha YZ450R at 1.694, Honda CRF 450 at 1.643.
It would seem there is no 'perfect' rod/stroke ratio, all kinds of combinations can be made to work, and in the past convincing arguments have been made for both long and short versions.

The V8 fraternity would seem to have the greatest difficulties in fitting a rod much longer than the standard. Easiest has to be single cylinders, for accommodating a longer rod is often little more than a spacer under the cylinder.

I would say that any Commando owner would certainly do well to take note of Jim Schmidt's recommendations regarding rods suitable for different engine configurations. Over his years of being involved with the engine (s) he must surely have covered all the obvious possibilities, and probably a good many of the less obvious also.

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And of course Honda's acknowledgement of the unfavorable thrust angle created by the short R/S ratio was to shift the bore forward, thereby restoring a more favorable engine geometry and thrust angle. Now there's a job for somebody that isn't busy enough, shift the cylinder forward ~ 1/4" on a Norton!

http://world.honda.com/CRF250L/engine/page02.html

We all know myriad tomes exist that recite the pros and cons of various R/S ratio. Although the following 2 links are more of the same, they are short, easily understood, provide insight on useful alterations to compensate/adjust parameters to suit a particular R/S ratio, provide critical rod angles for a variety of engines and provide a significant body of data on R/S ratio for specific engines. The 1st link provides automotive related examples and the 2nd link provides motorcycle related examples. Have a look and I think you'll enjoy the data tables as you view a variety of old stock and muscle car engines we were enthralled with growing up. You'll also see some real stinkers in the crowd, e.g., std 400 small block chev with a R/S of 1.45 - pew!

http://victorylibrary.com/mopar/rod-tech-c.htm

http://victorylibrary.com/tech/crod-c.htm

Another aspect of the Rod to Stroke (R/S) ratio debate is that the greater cylinder wall thrust associated with a smaller R/S ratio results in greater cylinder/piston friction. With greater friction comes greater power losses and resulting heat. In my opinion, heat would be the significant factor of the two.

If our Norton clunkers need help in anything its a good bit more rpm tolerance with least friction. Its very old school method to shift bore forward of crank for slower piston jerk down for a bit more rpm tolerance there. Similar to off centering piston pin forward not backwards like Norton did to expand engine. The fastest power to weight craft I've had in order of G forcing ahead was '68 P!!, '78 Chevy cargo van 400 V8, Mercedes 6.9, '65 Corvair Old's Vista Curiser wagon. I had the 400 V8 built like a propane jet boat engine so made 449 ft lb @ 4200 and 449 hp @ 4800 on its pewie b/s ratio. Point being b/s ratio is fairly minor influence across scope of engines compared to everything else. Ole Blue van woke up even more when Holley 2 bbl TBI replaced the Quadrajet to point I'd tease with the Houston sports cars so know how nice EFI in old clunkers can be. Peel has JSM kit so part of his choir already. You are missing a significant chuck of Norton lore and power insights if ya haven't read Jim's Race Book.

chris plant said:

bears?

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There are not many BEARS participants, so they usually all run off the same grid - moderns, oldies, watercoolrd , 4 valve. In any case the Japanese also make air-cooled two valve twins. The class needs to be specified on the basis of technology. Nortons and air coolrd Ducatis and Harleys are pre 1960 technology, everything ecept the Rudge was tewo valve and two strokes and watercooling were rare. If you look at the commando as a TYPE of motorcycle, there are some interesting comparisons. The main thing is that the motor should be less than four cylinders with only two valves per cylinder. Then the playing field would be level, and I believe the commando would be at the pointy end. If you think about the technology limitation, NORTONS ARE VERY GOOD. It is about BMEP.

Dances with Shrapnel said:

Another aspect of the Rod to Stroke (R/S) ratio debate is that the greater cylinder wall thrust associated with a smaller R/S ratio results in greater cylinder/piston friction. With greater friction comes greater power losses and resulting heat. In my opinion, heat would be the significant factor of the two.

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I suggest angularity is important in short stroke engines. I believe that the main reason for the lack of torque from my 63mm stroke Triumph motor was the long 650cc rods. The Daytona 500 used much shorter rods, however Percey Tait said it was difficult to ride. Mine was an absolute bastard, exciting however really nasty - always waiting to bite hard. It taught ne how to stay alive - the hard way.

I've got a question about Jim's light pistons. Are they made in a machine - spun cast then forged as are Japanese pistons ? I suspect the technique might have originated in the US. Years ago the Mahle forged pistons were the ultimate for strength and lightness. Thanks for the tip about the Honda Shadow pistons - very useful info.

At what part of its travel , rotationally or linearly , would the piston be at its Maximum Kinetic Energy , or is that Dynamic energy . If you would , sir .

Matt Spencer said:

At what part of its travel , rotationally or linearly , would the piston be at its Maximum Kinetic Energy , or is that Dynamic energy . If you would , sir .

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Maximum Kinetic I would say at the point of combustion, or shortly there after. Anywhere else it's just going for the ride.

I might think that dynamic energy lives in the rotation of the crank and depends on its speed and mass, relatively speaking.

You didn't say who you were asking. You just said "sir".



Regards,
Sir Pete

Matt most kinetic piston energy is at piston's point of fastest motion, somewhere near 90' ABDC & ATDC. The highest potential energy the piston and bores and rods and crank must stand is the acceleration at TDC of intake stroke with suction resistance adding to jerk down and tipping. One reason for multi cyclinders is less piston size jerk down mass per over all engine displacement but then loose some chamber area for valve area.

hobot said:

Matt most kinetic piston energy is at piston's point of fastest motion, somewhere near 90' ABDC & ATDC. The highest potential energy the piston and bores and rods and crank must stand is the acceleration at TDC of intake stroke with suction resistance adding to jerk down and tipping. One reason for multi cyclinders is less piston size jerk down mass per over all engine displacement but then loose some chamber area for valve area.

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I was thinking about that 90' degree number too. But after the big bang, isn't it just a coast till the next big bang?

acotrel said:

I've got a question about Jim's light pistons. Are they made in a machine - spun cast then forged as are Japanese pistons ? I suspect the technique might have originated in the US. Years ago the Mahle forged pistons were the ultimate for strength and lightness. Thanks for the tip about the Honda Shadow pistons - very useful info.

Click to expand...

Forged from a billet of 2618 aluminum, not from a spun cast form.

Ken

I think that the small changes in rod length that can be done within an existing engine design are not going to make that much difference. Probably the main reason the JS rods are longer is that the lighter pistons are all shorter so in order to fit the kit into an engine it was easier to lengthen the rod than shorten the barrel. The main reason this kit makes the Norton engine run well is the huge reduction is reciprocating weight. In the case of Dan Smiths short rodded Vincent it is fast, but it may not even be more powerful of fast than John McDougals bike that has standard Vincent rods. I think Johns bike actually recorded a higher top speed than Dan's bike, when they both visited the Isle of Mann. I think maybe Dan's bikes 5 speed gearbox gives it the advantage in a sprint. John tends to believe that Phil Irving did get it right! To make one of these engines powerfull it makes sense to copy the Goldstar. The DBD34GS had 39 top 42 HP from a 500 single that is still a lot. That level of tuning would not be good for a road engine though because the huge carbs and high level of overlap cause very poor fuel economy.