Advantages of longer rods in Nortons

For clarity:

Stock Commando rod - 5-7/8" or 149.225mm
Stock Norton Stroke - 89mm

Rod/Stroke Ratio
149.225/89 = 1.68

Long Commando rod - 6.40" or 162.56mm
Stock Norton Stroke - 89mm

Rod/Stroke Ratio
162.56/89 = 1.83

And just for fun, the same for the factory short stroke 750:

Rod - 6.200"/157.48 mm
Stroke - 80.4 mm

Rod/Stroke Ratio = 1.96

Ken

The main thing I find online on longer rod length is the lessening of piston jerk down acceleration then 2ndaryily less piston ring rocking flutter and less strength of onset of carb intake pulse of piston plunge.

Jerk down force
https://www.google.com/#q=piston%20acce ... calculator

Ringing rings
https://www.google.com/#q=piston+rod+le ... ng+flutter

Sucks or not
https://www.google.com/#q=bore+stroke+r ... induction+

hobot said:

....................... and less strength of onset of carb intake pulse of piston plunge.

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Until you speed up the motor.

Maybe a good thing for a short stroke, eh?

Fun stuff to rest mind on. Comnoz and Canaga and others scold us to expect worth while power pressing piston friction speeds beyond what existent infra structure can support. I have no preference as not aware enough of differences in long vs short stroke Norton twin power characteristics, but going from your gut reaction response I now suspect my beloved P!! dragster with tach distinctly marked at 9000 was a short stroke. If so then boy howdy leave those long in tooth tractor stroke engines in the museums and devote to a life like Burt Munroe for on going thrills geting even more out of it. Now picture how to make an over square 360' twin from scratch with all we now know. Might work just fine with only 4 spd gearbox to boot. Feel it > BLATTT redlined before wrist can twitch WOT > out in public teasing for whip lashing show downs. I am glad JSM added more rod length while removing a good bit of oscillating inertia.

hobot said:

Comnoz and Canaga and others scold us to expect worth while power pressing piston friction speeds beyond what existent infra structure can support.

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Short stroke engines address much of the friction due to excessive piston speeds.

Duh that's what i said so ask ya about what rpm the short strokes shine above with their advantages?

hobot said:

Fun stuff to rest mind on. Comnoz and Canaga and others scold us to expect worth while power pressing piston friction speeds beyond what existent infra structure can support. I have no preference as not aware enough of differences in long vs short stroke Norton twin power characteristics, but going from your gut reaction response I now suspect my beloved P!! dragster with tach distinctly marked at 9000 was a short stroke. If so then boy howdy leave those long in tooth tractor stroke engines in the museums and devote to a life like Burt Munroe for on going thrills geting even more out of it. Now picture how to make an over square 360' twin from scratch with all we now know. Might work just fine with only 4 spd gearbox to boot. Feel it > BLATTT redlined before wrist can twitch WOT > out in public teasing for whip lashing show downs. I am glad JSM added more rod length while removing a good bit of oscillating inertia.

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Steve,

have you posted any pictures of your old P11 here? If so, could you post the link. I don't recall seeing any, but that doesn't mean much any more.

Ken

hobot said:

Duh that's what i said so ask ya about what rpm the short strokes shine above with their advantages?

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You assumed I actually read your posts. :roll:

The short stroke (at least my 750 build) shines throughout the rpm range. Pretty much same power and torque of a race tuned Norton with an 89mm stroke but with the benefit of an additional 1,500 rpm of ever increasing power. As mentioned elsewhere, what you loose in mechanical advantage with a shorter stroke you gain with greater piston to crank force due to larger bore so it is pretty much a wash.

Only photo of the candy apple P!! was lost some how in moves to various schools and moves away from various women. Just a basic P11 framed Woods or Axtel desert racer build to sit 2" lower in front by chopping fork and inch and adding inch to down tubes. Over some time clutch began to foul but I'd didn't have tools or know what to do about it w/o money to have shop deal with it so was touchy to get it started by roll off hop on and have clutch break free instead of smoking burn out roaring between lines of parked cars in curvy old neighborhood. Paget's shop Tallahassee Fla. that built it gave it enough attention to safety wire all it engine fasteners like an air craft. I have tired searches for that shop and drag records of '68 & '69 w/o success. Triple 2smokes digging deep into lower gears for some rpm power band didn't stand a chance with a snick down just to 3rd and off on iffy road tire hook up. Was not so powerful to top out in a 1/4 mile though so might of reached 150 but wind and fear and vibration pressure with a lingering will to live - I never did. I was told in prime on slick broke below 10.5 but no way to hook that up w/o drag slick. It had slow 600 idling, hands off throttle lugging in 4th and off idle explosive response engine I've experienced and had 30 mm Amals likely worked over but beyond me to know at the time. I'd be hard call not to toss Peel current engine a Drouin on the P11. i'd miss eating a day to afford to top off with hi test. The Rear Rules my Roost.

I would love to try to recreate the P!! old school engine even though would have to run special octane. Even a youth can only take so much noise and G forces so wouldn't need a big tank but if ya hung a gallon under head light could got further even faster. I still don't have that much respect for even good Combat power, ie: that can get a leap on wheelie bar'd Harley till 2/3 down strip and float front at end of 1st and 2nd road racing, so first two versions of my Combat was mimicking to blend in with quaint foot forward saddle baggers with huge K/Q buckets. Peel in her power prime was still only a front tire floater at end of 1st and 2nd, pashaw. Norton is famous for torque so that's Peel path. What engine could we stick in something like this on week ends?



This is much better than flaging off bars of vertical wheele. Its twice as powerful as the P!! but sensations identical till front no longer floated.

Snotzo said:

...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...

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My friend Rob Tuluie tried to race a Commando 920 with stock rods and harder rubber engine mounts (to improve handling). His muscles cramped up trying to hang on. By the end of the day his fore arms were visibly swollen. Vibration is why the Commando has rubber mounts. Those rubber mounts protect the rider - but not the motor. Its bad and needs fixing.

Dances with Shrapnel said:

hobot said:

Duh that's what i said so ask ya about what rpm the short strokes shine above with their advantages?

Click to expand...

You assumed I actually read your posts. :roll:

The short stroke (at least my 750 build) shines throughout the rpm range. Pretty much same power and torque of a race tuned Norton with an 89mm stroke but with the benefit of an additional 1,500 rpm of ever increasing power. As mentioned elsewhere, what you loose in mechanical advantage with a shorter stroke you gain with greater piston to crank force due to larger bore so it is pretty much a wash.

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With a short stroke motor, the valve train has to keep up with much higher revs, and often a harsher cam. The lack of torque makes the bike much more difficult to ride, especially if you don't have enough CR gears. What you pick up on the swings, you lose on the merry-go-rounds, however there is a huge difference in rider anxiety between the two types of motor when you are racing. I presume that with your 750, short stroke refers to about 82mm like an early Triumph twin - not what I would call short stroke ? The later (70s) Daytona T100 at 65.5mm is short stroke. It is just a pity that more were not produced, and with a reasonably priced CR 6 speed box. When you look at the Japanese bikes of the same era, the 6 speed box was not rocket science. It would have made a purler of a race motor.

ugh Dances its hard to avoid hobot on optimizing Cdo's going nutz subjects. A 9000 rpm 750 that don't give up much in mid range would drive me nutz again. Being able to stay in a lower torque advantage gear longer-quicker is another rpm advantage. I may be a closet short stroke lover w/o even knowing it. I am still fabbergasted by what was possible in late 60's so some day may have to be tutored on how to obtain that again. Sort of a race not to get too old and wise not to want to pull trigger on potent engine or why else bother. Normal engines go putt putt putt at idle while the potent ones goes PoP PoP PoP...

hobot said:

ugh Dances its hard to avoid hobot on optimizing Cdo's going nutz subjects. A 9000 rpm 750 that don't give up much in mid range would drive me nutz again. Being able to stay in a lower torque advantage gear longer-quicker is another rpm advantage. I may be a closet short stroke lover w/o even knowing it. I am still fabbergasted by what was possible in late 60's so some day may have to be tutored on how to obtain that again. Sort of a race not to get too old and wise not to want to pull trigger on potent engine or why else bother. Normal engines go putt putt putt at idle while the potent ones goes PoP PoP PoP...

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Potent one's don't idle at all.

hobot said:

ugh Dances its hard to avoid hobot on optimizing Cdo's going nutz subjects. A 9000 rpm 750 that don't give up much in mid range would drive me nutz again. Being able to stay in a lower torque advantage gear longer-quicker is another rpm advantage. I may be a closet short stroke lover w/o even knowing it. I am still fabbergasted by what was possible in late 60's so some day may have to be tutored on how to obtain that again. Sort of a race not to get too old and wise not to want to pull trigger on potent engine or why else bother. Normal engines go putt putt putt at idle while the potent ones goes PoP PoP PoP...

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And when they go pop, pop, pop, we really like it, and decide we'd like them to go pop, pop, pop, even louder, and we tip the can a little too far and then the pop, pop, pop, ends in a singular bang and we end up with a motor that is taller than it should be (see before and after below). Isn't the Grain Belt Pony case a nice touch?

I assume its settled then, that longer rods work a better treat in long-ish to short-ish stroke Commandos and Triumphs and BSA too. IIRC there are both longer and shorter stroke Norton cranks in use. Most bragged on recent power house Norton-ish winner was the 1007cc-ish stroker in UK with JSM kit. I remain confused on most efficient power route in Nortons, stroker increased cubes or de-stroker higher rpm. Suck-compress more volume in each pulse or more smaller sucks-compressed faster. All I know for sure is how rpm allergic the crank is and the rod cap bolts too. Base line crank loads of 750 @ 6000 is ~7000 lbs = ~3.5 tons and goes up faster from there. JSM light JE pistons out shine the longer rods for crank and bolts rpm tolerance. Still I suppose what blew my away most on P!! was the 30mm carbs velocity into hi CR, wild cam and port work out open headers. This is how I picture us playing with our expensive engines. Power to spare till loose as snot at ease while looking out for others.

acotrel said:

With a short stroke motor, the valve train has to keep up with much higher revs, and often a harsher cam.

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Not necessarily, the Commando head flows pretty good. Depends upon if you want ultimate power or just more power. We are only raising the redline from 7,000 to 8,500 which is a 50% increase on acceleration forces. The Commando valve train (with some work) can handle more. Really a non issue, even with a harsh cam.

acotrel said:

The lack of torque ...........................

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Your assertion above is without a solid basis. If you had a lack of torque on your short stroke, you were doing something drastically wrong. What you loose in crank moment arm on a shorter stroke you gain in bigger piston and more force.

acotrel said:

I presume that with your 750, short stroke refers to about 82mm like an early Triumph twin - not what I would call short stroke ? The later (70s) Daytona T100 at 65.5mm is short stroke.

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Ahem, we are talking Nortons here. My 750 USS Norton is 81mm bore by 75mm stroke. Runs like the wind with gobs of torque throughout the range. Very trackable and very fast.

hobot said:

I remain confused on most efficient power route in Nortons, stroker increased cubes or de-stroker higher rpm.

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Well hobot, no replacement for displacement - Engine Science 101. You missed the boat and should have gone with a 1,007cc, and then supercharge it and then add nitrous and then.........

No replacement for displacement only goes so far Dances especially boosted hi rpm, though don't ya know I looked at 1007 and sealing options years ago with M.A.P. Cycles. If Peels base 920 JSM rod engine is good for 80 hp and Drouin only adds 50% instead of advertised 58% more Drouin power that's ~140 hp around 7000. Do ya think with Maney cases, welded light nit-fried crank, JSM kit, Norris D cam and proven out of tach sight valve control head, Peel could stand 8500 and not just friction burn out? If so might top TC's nitromethane fueled 850 stocker engines. I expect Peel base line closer to Steve Maney's 100 hp 920's at 7200. Combat head flow and stock valve size is the weakest power link mystery. May be more than I can handle regardless and that's all I want any way.