Short Stroke ~ High Compression

NOW THIS is a REAL Bottom End . BRITISH TOO . 1968 I think , if its a real ASTON 5340 Crank . etc .



PROPPER Main Shaft load anaylisis for dynamic stability . they new all about it from Le Mans .





max contiuous 6750 rpm. max intermitant . 7250 third only . unless your Phil Hill and knit the valves at 7600 . at 175 mph .





wrong plug type ( Surtees Sponser ) perhaps . sevre thermodynamic missalighnment . at 200 degrees and 200 mph . :?

50 mm bore webers . ' RESTRICTIONS " removed from exhaust , and dual ' inlet ' GRIND cam tweak . 175 mph recorded . Phill Hill . Road & Track .



2nd fastest car they could find . and break . Even Phill Hill has to OBSERVE Red Lines . Even Fourth Engauged .

paartcularly in Model Ts going downhill is wet clay .

From memory with my 63mm stroke triumph crank, the big end journals overlapped the mains. In any case as Rohan well knows, if the shaft is correctly balanced the bow at the top end of the operating rev range is minimalised.

???

Since the balance factor weights can sometimes be in flywheels externally located,
I fail to see how some/many configurations of balance have anything to do with how much the crank flexes.
That has more to do with the pure physical strength of the crank, plus how its reinforced and strengthened
by all sorts of fillets and other tricks of the crank makers trade, plus factors which can reduce the rigidity.

And a lot of auto cranks have problems with torsional vibrations, which can be sufficient to even break them,
which is a whole another kettle of fish. Most motorcycle cranks aren't usually long enough to suffer these problems,
but they need to be borne in mind and guarded against.

acotrel said:

if the shaft is correctly balanced the bow at the top end of the operating rev range is minimalised.

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Further reflection would suggest that if the crank didn't need to be balanced - for rider comfort - it could well be stronger and flex less ?
Think about it....

if there is no imbalance at high revs, why would the crank be bowed ? I suggest that if you strobed a standard commando crank at 10000 rpm you could watch it grow in diameter.

acotrel said:

From memory with my 63mm stroke triumph crank, the big end journals overlapped the mains. In any case as Rohan well knows, if the shaft is correctly balanced the bow at the top end of the operating rev range is minimalised.

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The Commando crank, with 1.750" (44.45 mm) rod journals and 30 mm mainshaft would start to have the rod and main journals overlap at 75.5 mm stroke. With the stock short stroke dimension of 80.4 mm, there is a separation of 2.97 mm. With the stock standard stroke of 89 mm, there is a separation of 7.23 mm, making the longer stroke crank more susceptible to bending.

Ken

acotrel said:

if there is no imbalance at high revs, why would the crank be bowed ? I suggest that if you strobed a standard commando crank at 10000 rpm you could watch it grow in diameter.

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Because a Commando crankshaft assembly can never be balanced at any rpm. As mentioned here repeatedly, you can not get rid of the unbalance forces due to the reciprocating masses. You can only move the direction of the force vector around. There are also the forces from combustion and from ring drag that are exerted on the rod journals, which contribute to crankshaft flex. Even if you added balance shafts to put the total engine in balance, the crankshaft assembly would still be unbalanced.

Ken

If you balace the commando crank at 78% and the motor becomes smooth at 7000 RPM, doesn't that mean the forces are equal in at least one direction when it is spinning ? At lower revs the forces become unequal, however the loads created are proportional to the square of the accelerations, so that what it does internally at peak revs is more important. I don't believe balance shafts relieve internal stresses in the motor. Isolastics probably help a bit by removing the sudden stop when the internal loads are imposed on bearings and cases. The short stroke motor is much safer to race as far as self-destructing goes. That is the only reason I ever persevered with the 63mm stroke engine for all those years. It was a pig to ride, however it was always exciting, and in the long run less expensive to race than the 650cc Triumph engine. The joke is that after that experience the Seeley is a soda. I still cannot afford to race again at the moment, however things will change and I will have another go - probably next May.
For me, this discussion is interesting but pointless. We are not smart enough to encourage a 750cc capacity class for air-cooled two valve twins. I'd really like to be racing against similar capacity bevel Ducatis, Guzzis and BMWs - that would greatly improve my sense of well-being. AHRMA sound like they are a smart organisation, however I believe that limitations on development are not the way to go. The truth is that the rich guys can spend shit-loads of money and still not get to the front.

acotrel said:

If you balance the commando crank at 78% and the motor becomes smooth at 7000 RPM, doesn't that mean the forces are equal in at least one direction when it is spinning ? At lower revs the forces become unequal, however the loads created are proportional to the square of the accelerations, so that what it does internally at peak revs is more important.

Click to expand...

Nope. All engine balancing is done purely for rider comfort, even if it's at the expense of added stress on the crank.
If you wanted to reduce the stresses on the crank and main bearings to a minimum, you'd be using 65% balance factor with a wet crank (full of oil). You won't be happy with that as a rider, and it will cause some discomfort. The trade off is to use a higher balance factor to reduce vertical imbalance or or a lower balance factor to reduce horizontal imbalance (whichever is causing most discomfort for the rider).

Even at 65% balance factor, you'd have an imbalance force at 7000 rpm of about 3,200 lbs at TDC (from 2 rods and pistons, minus the counterweight effect), and much the same at around mid-stroke from the counterweight alone (acting either forward or backward). If you change the balance factor one of those must increase, either vertical or horizontal.

Matt Spencer said:

NOW THIS is a REAL Bottom End . BRITISH TOO . 1968 I think , if its a real ASTON 5340 Crank . etc .

Click to expand...

If Matt had really done his homework, he would know that 4 bolted mains
are helpful in preventing distortion and flexing in big hp bottom ends.

But that is car stuff, and this is a bike forum...

Once again, Aco/Allan is demonstrating that he doesn't quite understand engine balancing. ?
I put this here in case others are mislead by his jottings....

Again again again we repeat,
you can NEVER perfectly balance parallel twin reciprocating bits (the pistons and rods)
with an out-of-balance crankshaft whirling around.

And while can you more-or-less do it with balance shafts,
these are simply more out-of-balance bits
counteracting the other out-of-balance bits.

While the rider gets a smooth ride,
the engine bits are no less stressed than they were before.

V-twin, flat twin, triples and fours etc etc all equally have stressed bits,
but counteracting other pistons and rods make it FEEL smooth.

X-file said:

So the real extra potential of the short stroke engine (at equal inertia stress) is only an 8.9% improvement (not 18.7%).

You can guesstimate inertia stress by using the simplistic piston speed, but it's not almost as good. You could easily end up with an error of around 40% , and that's not really "almost as good".

Did I miss anything?

Click to expand...

Well, quite simply yes, a few things. For starters, at the risk of being pedantic, the dimensions are a bit screwed up. The obvious elephant on the table is the concept of inertia stress mentioned above when it is really inertial force, or better still, force due to acceleration. Yes the increased force is there as mentioned above but it is somewhat of a nonevermind with regards to the realm of practical four stroke engine building constraints we are talking about here. Maybe there is an acceleration limit but I believe it is bounded by the practical limit of piston speed…at this time. Case in point is with Formula 1 engines experiencing three (3) times the maximum piston acceleration of our beloved Norton twins, ultra short strokes and all.

So why is it important to keep the dimensions and units correctly stated? By example, for a given engine, just by changing from the factory aluminum rods to properly designed steel rods we have increased the stress on the rods – yes, increased the stress. Stress is a function of load and cross sectional area and for a given piston force the decrease of cross section of the steel rod will result in a higher stress.

People have been shortening engine stroke for maybe close to a century and have not hit a wall due to acceleration. From what I can see, the technical wall is piston speed, and I am speculating that is probably due to practical limit of friction loss, localized heat and perhaps maintaining adequate lubrication. Cosworth Formula 1 has gone to DLC coating on the piston skirts to mitigate power loss to friction. The Formula 1 example of piston acceleration and dealing with friction seems to support the contention that friction due to piston speed is somewhat of a limiting factor. So to say “the real extra potential of the short stroke engine (at equal inertia stress) is only an 8.9% improvement (not 18.7%).” although probably technically correct it is mixed up and misleading as it assumes a somewhat arbitrary limit that is in fact not a limit nor any type of rule of thumb or guideline for engine design. Saying “engine potential at comparable piston acceleration” would have been accurate.

Mean Piston speed is a generally accepted rule of thumb as a good indicator of the class and performance of an engine relative to its competitors. From what I have read, piston acceleration is not a generally accepted rule of thumb for same. Things such as ring flutter and crank loading, ignition timing, rod loading, cylinder port performance and valve timing should be dealt with and usually are dealt with. A good example is what was done with the factory short strokes. Going from a long stroke to a shorter stroke; some things tend to mitigate the additional forces to a certain extent such as shorter stroke resulting in a more compact crankshaft, longer rods (reducing acceleration), higher performance pistons. etc.

fpm m/s
1,673 8.5 Low speed diesels
2,165 11 Medium speed diesels
2,756 14 High speed diesels
3,150 16 Medium speed gasoline
3,937 20 high speed gasoline
4,922 25 high speed gasoline
5,906 30 Competition - ex. NASCAR, Formula 1, Top Fuel drag

Formula 1 engines employ strokes on the order of 38mm with peak rpm ranging from 18,000 to 20,000. Our Norton Commandos fall in the lower end of the high speed gasoline engine range as outlined in the table above. A very crude analysis of the above indicates the Norton twin is roughly 2/3 up the range of mean piston speed and only 1/3 up the range of maximum piston acceleration. Room for improvement, eh

X-file said:

acotrel said:

If you balance the commando crank at 78% and the motor becomes smooth at 7000 RPM, doesn't that mean the forces are equal in at least one direction when it is spinning ? At lower revs the forces become unequal, however the loads created are proportional to the square of the accelerations, so that what it does internally at peak revs is more important.

Click to expand...

Nope. All engine balancing is done purely for rider comfort, even if it's at the expense of added stress on the crank.
If you wanted to reduce the stresses on the crank and main bearings to a minimum, you'd be using 65% balance factor with a wet crank (full of oil). You won't be happy with that as a rider, and it will cause some discomfort. The trade off is to use a higher balance factor to reduce vertical imbalance or or a lower balance factor to reduce horizontal imbalance (whichever is causing most discomfort for the rider).

Even at 65% balance factor, you'd have an imbalance force at 7000 rpm of about 3,200 lbs at TDC (from 2 rods and pistons, minus the counterweight effect), and much the same at around mid-stroke from the counterweight alone (acting either forward or backward). If you change the balance factor one of those must increase, either vertical or horizontal.

Click to expand...

So true. For the Norton twins I have read and been told a variety of balance factors ranging from 53% to 83%. The driving factor was always rider comfort.

Why do you say 65% wet is the least stress on the crank. Are you considering peak load alone (which is really not that critical) or more important, the magnitude of the load reversal which has direct bearing on the crankshaft life?

Two issues being blended & confused. 1. power loss by friction & 2. power loss by parts failure. From the factoids covered so far it seems main Norton power benefit of shorter stroke is the friction & heat relief by less piston rub travel per revolution. Ring flutter and piston-rod parting is only an issue at TDC/BDC when/where the max peaks of acceleration hit, not speed in bores. Historical development of piston power, parts breaking by acceleration was first primary reason to shorten strokes. When better parts became available the friction factor of power loss of long stroke rpm appeared next. As shorter stroke lessens parts stress and friction *at same rpm* of longer stroke its easy to confuse what matters most, friction or parts parting. With same bore size If friction can be reduced by loose fit and special treatments then the longer stroke should kick the short stroke butt at same rpms by more displacement-bigger engine torque. TC proved its not the torque levels that restrict long stroke Nortons. There is nil ring-bore friction factor at the stand still points of TDC/BDC, inertial spikes called acceleration. Of course over heated ring/bore can weld ring to bore at TDC and it takes a bit more power to start a still ring set moving/sliding again but its not a power substractor issue its a parts parting issue. Aboveare is separate issue than short stroke flow advantage by bigger valves while restricting displacement to race class rules. Consider the winning 1007 cc Norton in UK that don't spin as fast as shorter strokers.

In engines like Nortons that simply can not takes 8>9000 rpm d/t parts breakage or not breathing enough to make more power plus are limited on how big the bores can be - the shorter stroke's less friction factor may be main factor over breathing advantage of bigger valve room. Here's an old treatise on the crank main bearing friction to take into account too. Do note -to avoid confusion-flack that hobot is contradicting those correctly reporting short strokes stress parts more than long stroke - this only applies in same displacement engines ie: short stroke creates more stress at same rpm only because it has a bigger piston-longer rod to handle to get same displacement-engine size.

http://books.google.com/books?id=pgZaAA ... e&q&f=true

Dances with Shrapnel said:

Why do you say 65% wet is the least stress on the crank. Are you considering peak load alone (which is really not that critical) or more important, the magnitude of the load reversal which has direct bearing on the crankshaft life?

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Yes, I'm considering peak inertia stress alone, because peak stress is what breaks things (in this case the crankshaft).
There will be enough cyclic applications of that peak stress until something finally breaks. If that stress was being constantly applied at the same magnitude and in the same direction there would be no metal fatigue, but it isn't.

You get close enough to full reversal of direction, if you consider the imbalance situation at TDC and again at around mid-stroke.

hobot said:

In engines like Nortons that simply can not takes 8>9000 rpm d/t parts breakage or not breathing enough to make more power plus are limited on how big the bores can be - the shorter stroke's less friction factor may be main factor over breathing advantage of bigger valve room. Here's an old treatise on the crank main bearing friction to take into account too. Do note -to avoid confusion-flack that hobot is contradicting those correctly reporting short strokes stress parts more than long stroke - this only applies in same displacement engines ie: short stroke creates more stress at same rpm only because it has a bigger piston-longer rod to handle to get same displacement-engine size.

http://books.google.com/books?id=pgZaAA ... e&q&f=true

Click to expand...

With modern materials an 89mm stroke small bore engine can easily be spun to 10,000 rpm without breaking - but the friction vs piston speed wall would still be there and would still make turning the engine beyond about 7500 rpm pointless as far as building more horsepower. Jim

X-file said:

If you wanted to reduce the stresses on the crank and main bearings to a minimum, you'd be using 65% balance factor with a wet crank (full of oil). You won't be happy with that as a rider, and it will cause some discomfort. The trade off is to use a higher balance factor to reduce vertical imbalance or or a lower balance factor to reduce horizontal imbalance (whichever is causing most discomfort for the rider).

Even at 65% balance factor, you'd have an imbalance force at 7000 rpm of about 3,200 lbs at TDC (from 2 rods and pistons, minus the counterweight effect), and much the same at around mid-stroke from the counterweight alone (acting either forward or backward). If you change the balance factor one of those must increase, either vertical or horizontal.

Click to expand...

I agree with the above. Reduce the extreme out of balance whether vertical or horizontal to make life easier on the bottom end. Find out for sure be making an actual measurement. My measurements show it to be in 60s% wet and I've made several tests. Others may get closer. Measuring can certainly be refined but my tests are shown here.

balance-factor-scratch-test-tool-t15134.html?hilit=BALANCE%20FACTOR%20SCRATCH%20TEST%20TOOL

I'd like to see the results of a more refined physical test. Note that the movement of the motor is only about .020" with lightweight pistons (much more with heavier pistons) so you need accurate tools to make the shaking measurement.

comnoz said:

hobot said:

In engines like Nortons that simply can not takes 8>9000 rpm d/t parts breakage or not breathing enough to make more power plus are limited on how big the bores can be - the shorter stroke's less friction factor may be main factor over breathing advantage of bigger valve room. Here's an old treatise on the crank main bearing friction to take into account too. Do note -to avoid confusion-flack that hobot is contradicting those correctly reporting short strokes stress parts more than long stroke - this only applies in same displacement engines ie: short stroke creates more stress at same rpm only because it has a bigger piston-longer rod to handle to get same displacement-engine size.

http://books.google.com/books?id=pgZaAA ... e&q&f=true

Click to expand...

With modern materials an 89mm stroke small bore engine can easily be spun to 10,000 rpm without breaking - but the friction vs piston speed wall would still be there and would still make turning the engine beyond about 7500 rpm pointless as far as building more horsepower. Jim

Click to expand...

Thats right. I've tested my stuff at over 8000 RPM and you don't get more power - you just have to grit your teeth and wait as the motor reluctantly spins itself up way beyond reason.

X-file said:

Dances with Shrapnel said:

Why do you say 65% wet is the least stress on the crank. Are you considering peak load alone (which is really not that critical) or more important, the magnitude of the load reversal which has direct bearing on the crankshaft life?

Click to expand...

Yes, I'm considering peak inertia stress alone, because peak stress is what breaks things (in this case the crankshaft).
There will be enough cyclic applications of that peak stress until something finally breaks. If that stress was being constantly applied at the same magnitude and in the same direction there would be no metal fatigue, but it isn't.

You get close enough to full reversal of direction, if you consider the imbalance situation at TDC and again at around mid-stroke.

Click to expand...

I am looking at it in terms of magnitude of the bending moments along the centerline of the crankshaft at 0, 90, 180 & 270 degrees of crank rotation. These changes in the bending moment translates to changes in stress throughout the crankshaft. Without seeing a bending moment diagram throughout the 360 degrees it is difficult for me to see this magnitude of change.

From first hand experience and as Jim C has stated above, really a non issue with properly designed billet cranks for the Norton Commando short stroke.