Short Stroke ~ High Compression

X-file said:

It pretty much goes like this: For a given cam duration, you'll get the same piston speed, regardless of stroke.
If you halved the stroke, you'd get twice the rpm and the same piston speed.

There would be much more stress on the engine with 1/2 the stroke and double rpm. Stress is proportional to the square of rpm, but only directly proportional to stroke, so the short stroke engine would have approximately double the stress at double the rpm.

Click to expand...

Isn't it normal to extend the cam duration when you shorten the stroke to raise the operating rev range ? When you speak of 'stress' on mechanical components, are you thinking of F=1/2MV ^2 or F=MA ? I suggest that in the short stroke motor, the accelerations on the pistons are usually lower. The angularity of the rods in relation to the crank journals and the bores, and piston mass are always issues. Longer rods are safer however deliver less twisting power. Super light pistons make most motors better, however short skirts let more rock happen, then you might encounter more ring flutter ?

Rohan said:

hobot said:

What is your evidence crank is not main rpm limiter.

Click to expand...

A lack of blown up cranks in this 'special engine' ?
And where is this valve gear, and pistons, that can survive high rpm buzzing ? - if they are not involved with this skipping rope crank....

Click to expand...

Lightened valve gear in a Triumph 650 situation will reliably withstand 10,000 RPM - enough for any 75mm stroke commando based engine. The whole thing is completely feasible. With a short stroke billet crank, it should stay together forever as long as the rods are not too short. If you can live with reduced torque, it would be OK - needs the six speed box, and a big heart.

Its the TWO cams, with short little pushrods that allows this 10,000 rpm in a Triumph though.

How would you fit this to a Norton ??

Has anyone actually tried to see what Norton valve gear will reliably rev to ?
Disconnected from a large whirring crankshaft/flywheel assembly, perhaps ?
The heavy valves must place some limits in there too ?

NASCAR pushrod race engines are good for 9000 rpm, how do they do it ?

Dances with Shrapnel said:

X-file said:

There would be much more stress on the engine with 1/2 the stroke and double rpm. Stress is proportional to the square of rpm, but only directly proportional to stroke, so the short stroke engine would have approximately double the stress at double the rpm.

Click to expand...

The above assertion can be misleading.

So as a guide, if an 89mm redlines a 7,000 rpm, a 75mm would redline at around 8,310 rpm. Really the same maximum stress but roughly 19% greater rpm thus 19% greater mass flow. You also benefit from being able to fit larger valves in for better breathing.

Click to expand...

I don't think my assertion is at all misleading, and there is a lot of truth in it.
Piston speed means almost nothing, and you can dismiss that as a wives' tale. When the piston is at maximum speed (about 75 degrees before and after TDC) there is no inertia stress on the piston.
Piston acceleration means everything, and that will create limits. Go too far, and you'll even be breaking rings. Piston speed alone is not a factor in acceleration. Stroke length, and moreso rpm will be a factor.

Let's say we increased the rpm from 7000 up to 8,300 without changing stroke length (still 89 mm) or mass of pistons and rods. Inertia stress would increase by a factor of (89/75) squared = 140.8% of previous. Valve gear acceleration would do the same, if the same cam was fitted.
Now let's change it a little and bring the stroke down to 75 mm. which reduces the the stress by a factor of
75/89 = 84% of previous.
The overall effect will be 140.8% x 84% = 18.7% more stress on the short stroke engine at the same piston speed. How can yo say that stress is unchanged?
The short stroke engine with the bigger piston is likely to have to have more piston mass, which will make it worse.
Reducing the cross section of rod or piston to reduce mass is not likely to help their strength, in coping with the added acceleration.

I will grant that valve size can be increased with a bigger bore, so valve diameter to bore diameter ratio remains the same, and that will allow increased airflow (even with the same cam) to feed the engine at the higher rpm.

There is no free lunch, and the short stroke engine has more stress at the same piston speed.

X-file said:

There is no free lunch, and the short stroke engine has more stress at the same piston speed.

Click to expand...

What if you dramatically increase the con rod ratio ?

Time Warp said:

What if you dramatically increase the con rod ratio ?

Click to expand...

Do you think that a change of rod/stroke ratio could possibly change piston acceleration by 18.7%? Well it could, if you had an infinitely long rod. Even if you doubled the rod length, it wouldn't change much.
Comparing 89 mm stroke and 5.875" rod, you would need a 377.23 mm rod with 75 mm stroke to get that sort of change. A 377.23 mm rod (13.277") seems to be impractical.

That is not what I asked.
The question was, would it reduce stress and the answer is yes.

Dances with Shrapnel said:

if we are running a 75mm stroke the redline will be generally governed by the maximum recommended mean piston speed. So as a guide, if an 89mm redlines a 7,000 rpm, a 75mm would redline at around 8,310 rpm. Really the same maximum stress but roughly 19% greater rpm thus 19% greater mass flow. You also benefit from being able to fit larger valves in for better breathing.

Click to expand...

Yes, there are variations to the acceleration/deceleration for various rod to stroke ratios but as you mentioned, within the practical length to stroke ratios, it does not amount to much. The general rule with a Commando twin is the shorter stroke varieties end up with greater rod to stroke ratios which reduce acceleration piston.

Your statement below is what I see as misleading:

X-file said:

so the short stroke engine would have approximately double the stress at double the rpm."

Click to expand...

In theory the above may be true in your context but as I stated above, in practice (practical limits) this is a non starter and non issue. A good corollary to your statement above would be "so an 89 mm stroke engine would have approximately four times the stress at double the rpm" . This is a true statement but once you reach the redline (7,000 rpm) based on say mean piston speed, it is moot. No one (other than hobot) is even contemplating revving an 89 mm stroke to 14,000 rpm.

acotrel said:

If you can live with reduced torque,

Click to expand...

If you have reduced torque you screwed up the build, the design and/or the tuning. This is true for any variety of builds with an 89 mm stroke. Reduced stroke (while increasing the bore by a commensurate amount to maintain displacement) does not equate to reduced torque.

Is it worth racing a bike which is extremely fast once wound up, yet hopelessly nasty in and around and coming out of corners. I persevered with my 63mm stroke 500 cc Triumph for about 12 years and I'm still carrying the injuries. A near standard commando would be a much better option. A 75mm stroke 750 would be lovely, you could rev it to 10,000 provided the piston weight was kept low and the rod length not too short, and it would still pull hard from relatively low revs. Your comment about tuning is off the mark - how many combinations of the variables do you try until you realise that the rod length is wrong and you need to make a set of barrels ? That was the point at which I sold the bike and built the Seeley = a far superior option and I sincerely regret not doing it while I was still young enough to enjoy it.

Keep in mind that same displacement engine long or short short are all almost identical in torque and hp curves, allowing that head and cam configured to adequately feed their slightly different intake and exhuast needs. Looking at Porsche threads on idenictal disscussions will find this a dyno power fact in expensive air coolled boxer engines that have similar red lines as Norton, ie: upper 7's lower 8 grand. Something fishy about xfliles clac's conclusion implying shorter stroke gives more stress at same rpm as longer stroke. Main innate Norton rpm limiter is the jump rope crank: ie: unsupported in the middle where the jerk loads are applied. The rest of Norton rpm issues have been $olved. Also realize that efficient engine is great for mileage but not so good for racing unless fuel capacity rule limited - so for most fun its better to watse some gas to cool and dump the excess waste heat that otherwise could be used to move along, if more slowly. Comnoz tests reveals the main rpm power limiter in Norton rpm is stroke/bore friction so maybe we need to reverse our concepts- that short stroke less friction is main advantage rather than more power to over come more friction.

Piston speed is easier to calculate so to get higher piston speeds in same time mean jerking on piston harder so max piston velocity is almost as good as pure G's loads caluclated. http://hre.com/discus/messages/2/10113.gif


Not only does the overall size of the engine matter, but the aspect ratio of the engine cylinders—defined by the stroke-to-bore ratio—also matters. To explain why, one must consider three factors: in-cylinder heat transfer, cylinder scavenging and friction. Simple geometric relationships show that an engine cylinder with longer stroke-to-bore ratio will have a smaller surface area exposed to the combustion chamber gasses compared to a cylinder with shorter stroke-to-bore ratio. The smaller area leads directly to reduced in-cylinder heat transfer, increased energy transfer to the crankshaft and, therefore, higher efficiency. - See more at: http://www.achatespower.com/diesel-engi ... n2fiw.dpuf
Engine friction is affected by the stroke-to-bore ratio because of two competing effects: crankshaft bearing friction and power-cylinder friction. As the stroke-to-bore ratio decreases, the bearing friction increases because the larger piston area transfers larger forces to the crankshaft bearings. However, the corresponding shorter stroke results in decreased power-cylinder friction originating at the ring/cylinder interface. - See more at: http://www.achatespower.com/diesel-engi ... n2fiw.dpuf
At Achates Power, we have conducted extensive analyses in all three areas in order to correctly identify the optimum engine geometry that provides the best opportunity to have a highly efficient internal combustion engine. In-cylinder simulations have shown that the heat transfer increases rapidly below a stroke-to-bore ratio of about 2, engine systems simulations have shown that the pumping work increases rapidly below a stroke-to-bore ratio of about 2.2 (because of the associated decrease in scavenging efficiency) - See more at: http://www.achatespower.com/diesel-engi ... n2fiw.dpuf








The most significant impact of rod to length ratio is how it draws the air into the cylinder. It's been uite a long time since i've done this, but longer Rod:Stroke ratios will suck better after 90aTDC (that is better than a shorter R:S rthe key is to get a pressure peak as late as possible without losing overall work. As this means that more pressure is going to make torque, and less is going to bend the crank. However it's also interesting that this becomes largely meaningless at high RPM, as the torque output is determined by the inertia of the crankshaft. note it acutally sucks harder over the whole cycle). Shoter rod to stroke ratios suck better between TDC and 90aTDC. the key is to get a pressure peak as late as possible without losing overall work. As this means that more pressure is going to make torque, and less is going to bend the crank.

1/Z is the rod to stroke ratio (please not I think I messed up and labelled this graph wrong, the blue line should be 1/Z = 6). The greater the Rod to stroke ratio the closer to sinusodal motion you get.







http://ftlracing.com/rsratio.htm

Jeas, louise how would you balance this sucker, high revving short stroke, but still a long stroker twin cylinder, without counter balance shaft and or changing the crank angle???
The mind boggles!
Burgs

hobot said:

Something fishy about X-file's calc's conclusion implying shorter stroke gives more stress at same rpm as longer stroke.

Piston speed is easier to calculate so to get higher piston speeds in same time mean jerking on piston harder so max piston velocity is almost as good as pure G's loads calculated.

Click to expand...

There is nothing "fishy" about my my estimations and calculation of inertia stress, and it should be self-explanatory. If all other factors like rod/stroke ratio and rod/ piston mass are held constant, those calculations are accurate. Any changes you make to those "constants" are only likely to be minor, in any case.

I have not said that a short stroke engine has more inertia stress at the same RPM.
At the same RPM, the 75mm stroke would only have 84.3% as much inertia stress.
I HAVE said that the short stroke engine would have more inertia stress at the same PISTON SPEED, and it would be 18.7% hgher.
To maintain equal inertia stress the short stroke engine would have 8.9% more rpm, but 8.2% less PISTON SPEED. That would also mean a reduction of camshaft duration of about 11.5 degrees to suit the lower piston speed.
Sorry acotrel, but cam duration is related to piston speed (not RPM). That's why you had so much trouble with lack of low-rpm and mid-range torque on your short-stroke Triumph, and the rod length hardly mattered. Still, a rod-stroke ratio between 1.8 and 1.9 is about as good as it gets, but the effects are minimal.

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?

Ok x-file English was designed from the start to confuse folks. Now you've re-stated it w/o fishy scales, we are both saying the same thing, me by saying less inertail stress in short stroke at *same rpm* as long stroke, while you say short stroke has more ineria stress at same *piston speed*, but left out the key note, that the short stroke is able to spin faster to reach same piston speed as long stroke. After reviewing www it seems the main power advantage of short stroke of same displacement as long is the lowered bore rub fricton lenght & speed. Norton short strokes may *free up* more power while turning same of faster rpm than long stroke but d/t the jump rope cranks they are not known for any more rpm redline than factory stroke. I hope to be corrected if I'm stating this wrong.

hobot said:

Norton short strokes may *free up* more power while turning same of faster rpm than long stroke but d/t the jump rope cranks they are not known for any more rpm redline than factory stroke. I hope to be corrected if I'm stating this wrong.

Click to expand...

As long as we're getting all technical about it, you might want to factor in this tidbit. The short stroke crank will be stiffer in bending than the long stroke crank, because the crank pin and mainshaft centerlines are closer together. Now that one-piece cranks are available from several sources, it would be possible for someone wanting to experiment with a short stroke engine to have a crankshaft made with a larger diameter main bearing and larger diameter rod bearing. You could get a significantly stiffer crankshaft that way. The real question would be whether that gave you enough advantage to make up for the increased friction losses with the larger bearing sizes. The load capacity of the stock rod journal is already more than enough for anything that doesn't involve lots of nitromethane or nitrous oxide, so there's no real reason to go larger except crankshaft stiffness. Several people have already experimented with custom Norton crankshafts with larger main bearings, and some have tried larger diameter rod bearings, and I know of at least one builder who has done both at the same time in a short stroke race engine.

Having said all that, I think that in general a modern one-piece crankshaft in a long or short stroke Norton 750 is more than durable enough for any rpm at which the engine still makes reasonable power. I think you'll run out of air flow through the head before you run out of crankshaft endurance. Again, if your fondest desire is to build a 1007 cc monster, fit a turbocharger, and run it on 90% nitro, all bets are off.

And another caveat. All this only really applies to seriously built engines for race use. The stock crankshafts, that Steve likes to call "jump rope cranks", are just fine for the typical Commando street bike, as long as you don't think you should be able to run them at 7,000+ rpm all the time. Start modifying the engine seriously, and flogging them really hard, and you might find the limits of the crank's durability, but for most normal use, they are just fine.

Ken

X-file,
I think you are correct abot the cam duration aspect of my 63mm stroke Triumph engine. The cams were of very long duration and relatively low lift. With separate pipes and megaphones the bike was unrideable on short circuits, however towards the ends of the straights on long circuits it was extremely fast. It seemed to wind out forever. after I fitted the two into one pipe and adjusted the cam timing, I started to get decent lap times. The problem was that I had painted myself into a corner by fitting needle rollers to the cams, so I never changed them for something milder. a friend has the bike , still in the next town and now has megacycle cams fitted. I will get it going again and try it to see if it is any better. the power band with the old cams and separate pipes was from 5000 RPM to about 10,500 RPM and when it came on song the bike tended to go sideways. It turned me into an instant dud where I was a fairly decent rider. I never knew it's full story until recently, however a friend of mine crashed it at Bathurstin about 1958. He had been an excellent rider, however did not race for about 20 years after getting injured on that bike.

Gosh don't any of us dwell on women anymore : )
Sir Eddie knew and tried to apply all the old and new school tricks to attempt over 10 grand land speeder. Its a hi CR short stroke worth a review. Street bikes are cute and all but the real excitement for some is turning engines into elastic cartoon characters that spring back every time. Besides crank flex cam shaft can flex too. Head flow starving takes special fuels and boost to get past but then may blow up. Best I can find its by far the crank spin sling factor rather than combustion torque pressure that hurts Norton so only more torque can get much more out of old design. Some day may try a planetary gear crank/rod long stroke nil bore friction device and make stock cam into a small crank shaft the works pedding rods on a demo-like OHC.

Guess how a Norton twin sound inclosed tin shed as it passes 11 grand...
Ripping Fabric of Space percussions as rest of orchestra falls through the rafters w/o missing a beat~! A sound never want to experience again yet one of my life's peak experiences.

Perhaps it should have been pointed out A LONG TIME AGO HERE that ALL cranks flex at rpm,
its really only a matter of how much.

One of the lead-in subjects to Engineering 101 goes something like this.

All structures flex when a force is applied to them.
A fly lands on the Golden Gate Bridge.
Now calculate how much it flexes.

[By the time you are out of here*, we expect a valid answer.]
* i.e. finished this course.
** the answer nothing would get an automatic fail.

Dimensional Displacement ( missalignment )

Some More Than Others .

one might say that the TOLERANCES of Alignment vary with Construction - Configureation .

first THINGS A THING CALLED overlap .


THEN WE MIGHT EVEN HAVE SYMETRICALLY DISPLACED MAIN BEARING LOCATION

about the ceter of apllied force . at the source , so it might not ' walk out of bed . '

TORSION OSCILLATION STABILISEATION via ' crankshaft pulley torsion vibration absorber ' .
EVEN a model T had a One Piece Crank . but you need three mains with twin parralel cylinders

fore accurate dimension stability , both themral displaement ( expansion )

and dynamic primary motive force - combustion cambers . The whole 414 + the 41 at 10 ; 1 C. R. .

which is about two DUCATI 450s . they made them in 1970


so you could get two jugs there , with a NORTON .

unfortuanately our pre war prefabricated crank thus is more AKIN to thas.


still confusion in the ranks , in the 70s .