Heavy flywheel equals more torque? (2014)

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A simpler bike related example of momentary flywheel torque is when you rev the zinger out of it,
and drop the clutch. From standstill, or on the move even.
Guaranteed to get your attention if you are not pointing straight down the road,
and watching that the front wheel doesn't lift - over your head.

Did you know that a number of bikes these days have rev limiters (set quite low) on the lower gears,
so your bike doesn't do a head-over-heels and crunch you into the tarmac...
 
I'm sure I've seen that 'stump pulling torque' used in bike advertising somewhere ?

A Panther, perchance...
 
Discussing these technical queries over the net does help to make them clearer. Flywheels definitely have momentum once spinning - heavier more so than lighter, each having advantages and disadvantages. It makes sense to me a spinning flywheel produces torque at its axis (T = F x d) but technically I may be incorrect. Long stroke engines will definitely produce more torque than short stroke everything else being equal - again advantages and disadvantages with each design.

At least I'm happy in my head that the lighter flywheel is not going to alter too much the engine characteristics of the Commando. I'm looking forward to getting the bike on the road at the week-end but it looks like I'm going to miss the track day. Another assembly problem and probably my fault. I use centre less dowels to fix the crankcase and gearbox shell to the cradle and now the gearbox mainshaft is out of concentric with it's chaincase oil seal by approx 0.70mm (I should have checked prior to drilling/reaming the cases) so I need to turn up a pair of eccentric dowels to reposition the gearbox and it's possibly one delay too many. Which brings me to how I got here in the first place. Back in 2008 a genuine AN 15,000 mile old con-rod bolt broke and wrecked the engine. Original rods and bolts had run for 100,000 miles. Replaced parts as required and then in 2014 genuine AN crankcases crack at 34,000 miles. Original cases had run for 115,000 miles until rod bolt broke. So I'd really like to thank everyone at AN responsible for producing the mentioned components (and other parts) and for giving Commando owners the opportunity to spend so much time in the workshop/garage.

Things aren't all bad by a long way - important to keep things in perspective - Britti rally in three weeks :D
 
Al-otment said:
Long stroke engines will definitely produce more torque than short stroke
Click to expand...

We haven't fully had this discussion yet. !
Sure, long strokes have the reputation of making good torque.
But not necessarily..
And its going to depend on what you call "more torque" (what rpm, for example, or that statement is meaningless.)

However, one of THE MOST TORQUEY ENGINES of recent times was, oddly enough, a short stroke engine.
A 1980s Suzuki 4 cylinder 4 valve per cylinder 1100cc, the GSX 1100 EFE (?).
On the dyno, it put out MORE TORQUE low down than the same year HD big twin (1340cc).
And revved to twice the revs, still making good torque all the way. And thus good power.
One of the flattest, and fattest, torque curves you will ever see.
That is going to take some beating.
From a 4 cylinder short stroke, with Suzooks TSCC - Twin Swirl Combustion Chamber.

The sidecar guys luv em.
Plain bearing crank, so nothing to twist, or need welding, easy to put bearings in.
And can be bored to 1500cc, and still be unburstable.
Great engine - but the bike it went into was pretty ordinary.
(Later engines lost some of the low down stuff, in the quest for more top end go)

And I already mentioned the 1920s JAP 1000cc engines, where the long stroke was the revvy race engine,
and the short stroke (more square actually) was the torquey touring engine.

So there are short strokes, and short strokes.
Short strokes are usually tuned for top end power.
But not necessarily....
 
In this posting the phrase " to keep the mass moving" is the whole reason engines have flywheels...going back 300 years !
without weight added to the crank the engine would simply stop at the bottom of the firing stroke..thats it!


texasSlick said:
Well, I put in my thoughts earlier, then stepped aside . Now the old mechanical engineering professor in me just has to chime in again.

The basics:

Burning fuel in the combustion chamber generates pressure.
Pressure acting on the piston area generates force. F = p x A
Force acting on the con rod, and crank throws generates torque. T = F x distance
Torque x RPM = horsepower (simple formula - conversion factors left out)

The flywheel does not enter into any of the above. The flywheel, per se, does not alter or affect pressure, force, torque or horsepower.

A spinning flywheel (think of one suspended on pillow block bearings, not connected to an engine shaft - forget how it got to spinning) has energy by virtue of its mass and rpm. More mass or greater rpm = more energy, but no torque, unless you want to quibble about the friction in the pillow block bearings which would cause the bearings to rotate if they were not restrained. Now here is where some may be getting confused: What if this spinning flywheel were to be suddenly clutched into a transmission shaft? It is intuitively obvious that the transmission shaft would have a torque applied to it for at least a brief time. But to say the flywheel generates torque is like saying matches generate forest fires....it is only the manner in which the energy of the flywheel, or match, is harnessed, that generates the result.

The function of the flywheel is to smooth out the inherently non-smooth pressure fluctuations in the cylinders, and the non-smooth reciprocating mass. In some cases, particularly single cylinder, 4 strokes, the flywheel provides the energy to keep the mass moving until the next power pulse occurs.

In general:

heavier flywheels = smoother engine operation, less "bucking" or surging.
lighter flywheels = faster changes in rpm, either up or down.

The above is basic physics.
I defer to those with racing experience, for specific effects of altering flywheels.

Prof Slick
Click to expand...
 
john robert bould said:
In this posting the phrase " to keep the mass moving" is the whole reason engines have flywheels...going back 300 years !
without weight added to the crank the engine would simply stop at the bottom of the firing stroke..thats it!
Click to expand...

On a combustion engine as per a motorcycle,reducing the outer flywheel mass or counter weight of the crankshaft will eventually affect the ability to idle which will increase (idle speed) as the weight reduces even if it is a 3000 rpm idle (ie multi cylinder race bike)

Obviously large low revving stationary engines have large heavy flywheels for that reason,flywheel not being confused with the actual crankshaft.
You can have a light crankshaft with a heavy external flywheel as long as it does not induce whip into the outer shaft resulting in main bearing failure.
 
john robert bould said:
In this posting the phrase " to keep the mass moving" is the whole reason engines have flywheels...going back 300 years !
without weight added to the crank the engine would simply stop at the bottom of the firing stroke..thats it!
Click to expand...

As well as the high idle speed business, a no-flywheel engine is actually more likely to stop coming up on compression.

If you've ever tried to kick over a hi-compression big single, stopping on compression can be a problem even with a heavy flywheel. !
And you pray it doesn't kick back, and do your leg an injury....
 
Time Warp said:
You can have a light crankshaft with a heavy external flywheel as long as it does not induce whip into the outer shaft resulting in main bearing failure.
Click to expand...

Guzzi were famous for their flat single racebikes, with external flywheel (and on their roadbikes).
When Nortons tried to copy this external flywheel on experimental Manx Norton race bikes, they had a whole slew of crank breakages.
And decided this wasn't such a good idea to copy.

Many bike makers have had an external flywheel over the years.
Douglas in particular, for many decades.
Keeping it inside the cases keeps it out of harms way, it must be said...
 
Rohan said:
Al-otment said:
Long stroke engines will definitely produce more torque than short stroke
Click to expand...

We haven't fully had this discussion yet. !
Sure, long strokes have the reputation of making good torque.
But not necessarily..
And its going to depend on what you call "more torque" (what rpm, for example, or that statement is meaningless.)
Click to expand...

I did say 'everything else being equal' - the long stroke has the bigger lever (distance between big-end axis and out put shaft axis) than a short stroke so for the same force there's more torque.
 
Short strokes and long strokes can't have 'everything else equal' ??

Short strokes generally have room for (much) bigger valve area,
and that alone can have them get much better cylinder filling = torque.
And if the designers choose to implement that at low rpm, rather than upper rpm range.....
 
Is it time to back the car up to where I mentioned conrod length.
A quick google revealed the Norton twin conrod length remained the same at 129.274 mm from the 650 right up to the 850 and would think the stroke changed.
129.274 ÷ 89 = a 1.453 conrod ratio,quite low by modern standards where 1.8 + is common,ok for some mid range perhaps.
 
Rohan said:
Short strokes and long strokes can't have 'everything else equal' ??

Short strokes generally have room for (much) bigger valve area,
and that alone can have them get much better cylinder filling = torque.
And if the designers choose to implement that at low rpm, rather than upper rpm range.....
Click to expand...

I was talking hypothetically.
 
The Weslake 4 valve head doesn't do much for a 650 Triumph even though the valve area is bigger. On the Jawa speedway motors, a good two valve is almost as fast as a four valve motor, from everything I've encountered I believe there is about a 10% increase in top end power if the two valve head is replaced by four valve. I think the advantage of the short stroke lies in the number of firings per unit time, a long stroke motor generally doesn't have as many. I believe the reason that the smaller ports in a commando engine provide more torque, lies in gas speed and cylinder filling. If you are not using dramatic cams and high revs, the smaller ports are better.
I suggest a good way to get a commando engine going, might be to copy the set-up of the 1958 7R AJS. It was the best British 350 single of the fifties. The cam is the same configuration as the commando.
 
Time Warp said:
Is it time to back the car up to where I mentioned conrod length.
A quick google revealed the Norton twin conrod length remained the same at 129.274 mm from the 650 right up to the 850 and would think the stroke changed.
129.274 ÷ 89 = a 1.453 conrod ratio,quite low by modern standards where 1.8 + is common,ok for some mid range perhaps.
Click to expand...

The rod to stroke ratio of Nortons is bad, but not that bad. 750 and 850 CDOs have a R/S ratio of ~1.68. The manual has the rod length listed as 5.875" and 129.275 mm, which suggests something is amiss, since 5.875" is 149.2 mm, so perhaps a typo. But point taken that the R/S ratio is short.
 
Don't know what manual you are looking at, but the factory service manual I just pulled off the shelf shows the rod length as "5.877/5.873 in. (149.275/149.535 mm)"

FWIW, the factory short stroke 750 rod is 6.200 in. center to center. With it's 80.4 mm stroke, that gives a R/S of 1.96.

Ken
 
lcrken said:
Don't know what manual you are looking at, but the factory service manual I just pulled off the shelf shows the rod length as "5.877/5.873 in. (149.275/149.535 mm)"

FWIW, the factory short stroke 750 rod is 6.200 in. center to center. With it's 80.4 mm stroke, that gives a R/S of 1.96.

Ken
Click to expand...

Ken, how's about posting some of your stories re fast guys, who lapped slower - despite thinking they were going faster (re the light vs heavy crank topic)?
 
lcrken said:
Don't know what manual you are looking at, but the factory service manual I just pulled off the shelf shows the rod length as "5.877/5.873 in. (149.275/149.535 mm)"

FWIW, the factory short stroke 750 rod is 6.200 in. center to center. With it's 80.4 mm stroke, that gives a R/S of 1.96.

Ken
Click to expand...

This one.

Heavy flywheel equals more torque? (2014)


I have 2 "identical" manuals, one lists the correct rod length and one lists the incorrect rod length.

Heavy flywheel equals more torque? (2014)


As always, such misinformation manifests itself in other locales as well.

http://tinyurl.com/m82m2y9
 
Fast Eddie said:
lcrken said:
Don't know what manual you are looking at, but the factory service manual I just pulled off the shelf shows the rod length as "5.877/5.873 in. (149.275/149.535 mm)"

FWIW, the factory short stroke 750 rod is 6.200 in. center to center. With it's 80.4 mm stroke, that gives a R/S of 1.96.

Ken
Click to expand...

Ken, how's about posting some of your stories re fast guys, who lapped slower - despite thinking they were going faster (re the light vs heavy crank topic)?
Click to expand...

You can often do a lot of very hard riding on a difficult bike, to get the same result that you get when the bike helps you to do it easily. There are plenty of situations where you can be deluded into thinking you are really flying, when your performance is actually average. Often the harder you try to go fast, the slower you go. If the bike puts the power down strongly and safely and helps you around the tight corners without scaring you shitless on high speed bends, that is good. Then winning comes down to your own mental state and experience.
 
You can often do a lot of very hard riding on a difficult bike, to get the same result that you get when the bike helps you to do it easily. There are plenty of situations where you can be deluded into thinking you are really flying, when your performance is actually average. Often the harder you try to go fast, the slower you go. If the bike puts the power down strongly and safely and helps you around the tight corners without scaring you shitless on high speed bends, that is good. Then winning comes down to your own mental state and experience.
Click to expand...

Amen Alan. Like you say, a dangerous bike just teaches working harder not to crash while a good handler tests your plotting ability o finish a run out ahead with the limited horse power on hand. A heavier flywheel makes a cycle easier to handle and more resistant to fast change in tire speed and lean angle changes which can be a good thing - up to a point - locked on rails - buta a great cycle to me - is more like an unstable fighter craft that's almost too easy to change lean, tire spin rates that combine into about full range fork snaps w/o tank slapping. I guess it also boils down to how close to crashing out of control to ride ya like vs how close calls ya like crashing into control.
 
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