calculate how much we can take off of a flywheel

[coachgeo]

Not sure if this is the right forum to post this question...

how does an engine designer decide a diesel lumps flywheel's weight to be? As in what factors do they consider and... how much is our use in bike change those factors allowing for a margin of weight reduction off what the designer came up with.

Eee gad did that make any sense at all?

Maybe better said..... how do we determine how much weight we can take off a diesel lumps flywheel without causing performance and reliability issues with the engine.

For example.... what I've been told is HDT on it's diesel bikes is trying to go with basically little to no flywheel at all.... and this is why their diesel bike itself is a basic failure. It shakes itself to death and is inefficient. Gott think... gee the flywheel has got to be there for some damn reason.

I reallllly need to lighten my bike as I build it and turning down the flywheel is one possible area.

[sbrumby]

There has been a lot of talk and posts on this subject. But no manufacturer would make a flywheel bigger than nessesary.
They are there for a reason. If a magnum gun weighed 5oz it would probably break your hand. Every action has an equal and opposite reaction ( basic law). The Ruggerini 2 cyl has a big flywheel because the pistons are at a 180deg and the two fireings are close together followed by a lot of nothingness. On the other hand the more pistons you have the smaller the flywheel can be relatively. A friend of mine built a narrow boat and fitted a leyland 3 cyl which was marineised. what this meant was to fit an extra flywheel, this give the ability to slow run probably 400rpm but still smooth. I have a JCB 3C this has a leyland3.8 4 cyl in it. in the sump there is a balance shaft. The same engines were fitted to leyland trucks but minus the balance shaft. Another mate fitted a truck engine to his JCB when that went up the spout, it lasted a couple of weeks then the JCB vibrated to bits. In the truck the engine is rubber mounted and it stands on suspension. Nothing in the JCB.
The point I am trying to make is if you lighten the flywheel it might prove disasterous for the engine and also the bike.

[coachgeo]

From what you just wrote... I come to a different conclusion. Change is needed not..... should not be done at all. You speak of Specificity... and engine has the weight of the flywheel geared toward the general purpose of the tasks it is most likely to be used. Also, the boat vs truck example of your ... to adapt the truck diesel to slow moving boat use EXTRA weight AKA balancing was added to flywheel. They adapted it.

This tells me reverse would be true in our case in general. In my case I am using say a tractorish lawnmower engine. One whose specific design was low to mid range rpm usage. Means then a combination of rubber engine mounts (Im assuming) and heavy flywheel was probably part of the design specificity of that arrangement.

But a bike..... no low RPM cept sitting at idol. You spend most your time at 3000rpm. This is out of the specificity design of the motor. That tells me that some changes are of course needed to bring into compliance with this new specific application.

[sbrumby]

My thoughts are not to stop you trying far from it. I would be happy if you succeed. On the point that you spend most of your time at 3000rpm this is true for me on the motorway. But in town mostly 1500rpm in 6th gear, and it pulls nice at these revs I am doing 30mph which is the legal limit. To change speed lower I just change gear,to go faster out of town, just turn the throttle 3000 is 60mph 3600 is 70.

[coachgeo]

Thanks Sbrumby for some data to consider. Most helpful

[Mouse]

I've taken 30% off my flywheel and and plan on taking more off after I've got the bike running and I can make a further guestrimate and the next cut.

There has been a lot of talk and posts on this subject.

I did a bit of research on this and didn't come up with any definitive answers. The flywheel of my German V twin is 35kg which is about a third of the total engine weight and also looks ubsered on the engine which is why I want to lighten it.

I suppose the flywheel needs a minimum of enough rotational momentum to reliably turn the engine through to the next combustion cycle at minimum tick over speed. I can do a bit of calculus but know nothing of the thermal cycle of the diesel engine so cant even begin to dream about working it out using science.

So after my epic Gooling session I found that the general school of thought behind a flywheel for a stationary engine is....

a] It needs to run at constant speed, better with a heavy flywheel.
b] The governor needs to be simple and reliable, better with a heavy flywheel.
c] It needs to start up and tick-over under (full) load, better with a heavy flywheel.
d] losses through increasing and decreasing speeds is not a consideration, flywheel weight irrelevant.
e] time to alter engine speed is not an consideration, flywheel weight irrelevant.
f] Weight is not a consideration

All of these lend them selves to a very heavy flywheel,much heavier then the minimum to make it run.
The engine manufacturer also doesn't know the final application so would probably err on the side of caution ie he larger the better.


And the same for an automotive engine
a] It needs to run at constantly variable speed, better with a light flywheel.
c] It will only ever tickover under no load and wont need the extra momentum.
d] losses through increasing and decreasing speeds is a huge consideration, better with a light flywheel.
e] time to alter engine speed is a huge consideration, better with a light flywheel.
f] Weight is a huge consideration, better with a light flywheel.

The complete opposite of a stationary engine, ie as light as physically possible but still run.

But no manufacturer would make a flywheel bigger than nessesary.

I think it would be more accurate to say "But no manufacturer would make a flywheel bigger than necessary for the intended application." And we the diesel bike builder are changing the application from stationary to automotive and have to reconsider how appropriate the existing flywheel is.

[Crazymanneil]

I am with you on your thinking for weight vs application. I worked on gensets for a couple of months, putting them together on a production line. They all had chunky flywheels on the engine to keep ticking over at a set speed. Your results should be interesting anyway.

N

[sbrumby]

I agree with everything that has been said. But our engines have a very narrow rev range and all of this is needed . With a gearbox designed for a petrol. If by lightening you then have to increase the tickover, but more importantly it would be the vibrations I would be more concerned about. It would be usefull if you could get some indication of before and after vibrations.

[Crazymanneil]

Sam is right, you can't really be scientific until you have measured something. To measure engine vibration you could use an accelerometer? There are a few on ebay that have serial port output for a computer. They measure up to 6g, not sure if thats enough or not. You'd need to make sure engine is mounted the same way for each test. At least then you could quantify the effects of any changes.

N

[oldbmw]

Whist I accept the advantages of a light rotating mass for gearchanging and faster engine acceleration, please remember also in undulating countryside a bigger flywheel will allow you to keep in top gear going up little hills that would otherwise call for a gearchange. The heavier unit will also run at a slower speed so widens the range of available power. These things tend to make a bike easier ie more pleasant to ride.

[Crazymanneil]

What about the gyroscopic effect of a big flywheel? Would that impact a bike handling? I guess the wheels already do in some way so maybe not an issue? I know that most petrol machines have little or know flywheel, at least transversely mounted ones anyway...

N

[pietenpol2002]

If the flywheel were of sufficient mass on a north/south engine there could well be some impact on handling. Remember the rotary aircraft engines of WWI (Gnomes, LeRhones, Clergets etc.) where the crank was stationary and the engine spun around its axis (think massive flywheel). They would turn on a dime to the left but resisted any effort to force them to the right. Dogfight strategies were developed accordingly. You always knew which direction a rotary pilot was going to turn to elude you. The most famous quote as to the rotary's gyroscopic turn was that it would "put both eyes on the same side of the nose". Would seem to offer certain benefit to the circle track lads if one had the correct rotation.