Heavy Crankshaft....

Here is a picture of one of my 125's crankcase - I think Rob's crank will fit!

10 hours later and the feeling is I should have been a bit more bombastic in my reply to the unpleasant comments and also in replying to the comment , "My god can we pick a subject more boring than a wet lettice..." ????

Fact is Derek, I was asked to continue this subject by a Bantam racer at Cadwell and I feel sure there are a couple more Bantam racers would appreciate a bit more clarity on an obscure subject -- so I shall add a bit more in a hope that it becomes more understandable...

If you imagine the total flywheel mass, `m´, to be acting at a single point on the flywheel rim at `r´ distance from the centre when it is spun the flywheel will have `m x r´ amount of torque BUT that is imagination & THE mass is NOT at one point, the mass is spread all around the whole circumference of the flywheel at what is known as the "Radius of Gyration" which is NOT at the rim but at some other ring of circumference dependant on the cross-section shape of the flywheel:--
For a DISC the Moment of Inertia is "m x r²/2" which is close enough for comparing motorcycle engine flywheels.

INERTIA is the tendency of a weighted object to move and for a weighted object not to slow down once it is moving
...
Here are two of the flywheel cases (from 4) I chose to study plus that of Robbie´s---
(a) is typically 125 & (b) is possibly a typical 175 :-

Theoretically taken as Disc the Moment of Inertia is for each below here v ...
Flywheel (a) 3.8" dia x 5.3lbs (96mm dia x 2.4kg) --- (a) O.066lbs.ft²

Flywheeel (b) 7.9" dia x 9.9lbs (200mm dia x4.5kg) --- (b) 0.533lbs.ft²
& Robbie´s at what appeared from the photograph to be 13"...:-

Flywheel(c) 13" dia x 10.8lbs (330mm dia 4.9kkg) --- (c) 1.585lbs.ft²

The Moment of Inertia expressed in lbs.ft² can be likened to a torque of `x´ft.lbs x a lever arm one foot long!

These values multiplied by the factor of a given speed, `N´rpm, as [(2Pi/60) X N)²] will provide the energy (generated per second) that is required to spin the flywheel and is the energy absorbed by the flywheel at that speed...

Providing there were no arithmetical mistakes here (and there might well be, I am human***!!) it can be seen that the INERTIA of the three flywheels varies as follows: (a) is 8 times smaller than (b) and (a) is 24 times smaller than
(c)...

... Or put another way, it is that (c) requires 3 times more energy to spin up to any speed than (b) needs. And, of course (c) has 3 times more stored energy than (b) at any given speed!!!???

From this I wonder if Robbie´s flywheels are just a bit TOO large!
I could be wrong, of course, and am here to be shot down!!


***contrary to some belief on here!!

Bigger and bigger -- and more boring than ever...

Hey Mike, what is that fracture hiding behind the crankcase? and are Robbie´s rotating bits REALLY THAT LARGE??? I feel like singing -- "Oh my name is Sammy Small --- S-a-a-mm-y Smmawwwll, Sammy Small -- and I only got one ... Uuuh!!!

The fracture looks to be an overstressed section of a shaft -- but which shaft??

BOF!!

[quote="john bass"]I should have been a bit more bombastic in my reply to the unpleasant comments "My god can we pick a subject more boring than a wet lettice..." ????

Fact is John

Hi John seems we agree on something,!!!
I would like to see the "moment" effect on excelleration between gears per lap, I "feel" that robbie is clearly losing out on excelleration as i'm sure he does not slip the cluth between 2nd and top or in Top ? .

"Look" some people may well find this subject good or even interesting, (and that is good as i would prefer to whatch paint dry than, as i get as much stimulation from this as reading the same things being said over and over and on a subject that is on 1% effect "the start well first 80 meters", but I feel we have talked enough on this subject, I'm only suggesting in a "brummy way", yow could move to a better topic.

Like "transfers" or "exhaust pipes", "where the real power is produced",

I too had a Bantam 193cc sprinter, and for the record "I do not agree with Brian" "On a 175cc needing large cranks, especially a racer, but my thoughts are only a "feeling" that had, your calcualtions on the above could prove me wrong and show the excelleration is aidded over a distance 0r even more "up the large hill" - cadwell, if your calc's will show this.

"O" and John please accept My appology "I never new your were the "iddiot" who tryed to get fairings banned, way back in the day,? but I agree they should look period ! unlike some of the ? monstrosities currently being used.

"I do wish for you to keep posting, after all John, I "feel" you are probably the only one capable of doing the calc's that could/may !! prove my "feelings" to be wrong.

best regards
Derek

mjpowell wrote:

Here is a picture of one of my 125's crankcase - I think Rob's crank will fit!

Hi mike you going to show us the other half then ?

Derek

Thank you Derek!
Ok so this may be of no use to anybody out there with a square motor and very small flywheels:-

This is a little bit of my experience. My engine man & mentor (although DN didn´t race he could
tell where my riding style was going wrong -- like `...leaning the wrong way, as if on grass...´) reckoned it would be pointless going short-stroke because of my gross body weight!! .

To my knowledge the flywheels of Icarus-1 were always original (200mmdia, I think!?) withaluminium padding. The bore & stroke standard. The engine was never dyno-tested and except for piston scuffs very reliable after initially discovering a big-end boob which I mentioned earlier on here. Derek estimated Icarus-1 had 12 horsepower at 8,400rpm...!

Icarus-1 revved to 8,400rpm under load and went on to about 8,700 under no load -- and it often felt as if the pipe-power came in at 5,000 rpm. I used a high top gear and slipped the clutch (on just about every corner on every track) to use it as a torque-converter.

It could well be that I might have done better in a race if I had used a lower gear as seemed obvious during one time at Brands --where I had gone from (Rear & G/box sprockets) 46 x 15(6.355:1, 84mph in top at 8,400rpm) to 50 x 16 (6.984:1, 83mph...) in practice -- and then 47 x 15 (7.002:1, 82mph) for
the race... Just 1 mph in top... difference each time -- which I was NOT measuring, of course, but I did know from our Niffy that the first change of final ratio had taken me from 17th to 14th place in practice. During the break between practice & racing Niffy asked why I was changing the final ratio yet again and I spitefully snapped at him because of what he´d told me and why didn´t he stop chewing the fat and help instead of making silly noises...
I remember that well because as you all know Niff bought it later in the Isle of Man and I´ve never been able to forgive myself for swearing so harshly at a pleasant man who never could have kicked a cat in his life.
So, in the race I finished 4th... I can remember changing down viciously for Druids, diving inside Dave Coombs and slipping the clutch all way round Druids to pass him coming out and then changing up and up and taking South Bend flat ... thro´to the cut off from main circuit to Club... on full elbow, then braking viciously and changing down again before the kink into clearways ... All round Clearways, I´d be slipping the clutch to accelerate and change back into top and still be slipping the clutch....
After that discovery we reckoned with a good fling out of clearways Icarus-1 would get to 8,000 just after the start and reach max revs just before Paddock, meaning that my speed would be just about 82mph and that Paddock could be taken flat....
???

The above might explain how the difference of a heavy to light crank can be a benefit if the clutch is used more frequently and on some circuits --like Cadwell
I used the clutcgh slip more. So much so that I was constantly putting in new clutch plates,...
I was very happy to be aboard Andy Boyle´s 5-speed 250 ABS which
had a Greeves Silverstone clutch which didn´t need its plates changing and which I didn´t have to slip anywhere near as much as the Bantam .

I hope that this is of some use....

Derek! That was the reason for asking riders -- and Brian White in particular -- how many seconds was the slipping of the clutch
from a standing start and between gear changes , when racing & Sprinting, to which Brian retorted (quite correctly) the above
(in the title).
In answer to how many seconds between gear changes with a heavy crank as compared with a light one, --- it is only a small
amount -- fractions of a second as a free engine -- but when considering total weight being accelerated the wheels have moments
of inertia that dwarf the rotating engine parts Inertia.
i.e. If including the clutch ... the typical 125 would be, 0.176lbs.ft² & the typical 175, 0.318lbs.ft² which are both small relative
the Moments of Inertia of both road wheels. Here for example: -- front (Peter Tibbitts wheels) at 12.3lbs.ft² and the rear at
12.5lbs.ft² -- making a total of 24.8lbs.ft² of wheel inertia -- unless the front wheel is off the ground, of course!!

That sort of suggests the heavy to light flywheel goes poof! out of the window relative acceleration but I would put it this way:

If say the final gear ratio in bottom gear is 10:1*** and the 175, very big flywheels (Robbies) has 24 times more stored energy
than the smallest 125 flywheel -- and that energy is then being multiplied by 10 time more on its way to the back wheel then there
is a considerable advantage in acceleration when slipping the clutch with high revs but a definite lagging when accelerating engine
with the clutch fully engaged .... !! which goes for pulling away and changing gear.

Each time I look at this... I fiind out more that I don´t know --- and more that I would like to get to know.

Can no one among the Bantam racers estimate how long the clutch-slipping is from standing start and how long between gear changes?


***might be more, I have no idea -- I only know top gear ratios....

Hi John actually that is as I said small is better than large as it seems the clutch is engaged a lot longer than being slipped or well the only advantage "the start" Ok you win now can we pick another subject to discuss like what are your opinions on transfer ports volume of a 175piston ported over say a 125 with reed valve.

where would you begin, we have gone for an alloy barrel 125 and 175, but with the end result is very different, completly different.

if you we starting first what would you do fist lets say you have collected a rolling chassis wheels front end tank seat etc, all sorted bar the engine where would you start. ?

derek

I haven´t won anything Derek.

With my most humerous attitude pushed forward I say I am certainly not rying to win any argument over any other person on here.
Far far from the objective. All I am doing is putting up experiences I know and asking for comparison with others´ personal observations
with Bantam racing and bike preparation.

This way I am also learning....

I am not finished with heavy flywheels -- you asked about time between gear changes and that is difficult to answer because of the
many variables, bike to bike rider to rider but there is the big factor in this debate about how long is the time taken to accelerate
from a low speed to a high speed with different size flywheels and I have done that as a theoretical exercise which should give a good
idea of what is happening practically.

Peter has these calcs at the moment and I await his reply as to what mistakes have been made but I´ll expose myself here by stating
the times taken to accelerate -- FREE ENGINE -- from 1,500rpm to 9,500rpm of the smallest (96mmdia x 2.4kg) and largest
(200mmdia x 4.5kg) cases I looked at:-

The small (96mm x 2.4kg) takes 0.035sec and the largest (200mm x 4.5kg) takes 0.242sec.
Which suggests the largest does cause a lag in the engine response which when multiplied by the particular gear ratio comes out as
a substantial amount!!

Now that is theoretical and ignoring the bearings and air drag that must go with the spin rotation but it ought to be backing what I said
about driving a light flywheel, clutch coupled -- and driving a heavy flywheel, clutch slipping....

I know very little about gas exchhange: ports and reed valve and would be only quoting others´ tuning data if I tried to write about
them on here....
.

...

Last Post (by JayBee) on this subject sounding across the silent plains!!
We bury the heavier crank/flywheel NOW!

Looking back thro´ the postings, the various answers to Mike´s questions (why Rossi wanted a heavier crank in the Ducati) were actually there -- in scores of words, the answers were there --with `wicked Mike´ constantly wafting the Wind-Up flames.
In his 003 he has big flywheels (or just a big crankcase??) and obviously KNOWS the WHY & HOW of their function, advantages and massive disadvantge....

But would Rossi want to ride in the difficult manner of a Bantam racer (JayBee in particular with Icarus-1) keeping the revs up and slipping the clutch because of
the heavier flywheel???

Ta taaa! Ta taaaaa -- phew!!

Derek! you ask where would I start?

I´d certainly not start in the way I did in 1968 -- by accident -- having got into it with two other enthusiasts just wanting to BUILD a racing Bantam thinking we´d find ourselves a `jockey´, doing it all itssy-bitzzy as things cropped up and then finding out I´d got hooked and just-had-to-be the rider.
I´d certainly NOT spend so much time buzzing around Brands (although that was fun) proving a resonant pipe and other things and "running-in" pistons that could be -- would be -- done on a dyno.

I´ve just realised what you most probably meant here, which just knocked my notion of burying this topic on the nut.

Do you mean; Where would I start when looking at the discrepancies that have shown up here both with 125 and 175 re heavy or light flywheels???

If you do then I´d use the dyno to see what the engine, `Time to accelerate thro´ 8,000 rpm*** reponse -- UNDER NO-LOAD (FREE engine)´ with just a heavy flywheeled 175 compared with a light flywheeled 125.
Should show up immediately.

Determining the optimum dia & weight flywheel for each could be a lot of work amd quite costly but in the end it might be worth it because there must be an optimum
flywheel for both engines and that would only show up when incorprating road test in with the dyno work.

Cheers!

*** as I mentioned earlier, calculations for free spinning flywhels show a lagging
acceleration with the heavier flywheels. !

I am not evading the questions you put to me Derek I just think there is a new contributor on board here whom I think is a better consultant on the questions such as you put and he could do a better job of answering them than I ...
I mean your bit about alloy barrels on 125 and 175 are just way beyond my knowledge & experience and having a `rolling-chassis´*** along with a few bits
left me somewhat confused when asked where would I start?

But then I think I must have been a somewhat confused 45yr old father of two
to have ever cocked a leg over a racing Bantam saddle in 1968.

*** I take it this means a Test Track Hack which you use as a test vehicle obn some available track somewhere ...?

Cheers!

now can we pick another subject to discuss like what are your opinions on transfer ports volume of a 175piston ported over say a 125 with reed valve.

where would you begin, we have gone for an alloy barrel 125 and 175, but with the end result is very different, completly different.

if you we starting first what would you do fist lets say you have collected a rolling chassis wheels front end tank seat etc, all sorted bar the engine where would you start. ?

derek[/quote] Derek said that above and I didn´t know how to answer ... perhaps some of you can!!??

Maybe I´d be wrong to say the bigger the ports the better but there is a point Trevor raised that we would really like to get some swirl into Bantam combustion chambers and that means more cunningly shaped ports. A glance at Brian White´s ports in BWT tuning on the front page and you can see what big ports look like. I don´t think you can compare a 125 reed valve system with a 175ported because the reed valve changes the characteristics of crankcase compression such that this will effect the loop scavenging cycle considerably. Certainly comparing Peter´s 125 power curve and that of Jimmy´s 175 shows a completely different shape although Peter says he has (Nick has now) more torque than other 125s´.

I am wondering about alloy barrels -- no experience der you see! Perhaps the end
result is different because of heat loss -- somwhere? Aluminium had more heat conductivity than steel or cast iron with less emissivity than cast iron --

-- I´m floundering so someone out there, please step up and -- help!!

Hi John,
Just a small clarification , what we need for rapid burning is high turbulence , swirl in a t/s chamber won`t necesarily provide that .
Also , swirling action , spinning at 90* to the cylinder axis , with the exhaust port still open and with an attendent high pressure gradient
will result in excessive charge loss . Best to keep charge and ex port as far away from each other as possible , with an efficient loop .

Big ports , low rpm at the base of the powerband, big carb, less than optimum air flow through large ducts , retarded ignition at low rpm ,
big chunky crank to persuade to rev up ,to say nothing of unresponsive Avgas with it`s high end distilation temps, would worry me if i
was contemplating a 175 + engine, even having 50% more capacity does`nt seem to help , So, whats to be done then ? How can we
overcome all of this ?

Gonna get thinking on this one ! Trevor

Hi trevor

your saying from what I can make out.

a 175cc is better to have :-

small ports, base of the powerband,
smaller carb, for optimum air flow through smaller ducts ,
advance the ignition for "low rpm" ?
smaller crank to persuade to rev up ,
all to get the best from unresponsive Avgas,

sorry if this come across the wrong way.

what about comp ratio

regards Derek

Both compression ratios to consider, Derek!
You asked Trevor about compression ratio and I am poking my ore in that there´s crankcase compression to carefully consider as well a that above the piston.
Roy Bacon in his "Two-Stroke Tuning" book says the crankcase compression ratio should not excceed 1.5:1 and I am positive there are many now -- since Roy´s time -- well exceeding that ...

Trevor!!???

As far as the upper CR is concerned OVER 7:1*** on less-than-100octane petrol is asking for trouble and fuel of well-over-100octane necessary for above 8:1 and
definitely a methane fuel for 16:1 as with thevWalsh Bantam.

Trevor?

***measuring the stroke from the top of exhaust port.

Hi John,
You must have an intuitive feel for these things !

The two ratios , as you saw Mark race , are ------ crankcase ccr , without transfer port volume , is just over 1.4 : 1
from ex closure 7.3:1 and running on Avgas with lead and 100 octane !

Hope this helps , regards Trevor

Cor Trevor!
Thanks!

I thought I´d add that`"g" as the force of gravity was not known before Isaac Newton invented it -- but that is not true, Greek chariot racers discovered it when their chariots did a high-sider. It was that they just didn´t know how to measure it and felt too embarrassed to mention they fully understood the `force of gravity´.

Bit like my old old buddy on grass tracks, Slasher Edney, of Rudge fame he´s say
"Ett ain´t gonna last John Boy -- hear them `stones in a coke-tin´ sound...?" and he´d be right ....

Strictly Bantam:- I still wonder what happens when the transfer ports open and the incoming mixture rushes into the cylinder, as to what REAL temperature and pressure exist at the start of compression...??

Hmmmmm!?

Hi John /Trevor

on Johns comment in his last post - that "the transfer open and gasses rush in" ! I do feel this is the ideal -

But in realerty not true !, Mick scutt mentioned 2bar in the cylinder at exhaust closing,

do you think the treansfers could be going the other way I.e. the cylinder pressure will start back flowing down the transfers !!, for sure we would like the exhaust to open and evacuate enough to drop the pressure to allow this not to happen. ?


Trevor with the greatest of respect- the information on your primary comp ratio 1.4 to 1 without the transfers! my opinion is it's to missleading, while i understand you not wanting to share with us all your secrets, I dont like carrots that are too far away. ! not ideal for a forum thats here to increase knowledge and interest, besides there are enough variables, I think you should post some pictures of your barrel /crankase inners to make up for this small indiscretion !.

Best regards

Derek

Well spotted Derek !

Trevor - interesting, how do you measure crankcase compression ratio without the transfers ?

There's a fair bit of volume in the reed housing - is it completely sealed by the piston at bdc ?

We have 1.38 primary cr measured by an old piston with a hole in the crown and set at bdc so the oil goes most of the way up the transfers. When measuring, the piston skirt has a thin smear of grease to stop oil leaking past the skirt and out of the inlet port.

Cylinder cr is 7.38 measured from the top of the exhaust port and is running on avgas ( one of the best things we did ). For best part of a year before switching to avgas we had no end of siezures - we would have a race where it ran perfectly and in the next it would barely do a lap and sieze and that was with lower compression ratio.

A higher primary ratio certainly improves power output but I don't think its worth going overboard with stuffers and tight clearances to achieve it - effort is better spent on the exhaust pipe.

Mick

Oops , sorry all , the text should have read with transfers, my software provides for both with and without, so , whilst the number is correct
the wording is in error ! Had it been otherwise the ccr would have been stratospheric , with dreadfull consequences for scavenge flow.
And , yes ,the reed block is included as we have 360* opening of the inlet port . Hope that has been clarified , and , i feel duly chasened !

Mick , while i have you here , at what sort of rpm level are you regestering these revised numbers , i can approximately match them
in my simulations but at a certain rpm and exceed them at peak. Are you doing the same, it could be an interesting and absorbing
exchange of information , and bring others along with us , everyone becoming a winner !

I joined the BRC in 64 , now i recon you must preceed me by a couple of years and could make you the longest serving Bantamite
on the forum . I could ask of Bassy but he`s a bit sensitive about this age business so i won`t upset him by suggesting he`s the oldest !!

All the best for now, Trevor

Pack it in you three --- we were supposed to be on another topic than flywheels.

I´ve been bashing my nut against the wall trying to prove something to do with the McScutt engine (on the "Pipe design" topic?) where the theory doesn´t fit the practice -- and here you are debating it on here -- in the topic which was supposed to be the buried post.

Joking aside I think you should go over to the other topic where I have either proved I am mistake-prone, that theory is all bullshine as Nick B... said or there is a good reason why the McScutt Special does not follow theoretical rules -- the first thing I see on this topic -- over here -- is that Avgas is now in use and over there I had taken iso-octane, C8H18, for the exercise. Avgas might well be the reason the theory doesn´t fit the practical....

Hi Trevor

Yes, my figures come from a simulation and are taken from peak power rpm - 10,000 in this case. Peak torque is only 200 rpm lower but thats not a problem as the power is still good after changing up.

I started racing with the BRC in '65 at Cadwell and can't be sure but probably joined in '64. Whatever it is, between us thats a lot of two-stroke years and hopefully we can help the newcomers and others go a bit quicker ( yours is quick enough already <Grin> ).

Mick

Hi Mick, Trevor, john,

I can not hide my delight at the prospect of more of these topics appearing.

It does nothing for any sport or class to continually have an individual or the same individuals disappearing into the distance.

I have to admit I have resorted to all kinds of un-popular, rude and insulting attempts, first asking before resorting to "well you have all seen the carbage". Anyway some of you have been restrained, Imost humbly applogise for any remarks i have made to all individuals, or character assasination, anyway a few have been in on this, and responded when asked to,

John can you put your last technical post into layman's terms, please I just can not understand it, way above my knowledge.

The rest of what I read this evening is indeed music to my eye's, I wish to go quicker and need just a little help, roll on next year.

Mick -another time zone - can not believe your rersponding after 3am!!!!

On Ian - "here" "here"! he does'nt just have a fast bike either, I could see this at the first cadwell meeting.

regards Derek

Hi Trevor
I think I'm missing a post here -
in your last post on here you ask Mick you can match figure but exceed his figures at peek RPM ""hmm ! what figures are you refering to and what are yours at peek, I think I have missed a post have I !!

regards Derek