Heavy Crankshaft....

Ignoring friction and the connrod assembly ...
Just comparing Flywheels being accelerated from 2,000rpm to 6,000 rpm:-

Time taken, 2k to 6k rpm --
... 125 Bantam, with 0.096m dia and 2.4kg weight... is 0.246 of a second
... 175 bantam, with 0.136m dia and 3.8kg weight... is 0.812 of a second.

Comparing above flywheels in "Model" (parameters given previously) bike of 410lbs to be moved the accelerations of total machine + rider:--

[Note: this is ignoring the clutch which also acts as a flywheel within the gear train.]

... With aTop Gear ratio of 7.002:1 (engine/rear wheel):-
... 125, is 1.944 ft/sec².
... 175 is 1.858 ft/sec².

... With a Bottom Gear ratio of 13.2:1 (engine/rear wheel):-
... 125 is 4.705 ft/sec².
... 175 is 4.5 ft/sec².

...in the latter case the difference is 0.205 ft/sec² which means if the two of you you are coming out of a corner the one with the lighter flywheels is going to be
"... inches ahead..." -- 2 & a Half inches inches in fact!

Just to finish up (and forget) this exercise I calculated the difference of time for the two Bantams starting from zero and slippinng their clutches up to full engagement at 7,500...
Why 7.5k you may ask? -- well, it is the only figure of torque I have which is from the Jimmy/Ian graph at 10.85lbsft....
From my calcs the 125 needs 12secs...
-- and the 175 needs 12.6secs.

Which is a load of cock if the 175 is producing more torque at the lower speeds and can pull a different gear anyway.

Looking again at the whole concept -- in the higher gear ratios the effect of the rear wheel acting as a flywheel is more pronounced and "floods" the effect of the engine´s flywheels whereas in the lower gears the engine´s inertia is the greater amount of force -- obvious, I guess by the gear ratio (engine/wheel) being squared and multiplied by the engine´s Moment of Inertia.

Rossi probably has something like 45lbsft of torque compared with a Bantam´s of probably 2.5lbsft at the same low-down speed -- and I wonder what the rear wheel of his Ducati weighs?

I hope this might ease some minds over whether or not to go for a heavier flywheels
-- remembering what Trevor said, "... going bigger in dia can have the same effect as staying small and being heavier..."
Maybe not exactly Trevor´s words but that is what he meant -- didn´t you Trevor??
Cheers!

Hi, John,
Is this ever going to go away!?
What i did was to simulate a sort of racing situation where the two cranks were the same except for weight and dia.
I spun them up from 6000-10000 rpm in 2.5 seconds the D7 cank absorbed about 4x as much torque as the smaller
and, of course, in slowing down also.
The whole crank business is one of trading potential energy with kinetic energy . When you factor in each gear change
where rev rates drop and have to be recovered ,each dip of the throttle and braking event ,the big crank inexorably
loses out . Taken over a whole race distance this could be significant !
The longer crank throw of the D7 provides a greater turning moment to the discs and long stoke engines usually provide higher
torque values, at lower rpm ,than short strokes .However, all of this advantage is negated by the long 125mm rod the 175 boys are forced to use,
which linger for a long period of crank rotation at tdc and bdc . There are two elements to rod action , one vertically down and one at 90 degrees
to this . The long rod has a dominant down thrust which absorbs power in trying to stuff the crank out the bottom of the crankcase.
What is needed is an early tangential push to the crank discs to accelerate them away, this doesnt happen in long rod situations.
So, not only is weight a factor ,so also is the moment of inertia and rod/stroke geometry , all of which conspire to soak up energy
to no benefit of output to the crank axis where torque is measured .
The plight of the 175 boys is not an easy one i fear .
This tour de force of yours is great to behold but you`d think the instigator of it all would chip in with a thought or two. where are you Mike ?

Happy rotations guys , regards Trevor

Agreed Trevor -- the instigator is probably studying his Lay-Shaft -- any problems Mike?
According to my calcs the difference of KE of 175 to 125 cranks is only a small amount per acceleration but taken over the number of times accelerated thro´ the gears during a race the amount becomes significantly large.

I think the eye-opener might be some figures for torque at lower speeds from both engines...

As much chance of that -- I suppose -- as me riding round Cadwell again...

Cheers!

yes trevor I agree, SO come on Mike. "your stage" . ?

regards Derek

John, Trevor, Derek... I can't red through all of it its to much - hence i've lost interest!

John keep your stuff short and i'll read it... if it looks like 'War and Peace' i won't attempt it as i'm not yet retired.... (no offence ment)

Generally I think 125 light small crank 175 heavy large crank....

Right has anyone rumbled my 003 crankcase half with no packing rings yet??

Here it is again -

Hi all,
Tend to agree with Mike lots of waffle ,maths , theory and dare i say it bullshit.
the biggest contributing factor i feel is the rider just look at the front boys winners all of them ,even more proven with the return this weekend of Mark the type of rider i have come across not to offen .sensored or bust i see him as bantam racings simoncelli a breath of fresh air .thats not saying any of the other front runners arent a match just have a different approach to their craft.
By the way John ,Trevor i am not saying that all your posts arent good info but some of it i feel is OTT for us mere mortals.
Nice to see marks bike right on the pace is it one of your motors Trevor a bit quicker than icurus 1 John.
think ive gone of on one but what a was going to say is that there are no packing rings in my reed valve motor either Mike.
No hard feelings chaps , Nick

Yes, of course its bullshine ...

And what´s the point of having a lot of Maths (actually ONLY arithmatic!!) and lots of words when you know by the senses whether heavy or light flywheels are the best for the Bantam?

I think Derek said that anyway.

To finish up and forget this subject for evermore an interesting aspect showed
up in the curves...

No! forget that -- this is too long already...

Cheers!

The argument -- or debate -- or whatever you want to call it -- was NOTHING to do with the rider´s
(except that he is a part of the MASS to be accelerated) ability.

"Biggest contributing factor ... the riders up front... " -- that IS what I shall call "Nick waffle & bullshine" Nick...

The question was, would a heavier flywheel/crankshaft be an advantage in a Bantam?

AND it is NOT for either 175 or 125 -- I have proved it to Myself, conclusively (without waffle or bullshine) -- I think Trevor has proved it to Himself also.

Fin & OUT -- no hard feelimgs Nick, just pray that more Mark Carkreeks come along because your BRC racers -- real racers -- are certainly thin on the ground.

Hi, Nick B,
As you asked , yes it was my engine that Mark used at Cadwell . I began it in 1994 and it was in a state to be run in
in early 97 , in testing a few niggles appeared these were soon corrected, later that year Mark went to Lydden and
did very well ! He continued to race this untill he moved on to 350 TZs in 2001/ 2 , the machine has remained with Steve
ever since .
I did a new liner in 2005 , to revert to the original 54 bore, and to include some minor porting mods. and, in 2007 fitted
the RS ignition . In 2010 Mark paraded the the machine at Lydden to bed in the" new " bore and run with retarding ign for the first
time.
Remembering that Mark had`nt raced for eight years and the bike for nearly ten , i recon they both went pretty well !
, Hope this helps a little.
regards Trevor

Tut tut Trevor!
Too much of a mouthful for this topic. And you wuzz way off topic!!
One liners from now on -- be quite a challenge for me but I have never been known to shirk a challenge.

Comparing advantages/disadvantages of Heavy crankshafts when considering 125 & 175 Bantams as subjects is onerously wrong for the very reason the 175 can have 4 gears which gives the opportunity of acceleration way in excess of the lighter flywheeled 125: that is when using standard road gears, of course, and not CR ... but who is to know??!!

The 175 formula disadvantages it over the 125 without taking the heavier crankshaft into account along with the fact that its larger diameter piston increases the mechanical losses by the larger circumference of its piston ring(s).


Cheers,

Old Hat Waffler.

Good Evening all,

i must admit that i do read all of this and it does make my mind swim but the second from last post that trevor posted made sense to me.



Eddie

Steady on John,
Don`t depress the 175 guys any more with technical waffle and bullshine they might all feel
it`s just too much for them and pack up on mass.You`re dead right of course ,or , should i say,
as usual !
No one, as yet, has mentioned the huge crankcase volume at transfer release on the 175 engine ,
at that point, cylinder pressure is bound to exceed that in the case and that is not good!
Am i cross threading here again John, sorry ,but it is only a few lines .

Just trying to help, regards Trevor

Hello Trevor, i don't suppose you'd like to change the subject a little and clarify your minor porting mods! I think i already know your answer to that question. It was good to see Mark out on your bike again ,even though i only saw the back of them. Any chance of seeing you at a meeting soon. Would love to talk again.

Derek asked for it: clarificatiion, I mean and others don´t want it or find it heavy going so I´ll quickly explain (for Derek even if he doesn´t need it) that the piston ring to bore is the major source of power LOSS in an engine attributable to friction and its LOSING Length is 3.142 X dia of piston which for a 54mm (125) = 169mm and for a 64mm (175) would be 201mm -- which is 32mm more of piston ring length causing 19% more friction which is a square function of engine speed -- getting worse the faster your 175 engine goes, Phil...!!

OK! so I pack in with the clever stuff -- hadn´t had to bother with it for 4 decades and quite enjoyed it...

Mike would not have read this far. Far too long so maybe someone can explain at Lydden that JayBee is only making jokes from now on....

Ahhhhmennn!!

Not quite so fast John,
These guys need to know that crank angular velocity is far from constant , and in a typical 125 race engine
can fluctuate up to +/- 500 rpm . This may have consequenses for bmep and power if it occurs at crucial
port timing events. I dont know about you John but i did my calcs assuming no losses for crank rigidity ,
if you have a bendy crank or have sacrificed a bearing for a miniscule reduction in friction, or indeed for
any other reason , the result may be a net loss of power due to flexure . As far as cranks are concearned ,
lack of rigidity equates to a loss of torque , no way round it!

Sorry mate didnt mean to prolong the agony , but some things just need to be said--- have i said too much ?

Think i`ll uncap another Bud ,cheers Trevor.

No Trevor -- I don´t object to being corrected. In my treatise of Bantam crankshafts I ignored all those things that would make the ARITHMATICS more difficult, just kept to some good old engineering equations that might be Old Hat*** but are still used by the top scientists of the world -- but that amount of cyclic irregularity seems high to me???

Well, since I am communicating with you Trevor I´ll go to plus-two lines and ask about the effect of going to too high a compression-ratio -- when using petrol -- on volumetric efficiency. I have here a Ricardo research curve -- done on a single cylinder experimenta/reseach engine -- that shows the volumetric efficiciency drops 8% with a change of CR from 4:1 to 8:1!!
Now, -- if that curve were continued from 8:1 to 12:1 it would mean that the benefit of a higher CR would be lost to inefficient suction.
Of course 4:1 CR is unrealistic but do you have any thoughts or arguments about
A HIGH compression-ratio having a bad effect on volumetric efficiciency?

Certainly going to a higher CR makes for more shaft-twisting and that sort of grunt-thing...
My 500cc JAP speedway engine was on 16.5:1 CR but I was using Victor Martin Track Fuel (Methanol with 0.2% Puridin!!) -- a good natured 4-stroke, of course!

So Mark Carkeek was on your Bike was he?? Kept it quiet beforehand ey! With Peter T... packing it in Mark is needed.

Waffle waffle waffle...

***apologies to Derek but I rather like OLD HAT -- it being the name of a doubtful Whiskey....

Hi John/Trevor,

i was going to ask about balanced crankshafts but as i dont understand all of the info so far then i would probably be digging myself a bigger hole!


Kind Regards



Eddie

Hi John,


Thanks for the info on the balancing, im sure you will work out the photo scenario soon.



Eddie

Trevor mentioned the variation of angular velocity (crank speed) being +/- 500rpm and I promptly -- very cocky-clever-like queried it straight off, as it looked to be too much.

The flywheel is there for the reason of reduction-and-smoothing-out of crankshaft-speed-fluctuations.

What Trevor was talking about was that during one rotation of a crankshaft the instantaneous speed of the crank can change by as much as plus or minus 500rpm during the 360° rotation. I have since found out that it can be more -- with no problem....

What that means is this:- Say the rev counter tells us that the engine is running at 6,000rpm -- during the compression stroke (as a fraction of crank rotation) the crankshaft could be at 5,500rpm and then at expansion stroke the crank speed is 6,500rpm -- which the rev-counter reads and records as a mean speed of 6,000revs/min....

I´d forgotten this aspect of "The Heavier Flywheel" -- thanks Trevor.

By the way, are you going to go on with Balancing? I am sure Ed wiould like to know more -- particularly the bit about the reason for counterweights --
-- or do you know enough about balancing piston-inertia-forces already Ed?.

Hi John,

I dont know anything about these kind of things but as i have probably been driving people mad with asking questions so i have removed alot of recent questions which are probably straightforward for others.



Eddie

I assume, Ed, you would like to know...

Counterbalance weights are on the crankshaft of any piston engine to counteract the out-of-balance inertia-forces caused by the acceleration of the piston(s).
Without counterbalance weights severe-destructive forces would be created....

Force = ma -- that is a force is created when a mass (weight/g) is moved from one speed to another ... ie is accelerated.

The piston stops at TDC (top dead centre) and BDC (bottom dead centre) so when it moves from Zero speed (at BDC) to maximum speed (roughly halfway up its piston stroke) it accelerates -- and from there, at its max speed, it decelerates (negatively accelerates) back to Zero speed when it gets to TDC.
...
This negative acceleration produces a hefty force which tries to throw the piston away from its gudgeon pin. This force has to be balanced on the crankshaft and counterbalance weights -- positioned opposite the big-end -- are used for this puropose.

These counterbalance weights are creating a centrifugal force and when the crankshaft rotates such that the counterbalance-weights are opposite the big-end -- with the piston at tdc -- the rotation has created sufficient opposing force to achieve an almost*** balanced system.

The reason for the short-stroke motor being favoured over the long-stroke ...
is that the shorter stroke means a lower maximum speed for the piston and hence lower acceleration, which in turn means lower inertia force....
Or in other words, the engine can be run at higher speeds for the same level of inertia-force created by the piston.

The same theory applies to 4-stroke engine valves -- 4 small valves per cylinder can run -- with reliability -- at faster camshaft revs than two-valves per cylinder ....

And in the early days of Japanese racing 50cc we were amazed at their maximum speeds being in the region 22,000 to 24,000 rpm. Pistons were egg-cup size....

*** that´s not the full story -- gets a bit more complicated later....

I do hope I made this bit understandable.
I had hoped Trevor would do it -- of course!
Cheers.
JayBee.

Hi John,

yes i did want to know, thanks very much for doing that and making it understandable for someone like me.


Eddie

Skyving off ey Trevor!

Ed is keen to learn and there you are sneaking off working in secret on a devastatingly quick Bantam somewhere...

All the best,

Cheers!

JayBee.