Crankcase compression debate

Ref my earlier post Air cooled lightweight bantam (which has been edited now) should of read 72kg not 62kg!
Think we'd struggle to get to 62kg :-)

Mike

Mike just to correct you - I never miss quoted you !!!

bettsd wrote:

Hi Trevor
, Andy Moldens bike went resonbly well, as Mike says,
Derek

Hi Mike ,
Exactweld , great bike and in it`s day the fastest 250 in Europe , such a shame the money ran out. The twin carb , per cyl , set was also tried by
Kriedler back in the sixties , but seems to be lacking in any clear advantage . In the case of Exactweld , with four carbs , setting them up must
have been a real headache . Fundamental research by the Aprilia team has proved that a disc set up needs a specific case volume to produce
the power when relying on the diff to pull the fuel through . Being able to predict and control the most favourable inlet timing , the exhaust can
be maximised to work in concert with that and , having a case volume that is not too big or small , masses of gas can be inhaled . Stick in a
massive ex port with power valve control and the whole package works with perfect harmony . However , under or over the clearly defined
power band and the whole event goes haywire , combustion residues go out the carb bell mouth with reverse flow and , exceed the over rev
limit and the engine just stops effective running .

The change from side to rear discs throws up some interesting points , the side job , RSW , has to induct through the case transfer duct and
the con rod clearly obstructs flow into the engine . The rear disc ,RSA, eliminates both , now the RSW used a 115 rod and the RSA a 120 ,
in countering the longer rod , and supposed increase in case volume , the side disc had beveled crank wheels and the later , rear, had straight .
So perhaps Jan and the team hit upon a working volume and adjusted the geometry to accomodate it ? Bulk flow into the engine via the rear
disc must be superior but driving the opperating mechanism equally must absorb power , especially when spinning a dizzy 13500rpm , from
the dyno graphs that i have seen the RSA only produces a little more power than the RSW but maybe thats all it needs to keep winning !

A piston port nor reed can , for differing reasons and in a three speed environment , use much of this technology , it seem still that we carry on
in the time honoured way of the garden shed tuner , picking up on snippets that may or may not take us forward .

Take care out there , Trevor

Hey Mike ,
A 125 rod on a 54 stroke , now what can the radical thinking behind that one be ? The Bantam world is on tenter tenter hooks !
The implications leave me confused and worried , not a good thing at my age !

Regards , Trevor

Uh Oh! Come on Trevor? -- you know already...! --
"...it´s all to do with time taken...," you said -- might
not be an exact quote but the meaning was there.
To be or not-to-be... that is the question...

...Quicker thro´ BDC or slower thro´BDC...??
... Slower thro´ TDC or quicker thro´TDC...??

Leave it until the lads get back from racing...

Cheers!

JayBee -- ever trying to behave ....

Trevor and everyone, the rod lenght wasn't a radical decision it was a long rod that was needed to get a reed valve assy
mounted on the left crankcase. As i don't at present have an air-cooled motor i asked Tom if i could borrow his spare sprint
engine to race in the Fred Lauchbury at Lydden. However Michael Brown blew his motor up at Snetterton and replaced
it with the Tom's spare sprint motor. But Tom offered his current sprint motor - side mounted reed. We managed to fit the
engine and pipe by 'lowering' the engine in the frame(different engine plates) but the fairing wouldn't accomadate the carb
or the exhaust pipe, so a half fairing was knocked up with previosly crashed fairings. No testing was done the plan was just
to race it in the Fred race - one shot... The engine went well but being on a guessed gearing(14x54) being a lot taller than the
normal (13x56?) and running 26/21 output pair the gap between 2nd and top was a gap to far...

But it created a lot of interest in the paddock amongst bantam folk and spectators alike (the assumption it was a disc valve until they looked closer!)... and perhaps has got people thinking? ..... Side Reed Valve/Exhaust out the back ... that sort of thing?

Many thanks to Tom and Pete Styles for there input on the project... Between us I reckon about 35 manhours for 7laps of Lydden!!

The Side Valver Sprinter will be out at the Ramsey Sprint IOM (ManxGP) with its normal pilot....

Regards Mike

Hello Mike ,
Great effort by all the team to get that interesting engine to the grid , helps to keep things exciting and perhaps promote discussion ?

No ididn`t feel the rod to be radical , odd maybe , but there were enough clues to make a reasonable guess , welded cases with a plank
on top means height in needed , a long rod , but why , space for what ? A rear reed doesn`t need a 125 rod , but a side job to breathe
under the piston cutaway does, plus the interest in the side inlet Aprilia was telling . None of your pics revealed the carb area , another
pointer , in all , a nice piece of subterfuge and great fun , would be great to see it out again !

Do you really run 13/56 at lydden , I just checked with Steve to confirm that Mark always ran 14/54 and is typically in the 48-49 secs
bracket , but significantly we don`t have a 13t sprocket ! Does that say something about relative power characteristics , or just riding
technique ?

Stay in tune , Trevor

I don't know the in's and out's of the motor - but i'm guessing long rod to try and get the reed assy in nearer (hence shorter)?
Being made generally from 2nd hand previous race parts, and being used for sprinting things were a little different from the
'norm' - heavy crank and the later D7 output pair gears if it was to make another return to racing I think those 2 items need changing.
Ref gearing it had 14x54 on it for Lydden but for doing its normal 'standing quarter mile' (or eighth's)sprints it has 13/56 or the like on where the drop from max rev's in 2nd to top is not a problem!

We wanted it to 'break cover' so to speak? at Lydden and thats why i was trying to 'ramp up' conversation on it to create interest! Judging by people coming to chat and saying they had read 'bits' about it be it on here or on facebook. Anyway nobody has shouted 'Ban it' or 'Unfair' which is a bonus.
I've pic's to post on here but can't do it at the moment as can't post on photobucket and then to Forum?? There's a short clip of it running last week on the facebook page.

Mark was generally a couple of teeth taller geared that me at most places..(braver?) Don't know what the fastest bantam lap of the weekend was but would be very surprised if it was sub 50sec (I've only been under 50s 3 times in 30years!!)

Regards Mike

Pic's of Side Valver Bantam...

Hey Mike ,
Just a small point , did you race the bike with the carb bell mouth exposed as per the pics ? If so , the high speed air , accelerating over
carb face will lower relative , local , air pressure and mess things up , best you get an air box on there , it could help a lot .

Great bit of kit though ! Trevor

Hi Trevor and all, yep bike was run as in the pic's Carb has got a tea strainer type filter which may of helped
but i was keen not to use it so people could see the slide going up and down....

Lap times Fastest - Nick B 51.08 then Ian S 51.12 and me 51.21 so not anywhere near the lap record but good
close racing at the sharp end producing each of us having wins!!

Regards Mike

Mike ,
Those times , iron engine , all ally , and water cooled , yet the lap times are all within a fraction of each other , just goes to show the options
are pretty open and that makes for exciting , close racing , terrific , well done guys !

As we are still on with the ccr debate , it might be pertinent to learn how the big case vol boys got on , ace , race and on the pace i hope ?

I had a nice PM last night asking about adverse air flow over carb bellmouth and it`s range of influence , now that is a huge subject but just
a few pointers may help . Perhaps the most important is on the air correction system . As air flow and speed pick the main jet will provide fuel
that is progressively rich , introducing air to the jet block alters the mixture to the correct ratio . Obviously , a negative pressure robs the jet
block of correctig air and top end mixture goes walk about . Most carbs have the correction access at the bell mouth , breathing air at atmostpheric
pressure and that needs to be a constant , assuming of course that you don`t run a pressurised air box !

Then there comes the funny bits like fuel stand off and pressure pulses , running from the open case and off the closed reeds , that continually
alter sign , and i noticed that on Mike`s video that at about 1/4 throttle , gas can be seen exiting to atmostphere , on a stationary bike and it`s
all very intriguing !

Trevor

I'll be starting a collection to raise funds so that many more bantam racers have a pot/cup/plate? etc.

So post on here your donation, (i'll pm you my address) or see me at Darley or Cadwell.

Thanks Mike

Whoops posted in wrong place!

Right, Mike, if your address is still the same´I shall send a note by deutsches post
and royal mail. Won´t be a big one `cos I am an underpaid OAP.

Keeping to the topic. I see Trevor has avoided answering my query concerning
the advantage of LONG rod over short rod and that it´s all to do with TIME which
he kept ranting on about.

Let´s start: there´s a long and horrible equation -- which I shall never use on
here -- which tells how much `dwell´ time the piston sits at TDC and BDC. which
means there is a rod length which with the bore size used has the dwell time at
TDC and BDC the same. Longer than that the dwell time at TDc is longer than it is
at BDC...

So is it better to have a longer rod ...?? when the shorter rod provides a longer
time for air-charge flow into the crankcase??


Cheers!

Hi John ,
There are of course pros and cons on the rod length issue but for me , more significant , is the rod angularity , especially from tdc . For anyone not
wanting to do the maths or have no access to CAD then , a sheet of paper , pencil , rule , protractor and draughting compass will produce a
drawing to make the point . The longer the stay at tdc and just beyond , the more the push from combustion is absorbed by trying to thrust the crank
vertically downward , the shorter rod , for a given stroke , will have a greater tangential thrust and propell the crank around , not down . Maximum
piston acceleration will be earlier in the stroke and by the point of max angularity the piston is slowing down , so friction losses are reduced at this point.
With a long stay at tdc more heat energy is lost to the head , reduce temp and you reduce pressure , less pressure then remains to shove the piston ,
and more power disapears . Heat is power is heat , no way around it !

Another consequence of a long rod is the old case volume issue , assuming you can get the motor to rev reasonably well , then the issues are not too
significant . However , at part throttle , or low engine rpm where the pipe gives poor response and the case doesn`t pump and you can`t drop three
or four gears to get the revs up what do you do ? Over the years i have built a lot of engines of different bore and stroke and rod lengths , and with
out exception the crisper running and sharper throttle responce ones all had " normal " ccr . Long rods force the use of long transfer ducts , from the
crank to the combustion chamber , to get that long , inert , collumn moving and delivering a meaning full charge mass takes a lot of time , and , if the
ducts are subject to flow reversal there will be even less time available to achieve a decent transfer of pure , fresh charge to the chamber , not very
helpfull .

Take a sealed tea chest and a sealed shoe box , squirt an aerosol in both for one second , fix an extractor fan on both and suck for one second ,
from which volume could you extract the most molecules of the aerosol ? Simplistic i know , but it does make a point !


Stacks more stuff to consider , but my evening meal awaits , Trevor

Hi all..... some more pic's of the side reed motor technical inside bits....

Trevor - will measure my ccr in the future and post but its all together and will be until Darley meeting is over...

Mike ,
Thanks for those pics , a lot to analyse , thats a big , heavy rod in there , it must cost flow rate , but take a file to all those sharp edges around
the transfer cut aways they will produce a lot of turbulence and interupt flow . Sharp edges inside , where flow takes place is an absolute
no , no , make everything smooth and rounded , and the higher the revs go the more critical this becomes , it will pay off !

Again , a great , innovative engine , a credit to the design and development team , you all deserve much success .
Can`t wait to see more !

regards Trevor

HI MIKE THIS LOOKS LIKE TOMS SPRINT ENGINE. ITS VERY GOOD AND IT FAST TO

HAVE A GOOD DAY

what about a picture into the base of the barrel Mike. !

I seen this engine when it had only one fly wheel in Tom shed, what an idea, I see it now has one large flywheel and one small, I would love to know the thinking behind this change and why one small and one large ? only thing Ican see is the extra room it creats for the inlet charge not being interupted, but there must be other reasons ?.

Derek

Neat reply Trevor -- thanks!

I keep seeing BIG flywheels ---

uh -- uh! Thers´s a flywheel
running across the floor towards
my computer tower ---

-- and its BIG -- HUGE!

Aaaahhh!

Yes it's Tom's sprint motor as mentioned in previous posts..

See what you mean Trevor ref rod and other info, perhaps a RD400 rod 115 between centres may be better.
But it has been built using 2nd hand surplus race bantam parts - the rod is the old Jimmy/Snowy engines.
I dare say Tom will get wind of these posts and should a better rod appear use it. I think the large crank wheel
needs reducing a bit in diameter at least for 'Bantam Racing' but may be not sprinting. Spoke to Tom yesterday
and for the Ramsey sprint its having 13/60 gearing with the 26/21 gearbox output pair but its over an eighth of a
mile - should wheelie!! Thanks for your input...

Regards Mike