Torque talk Trevor...

Fascinating -- what did it rev to...?

Here are a few details. bore and stroke.....34 x 27.4 giving 49.75cc , 13 hp at 20,000rpm , 9 speed gearbox and very strangely, calliper front brake, as on a push bike? The high level megaphones enabled a slimmer fairing and reduced frontal area! Unconfirmed rumours suggested Honda were even developing a 3 cylinder engine?

Trevor

Tommy Robb won the first GP on Japanese soil, using the 50cc twin. The Honda 50 single cylinder GP bikes had proved not quick enough in practice, against the Suzuki two strokes, and they decided to wheel out their new, untried twin. In the race Tommy found the bike just about quick enough to live with the Suzukis, using the rev limit suggested by the bike's designers, so he revved it til it wouldn't rev any more - literally, off the clock.

Soichiro Honda was ecstatic. He had the engine removed while it was still hot, placed it on the back seat of his limosine, and took it back to the factory, so the technicians could test it and find out how high it had been revving - somewhere North of 20,000rpm!

I was talking to a German recently and he would not believe that the Japanese 50ccs engines were revving over 20,000rpm.... I am trying to find coinfirmation in press-writing somewhere ....

Cheers!

John-Boy!

Well John,
Perhaps you would like to inform your friend that the last of the non-turbo F1 engines were capable of 19,000/20,000 rpm and had the same stroke as a single cylinder 2T GP 50cc engine at 39mm, but that little 50 job would not rev above 16,500 rpm? In the context of these engines, 20,000rpm from a 50 twin should be no problem!

Don`t know what you think John but that seems a very high piston crown centre for such a tiny cylinder, effectively separating the combustion chamber into two sectors, not the most efficient layout?

Cheers, Trevor

Definitely two combustion chambers formed by the 4 valve pockets per cylinder head with no regard to VIB (velocity in bowl) numbers.... Valves like 4" garden shed nails...??

Them Japanes again Trevor -- oval pistons and awful combustions chamber shapes -- just before the anxiety about exhaust emissions that changed all that...

The boring bar must have been quite special for oval bores....

Much remembered was Chris Newport lending me his 6-cylinder, 150cc Benelli(was it??) at Lydden when I arrived late and neither Icarus-1 Bantam nor Andy´s 250ABS would start. Don´t let the revs go over 11,000 and I wheelied off the start with 14000 showing on the clock before I had any idea of gaining control.....

The Italians could do it as well -- why couldn´t we?

Yeah! I know -- the Brit government .... Daft lot only interested in investment....

Cheers!

John.
       I found the following quote contained in an article nominally about exhaust systems; " Unless some British manufacturer is prepared to employ, or to consult, theoretical experts then they cannot expect to compete seriously with the Japanese who do so in the biggest possible way."
A startlingly prophetic statement made way back in 1966 by......Gordon Blair, he of QUB fame, pre-empting the terminal decline of the British motorcycle industry!

Trevor

Morning Trevor, talk , torque.. got myself all revved up again this morning... not sure it i should be posting under this topic as im interested in the impact of dwell at both bdc and tdc and if it has any measurable gains with regard to engine torque and the conrod length which would be best for each end of the stroke.... hope you understand my question, as im having trouble with semantics( my new favourite word ) at present. thanks nigel... good moto 3 win looks like we may have a winner at last

Long rod: -- more time at TDC, Short Rod -- more time at BDC ....

hmmmmm? ha, one moment ....

Trevor! -- is it?

I remember there was a lot of chat about the longer rod and also the offset to piston-centre-line con-rod....

they both have their effect but I wonder how much of an advantage on a Bantam?

Trevor !?

CheerS!

afternoon John,
so would you think that a shorter rod stays stationary longer at bcd?, if your transfers were rubbish and ( not able to get the required area) this longer dwell time would help provide an answer to this problem? time /area.

Sorry Nigel I jumped in there where Trevor should have trod...

He wrote something about this subject on here some time ago -- perhaps he will revive it....

I MIGHT WELL BE WRONG BUT I SEEM TO REMBEMBER THE LONGER ROD GIVING MORE DWELL AT TDC ...

Big point about ports and exhaust resonance -- is if they aren´t right then the amount of dwell becomes irrelevant... I think!.

Cheers!

John. no problem... ive found some info as you say from the time area thread... had thought that holding the piston longer at bdc before it began to rise and close the transfers would be of some benifit perhaps enable longer blowdown keeping the transfers low but making up for smaller transfer port size(area) by increasing the time the transfers were open ...

For a given stroke, a long rod will have a longer dwell at tdc, and a shorter dwell at bdc. The short rod dwells for a shorter period at tdc and a longer dwell at bdc.
For a given transfer angle/area a short rod gives a longer, real time, open period which can be a great help in mass transfer mixture flow. The short rod provides for a shorter dwell over tdc and a greater angular thrust to the crank, and the point of maximum piston acceleration is earlier in the stroke than with the long rod.
The piston, cylinder head and bore have less time to absorb combustion heat which means more heat energy is available for gas expansion and more heat energy is available to be used to good effect in the pipe. The reverse is applicable for a longer rod under the same conditions, so the 175 engines are at a real disadvantage, over their 125 cousins!
The one possible down side to a short rod over a longer one, is the maximum angularity at 90* crank position, and a resulting friction penalty. As the piston is actually slowing down at 90* because the maximum acceleration point comes earlier, then the actual loss is smaller than might at first imagined. Con-rod weight must also be factored in and the top half of a short rod is, usually, less than that of a longer one. It is the rod top half that is used in reciprocating stress calculations for engines, and is also acting at 90*.
It might be argued that a big case volume is difficult to achieve in a short rod engine. All Bantam race engines produce low bmep, 8, perhaps 9 bar being the best, big case volume is non-applicable in low power scenarios. A good 125 RS reed valve engine will produce at least 50-60% higher bmep than the best Bantam and uses a very short rod, that engine then begs the question, what sort of case-ratio is necessary for 25 Bantam horse-power, so does computing the plusses and minuses give an overall benefit? Low rpm and big case volume plays hell with carburetor jet fueling signal strength and the 3 speed box only compounds this, together with long inlet timings and wide throttle openings.

This topic is just the sort of thing the new manual needs to help Bantam racers make an informed choice for their engine construction.

Trevor

Trevor, is there any advantage to being able to a vary conrod lenght during the 2stroke cycle

This might enable the differences to be seen, in exaggerated form for clarity and emphasis, the only constant being the angular duration!

I recon there are Nigel but how on earth could you do that at 10,000 rpm, damned interesting proposition though?

Trevor

Good example Trevor!

Anyone know anything about what happened to Dr Joe Ehrlic´s (4 cylinder, 4-stroke) `e-3 engine´ where the con-rod articulated during the cycle of ops...?

Last heard was that GM -- or some other big auto company -- had taken it over and it vanished from publicity about 7 years ago.

I´m still convinced that a later (after TDC) combustion is possible giving the crank arm more advantage and hence increasing torque for saem amout of fuel-air use...

... then there´s this moving-cylinder-head concept ....****

Cheers!

**** definitely would have JayBee drummed out of any Bantam Racing Prep....

Well... IM sure a patent has been applied for recently along with two others, sadly no details of the concept as yet...

http://www.revetec.com/Technology_Overview.htm i know this is 4 stroke engine design, but the crank set up seems 2 stroke possible... maybe ..

John,
Combustion after tdc already happens in 125 Bantam engines, courtesy of retarding ignition systems. With initial spark occurring at around 5* btdc the meaningful combustion is post tdc and is then over in about 40*; plenty of advance is available at low rev, low pressure, low cylinder filling that needs a long duration to get decent burn.
Sadly this progressive innovation of last century is not available to the very engine it will benefit the most; the 175?

Trevor

with good reason I guess Trevor, otherwise I doubt it'll be worth turning up on a 125 !

Ain`t that the truth Dan, makes you wonder just who came up with a daft piece of regressive regulation that has no relevance to the present situation, and even less when looking forward?

Trevor

Good point Trevor -- we actually did it -- in Montreal -- with a diesel by having `Pilot injection (a tiny squirt)  followed some degrees later by `Tailored Injection Rate´ (the squirting quantity increasing to a late maximum rate ...). As the fire spreads more fuel is injected and then as the pressure increases the max combustion pressure comes as the con-rod gets to an advantageous effective length***....

Unfortunately only  in laboratory conditions at a very narrow speed band -- which was considered failure for a highway vehicle  but come to think of it now with Hybrid -type electric vehicles we could have it running a generator whose electricity drives the vehicle...

Enough of my prattling ...  down to the real business of the Manual...

Am I correct in assuming:

The Bantam Racer as she is now,  needs con-rod, piston and crank?(??) which have to come from Foreign motorcycle makers -- there being no British?

Hence the new-boy has to go looking -- would be exactly one of the reasons you spoke of Trevor for having a Modern Manual  -- parts lists with Firm´s name and part number would be a marvellous innovation....


*** was it not something the vicar said....?

Just an academic question before I shut up (we live in hope, came the voice from above...) what are the practical  maximum and minimum  lengths of a Bantam 125 con-rod?  

 ...  114cm being popular...?

Cheers!

John,
For most Bantam applications a ratio of around 2:1 stroke to rod length is ideal, so for a 58mm stroke a rod length of 115mm is fine. The traditional 125 rod is just too long, in both the, nominal, 52 and 61.5 bore engines, and the down-side of that length has been well documented here on the forum and will doubtless be part of the manual!
The square 54x54 engines in the main use the 110mm rod length. The short stroke 50.5x56 engines have used 100, 105 and 110 rods in various engines. Problems can occur in all engines with too high a case pressure with the shorter stroke/rod length options. With poor control over the scavenge flow velocity level and "s..t" duct geometry, the scavenge flow can simply divert to the lower pressure area of the exhaust duct. As an example the old 350 TZ could respond to modification far more responsively when the original 110 rod is replaced with a 115 rod and an under-cylinder spacer for height compensation.

Case volume is a complex matter, compounded by stroke, rod and crank design and exhaust system combinations, all vying to perplex the engine designer. Reed valves bring their own demands and responses on top of all the other variables?

Between them, Yamaha, Suzuki and Honda have rods and pistons to cater for most applications, along with after-market companies catering for cheaper copies. Cranks tend to be home-made, or at least home designed and made elsewhere.

Trevor

Thanks Trevor!

Not as if I need it.

There´s a 99 bíllion to one chance, though, that I´ll winn the lotto*** and if its a tidy sum I´ll sponsor a likely lad and build a pulkka Racing Bantam to all modern criteria -- if it is in millions I´ll have several Bantam Racers built and even come over to Blighty to see them raced....

Aoh oh! John-Boy you wuzz always the dreamer....


*** Lottery to you....

http://www.revetec.com/Technology_Overview.htm. i know its 4 stroke but i like the crank set up