[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 3) (Locked)

Technopro a écrit:

Frits Overmars a écrit:

...And remember: the softer the reeds, the leaner the air-fuel mixture will become, so with every reed change you will have to reset the jetting.

Frits , could you explain why softer reeds are leaning the air-fuel mixture ?

Stiff reeds will not open until the crankcase pressure has dropped a fair bit below ambient pressure, which means that once they do open, air flow through the carburettor is accelerated fairly fiercely and that generates a strong suction pulse at the jets. Softer reeds generate a weaker suction pulse.

  Roger that .

Frits Overmars a écrit:

Technopro a écrit:

Frits Overmars a écrit:

...And remember: the softer the reeds, the leaner the air-fuel mixture will become, so with every reed change you will have to reset the jetting.

Frits , could you explain why softer reeds are leaning the air-fuel mixture ?

Stiff reeds will not open until the crankcase pressure has dropped a fair bit below ambient pressure, which means that once they do open, air flow through the carburettor is accelerated fairly fiercely and that generates a strong suction pulse at the jets. Softer reeds generate a weaker suction pulse.

There is no such thing as a SUCTION only atmospheric pressure that is greater than the pressure in the crank case so in reality the air is being forced through the carbs. On a side note with regard to long rod ratios, I have found out the hard way that one must be carefull with too much leed on ign. timing as the piston sits in dwell for a longer period of time verses a short rod ratio and detonation can become a problem. Once the plug fires the mixture and the flame front starts to propagate across the chamber and the piston is not moving things can go bad.. I have found 11 or 12 deg. is pushing it, of course this depends on fuel, how well its atomized by the carb,comp ratio and head and chamber design/pipe charging and the RPM..

frits i was wondering something today.  since its cold winter now and i dont do much work in the garage this time of year so i can be in the house and think how to make and operate a 24/7 valve. then in a few months when its warmer i can try to construct it.  

my bike(s) are only used for drag racing, im sure your familiar with this type of racing. mostly i just rev the engine to near full rpm and let off the clutch and proceed through the gears while still at full open throttle. i use only close ratio transmissions so the engine stays in the powerband at each gear change. for this kind of scenario i was thinking to have the flap manually operated by a cable mounted on the handle bars with a lever or even attached to the throttle housing so the flap goes full open when the throttle handle is turned near full open. really im not concearned with what happens below the powerband because the engine is never used in that lower rpm, and thats why im thinking a manual cable could work instead of a servo motor. plus it would be much easier for me to make.

so heres how i invision it to work: the flap would be closed to start the engine and get the bike to the starting line, then rev it to near full rpm (im thinking this would be accomplished with about 1/8 to 1/4 throttle or so because the engine isnt under any load yet), then simultaneously release clutch and fully open the throttle ( the flap would now open via the cable), then proceed shifting gears while maintaining full throttle. do you see any reason why this idea wouldnt work ?

also if i used a single flap design do you see any difference if the flap opened toward the top roof or toward the bottom floor ? or perhaps it should open in the middle and one flap would go up and the other flap would go down ?

SPEEDSLUT a écrit:

Frits Overmars a écrit:

Technopro a écrit:

Frits Overmars a écrit:

...And remember: the softer the reeds, the leaner the air-fuel mixture will become, so with every reed change you will have to reset the jetting.

Frits , could you explain why softer reeds are leaning the air-fuel mixture ?

Stiff reeds will not open until the crankcase pressure has dropped a fair bit below ambient pressure, which means that once they do open, air flow through the carburettor is accelerated fairly fiercely and that generates a strong suction pulse at the jets. Softer reeds generate a weaker suction pulse.

There is no such thing as a SUCTION only atmospheric pressure that is greater than the pressure in the crank case so in reality the air is being forced through the carbs. On a side note with regard to long rod ratios, I have found out the hard way that one must be carefull with too much leed on ign. timing as the piston sits in dwell for a longer period of time verses a short rod ratio and detonation can become a problem. Once the plug fires the mixture and the flame front starts to propagate across the chamber and the piston is not moving things can go bad.. I have found 11 or 12 deg. is pushing it, of course this depends on fuel, how well its atomized by the carb,comp ratio and head and chamber design/pipe charging and the RPM..

Semantics...

Pulse is the operative word. ( noun )

Suction is the modifier ( adjective ) and is appropriately graphic without a chart !

brokedown a écrit:

frits i was wondering something today.  since its cold winter now and i dont do much work in the garage this time of year so i can be in the house and think how to make and operate a 24/7 valve. then in a few months when its warmer i can try to construct it.  

my bike(s) are only used for drag racing, im sure your familiar with this type of racing. mostly i just rev the engine to near full rpm and let off the clutch and proceed through the gears while still at full open throttle. i use only close ratio transmissions so the engine stays in the powerband at each gear change. for this kind of scenario i was thinking to have the flap manually operated by a cable mounted on the handle bars with a lever or even attached to the throttle housing so the flap goes full open when the throttle handle is turned near full open. really im not concearned with what happens below the powerband because the engine is never used in that lower rpm, and thats why im thinking a manual cable could work instead of a servo motor. plus it would be much easier for me to make.

so heres how i invision it to work: the flap would be closed to start the engine and get the bike to the starting line, then rev it to near full rpm (im thinking this would be accomplished with about 1/8 to 1/4 throttle or so because the engine isnt under any load yet), then simultaneously release clutch and fully open the throttle ( the flap would now open via the cable), then proceed shifting gears while maintaining full throttle. do you see any reason why this idea wouldnt work ?

also if i used a single flap design do you see any difference if the flap opened toward the top roof or toward the bottom floor ? or perhaps it should open in the middle and one flap would go up and the other flap would go down ?

Its here somewhere and tested by a german guy. I recal.
It works but no magic to be expected

SPEEDSLUT a écrit:

There is no such thing as a SUCTION...

How about breast-feeding? .

brokedown a écrit:

... i use only close ratio transmissions so the engine stays in the powerband at each gear change.

It's important that the bike not only stays in the power band, but also on the pipe. That means you should neither close the throttle nor cut the ignition while shifting gear. As you are involved in drag racing you will doubtlessly know the Weismann dragbike trannie with beveled gear dogs; you'll need something along those lines for shifting under full power.
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Citation :

do you see any reason why this idea wouldn't work ?

Not really. But it may take some fiddling to get good carburation both when the engine runs in reed mode, and when switching it to 24/7 mode.

Citation :

also if i used a single flap design do you see any difference if the flap opened toward the top roof or toward the bottom floor ? or perhaps it should open in the middle and one flap would go up and the other flap would go down ?

By all means keep it simple and use a single flap.
I don't think it will make a lot of difference which way you open it. There may be slight differences in flow towards the transfers and in flow aimed at the big end.

Frits Overmars a écrit:

It's important that the bike not only stays in the power band, but also on the pipe. That means you should neither close the throttle nor cut the ignition while shifting gear. As you are involved in drag racing you will doubtlessly know the Weismann dragbike trannie with beveled gear dogs; you'll need something along those lines for shifting under full power.
.

yes ive seen those type of tranmissions in action. they shift at full power with no clutch and no ignition interupt. it appears they cut the gear dogs so theres a ramp. and perhaps they modify the shift drum also. ive been trying to find more info about them to see exactly how they work. from what i was told,  most any transmission can be modified in the same way but i guess the gears have to turn a specific direction on the shaft. like i said im still trying to learn more about how they work.  where i ride theres alot of guys using them on the banshee engines and i think atleast 5 of their gears can be made to shift at full power. i guess on some of the honda mx engines only 1st-3rd gear can be modified becasue 5th and 6th dont turn the right direction so they have to be manually clutched or thats how i understand it anyways.

so far ive just been using standard transmission and i slightly tap the clutch which releases the torq for a split second and allows the gear change, it works ok but a real drag race transmission would certainly be better.

frits when looking at a dyno paper is there a for sure way to see when the engine goes on the pipe ?

This zeroshift gearbox is the smartest I've seen. Dual springloaded dogrings is the key.

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I have no experience in dragracing.
Other than beeinng first at the next intersection.

Iff i would have to choose i would get a snowmobile and shit myself every run down the quarter.   
Also easyer to try the flappy reed valve.
And seamless all the way.

Sander

Mic a écrit:

This zeroshift gearbox is the smartest I've seen. Dual springloaded dogrings  is the key.
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I talked to the Zeroshift people on a couple of occasions because I liked their principle, but now it seems they're out of business. Xtrac produces a nice seamless shift system too.

Sanderhoutman a écrit:

If i would have to choose i would get a snowmobile and shit myself every run down the quarter. Also easyer to try the flappy reed valve. And seamless all the way.

Yes, a continuously variable transmission is the way to go. Too bad it is still illegal in most branches of motorsport.
If money was no problem, I would choose a Tbox double roller toroide variator: compact and efficient.
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Nice indeed.

Annybody want to massproduce them for a 50cc?  

zeroshift website is still working. maybe theyre back in business ?


but i think this transmission is more than my budget allow   

Dear Frits

About the 24/7 : Everyone heard the story of pilot who stops at pitstop and the engine stopped will not restart: the rotating disk was broken / missing. And the pilot said he had not noticed.
In other words, he didn't feel improvement of performance: neither negative nor positive.
suddenly I'm wondering interest to search the system 24/7 ....  

Thank you for all the things you do for 2 strokes maniacs  

Pierre

My pleasure, Pierre  .

 MERRY CHRISTMAS EVERYONE
    AND A HAPPY NEW
             YEAR

frits let me run something by you again. theres no rush but whenever you get a free moment to comment.

couple pages back i mentioned the 72x72 that i wanted to install longer rod (closer to 2:1 ratio,stock ratio is 1.73:1 ) and even make a pipe with the dimensions provided by this FOS underdog calculator but we wont worry about the pipe right now  

you said the importance of domed piston for the cooling. so i did some investigating to see what dome pistons i could use (original piston is flat top).

it appears there is no perfect option but rather several options with many compromises.

1-  i can get the ratio to 2* but the piston (slightly domed) ill have to use has huge side skirting cutouts which will certainly cause the same problem as the old ktm engines, a large escape route from the crankcase to the exh via the aux ports near TDC.

2- my other option is increase the ratio to 1.875*, keep the stock flat top piston and not have any shortcircuits, except for the brief moment when the piston pin hole runs over the transfer and aux exh simulataniously but this will happen with every piston  unless i use a pin plug, and a plug is really out of the question.

3- also theres a 3rd option. increase ratio to 1.875*, use slightly domed piston, but it will likely have 2 escape routes because of its design. one large escape path when the wristpin area passes over the transfer and aux ports because the side of the piston at the pin area has a large amount of material machined out for weight reduction, and another escape path from crankcase to aux exh because the large side skirting cutout

option 3 seems like the worst because 2 seperate escape routes.

i know you said dome cooling is paramount but is it not as important to elimate all shortcircuit as much as possible or no ? im sure a small shortcircuit wont cause a huge loss of power but what about when the escape hole becomes larger, does it still not have as much importance as the piston dome cooling ?

  honestly im leaning toward option 2 mostly because i can make the aux ports very large with no escape path,  but ill only have a ratio of 1.875* and flat piston

This is an easy one, Brokedown. Option 1 and option 3 are out, so that leaves you with...right!

thnx frits and merry christmas   . i guess in reality there is a 4th option but it may be well beyond my budget.

have a special piston made with a dome and proper side skirting and pin location, to allow use of the 2* rod.

wiseco has a 12 order minimum so thats out of the question.

perhaps one of the other manufacturers can make 1 or 2 pistons, based on one of their existing piston designs but with slight modifications for my specific application. still i dont think this would be cheap

does anyone have experience having 1 or 2 piston specially made ? other wise i better stay with option 2 from the previous post

brokedown a écrit:

does anyone have experience having 1 or 2 piston specially made ?  other wise i better stay with option 2 from the previous post

You could try VHM in The Netherlands [Vous devez être inscrit et connecté pour voir ce lien] they make 2 stroke pistons from billet.

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Mic a écrit:

brokedown a écrit:

does anyone have experience having 1 or 2 piston specially made ?  other wise i better stay with option 2 from the previous post

You could try VHM in The Netherlands [Vous devez être inscrit et connecté pour voir ce lien] they make 2 stroke pistons from billet.

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I asked VHM to make a batch of 20 pistons (I think that is the minimum) diameter 72mm (Bultaco 250, 72x60 engine) and VHM does seem manufacturing to 54mm piston diameter or less (50cc engine).
For higher diameters their should make all new toolings!
But you can ask about it them  

You can buy a piston to flat head and machined it a corner to avoid problems of temperature.
This was used by KTM 125 GP
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Piquer a écrit:

For higher diameters their should make all new toolings!
But you can ask about it them  

If they start from a forged blank and then fully CNC machine all surfaces then of course new tooling would be required, but if their starting point is normal billet material then I can't see the problem.

Mic a écrit:

If they start from a forged blank and then fully CNC machine all surfaces then of course new tooling would be required, but if their starting point is normal billet material then I can't see the problem.

For high precision machining and/or specific geometries it is necessary to tie the piece with specific tools for the piece remain perfectly centered and strongly tied without having to centered on each operation or each piece