[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 2) (Locked)

In regards to the use of a cog type belt for the primary side drive. I have experiance with a similar application using the HTD style cog belts on the primary side of a race kart jackshaft. The HTD belts are very efficiant, but large ratio changes between pulleys that have close centerlines can cause relability issues. Reason being that as the centerlines are brought closer together a loss of teeth in mesh with the primary drive pulley occurs. Most often when the belts fail the cords first tear internaly and then the teeth are sheared from the outer belt carcus. The belts seldom actually tear in 1/2. Tight radius's, low number of teeth in mesh, and angular mis-alignment are the biggest contributing factors to belt failures. Workable solutions are... Use the largest pulleys that will fit the application, apply an idler on the non torque side of the belt, and increase the width of the belt.


We have been building this unit for about 15 years. It is very popular in the USA for hi bhp oval kart racing. It will reliably handle engines with 35 ft pounds of torque and bhp's as hi as 70. Most commonly the primary ratio is 2.65 to 1 .

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I have been following this forum for quite some time, but have not made a formal introduction of myself. My name is Kerrmit Buller, in 1959 at age 4 after hearing and smelling a 2 stroke model airplane engine running my destiny was set..... modifying 2 stroke motorcycles for dirt bikes began by the early seventy's. Then in the 80's 2 stroke kart engines became a passion. In karting I have served as IKF's 2 stroke tech committe chairman, as well as chairing the North American Kart Tech Commitiee , which oversaw all facets of the homoligation process of Kart engines for the US.
Currently I own a business that manufactures varoius race kart products as well as a 2 stroke engine.

Manuel Rainer a écrit:

Hello

Jan or Frits can you tell me what is the better shape for the exhaust if the only structural restrictions is to make a 90° angle.thanks Manuel

somone please correct me if i'm wrong ,

i would have thought a gas/signal under pressure would take the shortest path posible,

example ; with a tight bend the gas path will be straight across your tight bend,

this would or could shorten your pipe needed pipe dimensions as well as disrupting a "clean" flow,

The pipe with the more gentil curve would be more dimensionaly stable as well as more stable for the signals.

Jan Thiel a écrit:

I would not not know without trying!

hello

the shape of the RSA exhaust seems to be more like the 2nd form. it was this that was the only possible shape or where other forms tried?

thank Manuel

Manuel Rainer a écrit:

Jan Thiel a écrit:

I would not not know without trying!

hello

the shape of the RSA exhaust seems to be more like the 2nd form. it was this that was the only possible shape or where other forms tried?

thank Manuel

It was the only possible shape, so no other forms were tried.

Jan

Do you think RSA would be better with a straight exhaust of course if it had been possible (like RSW) your advise could be a begining of answer for Manuel

Pierre

Jan Thiel a écrit:

Manuel Rainer a écrit:

Jan Thiel a écrit:

I would not not know without trying!

hello

the shape of the RSA exhaust seems to be more like the 2nd form. it was this that was the only possible shape or where other forms tried?

thank Manuel

It was the only possible shape, so no other forms were tried.

thanks Jan, pierre95 and rgdavid

rgdavid I also thought so and that's why i have drawn the exhaust like this.
I was just wondering because I have seen that the RSA exhaust is different. but since there was no other way, it is clear why he so.

Thanks Manuel

better or not i'd like an answer, perhaps it doesnt even matter ?

the shape of the bike/Kart/boat/sidecar/lawnmower or whatever dictates the space available & thus the curves tight or not,

pierre95 a écrit:

Jan

Do you think RSA would be better with a straight exhaust of course if it had been possible (like RSW) your advise could be a begining of answer for Manuel

Pierre

Yes I think a straight exaust would have given more power.
Also the necessary long tailpipe cost us some power and caused some detonation.

Jan Thiel a écrit:

pierre95 a écrit:

Jan

Do you think RSA would be better with a straight exhaust of course if it had been possible (like RSW) your advise could be a begining of answer for Manuel

Pierre

Yes I think a straight exaust would have given more power.
Also the necessary long tailpipe cost us some power and caused some detonation.

It is interesting that the length of a tailpipe could cause such a problem!

rgdavid a écrit:

somone please correct me if i'm wrong ,

i would have thought a gas/signal under pressure would take the shortest path posible,

It's not that simple. For acoustic signals, provided the pipe diameter is not too big relative to the radius of the curves, the waves propagate around corners just fine

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However as Frits has told us many times, the conditions in an exhaust are far from those that validate the acoustic approximation. I would guess the signal is less coherent and that you'd do better by putting the curvature where the diameter is smaller, but it would just be a guess. Frits is the only one here equipped to answer, I suspect.

XpTpSMTT a écrit:

Jan Thiel a écrit:

pierre95 a écrit:

Jan

Do you think RSA would be better with a straight exhaust of course if it had been possible (like RSW) your advise could be a begining of answer for Manuel

Pierre

Yes I think a straight exaust would have given more power.
Also the necessary long tailpipe cost us some power and caused some detonation.

It is interesting that the length of a tailpipe could cause such a problem!

Hi all , having been an exhaust manufacturer for over 30 years and involved in superkarts where we have been continually restricted on noise and then compulsory minimum silencer lengths and then minimum length link pipes
it has been a major part of my developement program to find suitable stinger,link pipe and silencer perforated combinations. It is interesting how big the affect is after the stinger not only on power but the detonation that can occur. Even tapered link pipes have been tested.

Pressure waves are of more interest than acoustic waves

Institute of TwoStrokes a écrit:

Pressure waves are of more interest than acoustic waves

Well, yes, that's more or less what I said.

Acoustic waves are pressure waves... but they are an idealisation/approximation which depends on certain assumptions (small variation in pressure relative to the average) which don't hold inside a 2-stroke exhaust.

I believe that for a wave to follow the shortest route through a bend,it could only happen(maybe) if it was a shock wave propagating in a medium,and would also move in a straight line and reflect of the walls.
I don't think that's the "good"effect of the pipe.
Engine exhaust is more like sound,ruled by the chamber environment which is not constant and very distinct from open air.
Though sound mostly moves in straight line is also able to be carried,diverted by air draft or favorable pressure differential,like happens on a loud speaker on open air without a box or baffle will "short circuit"and unable to sound out loud despite huge amounts of electric power being dumped in it.
However the higher the frequency,the bigger the imunity to this "s.c."

On the other hand,the Helmhöltz ressonance "ignores" the position of the outlets of the chamber,as a 2nd entry is redundanct.
This is a "war" between elastic and inercia forces based on volume and weight quantities.
Calculating a ressonant box with the standard formulae with more than 1 entry with diferent charactristics can give false results,unless all variables are accounted including friction att.

All that said,it ocurres me that a good pipe will combine wave speed propagation with ressonance to maximize effect,and that though direction changes will bear a light attenuant effect,if the bike design allow for a straight pipe and if the last h.p. is relevant,why not make use of it?

It's also pretty evident (to me) that ,like all on 2 stroke gas dyn.,the best results can only be achieved by "cut and try" and that attempts to establish rules or reason exactly what's going on,are very limited or even futile as the involved variables would overload a computer.

Off course like everyone else I too use the available data and software as a base,since I'm just a private hobbyst.

Yes,all the programs,simulations and...theories regarding two storkes cannot
substitute intuition and a true feeling and "grasp" of their function,
and of course CUT and TRY!or at least many times Cut and....fail...
That is what it has come down to.At least to me.
And as Jan said
"I would not know without trying"

Hi all,

I am just brooding about vertical transfer angles and what parameters they are depending on.
My assumption is: The more rmp, the steeper the angles and the more stroke, the steeper the angles or short: The more piston velocity, the steeper the angles. Am I right? Please correct me if not :-)

The Aprilia drawings show, that B flows with 6,82° on the upper side and 7,42° on the under side and A flows with 28,16° on the upper side and 16,45° on the under side. I imagine that someone said these drawings were old and I also imagine that someone (Jan or Frits) once said, that the transfers had the same angles on the upper and on the under side. That would confirm, that the drawings are old.

So let's say the RSA B-port flowed with 7°. And the A ports?

And back to the context "vertical angle - piston velocity": Is there any simple connection to scale the RSA angles down for less stroke and let's say the same rpm? (provided my assumptions at the beginning of this post are right)

Regards,
Daniel

The axial angle of the A-ports was 25°; for the B-ports it was 10° if I remember correctly.
If you take the piston velocity as a starting point, you need no longer worry about less stroke and same rpm or vice versa. And like you say, the trend is towards steeper angles if the piston velocity rises, but the connection cannot be described in simple terms.
Not too long ago I posted a little story here: [Vous devez être inscrit et connecté pour voir ce lien] . That story and the subsequent discussion may contain answers to some of your questions.

This detonation mentioned I remember a case we had many years ago with a 50cc tzr bored out to 80cc and 21mm Mic

The bike ran fine with a metrakit exhaust,but a new (Athena?)Kit chrome cylinder was tried with better ports design,also bigger reeds and 28mm carb. and then it blew from detonation on the main straight at full throttle.

after rebuilt,as I remember ignition advance was irrelevant for the deton.problem,only lower compression and richer mix made it work but performance was down.A colder spark plug helped but didn't last long as it would soak in oil.

We reasoned that this new cylinder kit moved a lot more air than the original bored yamaha,despite the same 80cc and I wanted to cut the brand new Metrakit exhaust,but was not allowed.

There was no time nor will to build a pipe from scratch with a bigger size.

So instead we took that Metrakit pipe dimensions and volume,and had another pipe made out of old scraped pipes,exactly the same length,angles etc but with a fatter belly and wider stinger,both enlarged by the same %,don't remember maybe 20%.
The pipe capacity/bleeder section size relation was maintained
Theory was this would permit to pass more air while keeping the same Metrakit RPM power band,as we all agreed this needed no change.

The silencer was a much bigger old used 125 MX Proma we borrowed .

This was the right choice and solved the deto. problem,despite looking very bad rusted and welded like frakenstein,indeed it turned out to be a very reliable powerful pipe,so compression and mix could be restored and no knocking.
Powerband was the same.

Metrakit was sold to rival team,later seen on a hi power 50cc.

All this to say that I'm almost sure a pipe or silencer can choke the engine,and cause full throttle pre ign.

also I don't use the silencer to tune a pipe,I make sure the silencer section is bigger and not influence performance,but noise level was liberal on amateurs race play


Mr. Jan
Could this detonation/power loss caused by a longer pipe be a choking effect ?

Why not increase diameter then?

Thank you

That was the best answer I could get! Thank you very much Frits

Often this stinger / muffler problem is over looked. In older twostrokes there is often carbon build up around the outlet pipe area causing a slow restriction over time. Slowly the bike is more prone to detonation at top end but because it is a slow buildup process this problem is over looked and other "reasons" for the detonation are found. Old TZs and the like. Make sure this area is kept clean and carbon free. I think a vairable nozel at this point would be a great device.

Or a frequent cleaning

Sorry I don't mean to point out the obvious, just I've seen it happen many times and get overlooked.

Manuel Rainer a écrit:

Hello

much earlier someone was talking from a detonation sensor in the RSA. what is it and how does such a sensor works.


thanks Manuel
[left]

Firstly, I'd like to express my overwhelming gratitude for the contributions of Jan, Frits, and the many other knowledgeable people who are participating in this thread! It has become an addiction.

I'm going a little off of the RSA here, but since it's " All that you wanted to know on Aprilia RSA 125, and more" I figured I may as well ask.

I have been doing a little bit of reading on detonation sensors, and one of my friends mentioned a "det can". It is basically a copper tube that is flat on one end with a hole in it to bolt it to an engine. The other end is connected to a stethoscope, so that the tuner can listen for detonation.

As a "budget tuner" this idea appeals to me. Has such a method been used by anyone here on a 2-stroke? It seems like the car guys use them a lot, but I don't see reference to using them on a 2-stroke engine and thought I would seek opinion and advice from the fine fellows here a pit-lane. :)

I'm going to start working with my first programmable ignition soon (Ignitech). My ears aren't all that great and I fear that over the noise of my loud race bike and all the fans on the dyno, I may not hear detonation very easily. I haven't heard of a "det can" before last week, but I figured that if I'm going to tune this ignition system to get the most out of it, that I may want to give it a try. Here are some links - google reveals a bit more, but I don't see 2-stroke guys doing it.
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As anyone had experience using this method? Or am I perhaps greatly over-complicating this, and would be better off relying on naked ears and careful checks of the piston crown and plug?

All the best, from the USA,

~Josh

Hey Josh,

As suggested elsewhere ... why trouble yourself with that half-accurate method?
You could get one of these [Vous devez être inscrit et connecté pour voir ce lien] , plus a used knock sensor off ebay.
It 's a good investment if you into this level of tuning!

Vagelis a écrit:

Hey Josh,

As suggested elsewhere ... why trouble yourself with that half-accurate method?
You could get one of these [Vous devez être inscrit et connecté pour voir ce lien] , plus a used knock sensor off ebay.
It 's a good investment if you into this level of tuning!

Vagelis,

Many thanks for the hasty reply! Apologies also, as I must have missed the reference to the Avejas knock gauge. I will get one of these - thank you for bringing it to my attention!

Cheers,

~Josh