[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 1) (Locked)

really fantastic

Marc a écrit:

Un petit break avec les dernières photos de Francis Payart...


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Can you show the other side of the carburator?

Seems diferent from the others bell shape intake

Très belle moto!! Je crois que Fontana est réputé pour toujours préparer des motos très propres.

Je pense qu'il manque un zéro...

be diffident from subito.it, here in Italy is known for some post made by non serious people....

Hello,my name is Peter,I have been watching this forum-mostly this specific thread-some time now,unfortunately i do not know french,sorry,but luckily(?!) Mr.Overmars and MR.Thiel dont write it either!
The rest is...Google Translator!
I am a motorcycle mechanic,my passion are two strokes,unfortunately there are not many left but
off roaders seem to still like them so I still lay my hands on some,exept off course my two 2strokes with I fidle with all the time...
There is so much information in this thread (those ct scans of the RSA Ports are worth a billion words themselves ,nevermind the priceless comments of two stroke gurus)to make archive and go next to my G.Bell and...Blair books.
A question I would like to make is regarding the development of this engine.

It all started with the Rotax engine"design theme" and I see the power valve design hasnt changed much either through the years,isnt that aspect of the engine important?it was good from the begining?.Did Mr.Jan Viteveen have anything to do with the development of this engine?

Hi Peter,
The guillotine blade-type powervalve of the current Aprilia engines looks the same as the original Rotax valve. The big difference is, that the Aprilia valve is electronically operated whilst the original Rotax valve was pneumatically operated through exhaust pipe pressure. That was a cheap, simple ystem, but with some severe drawbacks. When the exhaust pressure dropped, the valve would drop. Rev the engine past its power peak, and the exhaust timing would be lowered, something you definitely do not want. It was the same while shifting gear: close the throttle or cut the ignition for a gear shift, and the valve would drop; quite unwelcome since the engine rpm level after the gear change would require the exhaust to be fully open.

In addition to the guillotine type valve Aprilia tried the rotating Honda version, but contrary to expectations that did not function as well as the guillotine.
Witteveen's influence on the development of the Aprilia engines was bitterly felt: he severely slowed down development of any idea he did not understand.

Frits Overmars a écrit:

Hi Peter,
.....
In addition to the guillotine type valve Aprilia tried the rotating Honda version, but contrary to expectations that did not function as well as the guillotine.
Witteveen's influence on the development of the Aprilia engines was bitterly felt: he severely slowed down development of any idea he did not understand.

Thank you,for reply.
So the guilotine and rotary valve was the child of rotax (beside the rave system)
inherited to aprilia and developed from there.
I am suprized of the reasons a rotating valve did not operate as well as the quilotine.
The best design I feel is the KTM enduro valve with one center large "flaper" and two side drums
for the auxiliary exh ports i feel it gives smooth exit/return of presure waves . Although the japanese in MX have mostly "returned" to quillotine type.
Looking the RSA engine it's like there is no effort to control the sub exhaust port gasses
or they are controled alltogether with the one valve of the center port-but they are all the same height so how is "blow down" managed with?The gases seem to burst altogether into the exhaust..
It is also very interesting to see eforts for for counterbalancing the engine even if it costs power(?)
Also since the bike developed a weight distribution broblem from the new engine how dificult was it to rotate the cylinder with the new rotary valve arangement?

Around 1994 mr.Viteveen was credited for the development of world championship winning bikes
like Sakata and Biaggi.He is also credited for the RSV 400 (witch was a failure to me ...)

XpTpSMTT a écrit:

So the guilotine and rotary valve was the child of rotax (beside the rave system)

The guillotine valve was the Rave system, so called by Rotax. But the rotary valve already existed in 1918.

Citation :

Looking the RSA engine it's like there is no effort to control the sub exhaust port gasses

That is correct.

Citation :

It is also very interesting to see eforts for for counterbalancing the engine even if it costs power(?)

Whatever gave you that idea?

Citation :

Also since the bike developed a weight distribution problem from the new engine how dificult was it to rotate the cylinder with the new rotary valve arangement?

Well, If you don't mind the exhaust pipe going straight through the inlet valve....

Frits Overmars a écrit:

Well, If you don't mind the exhaust pipe going straight through the inlet valve....

But I guess the question is, would it be possible to mount the inlet valve on the front of the motor instead of the rear?
Although given you need to fit a carb and allow it some space to breathe, that might be just as big an obstacle as the pipe...

@ GrahamB
Well seen, especially as the inlet temperature could be strongly influenced by the proximity of the radiator.
Besides that the position of the exhaust duct would probably have a less favorable position relative to the rotation direction ...

GrahamB a écrit:

Frits Overmars a écrit:

Well, If you don't mind the exhaust pipe going straight through the inlet valve....

But I guess the question is, would it be possible to mount the inlet valve on the front of the motor instead of the rear?

Jan Thiel has seriously considered putting the inlet valve at the front. But that would have meant incoming mixture flowing against the direction of crankshaft rotation. Tests had already shown that this would cost power.
We also talked about a primary transmission with a toothed belt instead of gears to reverse the direction of crankshaft rotation, but that idea never materialized. Just as well, because a carburetter plus airbox in front of the engine would have pushed the engine even further back in the frame than the front-exiting exhaust pipe of the RSA does now. And the present mass distribution of an RSA already is less than perfect (an RSW handles better).

Frits Overmars a écrit:

But that would have meant incoming mixture flowing against the direction of crankshaft rotation. Tests had already shown that this would cost power.

Ah, so that's one excuse for the TZ250 :)

In % terms, how different are the RSA and RSW weight distributions ? I assume the 125's need an extreme front bias to compensate
for the (relatively !!) heavy rider...

GrahamB a écrit:

In % terms, how different are the RSA and RSW weight distributions ? I assume the 125's need an extreme front bias to compensate for the (relatively !!) heavy rider...

Dunno. That's the downside with a large racing department: you don't get all the information
(in fact you didn't get any feedback if it was up to Witteveen ).

Please dont let me get misunderstood.
It is obvious the right decisions were made for the engine world championships prove that.
Its just that seldom one gets the
oportunity to "talk" with people close to a marvel like this litlle engine,ruling out other design options.
We JUST WANT TO HEAR MORE!!!
The TZR 3MA i think it was had foward facing carburetors and proved nothing.
Sorry,by "rave" i meant presure regulated.
Also I thought there is alot of space over the rotary valve...
What gave me what idea?That counterbalancing removes power or that there are counterbalancers?
I dont understand...I have not your knowledge on 2strokes....obviously.Im all ears:-)

XpTpSMTT a écrit:

The TZR 3MA i think it was had foward facing carburetors and proved nothing.

I think it was the TZ250A that was the reversed-motor TZ. I don't think it was a world-beater...

However I should take back my comment re the V-twin TZ: although the RH carb faces forward, it is still behind
the cylinder (which points down). They have the reed cage pointing against the direction of crank rotation
however, and the TZ (and post-Kocinski YZR) was never really equal to even the Honda for hp, let alone the Aprilia.

Except when Honda made a complete mess of the twin crank bike and OJ and Nakano ran 1-2, of course

GrahamB
I dont remember what model it was many years since then...

Mr.Overmars.Sorry I didnt notice the highlighted words of the reply about the balancer shaft.
So for the two strokes at max power rpm the balancer does not hinder max power-apart from the slight friction from two bearings and the gear(s) i presume.

"...Regarding the Aprilia RSW/RSA125 single, Jan Thiel told me that in seven years of testing the racing department had not been able to establish which was best: low or high inertia. I would choose low..."
does this have anything to do?

Peter, the additional inertia of a balance shaft will absorb a little engine power, but there are three great benefits.
First, a vibrating engine would mean that its centre of gravity is constantly moving about, which itself would cost power.
Second, the reduced vibration thanks to the balance shaft makes carburation much cleaner.
And third, it is better for the frame (aluminium is very sensitive to vibrations).

I would choose low inertia, especially for small engines. The loads on the transmission are highest during gear shifting (far higher than the load caused by the engine power), and then a low crankshaft inertia is beneficial.

O, and stop calling me Mr. Overmars. That was my beloved father. I am just Frits .

Yes i also approuve a low inertia i tested it on a TZ125 (94) with a titanium cranckshaft i did the result was quite surprising better rise in RPM more smooth when changing gear (without shifter) and much more max RPM but on this point it's maybe due to the different shape in did on the webs crankshaft (fully cylindrical).If i well remember the genuine TZ cranckshaft was near 2,3kg and the Ti one is 1,6KG .It was a very good fun to built that!! It have been made in TA 6V without any treatment

Frits Overmars a écrit:

..........

I would choose low inertia, especially for small engines. The loads on the transmission are highest during gear shifting (far higher than the load caused by the engine power), and then a low crankshaft inertia is beneficial.

O, and stop calling me Mr. Overmars. That was my beloved father. I am just Frits .

So Mr.Ov......sorry, Frits :-)
I understand,especially since there is no flywheel weight to take into account and no traction problems to face i should think.(like an 125 engine for enduro...)

So what "discovery"during research did you find impressive and or harder understand?
Any "wild" idea considered for testing(like the reverse rotating crankshaft)

XpTpSMTT a écrit:

So what "discovery"during research did you find impressive and or harder understand?
Any "wild" idea considered for testing(like the reverse rotating crankshaft)?

Those are two questions, both with the same answer. It was the fact that an engine can keep running in the powerband when the inlet stays open all the time. It was not hard to understand, but it was very impressive.
It led to the 24/7-principle that I have described elsewhere in this forum.

Frits, I broached this subject in my welcome thread, but you never responded, so I will bring it up again.

I think I have the perfect application for your 24/7 idea. Land speed racing. I work my way into the power band and never leave it until the run is over. In fact it is a deal breaker to try to get into sixth if I am not "on the pipe".

I am in the process of machining new reed boxes on new cylinders to update the old reed/cylinder combo I have now. I redesigned the weld-on reed boxes to a bolt-on design to accomidate a "flapper" reed box. I think it a better idea anyway, being able to change things on the fly vs welding and commiting a cylinder.

This is for an Yamaha RD400, BTW.

Having said that; I found your ideas, and this thread at a very opportune time.

This is what I am thinking:

What I will run with new reeds;

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And what I would like to try with the same cylinders;

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In the closed position The "flapper" is about 35 degrees from the face of the reed box. Open, the bottom of "flapper" will be flush with the top of the port in the cylinder. This is not the best prospect for airflow, but I can't think of a better compromise. I will run a mechanical linkage to the left handlebar to open and close, and a spring loaded detent to secure the position. I am concerned that the drastic angle of the "flapper" will break reeds and if I limit the travel of the reeds will not gain the RPM I need. 9.5k is about the start of the "power band". What do you think?

"Flapper" may not be the best word for this..."gate" may be better. "Dam" possible. What do I know, I am not a wordsmith, just a gearhead.

High and low inertia crankshafts were sometimes fitted between practice sessions.
The riders did not feel that much difference.......

Makr a écrit:

Frits, I broached this subject in my welcome thread, but you never responded, so I will bring it up again. I think I have the perfect application for your 24/7 idea. Land speed racing. I work my way into the power band and never leave it until the run is over. In fact it is a deal breaker to try to get into sixth if I am not "on the pipe".

I did not respond because, although land speed racing is a very good application for the 24/7-inlet, I am currently looking at an even better (and simpler) application: scooter racing. No need for gear shifts, so no need for ingenious ways to keep the gas dynamics in the exhaust pipe going while at the same time cutting the torque on the gears in order to enable shifting.

Citation :

I will run a mechanical linkage to the left handlebar to open and close, and a spring loaded detent to secure the position.

I would prefer to keep both hands firmly clinched to the handlebars. Why don't you use a servomotor from an exhaust power valve? You could even use the same the electronics, as the opening rpm's of an exhaust power valve and the 24/7-inlet valve may coincide.

Citation :

I am concerned that the drastic angle of the "flapper" will break reeds and if I limit the travel of the reeds will not gain the RPM I need. 9.5k is about the start of the "power band".

You can use quite a stiff reed. It does not have to flow very well, because once you are in the power band, it swings out of the way anyway. You may have to use the clutch to get into that power band, but then you may have to do that anyway with a close-ratio gearbox.