[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 5)

Marc a écrit:

Frits Overmars a écrit:

all other alloy engine parts were milled from solid.

In Italy ?

In Germany.

Citation :

Gorgeous ! Thank you Frits ! But please, stop laughing at paparazzi !

We should all be glad that paparazzi exist, so we can play April fool jokes on them all year

By the way Marc, don't you ever sleep?

Yes, a lot ! But not at usual times...

Hello Frits and Jan, Marc and guys
I just forgot to thank you for this Topîc, the wonderfull pictures of this beautifull RSA 125 motor, and your explanations and pedagogy ! This Topic is very well !
Frits, thanks for answers ; I have a lot other questions... but, I didn' dare explain these because I'm a beginner ("better late than never")... and I try to understand what you an Jan explain.
I have to read 4 other parts before many other questions...
When I was young, Yves made his own 80cm3 motor with very limited budget and using Huvo cylinder and modified Huvo crankshaft (but at the time (1985-1987) I just only looked Embarassed ...), and his 80 frame and swing arm too (TYL80).
He made 2 independant cooling systems (the second one with a very smal radiator) because he wanted to have very low temperature particulary around crankshaft (when he was racing with 50 Kreidler he saw it was too hot there, and performance was falling during races (particulary gp or International races) ). But effectively with a very big radiator...Unfortunetely, the 80cm3 were stopped in 1989... Sad
What is the 125 RSA water temperature ? I think remember you say the colder the better ?
Curiously, a few years ago, when I went to Moto Legend DIJON or events like that, to see them, I realy wanted to ride with them now .... !!!!
So I only began to ride on circuit 18 months ago with my old and simple Aprilia Futura 125 cm3 Rotax 123 prepared for circuit... I learn and enjoy...
For exemple, I put a 45° Calorstat and it's seems good for circuit, it works betwenn 45-50°. What do you think about it ?
Yves is working since months to make me a 125 prototype with Rotax motor ; it's not finished, very much work on it !
So, now, I try to understand... scratch
For exemple, I hav'nt understood the advantage of Top rotative disc with axis on beautifull RSA Motor compared to side rotative disc (like RSW, if I remember photography) ?
Thanks,
Stéphane

SLT a écrit:

Hello Frits and Jan, Marc and guys. I just forgot to thank you for this Topîc, the wonderfull pictures of this beautifull RSA 125 motor, and your explanations and pedagogy ! This Topic is very well !
When I was young, Yves made his own 80cm3 motor with very limited budget and using Huvo cylinder and modified Huvo crankshaft... and his 80 frame and swing arm too (TYL80). He made 2 independant cooling systems (the second one with a very smal radiator) because he wanted to have very low temperature particulary around crankshaft (when he was racing with 50 Kreidler he saw it was too hot there, and performance was falling during races (particulary gp or International races) ). But effectively with a very big radiator...
What is the 125 RSA water temperature ? I think remember you say the colder the better ?
For exemple, I put a 45° Calorstat and it's seems good for circuit, it works betwenn 45-50°.  What do you think about it ?

Stéphane, in the 50cc and 80cc eras 70°C was considered a normal cooling water temperature. Most 50cc machines had a radiator originally designed to heat a car's interior, so their riders were already happy when they could get the temperature down to 70°C.

"The colder the better" is absolutely true. At Garelli, for example, engines being tested on the dyno were cooled with cold tap water, straight from the tap. The difference for the Garelli 125cc twin was about 2 hp per cilinder.

With a 125cc engine, the target temperature was 40°C. Even colder would have been even better for power, but during the GP season some races were run with outside temperatures above 30°C and then cooling the engine below 40°C would have required a huge radiator, which due to its air resistance would have negated the power gain. For the same reason, the target temperature for the 250cc engines was 50°C.

SLT a écrit:

I put a 45° Calorstat and it's seems good for circuit, it works betwenn 45-50°. What do you think about it?

I was not familiar with the name Calorstat, so I googled it, and it appears to be a normal open-or-closed thermostat. The 45°C treshold temperature sounds fairly good for a 125cc engine, but the problem is that it will allow zero water circulation until it reaches that temperature. Because of this zero circulation, large temperature differences will develop between the hot and cold areas of the cylinder. And when the thermostat finally opens, very cold water from the radiator is suddenly admitted to the cylinder, which can lead to deformation ans seizures.

It is better to use a three-way thermostat that always permits water circulation through the cylinder, and switches over from a cooling circuit excluding the radiator, to a cooling circuit including the radiator, when the appropriate temperature is reached.
[Vous devez être inscrit et connecté pour voir cette image]

SLT a écrit:

I haven't understood the advantage of Top rotative disc with axis on beautiful RSA Motor compared to side rotative disc (like RSW)

With a side disc the incoming mixture collides with the con rod, so there is an unwelcome 'shadow' behind the con rod.
A second disadvantage of the side disc is that the incoming mixture flows over the crankshaft instead of between the crank webs, so the big end bearing is not optimally cooled and lubricated. The RSA is better in this respect.

But the rear-mounted disc of the RSA had two disadvantages as well. The drive shaft of the disc, with only 8 mm diameter, was too flimsy, so it broke a couple of times.
The second disadvantage was that, because of the rear disc, there was no room on the RSA for the rear-exiting exhaust of the RSW.
On the RSA the cylinder had to be turned around, so the exhaust exited at the front. This made it necessary to place the engine further rearward, away from the front tire, which lead to a weight distribution that was not as good as it had been on the side-disc RSW.
Many riders who were used to the handling characteristics of the RSW, had trouble adapting their riding style to the RSA. In short: the 'old' RSW was a better handling bike.

Frits,

Thank you this is good info. Further on the topic of side vs rear entry:

Has the difference between left and right hand transfer port flow been measured and if so was it noticeable? It seems the one side has a nicely shaped transfer duct entry in the crankcase while the other side it is "destroyed" by the inlet port.

So if there is a worthwhile difference that will be another advantage to the rear inlet.

Another disadvantage of the rear inlet was the curve in the exhaust
A straight pipe gave more power.
And also the necessary long tailpipe cost some power.....
There were some carburation problems when braking too!

Wonderful pictures Frits,
Is it possible you would explain the oil pump system in greater detail please,

Why not a front disc with the carb located in front of the bottom housing or in the best place to a good inlet feeding, the exhaust will be straight? is the carb position possible or not?
Dan

Frits Overmars a écrit:

but the problem is that it will allow zero water circulation until it reaches that temperature. Because of this zero circulation, large temperature differences will develop between the hot and cold areas of the cylinder. And when the thermostat finally opens, very cold water from the radiator is suddenly admitted to the cylinder, which can lead to deformation ans seizures.

I'm not sure... many thermalvalve (2 or 3 ways) have a "little" hole in the valve to let the water flow because the sensitive element need a water flow to operate as it is placed on the outlet of the system.
Maybe, I'm wrong but with commercial bikes, it is often made that way.

Calorstat is an old trademark. We, French, use this word but most of the (modern) time it is to talk about three-ways thermostats.

Vannik a écrit:

Frits, thank you this is good info. Further on the topic of side vs rear entry: Has the difference between left and right hand transfer port flow been measured and if so was it noticeable? It seems the one side has a nicely shaped transfer duct entry in the crankcase while the other side it is "destroyed" by the inlet port. So if there is a worthwhile difference that will be another advantage to the rear inlet.

This is a question for Jan to answer. To be honest, Jan is the best person to answer all Aprilia-related questions. And all Garelli-related questions. And all Jamathi- and Piovaticci- and Bultaco-related questions. Then I can take a little forum-sabbatical

Good evening everyone, hope you all are in good health condition.

While we are talking about old GP bikes, I have 2 questions for Jan, Frits or anyone knows:
1- (I think this one is for Jan to answer) About the 50cc Bultaco of 1978: this bike finished all the races of the year while the other brands were often breaking their engines. What was the trick that made the difference?

2- About the 250/350 Aermacchi/Harley that allowed Walter Villa to clinch 4 world titles, I've read in the litterature that the transfers were using a special trick that gave their engine an advantage over the competitors, anyone knows what this trick was?

carlovitch1 a écrit:

About the 50cc Bultaco of 1978: this bike finished all the races of the year while the other brands were often breaking their engines. What was the trick that made the difference?

Jan is of course the right person to answer that question, but I suppose he is sleeping now. What I can remember is, that his Bultaco had about the same maximum power as the Van Veen Kreidler, but with more low-down power and with a lower maximum rpm, which of course improved reliability.

carlovitch1 a écrit:

About the 250/350 Aermacchi/Harley that allowed Walter Villa to clinch 4 world titles, I've read in the literature that the transfers were using a special trick that gave their engine an advantage  over the competitors, anyone knows what this trick was?  

The 'special trick' mentioned in the literature was the fifth transfer port, fed through a hollow center bridge in the inlet duct.
But I doubt if it was really that helpful. I tried it myself, and after testing it on the dyno I filled this fifth port with Devcon and removed the inlet bridge.
The engine gave more power without that bridge and port.
On the picture of the Aermacchi cylinder below you can just see the duct and the fifth port, if you know what you're looking for.
[Vous devez être inscrit et connecté pour voir cette image]

Thanks a lot, Frits for your quick and precise answers, as always.

I wonder if a book exists that would explain history of all the racing 2 stroke evolutions, from the very beginning to the last racing engines. It is often good to know where the improvements came from.

I have another question, which is maybe trivial for some of the forum members, but not easy for me: I'm actually thinking of building a 50cc prototype for having fun on our local race track, and I have hard times figuring out which cooling radiator size I need to use to keep the engine within the specified temperature (the manufacturer of the top end says it needs to run at about 50°C). Any rule of thumb I could use to get at least an idea?

The Bultaco 50 did not rev so much, done to avoid friction.
I did that after seeing a Yamaha SAE paper...

Thanks for the explanation, Jan, it makes sense, but probably not that easy to do on a very small engine.

carlovitch1 a écrit:

I'm actually thinking of building a 50cc prototype for having fun on our local race track, and I have hard times figuring out which cooling radiator size I need to use to keep the engine within the specified temperature (the manufacturer of the top end says it needs to run at about 50°C). Any rule of thumb I could use to get at least an idea? 

I have never thought about a rule of thumb for radiator sizes, but the rule of thumb for coolant temperature is simple: the colder the better,
so I would go for 40°C rather than 50°C.
Here are some pictures of Freetech50 bikes that may give you an idea about radiator sizes.The first picture shows the actual European champion.
[Vous devez être inscrit et connecté pour voir cette image]

Thank you very much Frits !
As I'm on a budget, I've been frightened by the cost of so-called "competition" radiators without having any guarantee nor indication to know whether they will match the need.

carlovitch1 a écrit:

As I'm on a budget, I've been frightened by the cost of so-called "competition" radiators without having any guarantee nor indication to know whether they will match the need.

"Competition" in racing is more or less the same as "Bio" in a grocery shop,  where it means "costs three Euro more".
Unless you are planning to become the next 50cc world champion, you might take a look at a car or motorcycle scrap yard.
You may see radiators that are too big in size for your needs, but there is no such thing as too much cooling capactiy.

Yes, Frits, this is exactly what I intend to do, I will go to the closest motorcycle scrap yard and check what I can find. From the pictures you posted, the needed size is somewhat 300 x 250mm.

Hello Frits, and Thanks for explanations !

Frits Overmars a écrit:

With a 125cc engine, the target temperature was 40°C. Even colder would have been even better for power, but during the GP season some races were run with outside temperatures above 30°C and then cooling the engine below 40°C would have required a huge radiator, which due to its air resistance would have negated the power gain. For the same reason, the target temperature for the 250cc engines was 50°C.

ok, I understand why only one cooling system ...And I 've seen your picture about actuel race 50cc radiator size...
The RSA motor used only water in cooling system  ?

Frits Overmars a écrit:

I was not familiar with the name Calorstat, so I googled it, and it appears to be a normal open-or-closed thermostat. The 45°C treshold temperature sounds fairly good for a 125cc engine, but the problem is that it will allow zero water circulation until it reaches that temperature. Because of this zero circulation, large temperature differences will develop between the hot and cold areas of the cylinder. And when the thermostat finally opens, very cold water from the radiator is suddenly admitted to the cylinder, which can lead to deformation ans seizures.

It is better to use a three-way thermostat that always permits water circulation through the cylinder, and switches over from a cooling circuit excluding the radiator, to a cooling circuit including the radiator, when the appropriate temperature is reached.

Sorry For "Calorstat" : I just used this word because it's called like that on the Rotax Parts WebSite where I bought it ! There were 2 possibilities 45 or 58°  
So, I will try (as soon as possible...) on my Aprilia the bypass with 45° 3 ways thermostat : with your explanations, I understand it's better ! Thanks

Frits Overmars a écrit:

The second disadvantage was that, because of the rear disc, there was no room on the RSA for the rear-exiting exhaust of the RSW.
On the RSA the cylinder had to be turned around, so the exhaust exited at the front. This made it necessary to place the engine further rearward, away from the front tire, which lead to a weight distribution that was not as good as it had been on the side-disc RSW.

Jan Thiel a écrit:

Another disadvantage of the rear inlet was the curve in the exhaust
A straight pipe gave more power.
And also the necessary long tailpipe cost some power.....
There were some carburation problems when braking too!

Frits and Jan ; OK I only thought about straight exaust...  but have you got an idea of Power difference on RSA/RSW for this ? I saw 125 GP Exaust Pictures, and Tailpipe seems -to me- effectively to be rather short ones ?  Could you explain me what 's the impact of tailpipe lenght and diameter ?  
Thanks,  Stéphane

SLT a écrit:

The RSA motor used only water in cooling system?

I seem to remember that according to the Technical Regulations "only water is allowed as a liquid coolant, whether or not mixed with ethanol. Other antifreeze agents are not allowed. A maximum of 2 % anti-corrosive may be added to the coolant".
The reason for excluding antifreeze is, that while a water spillage on the track can quickly be cleaned up, antifreeze is slippery and does not evaporate, so it could make the track unusable for a longer period of time.

Jan Thiel a écrit:

Another disadvantage of the rear inlet was the curve in the exhaust. A straight pipe gave more power. And also the necessary long tailpipe cost some power.

SLT a écrit:

... have you got an idea of Power difference on RSA/RSW for this ?

I have no data at hand, but I think the difference was not very big. And in some cases a pipe with a curved header actually produced more power than an straight pipe.

SLT a écrit:

I saw 125 GP Exaust Pictures, and Tailpipe seems -to me- effectively to be rather short ones ?  Could you explain me what 's the impact of tailpipe lenght and diameter ?

The tailpipe of the RSA125  is about 10 times longer than the tailpipes of the RSW125, the RSW250 and the RSA250, while all other pipe dimensions are supposed to be identical.
[Vous devez être inscrit et connecté pour voir cette image]

There is a method to minimize the influence of the tailpipe length: place a restrictor between the endcone and the tailpipe, and give the tailpipe a clearly bigger diameter than the restrictor. On the exhaust concept below you can see what I mean. D5 is the restrictor diameter and D6 is the tailpipe diameter.
[Vous devez être inscrit et connecté pour voir cette image]

Frits I'm sorry if you said that here, but I'm curious about your opinion on the reasons for an engine with a compression ratio of 9: 1 for example, having better performance, I've used 10.5: 1 and it worked well, I have some thoughts about it, however your explanations always make more sense to me than my thoughts

my guess his answer will be :
less compression means less of the energy is used to push the piston down, so there is more energy left in the exhaust gases and they make your pipe work harder, pushing more fresh charge in the cylinder and more mixture in the cylinder makes the engine more powerfull.

but I'm not a good "guesser" so he may come up with another explanation ...

LeonardoMRF2 a écrit:

Frits I'm sorry if you said that here, but I'm curious about your opinion on the reasons for an engine with a compression ratio of 9: 1 for example, having better performance, I've used 10.5: 1 and it worked well, I have some thoughts about it, however your explanations always make more sense to me than my thoughts

JanBros a écrit:

my guess his answer will be :
less compression means less of the energy is used to push the piston down, so there is more energy left in the exhaust gases and they make your pipe work harder, pushing more fresh charge in the cylinder, and more mixture in the cylinder makes the engine more powerfull.
But I'm not a good "guesser" so he may come up with another explanation

Your guess is as good as mine, Jan