[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 4)

Thank you Jan

Jan or Frits : has it ever been tried to tune each cylinder on a twin differently, like one for more top-end and the other for more mid-range, to obtain an engine with a wider spread power-band , easier to ride and combine a bit of both worlds ?

No, I never tried this!
Quite the contrary, I tried all cylinders and pipes separately.
And made pairs of the most similar ones!

JanBros a écrit:

Jan or Frits : has it ever been tried to tune each cylinder on a twin differently, like one for more top-end and the other for more mid-range, to obtain an engine with a wider spread power-band , easier to ride and combine a bit of both worlds ?

O yes, it has been tried. But not by me. It would mean that one cylinder is always forced to run in an rpm area where it is not happy. It's more effective to give both cylinders equal broad power bands.

tnx. not that I was planning on trying it out - I don't have the funds to experiment, was just wondering.

JanBros a écrit:

Jan or Frits : has it ever been tried to tune each cylinder on a twin differently, like one for more top-end and the other for more mid-range, to obtain an engine with a wider spread power-band , easier to ride and combine a bit of both worlds ?

Harry(probably Stefan too) Everts would give you a kick in the shins for not remembering the famous Puch 250(disc and piston port induction)that Frits pulled apart and Harry won a world title on.  

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hi frits what do you think about this. i have 2 pistons with different designs for my engine. you can see one leaves a open area bewteen the wall and piston side at bdc and the other does not.  when that gap opens im sure the air is moving fast to reach the cylinder so do you think the air can jump that gap and continue upward through the duct without much problem ? i like the full circle piston better but the skirts are thinner so i think its more prone to cracking

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Those gaps not only leave an open area at bdc, they also create an undesirable connection while passing over the A-transfer ports and the auxiliary exhausts ports or the 'ears' of a T-exhaust port. So I do not like those pistons at all.
Does their brand name start with a W?

look at this one frits. i dont know what the wiseco engineers could of been thinking. the left one would be a very bad choice   

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frits i forgot to ask earlier but im wondering about the rsa125 bearings. if i recall correctly it used cylindrical roller bearings for the crank shaft. did it also use cylindrical roller bearings in the transmission, atleast the ones supporting the front sprocket and clutch basket ?  and what bearing manufacturer did rsa125 use ?  thnx

brokedown a écrit:

l im wondering about the rsa125 bearings. if i recall correctly it used cylindrical roller bearings for the crank shaft. did it also use cylindrical roller bearings in the transmission, atleast the ones supporting the front sprocket and clutch basket ?  and what bearing manufacturer did rsa125 use ?  thnx

You will find a lot of answers here:
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skf germany is a great bearing.  thats what im using in one of my engines. ive made it all cylinder roller bearings.  i have another engine with mostly japanese ball bearings but im going to exchange them for skf germany roller style    

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Hi Jan. I built a motor from various parts with a 75.8 stroke and an 85.0 bore
As you can imagine it shakes like a drunk alcoholiccrack addict in detox
I plan to install a balancer shaft arrangement. My question is:
Is it worth it to install two shafts (one in front/ one behind the crankshaft or is the added complexity worth the small gain? What balance factor would you recommend? I have heard everything from 50% to 75%. I would assume the balance factor of the balancer should be the same as the engine?
I would like to spin it up to 8500 rpm

Hi Howard

In a balance shafted motor the balance factor of the crankshaft assembly should always be 50%.

This value gives the minimum resultant unbalanced force, which is of constant magnitude
throughout the revolution of the crankshaft, although rotating in the opposite direction.
A constant rotating force can easily be balanced by another constant rotating force spaced 180°
apart. However, it is not as easy as adding another counterweight to the crankshaft, because this
force rotates in the opposite direction, and therefore can only be balanced by a counterweight rotating in the opposite direction. This is the reason that balance shafts rotate in the opposite direction to the crankshaft.

As you will know the balance factor is the percentage of the reciprocating weight (piston assembly plus little end weight of con-rod) that is balanced by the crankshaft.

The balance shaft does not have a balance factor as there is no reciprocating weight to balance out. It is just an unbalanced rotating shaft that contra rotates to the crankshaft, so that when the piston is at TDC the centre of balance of the balance shaft is at BDC (same as the crankshaft).

The balance shaft creates the necessary force to balance the reciprocating force when at TDC. At 90° rotation there is no reciprocating force but because the balance shaft rotates in the opposite direction the two centrifugal forces cancel out, leaving zero unbalance force through a full revolution.

I believe that the twin counterbalance shaft assembly is not worth the extra cost and complexity and the drive losses and any churning losses outweigh any benefit.

You may choose not to fully counterbalance as this can require very big and heavy counterbalance shafts (weights). It basically depends on what the driver/rider feels is acceptable and what happens in terms of the weight of the engine and the effect on power output.

Here are 2 images of our balance shaft assembly.

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Best Regards

Ian 

Frits Overmars a écrit:

You will find a lot of answers here:
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hi frits i hate to bother you but i have a problem. i tried to look at the rsa125 engine manual on the aprilia site and see if it says the crank bearing dimensions but the manual appears to be a xls file, which needs microsoft excel to open i think ?  i dont have excel on my computer and cant find a way to open the file.  do you know right off hand the bearing number and what its dimensions are ? width, OD, and ID of the bearing race. thnx for the help

@ Brokedown : if you google " free .xls viewer " , you'll find lots of links, including an official Microsoft oe :

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very good job janbros i got it to work   . but its in a foreign language so im not sure i read it correct.  6304 20x52x15  does that seem like the crank bearings ?

i look at it more but i dont think it says the crank bearing dimensions. COM02105 is the part # but it doesnt say anything else

Hello Frits,
The link you posted, [Vous devez être inscrit et connecté pour voir ce lien]
This requires a login password, Would you please PM me the login ?

RAW a écrit:

Hello Frits,
The link you posted, [Vous devez être inscrit et connecté pour voir ce lien]
This requires a login password, Would you please PM me the login ?

Copy and paste the full link?:

[Vous devez être inscrit et connecté pour voir ce lien] and Manuals/02_2-stroke

hi frits im wondering about the height relationship between the main exh and the auxillaries. from the rsw drawings the main window looks slightly over 2mm higher than the aux, im guessing thats around 2 degrees but not sure. im assuming the rsa125 was the same as for the exh windows being staggered the same. im wondering about a explanation for this height difference. i figured it would be best to have all the windows open at the same time but that doesnt appear to be the case. is it because the pressure drop would be too quick and possibly cause a brief stalling affect if the windows are the same height ? or is it something to do with a unfavorable situation when the pipe is pushing washed mixture back into the cylinder ? thnx

I hope this answers your question.

'The drawback of having auxiliary ports that are higher than the main exhaust, is that the bulk of the heat will flow through the aux. ducts, whose cross-section area / wall surface ratio is worse than the main duct's, so more heat is fed into the cylinder.'

I guess this is the same reason why you would lower them even more.

(Frits overmars said this here:
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is there no other reason besides more heat in the cylinder ?

brokedown a écrit:

hi frits im wondering about the height relationship between the main exh and the auxillaries. from the rsw drawings the main window looks slightly over 2mm higher than the aux, im guessing thats around 2 degrees but not sure.

No need to guess, Brokedown. The stroke is 54,5 mm; the con rod length is 115 mm for the RSW and 120 mm for the RSA, so you can do your own calculations.

Citation :

i figured it would be best to have all the windows open at the same time but that doesnt appear to be the case.

The staggering results from Jan's experimental work in which he was forced to adapt the cylinder to exhaust pipes designed by the Great Leader  .
I have seen good results from equal timings for all exhaust ports. Raising the auxiliaries to 196° allowed me to lower the main port to also 196°.
A word of warning for over-zealous tuners: for engines with lower piston speeds than the RSW/RSA (23,6 m/s at max.power rpm of 13000) I would keep the exhaust timing even lower.

Remember, exhaust timing is a compromise. About 192° seems optimal for exhaust resonance, but this limits the blowdown angle.area to such an extent that the engine can only produce its maximum torque at limited revs.
Widening the ports instead of raising them is hardly possible in conventional cylinders because the total bore area is already occupied by ports. In my FOS cylinder I have much more total port width which allows me to run very high piston speeds with 192° exhaust timing and 130° transfer timing.

Hello Frits
As you have raised your FOS cylinder / engine. How is it all coming along ?

RAW a écrit:

Hello Frits
As you have raised your FOS cylinder / engine. How is it all coming along ?

Much slower than I would like. I really need to win the lottery; then I'll be able to farm out some of the work  .