[2 stroke] Weight or balancing factor

Hy guys,
I need your advice concerning a 125 cc single cylinder crankshaft without conterbalancing.

i changed the standard crank from

Total weight crankshaft : 2705 gr.
Conrod:             106mm
Rod/Stroke ratio: 1,947
Weight nacked conrod: 117,20 gr.
Conrod part alternativ: 40,80 gr.
Crankpin+2silver+cage: 149,90 gr.
Conterbalancing weight Mx: 100,44 gr.
Weight piston complet: 174,90 gr.
Balancing factor calculated: 65,5%
to
Total weight crankshaft : 2345 gr.
Conrod:                     106 mm
Weight nacked conrod: 117,20 gr.
Conrod part alternativ: 40,80 gr.
Crankpin+2silver+cage: 131,10 gr. (crankpin with hole 6mm)
Conterbalancing weight Mx: 101,20  gr.
Weight piston complet:        161,00 gr.
Balancing factor calculated: 70,9 %

In order to reach this balancing factor i had to place the tungsten very outside of the crank, so the inertia is higher with less weight.

Now the engine works so much better from 9500-13000 rev/min, it revs up much quicker, the braked dyno marks + 2,5 hp in that range
but it seems that the engine has to much problems to "pass" over to higher revs, like against a wall and loosing more than 10 hp in the upper range 13300-14500 rev/min.

To much inertia or to less weight, what do you think?

Thanks a lot.

Hi Vortex

I get your crank balance factor as 70.37% in the second instance.

(101.2+40.8)/(161+40.8)x100 = 70.366

By changing the balance factor you alter the rpms at which the engine (and rider!) "feels" the worst vibrations. This is affected by the surrounding environment i.e. method of engine mounting, angle of cylinder axis to chassis axis and rider axis. Stiffness characteristics of the chassis, etc.

Hence there is no such thing as the correct crankshaft balance factor, it's just what works best for your engine/chassis/rider and the best solution can only be found by trial and error, but from previous experience we can make an "educated guess".

What I believe is probably happening with your engine is that you are now hitting a critical vibration at 13,300 that is affecting the carburation very badly and hence engine power, to such an extent that it can't pull through that period. The rider would usually feel that and hear a change in engine note.

I believe that a further increase in crank balance factor to 75/77% may prove beneficial.

In this instance I don't believe that crankshaft weight/inertia is a factor, unless crankshaft stiffness is also affected.

Perhaps if you name the bike, others will be able to be more specific from actual experience.

Best Regards

Ian 

Hello Ian,
Thanks for your reply.
The concerned engine is a 125cc shifter kartengine with
Your theory with the resonating shaft is not bad but if running the engine on my dyno i'm a least 1 meter away from the engine and i can't here no strange noise.
Very strange to is that it seems that the engine makes in this rpm range less noise, smoothy passing but loosing so much power.
The applicated force i'm using on my dyno is 275 N fixe.
With the standard crank factors no problem to pass over the 14500 rpm with that charge but with the modificated values no chance to pass 13400 rpm because the power falls quit fast from 45 to under 30 hp.
The same cylinder with a stdr. crank has more than 15 hp more in that range but loosing on the hole range more than 2,5 hp from 9500- 13000 rpm, the same with the tork.

Best regards.

Ian Harrison a écrit:

there is no such thing as the correct crankshaft balance factor, it's just what works best for your engine/chassis.

+1 (unless your engine has a balance shaft, in which case you should be aiming for a 50% to 60% balance factor.

Citation :

What I believe is probably happening with your engine is that you are now hitting a critical vibration at 13,300 that is affecting the carburation

+1 again. A simple test might be to stick some tire balancing weights (the sticky kind, for light-alloy wheels) onto your carb. This will change its mass and hence its resonant vibration frequency. Your problem may become either less or worse but at least then you'll know where to look.

Thank you Ian, no balanceshaft in the engine.

Thank you Frits, very good idea with the balancing weights, so the frequency will change an i may see if the
problem occurs in a other rpm range.

Again, thank you both.

Vortex a écrit:

To much inertia or to less weight, what do you think?

Thanks a lot.

What kind of dynamometer measures this result? Inertial dyno is unsuitable for this measuerement. Brake dyno only be considered by different inertial measurements.

gpracing a écrit:

Vortex a écrit:

To much inertia or to less weight, what do you think?

Thanks a lot.

What kind of dynamometer measures this result? Inertial dyno is unsuitable for this measurement. Brake dyno only be considered by different inertial measurements.

Hi GPracing,
A "through the gears" run time (let's say between 9,000 rpm in 2nd to 13,500rpm in 6th with gearchange at 13,500rpm) on a load control dyno set with a load that increases with the square of the speed (simulating wind load), will give a very good comparison. However, consistency of result will generally depend on driver/rider input also being consistent.

The name of the dyno you are really looking for is "race track" and the readout is in lap times, not Hp or Kw  But you should also listen to what the rider/driver says about rideability/driveability. It's more about 10 fast laps than 1.
Of course if your driver/rider is "finished" after 5 laps, he may have just had too much beer the night before  

1. I believe that crankshaft "weight" or "mass" on it's own is a red herring unless you are thinking about weight reduction of the vehicle as a whole.
2. I believe that ideally you want the lightest possible crankshaft, with the optimum inertia. The engine will only "feel" inertia, the tyres will feel weight. Traditionally weight and inertia went hand-in-hand, but with aluminium and tungsten plugs that's not (totally) the case. (It's all about weight x radius).
3. I believe that you want the best aerodynamics possible for the crankshaft/con-rod (including resistance/friction imparted by proximity to the crank chamber walls.
4. I believe that primary compression (crankcase volume)needs to be smaller and is more critical for a reed valve engine than for a disc valve engine.
4. I believe that the optimum inertia is the one that (all other things being equal, whatever that means!) gives the best lap times, which will be achieved with a trade-off, (in crankshaft terms), between weight/inertia/unloaded engine response (gearchange times up & down) and torque available at the point of selecting the next gear on acceleration.

Sorry, that's a very long answer to a very short question, and some may want to disagree with some or all of that!

Best Regards

Ian  

@Ian:
For sure, you're right. But comparing different crankshafts on braked dyno is for me one opportunity more trying to understand this complex stuff, ok?
@Gpracing:
My dyno is braked, i did apply 275N fix.

Thank you guys.

Ian Harrison a écrit:

3. I believe that you want the best aerodynamics possible for the crankshaft/con-rod (including resistance/friction imparted by proximity to the crank chamber walls.
4. I believe that primary compression (crankcase volume)needs to be smaller and is more critical for a reed valve engine than for a disc valve engine.

I think similarly. But i think the primary compression (e.g. compression ratio) is a static parameter, not well characterized a dynamic system. Preferably to handle like as electrical resonant circuit. Cannot be perfect volume, only good for requirements. Between rational values have no better, but other.
In addition important the position of crankase volume, but consequence this is for 3. point.

To braked dyno, i think true brake dyno, ergo the drive in and the brake torque is in an accurate ballance, with few operation point. But "through the gears" measurements is better than customary inertial method.

Best Regards

(I can't long answer, because I can't English well :) )

Vortex a écrit:

Hy guys,
I need your advice concerning a 125 cc single cylinder crankshaft without conterbalancing.

i changed the standard crank from

Total weight crankshaft : 2705 gr.
Conrod:             106mm
Rod/Stroke ratio: 1,947
Weight nacked conrod: 117,20 gr.
Conrod part alternativ: 40,80 gr.
Crankpin+2silver+cage: 149,90 gr.
Conterbalancing weight Mx: 100,44 gr.
Weight piston complet: 174,90 gr.
Balancing factor calculated: 65,5%
to
Total weight crankshaft : 2345 gr.
Conrod:                     106 mm
Weight nacked conrod: 117,20 gr.
Conrod part alternativ: 40,80 gr.
Crankpin+2silver+cage: 131,10 gr. (crankpin with hole 6mm)
Conterbalancing weight Mx: 101,20  gr.
Weight piston complet:        161,00 gr.
Balancing factor calculated: 70,9 %

In order to reach this balancing factor i had to place the tungsten very outside of the crank, so the inertia is higher with less weight.

Now the engine works so much better from 9500-13000 rev/min, it revs up much quicker, the braked dyno marks + 2,5 hp in that range
but it seems that the engine has to much problems to "pass" over to higher revs, like against a wall and loosing more than 10 hp in the upper range 13300-14500 rev/min.

To much inertia or to less weight, what do you think?

Thanks a lot.

hi guys

can someone explain pleas what is
Conrod part alternativ
Counterbalancing weight Mx

thanks Manuel

Manuel Rainer a écrit:

hi guys

can someone explain pleas what is
Conrod part alternativ
Counterbalancing weight Mx

thanks Manuel

Conrod part alternative
I would call this upper conrod weight. If you put the crankshaft on the bench so that the rod is level and the little end eye resting on a precision scale, this is the weight you will read of the scale. It's considered the reciprocating part of the conrod.

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See below:

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Counterbalancing weight Mx.
If you place the crankshaft on rollers or knife edges, it is the weight that you have to hang from the little end eye so that the crankshaft will stay at any position and not rotate. This is usually formed by a small cloth bag filled with lead shot tied through the little end eye. Just add or remove lead shot to adjust the weight. Another way is to use nuts on a string or wire. See below:

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The formula to calculate the balance factor in this way, is:

(Counterbalancing weight Mx+Conrod part alternative)/(Piston Kit Weight+Little End Bearing weight+Conrod part alternative) x 100 = Crankshaft Balance Factor

Best Regards

Ian 

hi

thanks Ian nou its clear. i was always only measuring the Counterbalancing weight Mx. in Italy we cal it "Tiro".

what is the reason of asymmetrical assembly of the Counterbalancing weight Mx? linke in the RSA 250. has ist to do with the angles of the cylinders?

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Thanks Manuel

Vortex a écrit:

Hy guys,
I need your advice concerning a 125 cc single cylinder crankshaft without conterbalancing.

i changed the standard crank from

Total weight crankshaft : 2705 gr.
Conrod:             106mm
Rod/Stroke ratio: 1,947
Weight nacked conrod: 117,20 gr.
Conrod part alternativ: 40,80 gr.
Crankpin+2silver+cage: 149,90 gr.
Conterbalancing weight Mx: 100,44 gr.
Weight piston complet: 174,90 gr.
Balancing factor calculated: 65,5%
to
Total weight crankshaft : 2345 gr.
Conrod:                     106 mm
Weight nacked conrod: 117,20 gr.
Conrod part alternativ: 40,80 gr.
Crankpin+2silver+cage: 131,10 gr. (crankpin with hole 6mm)
Conterbalancing weight Mx: 101,20  gr.
Weight piston complet:        161,00 gr.
Balancing factor calculated: 70,9 %

In order to reach this balancing factor i had to place the tungsten very outside of the crank, so the inertia is higher with less weight.

Now the engine works so much better from 9500-13000 rev/min, it revs up much quicker, the braked dyno marks + 2,5 hp in that range
but it seems that the engine has to much problems to "pass" over to higher revs, like against a wall and loosing more than 10 hp in the upper range 13300-14500 rev/min.

To much inertia or to less weight, what do you think?

Thanks a lot.

hi Vortex

in our Tm 125 Kart engines we have a
Conrod part alternativ: 46 gr
Weight piston complete:  170 gr.
an a Counterbalancing weight Mx: 70-85  gr.

it gives a Balancefactor of 53-60% and it works good. i think your Counterbalancing weight Mx of 100 is a bit to much, at all with your light piston.

Manuel

Manuel Rainer a écrit:

hi Vortex

in our Tm 125 Kart engines we have a
Conrod part alternativ: 46 gr
Weight piston complete:  170 gr.
an a Counterbalancing weight Mx: 70-85  gr.

it gives a Balancefactor of 53-60% and it works good. i think your Counterbalancing weight Mx of 100 is a bit to much, at all with your light piston.

Manuel

Hi Manuel, How can your counterbalancing weight be 70-85 grams? It will be a specific value and you should be easily able to measure within 2 grams on a good set of parallel beams or knife-edge rollers. (unless you are using different cranks/rods/pistons)

Of course a 125 is never too bad for vibration, but we have run balance factors of up to 77% on non balance-shafted engines in a kart chassis with good results.

Best Regards

Ian 

Ian Harrison a écrit:

Manuel Rainer a écrit:

hi Vortex

in our Tm 125 Kart engines we have a
Conrod part alternativ: 46 gr
Weight piston complete:  170 gr.
an a Counterbalancing weight Mx: 70-85  gr.

it gives a Balancefactor of 53-60% and it works good. i think your Counterbalancing weight Mx of 100 is a bit to much, at all with your light piston.

Manuel

Hi Manuel, How can your counterbalancing weight be 70-85 grams? It will be a specific value and you should be easily able to measure within 2 grams on a good set of parallel beams or knife-edge rollers. (unless you are using different cranks/rods/pistons)

Of course a 125 is never too bad for vibration, but we have run balance factors of up to 77% on non balance-shafted engines in a kart chassis with good results.

Best Regards


Ian 

hi Ian

i mean in the older engines were the cylinder is a little bit more inclined forward the counterbalancing weight is about 70g an in the newest with the cylinder nearly vertical the counterbalancing weight is about 85g.

Ian how does it has to be with a balanceshaft? what does it does? hat it to have the contrary factor or something of the cranc?

Thanks Manuel

Manuel Rainer a écrit:

hi Ian

i mean in the older engines were the cylinder is a little bit more inclined forward the counterbalancing weight is about 70g an in the newest with the cylinder nearly vertical the counterbalancing weight is about 85g.

Ian how does it has to be with a balanceshaft? what does it does? hat it to have the contrary factor or something of the cranc?

Thanks Manuel

Hi Manuel

Ah OK, I see now, that makes sense.

With a balancer shaft, the crankshaft is balanced to a 50% balance factor and so is the balancer shaft. The balance shaft rotates in the opposite direction to the crankshaft and in phase (just like the 2 crankshafts in a tandem twin).

The forces are always in opposite directions, so cancel each other out. However it is not usually possible to 100% cancel out the forces of the crankshaft (which is much bigger), so you would usually counterbalance say around 70% of the crankshaft forces. This makes a much smoother engine.

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A slight rocking couple is introduced due the distance between the crankshaft and balance shaft centre. Ideally you would have 2 balance shaft equally either side of the crankshaft, but this is becoming more complicated for very little gain.

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Best Regards

Ian 

Ian Harrison a écrit:

Hi Manuel

Ah OK, I see now, that makes sense.

With a balancer shaft, the crankshaft is balanced to a 50% balance factor and so is the balancer shaft. The balance shaft rotates in the opposite direction to the crankshaft and in phase (just like the 2 crankshafts in a tandem twin).

The forces are always in opposite directions, so cancel each other out. However it is not usually possible to 100% cancel out the forces of the crankshaft (which is much bigger), so you would usually counterbalance say around 70% of the crankshaft forces. This makes a much smoother engine.

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A slight rocking couple is introduced due the distance between the crankshaft and balance shaft centre. Ideally you would have 2 balance shaft equally either side of the crankshaft, but this is becoming more complicated for very little gain.

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Best Regards

Ian 

Hi Ian thanks a lot

if there is a balance shaft with the same diameter it would work with the same balance factor?

how can i measure the factor from the balance shaft?

has it to have the same counterbalancing weight like the cranc?

thanks Manuel

Manuel Rainer a écrit:

Hi Ian thanks a lot

if there is a balance shaft with the same diameter it would work with the same balance factor?

how can i measure the factor from the balance shaft?

has it to have the same counterbalancing weight like the cranc?

thanks Manuel

Hi Manuel

Perhaps you should read this article from Tony Foale Design. The crankshaft balances out 50% of the reciprocating force. The balance shaft (ideally) balances out the other 50% and cancels out the rotational forces.

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Here is a print from the 3D model our new 250cc single cylinder Superkart motor. You can see the left-hand balance shaft weight, which runs externally.

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Best Regards

Ian 

Ian Harrison a écrit:

Here is a print from the 3D model our new 250cc single cylinder Superkart motor.
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Nice job Ian. I wonder: is that a brake disk fixed on the secundary transmission shaft or were you just too lazy to draw all those teeth ? 

Ian Harrison a écrit:

Hi Manuel

Perhaps you should read this article from Tony Foale Design. The crankshaft balances out 50% of the reciprocating force. The balance shaft (ideally) balances out the other 50% and cancels out the rotational forces.

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Here is a print from the 3D model our new 250cc single cylinder Superkart motor. You can see the left-hand balance shaft weight, which runs externally.

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Best Regards

Ian 

hi

Thanks Ian i'm gonna read this article.

and nice work.

thanks Manuel

hi

Frits what is the reason of the asymmetrical assembly of the Counterbalancing weight Mx like in the RSA 250. has ist to do with the angles of the cylinders?

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Thanks Manuel

Frits Overmars a écrit:

Nice job Ian. I wonder: is that a brake disk fixed on the secundary transmission shaft or were you just too lazy to draw all those teeth ? 

The drawing skills are not mine, Frits and many thanks for your compliment  . I'm sure Paul would hit me over the head with something very heavy, if I were even to suggest he was lazy, after the 100's of hours that he spent putting my dreams into a form from which we could create the reality, including his technical input into the design. You can now indeed see that we don't have an extra disc brake!!   

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The cylinder is tilted rearward so that the inlet is raised higher above the (shrouded) crankshaft, to minimise any negative effect from the crankshaft rotating against the inlet stream.

A big benefit will be the front-mounted intake, lending itself to a ram-air system and rear-exiting exhaust, allowing a much better expansion chamber shape when installed in a Superkart.

Regulations do not allow a disc-valve intake system.

We should be dyno running next week and working specifically on exhaust and ignition, with a track debut of March 15th at Silverstone.

Cylinder/Porting design is inspired by Aprilia (who else!).

Beautiful CNC work on the cases from Harrison Precision Engineering (HPE)!
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Best Regards

Ian 

Manuel Rainer a écrit:

hi

Frits what is the reason of the asymmetrical assembly of the Counterbalancing weight Mx like in the RSA 250. has ist to do with the angles of the cylinders?

Thanks Manuel

Hi Manuel, thanks for your compliment on my engine.

I think that you are confusing "counterbalacing weight Mx" (which is the weight that you have to hang off the conrod so that the crankshaft will rest in any position on parallel bars), with the actual crankshaft balance weights (tungsten and alloy inserts, etc.).

On the 90 degree V-Twin if the crankshafts were normally balanced the forces would be acting at 90 degrees to each other, not cancelling each other out (like on a tandem twin), so the crankshafts are assymetrically weighted to achieve that result.

Best Regards

Ian

nice work ian. does the cylinder stud pattern happen to be similar to ktm 250 ?   

brokedown a écrit:

nice work ian. does the cylinder stud pattern happen to be similar to ktm 250 ?    

It's the same as the 2002-2007 Crankcase Induction Honda CR250, so the cylinder and crank also fit the CR250.

68 x 68.8 bore and stroke with 128mm rod.

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Best Regards

Ian