Two-stroke performance tuning?

[SS90]

No SS90, it's GOD....

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[TZ350]

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SS90 How did you bring this on yourself?

Here is something to brighten you up. http://www.youtube.com/watch?v=7__-s6VsmAM

I am off to somewhere saner for the rest of the evening.

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[Kickaha]

Now, I first read the Phil "slide rule" Irvine book when I was 18.

If I remember correctly, it was first printed around 1965.

It was first published in 1948 and then other editions in 1949,1951,1952,1956 and 1960

Doesn't everyone have a copy?

[SS90]

It was first published in 1948 and then other editions in 1949,1951,1952,1956 and 1960

Doesn't everyone have a copy?

Cripes!

Your right! things have moved on a bit since then.......... the basics are still the same, certainly, but really our understandings of different effects has improved somewhat.

Like I say, old book, old engine.

I must say, I am looking forward to Thomas's promised drawings/explanations!

[FastFred]

Like I say, old book, old engine.

The crankshaft goes round and round, the piston up and down.

I guess things have moved on and modern engines do it differently.

Can SS90 explain how, we are all waiting.

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[SS90]

The crankshaft goes round and round, the piston up and down.

I guess things have moved on and modern engines do it differently.

Can SS90 explain how, we are all waiting.

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Well Fastfred, your quite right...... round and round, up and down..... however it's not so much that those things have changed, it's more that our understandings of the effects of things like-: primary compression ratios,type of mounting systems,weights of flywheels,RPM,intended use,(even who is riding) angle of cylinders ("laydown" or "upright" cylinders) and to a certain degree....type of expansion chamber design (and even ignition type) .....have improved. And as such different balance factors are applicable.

I hope that speedpro kept his figures from the crank he did, because we can (very quickly) calculate his balance factor he ended up with (using the equation I posted last week)

Speedpro also correctly said that there is an element of "suck it and see" with balance factors.....

like many things with a two stroke engine, it's about a complete system, rather than "one for all", and experience is everything.

I have to get up early tomorrow and we are driving 200KM for the first Race meeting of the year, (some place called "Dettendorf"...thank god for GPS....)so I don't have time to elaborate, but perhaps on Sunday night I can write a few things from my experiences.
That's Monday morning NZ time, so something for the new week.

It's not exactly "revolutionary",(and I will certainly never claim the credit for these (or any other) tecniques.....I have actually found over the last few years that every time you think you have found something new, a little research shows you that someone else thought of it before you....a long time ago generally!

Like I say, this engine DOES vibrate a little (but less than the full circle crank does...which, by the way, vibrate TERRIBLY!)..........

Enjoy your weekend!

Cervus!

[bucketracer]

(60g+34g)/(60g+315g+10g)=25% (using Bucketracers suggested 3 decimal places method), other wise it would be 24.5%

Well Fastfred, your quite right...... round and round, up and down..... (using the equation I posted last week)

Just a little thing but, A+B=C is an equation a string of numbers added and divided is an arithmetic sum.

Knowing the difference is probably important, before trying to give us an insight into the science of balance factors.

SS90 which branch of classical mechanics will you use to paint the picture for us.

Will it be kinematics, which is not to be confused with another branch of classical mechanics: analytical dynamics (the study of the relationship between the motion of objects and its causes), sometimes subdivided into kinetics (the study of the relation between external forces and motion)

And just a reminder, it was not decimal places. I suggested working with the "significant numbers" just like the profesionals do, if you want accuracy in measurement and calculation.

We look forward to your insights.

[speedpro]

Balance factors - so simple on a single. A mass going up and down generating forces which we try to negate with something going round and round also generating forces. When you get down to it the whole idea is pretty stupid. It can't work except at 1 speed and on a single the rotating bit generates forces whose direction varies as the crank rotates. Engineers have lost their jobs for coming up with ideas like that. BMW had a better idea, they balanced their single with a more or less reciprocating mass.

When I balanced my MB crank on the jig I just measured the weights and divided one by the other and multiplied the result by 100 to get %. An old guy I met who used to race big singles suggested the method. If you get right into the maths it'll drive you crazy with vectors, mass, square laws, and on and on. Hence my little jig and scales. As SS90 says though, if you were to balance a TS100 motor the same as a MB100 you will get a different effect due in this case to the difference in cylinder angle. It'll still be balanced exactly the same if all the bits of metal weigh the same but once fitted to the bike it would feel different. That might be why Honda fitted a balance shaft - to get around the problem. A single is going to have forces generated at 90deg to the centre line of the bore by the crank that aren't opposed by the reciprocating forces. On the TS they are essentially horizontal whereas on the MB because of the sloping cylinder they will have a vertical component which will be noticed more by a rider.

[FastFred]

From what I can see SS90 has not brought up anything Thomas and his KISS method has not already covered here or on the ESE or can be found in Phill Irvings book.

I look forward seeing what SS90 has to offer in the way of new ideas.

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[k14]

Can SS90 explain how, we are all waiting.

Maybe he needs to go and ask Thomas?

[Buckets4Me]

Jeckle and Hide
or Thomas and SS90 ??????

which one is nice and who is the prat ?

[sonic_v]

Well, that's what i am trying to establish some figures for...... like the figures you posted.......a "modern" TZ250 is 1.40:1......an RS125 1.33:1

From memory (I have owned and raced both these models) A 1991 TZ250B revs till 11,000 (primary compression ratio 1.40:1) a 1995 RS125R revs to 13,000 (standard, A kits more), and have a primary compression ratio of 1.33:1

Hmmmm,

I see a pattern forming here!

Re-read Gordon Jennings chapter on Crankcase volume (he mentions the same SAE paper there)

He also gives some insight and opinion on the reasons people increased primary compressions in the first place.

Have you seen any modern literature that gives minimum figures for primary compression ratios (for different engine set ups (operating RPM,capacity exhaust design etc?)

The TZ250B v-twin yamaha give peak power at 12,000 revs/min and could produce useful power to close on 13,000. The fuel power jets did not even switch of until about 12,200 revs/min. Just concerned that you might be drawing conclusions based on 11,000 revs/min.

[SS90]

From what I can see SS90 has not brought up anything Thomas and his KISS method has not already covered here or on the ESE or can be found in Phill Irvings book.

I look forward seeing what SS90 has to offer in the way of new ideas.

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O.K

Like I said, these aren't "new Ideas".....but it would seem that it is new to (almost, certainly not all) people who contribute to these series of threads.

I'm not really sure where to start, but I guess the best place is the confusion over the selection of balance factors (Speedpro has covered the basics of the process you can all do at home as regards to calculating the balance factor, so perhaps I should touch on the chosen factor itself)

Maybe it would be best to use the crank design I posted pictures of as an example.

Thomas has claimed that the factor I ended up with was something like 80 to 85%...... I suspect he arrived at this (incorrect) figure through lack of experience.

Here is why.

When you take a "full circle" two stroke crank (let's use a CR125 as an example in this instance, because I have personally checked this crank for this particular feature....(Honda RS125's are the same by the way) the crankwebs are in fact, not a "solid web"......they are actually "hollow" around the pin area.

This is why some two stroke cranks (Cr125, RS125) have a thin steel outer "shell"
.... that is to say...... just because the webs look like they are spun from solid stock, they are actually "cast", and the area around the pin is actually hollow.......(this is not for ALL crankwebs, but is common on most "modern" full circle crank webs....I suspect that PERHAPS MB100's are NOT like this (due to the balancing shaft), but the truth is I don't know.....If the crank is "completely round" i.e no "wasted sections" on the inside of the crank web, then chances are that it is hollow around the pin area)

My experience is if the crankwebs have a thin sheet metal out cover (rs125 Cr125 etc) then they are indeed hollow around the pin area.

K14, do you have an old RS125 crank that is ruined? maybe you could remove the tin covers from the webs and post some pics?

This, is to have a "full circle crank web", AND still maintain an acceptable balance factor, while having the "full circle" webs increase the crankcase volumes to where they want it. (by the way, this is normally quite a high primary compression ratio, as the engine's are generally designed to run over a wide RPM range (but all be it at a lower RPM) (or in the case of a commuter bike, run with out an expansion chamber (just a muffler and header pipe), both of which scenarios require "higher" primary compression ratios.

Now, the picture of the crank I posted "full circle" crank (aftermarket tuning part, made only to increase the primary compression) IS MADE FROM SOLID STOCK...(It's cheaper to manufacture)....and as such, the balance factor is 25%.....(because it has more material around the pin....it's not hollow).

If, like (most) Japanese two strokes, the webs where hollow (around the crank pin)......the balance factor would be something like 50% (near enough anyway)

Fact is, it's not, so LUCKY LUCKY me, (through experience) the very thing I need to do (increase crankcase volume (achieved by removing crankweb material)) also puts the crankshaft balance factor significantly closer to where it needs to be!

42% (with the piston assembly I am using)

Lucky me.

All I need to do, is remove the material I want (to increase crankcase volume) from around the crankpin area.

This, like I say, is why I would remove MORE material from around the crankpin, except for the fact that I fear this would weaken the assembly too much.

OR, like Speedpro correctly pointed out, add some Mallory Metal slugs to the opposite side

The end result is the same

So, there is the answer......most modern "full circle" cranks are hollow around that area.

I am not saying that ALL cranks are like that (anyone gott a full detailed view of a Suzuki GP125 crank (split from it's pin?) if it is "full circle", I would suspect that it to would be hollow in this area, if not, it will be wasted around the inside of the web (around the pin area).....

Which also helps lubricate the pin area.

If anyone has a wrecked one (CR125 or MB100 or RS125R) then perhaps they could section it right the way through..... then we will know.

Now, selection of balance factors.

That really is "suck and see"

Thomas (and others) has claimed 60-65%...... does ANYONE have data to back that up

(no internet searches please.....real data)

I have NEVER seen a factory crank with that ratio.....they are normally from 50 to 60%.

Now, (thanks again speedpro), when I stated the figure of 42% (not what I would like, but rather what I got when I had the crankcase volume where I wanted it) he correctly pointed out that such a ratio WOULD GET LESS VIBRATION THE FASTER THE ENGINE SPUN.........

Ah ha!

So, if you had a balance factor of 65%, then low RPM would be quite smooth and high RPM would vibrate more ( good for a commuter bike), and a ratio closer to 50% would be the opposite.....vibrate in the lower RPM, but smooth out at the higher RPM ( good for a Performance Two stroke)

Don't forget that a 125cc aircooled performance two stroke generally operates from 7500RPM up to say 12,000 RPM, a commuter bike from 2,000 RPM to maybe 6,000RPM

Also, as I have covered before (in the ESE thread) higher primary compression ratios (too high ratios that is) increase pumping losses of an engine (energy required to do the work) in the situtation where you have such "pumping losses", your engine is subjected to more vibration........lower the primary compression ratio....lower the losses.....less vibration (therefore a smaller balance factor can be used)

So, less primary compression means smaller balance factor.

Also, peak pressure on the piston comes into play as well.

Like I have said, it's all about a complete system.

Not "one rule for all"

As a matter of interest, Honda where using this "hollow web" system since the 70's.....

What balance factor would Thomas suggest?

PS, a google image search under "hollow crankwebs" yeilded this result......

It's FOR A CHAINSAW! (they rev a bit don't they?)

It's not what I was looking for, but huh,still worth seeing.

Hopefully someone like K14 has an old crank to illustrate my point.

YOu may also be interested K14 (and other GP125 riders, just out of interest, no good for New Zealand circuits) the picture of the second crank is an after market Rs125 Honda crank....it has no tin plates on the webs, it is a complete "full circle crank"...this is a deliberate design, and done to increase the weight of the flywheel....(it does two things for a 125 GP bike.......)

1) increases the inertia of the crank (producing more torque),
.....and
2) allows the rider to "get on the gas earlier"....

The downside is that the heavier crank is no good for "short curcuits", (like NZ...the tracks are too tight in NZ) and only of benefit when it is used on the larger tracks here in Europe...........

To get the balance factor correct, you can see that the manufacturer has added "Mallory Metal" to the lower portions of the webs.

The crankcase volume on a Honda RS125 is quite large, and when running a true "full circle crank" the ratio is around 1.38:1 (from memory)

If the crankcases on the Vespa small frame engine I put the crank in where "larger" (internally) I would have been able to leave the webs complete, and, to get the balance factor correct, added Tungsten (Mallory Metal is a free machining alloy of Tungsten) at similar points.

I think you would find that while the weight of the two cranks in the pictures I posted are different........with the addition of the Malloy Metal in the second example, the "balance factors" would be quite similar.

Balance factors are simply ratios.
It doesn't matter what the whole assembly weighs.

That's another thing all together.
The last picture is of a Ducati 748 Crankshaft..... no Mallory Metal needed!

The principals are the same.
Oh, by the way.... none of this is new (or a secret)

[SS90]

The TZ250B v-twin yamaha give peak power at 12,000 revs/min and could produce useful power to close on 13,000. The fuel power jets did not even switch of until about 12,200 revs/min. Just concerned that you might be drawing conclusions based on 11,000 revs/min.

To be honest, I am not 100% sure ( however,I am fairly sure that my old 91 had a (factory) mark at 11,000, but I can remember revving it over that a few times, and yea, I am pretty sure it still made mumbo)

I think that it's the kitted stuff that revs to 13,000 (and then they don't last long!)

The concept was to illustrate that intended RPM is ONE factor in selecting a primary compression ratio for a particular situation, and "modern" bikes (using modern exhausts and ignitions run lower than the "old school" 1.5:1

I am not saying that ALL engines should run lower ratios (motocross bikes, commuter bikes with-out expansion chambers etc) need the smaller crankcase volumes. (higher ratios)

Similarly if you have an "old school" engine set up (high exhaust port time areas, non retarding ignition,wrong blow down times,old expansion chamber design (diffuser angle etc....) then this would be an example where something closer to 1.5:1 would be suitable.

That's my experience anyway!

[FastFred]

A lot of talk about different physical layouts of crankshafts but still no explanation or formula to explain the "Balance Factor" or how its calculated.

PS. SS90 talk does not equate to knowledge.

I am begining to suspect you don't know what a "Balance Factor" is, how to calculate it or even its purpose and as a hint it has nothing to do with primary compression ratios.

And by way of a reminder.

Just a little thing SS90, A+B=C is an equation a string of numbers added and divided is an arithmetic sum.

Knowing the difference is probably important.