Two-stroke performance tuning?

[SS90]

I got the idea these big Hp Scooters were bigger than 125 cc from the link you posted http://www.lambretta.it/eddy/edmonds.htm posted page 5 of the “Two-stroke performance tuning?” thread.

Where Charlie Edmonds talks about his 225cc 37Hp 07 BSSO Championship winning scooter and the “Vespa 180 VE Small Frame” tag on the dyno charts you posted. And amongst all the Scooter hot-up cylinders I’ve seen while following your Posts, they all seem to be “Big-Bore” and about 200cc or more and in the so called 30+ Hp range.

The 125 scooters you build, are they using the traditional 56mm bore or the more modern 54mm bore?

The Charlie Edmonds article was a very interesting read. I see he has trouble with heat in the air-cooled engines and this limits the power output. To help with cooling he uses air ducts and a Yamaha MX head with a head insert, I bet he wishes he had thought of using a copper head gasket/fin like Thomas, ESE’s Engine Tuner has.

The "VE number" is the number of runs you have done (starts at VE 001, and goes up from there)

(see attachment)

I assure you 125cc 30 PS is the standard over here.

140cc and 150cc 35PS

210cc-230cc ("big frame" engines) 40PS seems the level, but on the Vespa this is where the gear box breaks. (the shift fork)

The Lambretta's (when the gearboxes are modified (welded etc) seem to be able to handle it, others can also get them to 40 P.S

I have no experience with Lambretta (and very little on the "big Frame " Vespa.

Also, above 30PS, the small frame clutches have reached their limits, and it takes quite some work to get them to function correctly. But it is possible.

A standard 4 plate small frame clutch will take 15 PS, a basic modification will take 24 PS (and 20NM) and a little more work will hold 30 PS (or thereabouts)

Much over that is quite involved, but it is possible.

I must admit I was sceptical about what these engines could do when I started.

(tuned scooters? hahaha, you must be joking!)

It didn't take long for me to eat my words!

[SS90]

Or possibly from another Planet!!!



Hmmmm that full circle crank looks pretty much like most mid capacity Japanese 2-Stroke Cranks, but, what the heck, what would they know.

and such engines generally have balance shafts as well.

[SS90]

There is a lot of talk about "Balance Factors", folks how about putting some numbers to the talk and a small explanation about how they are calculated from someone who knows what they are talking about.

If SS90, FastFred or someone else can't do it I will ask Thomas to write something for us, he's bound to come up with something interesting.

.

Sure I can, I don't have huge experience, but I know SOME of the basics.

but I was hoping that other people can contribute to this thread (in an adult fashion)

I look forward to Thomas contributing.

What's his log in name?

[FastFred]

and such engines generally have balance shafts as well.

"Generaly" suggests more do than don't. Lets name them, Bet you can't name more that do than I can name that don't. I'll give you a head start Honda MB100 does. Yamaha RX and Suzuki A100 don't.

2:1 to me so far, now lts your turn, don't be shy, have a go.

[Chambers]

but I was hoping that other people can contribute to this thread (in an adult fashion)

For it to be adult you need to be adult yourself, your snide comments leave you open to a poke in return. You will have to sharpen up if you want to stimulate real adult discourse.

[TZ350]

Sure I can, I don't have huge experience, but I know SOME of the basics.

Some of the basics would be a good start.

.

[SS90]

Some of the basics would be a good start.

.

OK, the following is the data supplied to me from the machine shop.

( I would prefer to keep the modified specs to myself, but it's not hard to calculate)

Perhaps Thomas could explain it for us?

That's not a "snyde" comment incidentally.

It's quite an interesting subject.

Balance factor with original ("uncut" "full circle crank") used with the piston, gudgeon clips small end bearing supplied.

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

The crank was measured as a complete assembly, and not dissasembled to be checked (quite an involved process)

[Chambers]

The whole of the rotating mass is balanced and only a portion of the reciprocating mass is balanced.

From memory: Balance factor for V8's and other V configurations 50%, multi verticals, twins fours etc 75-85% and singles 60-65%.

This is an interesting subject, looking forward to hearing more.

[TZ350]

The crank was measured as a complete assembly, and not dissasembled to be checked (quite an involved process)

Thomas tells me (after a quick phone call) its not that hard, a 100% of the rotating mass is balanced, this is always assumed and does not figure in the calculations. Only the % of the reciprocating mass to be balanced is calculated and this is where the balance factor comes from and is usually greater than 50%.

That is more than 50% and less than 100% of the reciprocating mass is offset (balanced) by the holes drilled in the flywheel near the bigend pin.

So the more holes drilled or metal removed near the bigend pin balances more of the reciprocating mass. So SS90, by cutting away the flywheel like he did increased the balance factor from something like 60% to maybe 80%. Ie more of the reciprocating mass was balanced by the remaining mass on the other side of the flywheels.

Thomas says its easy to find the reciprocating mass and balancing is very easy to do on a single statically but on multies you get better results balancing the cranks dynamically.

Its easy to chose a balance factor and balance an assembled single crank using knife edges but you have to strip the crank and re-assemble it with bob weights if you want to balance it dynamically.

The balance factor is a trade off between the vibrations going up and down or backwards and forwards in a single, or side to side in a multi, its your choice, chose your balance factor carefully.

He says there is no "correct" balance factor, only general ideas about what works and the real trick is to chose a balance factor from experiance that works with your frame and other reactive components like handle bars and foot pegs, also vibration robs power as the energy that is going into shaking things about is energy that is not turning the back wheel.

Finding the reciprocating mass and balancing a single crank is easy chosing the right balance factor is the trick.




So there it is, from the man himself, as best as I remember it.



.

[quallman1234]

Will a balanced crank make the engine be able to rev more freely, or simply just reduce vibrations?
A bit of a hypothetical/dumb question. But nevertheless.

p.s I now have a tf125 (6 speed).

[SS90]

Thomas tells me (after a quick phone call) its not that hard, a 100% of the rotating mass is balanced, this is always assumed and does not figure in the calculations. Only the % of the reciprocating mass to be balanced is calculated and this is where the balance factor comes from and is usually greater than 50%.

That is more than 50% and less than 100% of the reciprocating mass is offset (balanced) by the holes drilled in the flywheel near the bigend pin.

So the more holes drilled or metal removed near the bigend pin balances more of the reciprocating mass. So SS90, by cutting away the flywheel like he did increased the balance factor from something like 60% to maybe 80%. Ie more of the reciprocating mass was balanced by the remaining mass on the other side of the flywheels.

Thomas says its easy to find the reciprocating mass and balancing is very easy to do on a single statically but on multies you get better results balancing the cranks dynamically.

Its easy to chose a balance factor and balance an assembled single crank using knife edges but you have to strip the crank and re-assemble it with bob weights if you want to balance it dynamically.

The balance factor is a trade off between the vibrations going up and down or backwards and forwards in a single, or side to side in a multi, its your choice, chose your balance factor carefully.

He says there is no "correct" balance factor, only general ideas about what works and the real trick is to chose a balance factor from experiance that works with your frame and other reactive components like handle bars and foot pegs, also vibration robs power as the energy that is going into shaking things about is energy that is not turning the back wheel.

Finding the reciprocating mass and balancing a single crank is easy chosing the right balance factor is the trick.




So there it is, from the man himself, as best as I remember it.



.

I see,

So (from the figures I posted of the "full crank", which showed 25% balance factor)

According to Thomas the crank I started with had a factor of 60%....however it has been measured at 25%. (measured at 12 o'clock position)

So Thomas is correct when he told you I increased the balance factor.......

........ from 25% to 42%

(not from 60% to 80%) as Thomas suggested

I was hoping Thomas could explain the effect of cylinder angle on chosen balance factors as well.

Of course I am not advocating balance factors so askew from the "norms" it however what I achieved when I cut the crank to get the primary compression ratio I wanted. (I thought I had made that clear)

The rubber mounting of the engine cope well. (It has been done before)

Is Thomas suggesting I should have cut the material from the side opposite the pin? (.......that's what I think Thomas is suggesting)

What does Thomas think of the primary compression ratios I am using/advocating (dependent on exhaust design)

What does Thomas think a good balance factor is for such an engine? (like I said, a full circle crank is 25%, so 42% is better....( but like I said, not quite right, and like I said, Tunsten (actually Speedpro quite rightly suggested "Mallory Metal" (an alloy of Tungsten that is free machining, but I'll wager quite expensive!)

For balancing purposes I would like to take more material off the crankpin area (but I fear that would weaken the crank too much.)

[SS90]

Will a balanced crank make the engine be able to rev more freely, or simply just reduce vibrations?
A bit of a hypothetical/dumb question. But nevertheless.

p.s I now have a tf125 (6 speed).

Without having any thing to back it up, my experience is yes, a perfectly balanced engine does spin up better.

Have you ridden a pre 1995 Honda Rs125R?

My god, (no balance shaft (full circle crank), they where physically exhausting just riding 10 laps.

By comparison the 1995 and later (balance shaft) where like a Toyota Corolla (comparitively that is) Yet, the crank weighed the same (well near enough I guess)

[SS90]

I had to ask someone but apparently an original crank (like the picture I posted above) is "around 50%" originally

Adding more material arouond the pin (like a "full circle") decreases this "balance factor"

Like I showed....to 25%.

[TZ350]

I see,

So (from the figures I posted of the "full crank", which showed 25% balance factor)

According to Thomas the crank......................................

So Thomas is correct when he told you...................................

-------------------not as Thomas suggested

I was hoping Thomas could explain................................

Is Thomas suggesting.....................................

.............................that's what I think Thomas is suggesting)

What does Thomas think of the primary compression ratios..............

What does Thomas think a good balance factor is...........................

I asked Thomas but he said he explaind what a good balance factor is and he can't do all the work for you.

He can't remember if there is anything in Jennings or Bell on crank balancing but there certainly is in "Tuning for Speed" and also there is plenty on the net.

You could do some reaserch of your own, on how to balance cranks. Thomas would be interested to see what you come up with.

I guess we all would.

.

[bucketracer]

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

So SS90 has drawn me into this again. SS90 I explained to you that professionals work with significant figures.

Thats Significant figures not Decimal Places.

I don't know SS90 if your being deliberately obtuse or if your merely stupid.

Mmmmmm you didn't pick up the gift I gave you on significance and accuracy in measurement and calculation, and again you'v involved me unnecessarily so I guess that makes you stupid.

.