ESE's works engine tuner

[Frits Overmars]

Top:-------------- Cranks 50% plus Balance Shafts 50% going down together cancels the Pistons 100% coming up.
Mid going down: Cranks 50% cancels Balance Shafts 50% as they are now pulling in opposite directions.
Bottom:---------- Cranks 50% plus Balance Shafts 50% coming up together cancels the Pistons 100% going down.
Mid going up:--- Cranks 50% cancels Balance Shafts 50% as they are now pulling in opposite directions again.

You understood completely.

To do this the Balance Shaft has to rotate twice as fast as the crank.

Or so I thought .

I don't expect to find a balance shaft turning at twice the crankshaft speed on a two-stroke single. And if I did, I would hope to find two of those shafts instead of just the one. Auto manufacturers trying to make their four-inlines run like straight sixes sometimes employ twin balance shafts turning at twice the crankshaft speed. But we like to keep things simple (and affordable).
Anyway, one balance shaft rotating at the same speed as the crankshaft, but in the opposite direction, will already do a lot of good. Let's forget about the con rod effect for now (assuming its length is infinite), then we can balance the total reciprocating mass by putting 50% counterweight in the crankshaft and the remaining 50% in the balance shaft.
A lot of fuzz is made over measuring this total reciprocating mass, but it's dead easy: see the picture below.
Note that the con rod is completely horizontal and that the gudgeon pin with its bearing, the cir-clips, the piston rings, are all fitted.
Now if you want a 50% balance factor, replace everything attached to the con rod small end with so much mass (a wire hook with a collection of nuts or washers, for example) that the scales read 50% of their original value. Attach that wire hook loaded with nuts to the con rod small end and then figure out the amount of material that you have to remove from or add to the crankshaft, so it will remain at rest in any position.


Now here's a little teaser. Suppose we've found the optimum balance factor for a certain single cylinder engine without a balance shaft. A lower balance factor would increase the vibrations, as would a higher factor.
Now we fit a lighter piston. This means that the balance factor will increase. But the engine will vibrate less...

[Martin1981]

Thanks for that Frits. Can you please add a picture of THIS procedure: "Attach those nuts to the con rod small end and then figure out the amount of material that you have to remove from or add to the crankshaft, so it will remain at rest in any position." i just cn not imagine how this would look like.

sorry frits, i think i understand the most of what you are explaining. but that crankbalancing still makes me a little sick

[Frits Overmars]

Thanks for that Frits. Can you please add a picture of THIS procedure: "Attach those nuts to the con rod small end and then figure out the amount of material that you have to remove from or add to the crankshaft, so it will remain at rest in any position." i just can not imagine how this would look like.

My pleasure Martin. Take a look at the above picture. Jot down the scales' reading, remove the piston from the con rod, let the con rod rest on the scales again and add a wire hook and a collection of nuts or washers to the scales until the reading is what you want it to be (50% of the original weight in the above example).
Attach the wire hook with the nuts to the con rod's small end and let the crankshaft rest on two beams, like in the picture below. First make sure that the beams are exactly horizontal by putting a piece of tubing (or a bottle) on them and adjusting their heights until the tube does not want to roll any more.


Now if we put the crankshaft on the beams, it will probably roll until the bob-weights are either at the highest or at the lowest point, in which case we will have to add or otherwise remove material from the bob-weights.
Let's assume that the bob-weights move to the lowest point. Then we add extra weight (some more nuts or washers) to the wire hook until the crankshaft will stay in any position on the beams without a tendency to turn. We have now created the torque that is necessary to keep the crankshaft in equilibrium.

Example: We have an engine with a stroke of 54,5 mm. And we had to attach 80 grams of extra weight to the hook in order to reach equilibrium.
This weight acts via the con rod on the crank pin radius (half the engine stroke), so it generates a torque of 80 * (54,5 / 2) = 80 * 27,25 = 2180 gram*mm.

The bob-weights had a tendency to move to the lowest point, indicating that they are too heavy, so we have to drill holes in them.
The total mass of these holes, multiplied by the distance from the holes' centers to the crankshaft center line, also generates a torque.
Let's assume that we can drill a hole in each bob-weight sidewall, opposite the crank pin, with its center 35 mm away from the crankshaft center line.
In order to generate the same 2180 g.mm torque, the total mass from these holes must be 2180 / 35 = 62,28 gram.

One hole in each bob-weight, that's two holes in total. One hole should then equal 62,28 / 2 = 31,14 gram.
The specific mass of steel is 0,00785 gram / mm³, so the volume of each hole must be 31,14 / 0,00785 = 3967 mm³.
Let's assume that each bob-weight has a thickness of 20 mm. That is how deep we can drill (through and through).
Then the cross-section area of each hole must be 3967 / 20 = 198,35 mm².
Its diameter must then be SQRT (198,35 / (pi/4) ) = SQRT ( 198,35 / 0,7854 ) = SQRT (252,55) = 15,9 mm.

Obviously the further away from the crankshaft center line you can drill these holes, the smaller they need to be.

sorry Frits, i think i understand the most of what you are explaining. but that crankbalancing still makes me a little sick

Piece of cake, isn't it?

Now how about that teaser? Any takers?

[cotswold]

Here is where TZ showed me how to balance my GP125 crankshaft after rebuilding it we used 50%

[Frits Overmars]

Here is where TZ showed me how to balance my GP125 crankshaft after rebuilding it we used 50%

Very nicely illustrated. I would have used those pics if I'd had them .

[Martin1981]

Thanks Frits and Cotswold!

Got it now.

[speedpro]

For a TS 125 Suzuki to rebalance for bucket racing, ie revving the nuts off it, take a look at the picture of the GP crank above. The TS crank is similar in that it has 2 holes in each web. In the TS case the holes are different diameters. All you need to do is press a slug of aluminium into the larger hole on each web. No maths, no measuring, just do it. I figured with my TS years ago that it vibrated at revs(bad enough to cause bits to fall off the bike) because the counterbalance was too large so I added weight opposite. Worked so well that as soon as we fired it up on Pete Sales dyno he commented without me telling him I'd done it.

[speedpro]

In my opinion it is such an inexact operation, in that all frames are different, cylinder geometry relative to the frame, and the "realities" of use that a good educated guess of which way to go is all that's needed. Give it a try and make another adjustment. So far so good for me. On 2 stroke singles anyway.

[wobbly]

The so called "heavy " cranks for an RS125 Honda be they from the A Kit or from VHM are not in fact heavyer - they are lighter.
But the added Mallory metal near the crank wheels circumference adds considerable inertia.
There are alloy plugs in the blank areas near the wheels middle radius,and this reduces the actual mass.
Here are the old Rotax 256 with porkchop cranks, and the later Aprilia - both will have around the same balance factor,but the Aprilia has way higher inertia
with all that heavy metal added at the circumference.

Re the lighter piston scenario - this will effectively increase the balance factor so the peak forces in the plane of the cylinder axis will be reduced.
Thus less vibration in the critical vertical plane of the handlebars - but, by increasing the factor it will now vibrate more horizontally - usually not so much of an issue.

[lodgernz]

Phil Irving tuning for speed........
google has it
http://tuningforspeed.com/files/Tuning_for_Speed.pdf.

Fascinating reading is the chapter on 2-strokes. Phil may not have realised it, but he was documenting the dawn of the modern 2T engine, the point when it all changed.
Reed valves, disc valves, expansion chambers, squish bands, boost ports were all being experimented with. All that appears to be missing from his list is the A and B-transfer design.
Port timings were rather different: Ex duration of 140*, transfer 120*. Not much blowdown.
Interestingly though, he already understood STA was important, although he didn't use that name and gave no details.

[senso]

Regarding the crank balance, what a beautiful explanation!
Now I sort of understand, so beware, I will bombard this forum with doubts, as I have been running my crm 50 engine without the balance shaft(the engine revs up and down a lot faster, but does it vibrate, already broke a lot of supports due to vibration )

Sorry for the long long delay in posting more photos of the cylinder cut-out, but here they are.
Around the A transfers the exhaust port has about 5mm of solid casting, there is 3-4mm of clearance between the holes that put coolant in the bottom of the exhaust duct, so I will enlarge mine to 7 or 8mm for better "flow", I hope the photos are clearer, but well they got out a bit blurry, I can take more if needed.

[senso]

And the rest of the photos, there is a good volume of coolant under the exhaust duct, but there is only two 5-6mm holes connecting them to the rest of the cooling jacket, I think there is some flow promotion because one of the holes is right in front of the coolant entry in the cylinder, so there might be some sort of venturi effect making it flow, so enlarging the holes to around 7-8mm might be a good idea:

[TZ350]

Eeeek I have gone blind, everything is black, help how do I get back to the light side??????

[Grumph]

Fascinating reading is the chapter on 2-strokes. Phil may not have realised it, but he was documenting the dawn of the modern 2T engine, the point when it all changed.
Reed valves, disc valves, expansion chambers, squish bands, boost ports were all being experimented with. All that appears to be missing from his list is the A and B-transfer design.
Port timings were rather different: Ex duration of 140*, transfer 120*. Not much blowdown.
Interestingly though, he already understood STA was important, although he didn't use that name and gave no details.

i've got his Two - Stroke Power Units, published 1967 which covers a lot more ground. When he was out here in the '80s i remember him being asked about updating his books.
I think the answer was along the lines of "I'm out of touch now, let someone else have a go" and Bell and Robinson carried it forward.

Next up, Thiel and Overmars......Please!! Failing that, someone do the collected observations of Chairman Wob....

[Grumph]

Eeeek I have gone blind, everything is black, help how do I get back to the light side??????

No idea what you were looking at but I was seeing it on a black background, now blinding white....Swap you.