750 crank balance factors

  - I've run mine with both seperate pipes and a 2 into 1 - both work fine. Looking at all the discussion and seeing the technicalities being explored does somewhat make me smile. When i started to get intersted in this sort of thing over 30 years ago - I just got on with it - there was hardly anyone else to consult - whatever the pros or cons - i always believe that - until you've tried it yourself - or ridden a converted machine -although it can make interesting reading - maybe all this probing discourages a lot of development - just my opinion - I do tremendously admire Reds attitude of giving it a go and hope to read about his results ASAP.  I chose the 90? option to give the best chance of having the smoothest motor with a really good power delivery - the effect of the more kindly power pulses that were liable to give the rear tyre an easier time and be able to get the power down earlier were a huge consideration too - not really an issue for road riding though. - Regards - Phil
 
Cam you are right!
I just had a little think about what I wrote and yes.

The point of maw piston speed is in fact not half stroke but simply plotted as a tangential line with the length of your rod from the crank pin centre to intersect somewhere on the piston pin (stroke) centre line,  actually just as I think Chris had drawn it.

THEIN,
You are referring to Crank pin velocity on it's own I think. We are referring to piston speed connected by a rod that changes angle.
At least that is what I deducted from your writing?

When a piston is connected to a rod which changes it's angle during it's movement cycle the piston changes velocity as it needs to move more or less distance depending on the length and angle of the rod.  Think of a water skier behind a boat. How can he ever ski faster than the boat that is pulling him if you are right?  But he can when he crosses the line drawn through the centre line of the boat, and moves off to one side etc, he can actually catch up with the boat viewed from the side, but still obviously be the same distance away due to the length of the rope (con rod) he is holding, somewhere during that "arc" he is at max velocity, but when directly behind the boat and exactly at 90 degrees he is at the same speed as the boat.  

For practical purposes I would phase it at 90 degrees,  and to be honest I think it would probably be the optimal.  Remembering that what you take of in degrees one side of the crank movement, you add to the other!  so you may be spot on max piston speed one side but you will be "WAY OFF" on the other side!    Stick to 90 and make your life easier!  it will probably be the best compremise anyway!

Julian.
 
julian said:
You are referring to Crank pin velocity on it's own I think.
Click to expand...

No, I'm referring to piston speed as mm travel per degree of crank rotation (the derivative dx/dA) which I calculate using the following formula:

Piston speed = -r.sinA - (r2.sinA.cosA)/√(l2-r2.sin2A)
where l = conrod length, r = crank radius (or half stroke) and A = angle from TDC

Red's red herring of offset gudgeon buggers up that formula entirely, so will have wait until I've had less booze!  :D

Remembering that what you take of in degrees one side of the crank movement, you add to the other! 
Click to expand...

Correct
Lets assume 76 deg phasing of crank pins. When piston 1 is at TDC, piston 2 is at 76 deg BTDC (max piston speed) on the up stroke. After the crank rotates 76 deg, then piston 2 is at TDC and piston 1 is at 76 deg (maximum speed) on the down stroke. All OK so far if you're trying to minimise inertial torque.
But it all turns to shit when you get to BDC. When piston 1 is at BDC, piston 2 is at 104 deg after TDC where its speed is 88% of maximum.

If you phase the crank throws at 90 deg, when one piston has zero speed at TDC or BDC, the other piston is at 97% maximum speed. Seems like a good compromise to me. As I understand it, this is the thinking behind the cross-plane crank.

I haven't read Phil Irving's comments about the 76 deg phasing, so I'm not sure what he's trying to achieve, or why.

Cheers,
Cam

 
Dellortoman said:
I haven't read Phil Irving's comments about the 76 deg phasing, so I'm not sure what he's trying to achieve, or why.

Cheers,
Cam
Click to expand...

I get the feeling that we are about to find out shortly!
Theory is great, but counts for nothing when the switch is flicked.

Very interesting thread, I like this sort of thing.
 
Dellortoman said:
julian said:
You are referring to Crank pin velocity on it's own I think.
Click to expand...

No, I'm referring to piston speed as mm travel per degree of crank rotation (the derivative dx/dA) which I calculate using the following formula:

Piston speed = -r.sinA - (r2.sinA.cosA)/√(l2-r2.sin2A)
where l = conrod length, r = crank radius (or half stroke) and A = angle from TDC

Cam,
I was refering to Thien's comments.

J  ;)
Click to expand...
Click to expand...
 

No, I'm referring to piston speed as mm travel per degree of crank rotation (the derivative dx/dA) which I calculate using the following formula:

Piston speed = -r.sinA - (r2.sinA.cosA)/√(l2-r2.sin2A)
where l = conrod length, r = crank radius (or half stroke) and A = angle from TDC

[/quote]

(happy1)



(clever)
 
while you lot are discussing what degree to set the crank at I just went ahead and machined the 76 degree insert for my crank fixture
is sort of a bit misleading in the photo, middle hole is 180 degrees, outer holes are 76 degrees and 284 degrees from zero
makes sense if you see my crank jig, insert locates on the 180 degree hole into the bottom plate, drop one inner web into the insert 76 degree hole using the big end pin and line next web up on the zero degree hole in the top plate using it's big end pin
easy as

we need to machine more metal off the little end section of the crank webs, current balance is 35%, will press crank apart in the morning and have lined up CNC time on Saturday, hope to get two sets of crank webs done Saturday, as in 2 complete cranks

House is getting the 76 degree crank as he does not use a computer much so is not following this forum and it is going to be a race bike anyway so certain to see inside this engine agin sooner rather then later, can change then if we have to

looks like CK wants a 90 degree crank which is fine as when I first made my crank fixture I had a senior moment and machined in a set of 36.03mm holes at 90 degrees so I already have the fixture set to press at 90. Excellent stuff up that one, worked out great

Greg can have a 76 degree crank as well, will be a retro style SFC replica with hot rod engine, shaping up well

BK, I have enough crank webs here to do your crank as well so let me know if you want 76, 90 or 360

76degreecrankinsert.jpg




 
Hi,

Ref. 17.10 18:31.
It was not the first time when I took a short cut with limited amount of knowledge.
(and not the last) :D,

Teuvo
 
hi ,all this talk is brill and i have a formula of my own it goes like this;
ignition + spark =bang
fuel & air +spark =bang
now for the good bit, add big right hand +  a laverda = BLISS :D
phils sf goes like stink and sounds ace  i know i followed around cadwell at the Bears,so 90 it is then.
best regards andy
 
I wonder if re-phasing a Zane engine might cure some of the fragility problems that seem to plague them. A fuel injected ,twin cam. 8 valve ,90 degree 750 might be an interesting concoction.
 
bob said:
I wonder if re-phasing a Zane engine might cure some of the fragility problems that seem to plague them. A fuel injected ,twin cam. 8 valve ,90 degree 750 might be an interesting concoction.
Click to expand...

Allmost a TRX except from that it had a 10 valve head.

I heard Phil's 270 degree twin in the early 90:s in his back yard.
Another guy in Sweden made his 750 a 180 but wasn't to happy about it. It run smooth but lost a bit of grunt.

I had a TRX about 10years ago and I liked it a lot, not quite the torque as in a 1130 180triple though...
 
OK, to answer Red's question about offset gudgeon (or offset cylinder, the geometry of motion is the same), here are some numbers based on a 5mm offset. I doubt that Red would go as far as 5mm (maybe 1 or 2mm), but I wasn't expecting much of a difference so I chose a largish number just to see what would happen. As it turns out, the effect is more than I expected.

Firstly, stroke is increased by 0.05mm, which means the engine capacity increases too. But don't get excited about a bigger engine, the difference is bugger-all in the scheme of things.

Secondly, the terms TDC and BDC no longer make sense because the upper and lower limits of piston travel don't occur at 0 and 180 deg. The new TDC is at 1.57 deg and BDC is at 182.65 deg (the strike-through of "DC" is intentional becuse it's no longer dead centre). That'll make ignition and valve timing interesting.  ???

The point of maximum piston speed becomes 78.24 deg from TDC, which is a couple of degrees away from a non-offset engine.

If I was building an engine, I wouldn't bother offsetting the gudgeon. As I said before, I'm not convinced that there is any useful advantage. A better idea would be to offset the cylinders by a couple of mm (I think Yamaha R1 has 2mm offset). That could be easily achieved with an eccentric bore in the cylinder liner (like Alan Millyard did with his Kawasaki V12, except he did it sideways to accommodate the side-by-side connecting rods). The liners may need to be pinned to the cylinder block so they wont shift during service.

Cheers,
Cam
 
Dellortoman said:
A better idea would be to offset the cylinders by a couple of mm
Click to expand...

i seem to recall in the Castrol 6 hour, the scrutineers use to check for offset bores ??
woz long time ago eh.  but bloody good racing.
 
You must log in or register to reply here.
Back
Top