Scooterotica

Ok ive just had a argument/discussion regarding port timings in degrees.

i said that if im using a 60mm crank, when the piston is half way between tdc and bdc ie 30mm down the bore it will be at 90 deg (180 deg port timing) .

He said no its not like that, its more or less than 30mm something to do with conrod length and angle ?? this sounded a load of b@ll@cks so i went back in to the garage to check and prove him wrong ! as it seems like basic geometry to me. ........however, i had to eat my words from tdc to 90deg the piston had moved down the barrel something like 32mm ?
so can someone please explain as my heads hurting trying to work out why

I don't think the relationship between degrees of crank rotation and piston movement is a s direct as that...

Take a look at the chart on p249 of Sticky's book and you'll see stroke and rod length also come into it, which I'm presuming affect the geometry and therefore linear movement, but I'm drunk and in the second day of a shitty migraine so can neither really focus visually nor concentrate...

lol yes im getting one just thinking about it
I understand the piston is moving fastest at mid stroke then slows down towards tdc and bdc .
If the angel of the rod at 90deg is (for EG.) 20 deg to the bore , then at 60 deg or 120 deg (30 deg either side) the rod angle would be the same angle , say 15 deg? so.... mirrored either side of the 90 deg mid stroke ? dose that make sense ?? so the piston travel would be mirrored either side of the 90deg mid stroke ???

its a maths thing

I think that is likely.

If you look at the figures for a 58 stroke 107 rod going up from 23- 15 deg the gaps are .24, .23, .22, .21, .21, .19, .18, .17

it loses/gain about 0.01mm / degree of travel, assuming these are rounded to 2 DPs on either side of TDC. This must be related to the difference between the rotational diameter at the big end as opposed to the small end and the relative linear travel thereof. I expect there will be people far smarter and less frazzled than I am who can actually work this out exactly....

Doesn't the con rod rod form the hypotenuse of a triangle with the long side being the distance the piston travels? I got B in my O level maths thanks to geometry.

if you use the centre of the big end pin and the small end pin and the centre of rotation of the crankshaft as the reference points, I don't think the conrod is always the longest never mind the hypotenuse, nor is there a right angle in all cases so hypotenuses are out...

Hypotenusii?

My Greek / Latin are rusty

Piston goes up n down......crank goes round n round.

Are you sure?