58 or 60mm crank?

[Rich_T]

I wouldn't get bent out of shape about the ratio of rod to stroke, after all many MX250's are 72 stroke x 125 rod. The whole thing is about overall balance of cylinder wall area, piston crown area and what you can do with port area. The differences, benefit or otherwise or rod angle between 107 and 110 are exceptionally marginal, the larger influence in that change would be port timing. For a given stroke a longer rod reduces port timing compared to a shorter rod, and an increase in stroke for a given rod increases port durations.

The relationship between torque, piston surface area and wall area is interesting ( for given timings and geometry). The higher the ratio the more BMEP you get ie bore to stroke ratio, square or supper square configurations. (F1 and LeMans engines are typically 90mm bore 30 stroke).

The relationship between stroke and RPM is interesting too as it essentially limits reliable piston speed. For example changing between 58 and 62mm stroke may significantly influence piston velocity enough to cause problems with little end, rod loadings or even for the piston to break up.

It is all a balancing act and there is a bit or a Darwinian mechanism at work over the last 3 decades which principally steers engine mods to "over square" configurations with percentage increases in the following orders bores 6%, strokes 3.5% and rod 2.8%. Timing combinations are dependant or the desired engine duty cycle, touring or race etc. The difference in rod to stroke ratio is around 0.015 compared with everything else it is minor in the pareto analysis provided there are sufficient mechanical clearances

[Adam_Winstone]

^... what colour do you like your plugs, chocolate or Rich_T?

Soz, laughing at my own joke, which I bet you've never heard before (lol).

I'll get my coat.

[Yanker]

^... what colour do you like your plugs, chocolate or Rich_T?

Soz, laughing at my own joke, which I bet you've never heard before (lol).

I'll get my coat.

Adam, has your account been hacked?

increases in the following orders bores 6%, strokes 3.5% and rod 2.8%.

Yes, as the stroke and rod were already at near optimal levels before this re-calculation, engineering tollerances and material consistency progress: those figures are a completely predictable consequence...... The Lambretta Rod/Stroke ratio deteriorates even further as capacity increases....innit