why long stroke cranks?

[bluebob]

just trying to get my head round the pros and cons of using longer stroke cranks.i don't know enough about them to decide if I should keep to 58 or go longer on a rebuild which will be based on a 225(Not sure which yet).So first,why do we use them,rather than a standard size.Is it simply to get extra cc and/or because it gives the tuners more options?i think it would be interesting know why they seem so popular.Or is standard stroke and longer conrod a better option depending on the main use of the engine.Can someone please try to explain the pros and cons in plain English.
Cheers

[rossclark]

There is an old tuning maxim goes 'there's no substitute for capacity'

Adding stroke will give you more capacity, about 5 - 6 cc on a 70mm bore taking your 225 to 230 or so.

It should also improve the torque out put of an engine.

A longer rod gives you added flexibility in adjusting the positioning of the ports and the related port timings by using the appropriate packers under the cylinder or on the head. It also lets you make use of a little extra stroke without having to worry about the piston hitting the casing at BDC. You do have to ensure that the piston doesn't poke too far out the top or recess the head to set the squish.

There are undoubtedly more scientific explanations which an adult will be along with shortly.

[Norrie Bodge]

ava read here will help m8-> https://www.mbscooters.co.uk/info/setti ... s+190.html

[bluebob]

thanks for the replies.

[Feersum Injun]

They also help reduce vibration...

[bluebob]

Now that would be a worthwhile improvement! Anything that reduces vibration on our lambrettas is good enough reason for me

[holty]

i think what was meant was a longer rod can help to reduce vibration, a longer stroke crank is probably going to create more vibrations than a standard crank, as a result of the crank pin being farther away from the center, however off-setting this with some lightening can still produce a smooth engine, i like the look of the gt cranks with plastic inserts which help to counter balance the crank, ive recently had a 64mm crank built from scratch by martin at chiselspeed, he drilled holes on the inside of the webs to balance it out, but he did need the exact weight of piston, rings and wrist pin, to get the balance correct.