Scooterotica

Hi, It seems alot of people are putting long stroke cranks in their lambrettas. I have read in a 2 stroke tuning manual that going for short stroke has alot of benefits, ie less piston stress, less piston acceleration less piston speed.
Has anyone tried using the 54mm crank?
Ive worked out if you use a 70mm bore and short stroke you still have 210cc (if ive worked it out correctly)
Thoughts please!

it will rev its tits off

Quite oversquare mate. Probably quite low on the torque front and you'll need the revs to get it to 'punch'.

Tony's right (that's one of his traits).

It seemed that during the 1990's, Japanese manufacturers were steadily reducing the strokes of their in-line fours to raise the revs. This is not too much of a problem if you have a number of pistons all working together, but in the single cylinder world, running with huge revs to overcome the lack of torque (or extra pistons) is not comfortable.

Short strokes were popular on the scoote race circuit in the 1980-90's (even I built one) but use on the track is not the same as use on the road. On the track, you only do one clutch start per race. How many traffic light clutch starts at 6000rpm can you stand before you wish you had a torquey engine.

Martin

anyway, I got a bit of experience with short strokes. My last grp4 proto thing I attempted to build was a 61 over 51. It had more torque than my 135 58x51 but not proportionate. Bhp was higher but it suffered from being oversquare. The vibration was a killer using such a big piston on a little crank. It needed to be kept buzzing to make it fly. Years ago whilst racing group 1 for Kegra, Graham and Keith developed the short stroke 100. This was a 55 over 43 or thereabouts. Again this really had to be kept on the boil all the time. But went well. I wouldnt want a road bike like this.
The falc motors are less oversquare but with modern thinking and a crank rework they work well.

Bob West (Taffspeed) ran a 70mm bore and a 54mm stoke in group 4 for a few years. I thought Kegra had tried some thing simliar too. The group 5 60mm bore 54mm stroke were little rocket ships, but did rev to 11,000 rpm.

DMC wrote:Bob West (Taffspeed) ran a 70mm bore and a 54mm stoke in group 4 for a few years. I thought Kegra had tried some thing simliar too. The group 5 60mm bore 54mm stroke were little rocket ships, but did rev to 11,000 rpm.

There were/are arguable advantages under Gp.4 rules, and Bob wouldn't have been the only one (though getting people to disclose details is another thing).

even though the bore and stroke are to be displayed on the side panel

I remember Bob West ... He was at least 7 foot 11 inches tall.......well he looked that big when you put him on a Lambretta.

Kegra did quite a few. Tim Hayday had one. Most recently Phil Bevis ran one but I believe he preferred the torque of the longer stroke motor. Dean Aylott ran the short stroke engine last year.