revtothelimit - motorbike forum

Rockburner wrote: This is my Dad's contribution to the ouvre:


JAP 400cc 'speedway' engine in a (IIRC) Slimline Featherbed.
Custom tank (double chambered),
BTH mag
Twin leading shoe Grimeca (IIRC)
Norton (I think) gearbox
etc etc



He'd seen someone racing a JAP engined Featherbed in the 50s/60s (when he was racing himself) and always fancied building one, finally managed it after 50 years.... It's more of a race bike than a cafe-racer, but it's all road legal.

Yeah it’s a Slimline frame, but he’s done something drastic to the front downtubes!

I remember that my old Honda CB77 (305cc parallel twin) had a 180 degree crank. Don't recall much side to side action. And so of course does the 350LC.

mangocrazy wrote: Sun Jan 10, 2021 3:16 pm I remember that my old Honda CB77 (305cc parallel twin) had a 180 degree crank. Don't recall much side to side action. And so of course does the 350LC.

Yeah, shoulda said I am referring to 4 strokes.

The 'side to side' action is a rocking about the longitudinal axis of the bike - so you're trying to make it lean one way and then the other. All these vibe effects of course get better/worse as you make the pistons bigger/smaller and rev highe/lower.

I do wonder why so many modern twins are 270° rather than 360°. It seems to be the modern trend doesn't it?

Less vibey than a 360 maybe, but without the lumpy power of a 180° crank (in a 4T engine). Maybe they just didn't make 'em much before because it was expensive, but with modern manufacturing technology it's more affordable.

Mr. Dazzle wrote: Sun Jan 10, 2021 3:11 pm I think 180 cranks are pretty rare. AFAIK all the common garden parallel twins (CB500 etc.) are 360' cranks. Brit bike almost univerally are are too. 360' cranks mean both pistons go up and down at teh same time, one is intaking while the other is on the power stoke.

180 cranks would be where one piston goes up while the other goes down. They rock side to side loads and have an uneven firing order.

The 'big bang' engines still have 360' cranks, they just change the cam timing so both pistons are doing the same thing at the same time.

EDIT: Here you go, neat little gif.

2 stroke twins were all 180. The Kettle was 120 and perfect balance. You could put a 10 bob bit on the tank and it wouldn't fall off

Mr. Dazzle wrote: Sun Jan 10, 2021 3:11 pm I think 180 cranks are pretty rare. AFAIK all the common garden parallel twins (CB500 etc.) are 360' cranks. Brit bike almost univerally are are too. 360' cranks mean both pistons go up and down at teh same time, one is intaking while the other is on the power stoke.

180 cranks would be where one piston goes up while the other goes down. They rock side to side loads and have an uneven firing order.

The 'big bang' engines still have 360' cranks, they just change the cam timing so both pistons are doing the same thing at the same time.

There may not be many 180 degree twins in production now that balancer shafts are common, but from early 1960s to late 1970s, all the twin carburettor Honda twins had 180 degree cranks, as did many other Jap four stroke twins.

Suzuki GS500 crankshaft:

3 cylinder engines aren't perfectly balanced, although you can get pretty close with a balance shaft.

2 strokes are gonna be different to 4 strokes cause - obviously - they've got half as many strokes. One of the big disadvantages of a 180° crank 4 stroke is that you get two power strokes close together, then a big gap, then 2 close together, then a big gap etc. With a 2 stroke you don't, you just get perfectly distributed power strokes.

180° cranks have perfect primary balance, so they would seem good for vibration. They do suffer from the aforementioned rocking effect though, but as above that's proportional to your piston sizes and weights. 2 strokes tend to be small (roughly half the capacity of an equivalent 4T after all) so they have lighter pistons and you just don't notice as much. That also tells you why small capacity bikes of yesteryear, which also rev quite slowly (compared to today) used them.

As with all things like this, is just compromises in lots of directions.

Yorick wrote: Sun Jan 10, 2021 3:26 pm 2 stroke twins were all 180. The Kettle was 120 and perfect balance. You could put a 10 bob bit on the tank and it wouldn't fall off

A 2T inline triple would be every bit as smooth as an inline 4T six cylinder, which has perfect primary and secondary balance characteristics. They also sound quite similar (unsurprisingly).

And all this stuff about 180 degree 4T cranks having a rocking couple - in theory they do. In practice (at least on a 250/305cc engine) you really don't notice it.

mangocrazy wrote: Sun Jan 10, 2021 3:32 pm

Yorick wrote: Sun Jan 10, 2021 3:26 pm 2 stroke twins were all 180. The Kettle was 120 and perfect balance. You could put a 10 bob bit on the tank and it wouldn't fall off

A 2T inline triple would be every bit as smooth as an inline 4T six cylinder, which has perfect primary and secondary balance characteristics. They also sound quite similar (unsurprisingly).

IIRC, the addition of all the other moving stuff in a 4 stroke, makes them 'slightly' less perfect.

mangocrazy wrote: Sun Jan 10, 2021 3:32 pm

Yorick wrote: Sun Jan 10, 2021 3:26 pm 2 stroke twins were all 180. The Kettle was 120 and perfect balance. You could put a 10 bob bit on the tank and it wouldn't fall off

A 2T inline triple would be every bit as smooth as an inline 4T six cylinder, which has perfect primary and secondary balance characteristics. They also sound quite similar (unsurprisingly).

And all this stuff about 180 degree 4T cranks having a rocking couple - in theory they do. In practice (at least on a 250/305cc engine) you really don't notice it.

Not quite - the 2 or 4 strokeyness of it doesn't effect the balance issues, beyond the differneces in piston size etc. The underlying cuase of the imbalance remains.

120° 3 cylinder engines also have a rocking motion about an axis through the middle of the engine. A straight 6 just puts two 3 cylinders back to back so their rocks cancel out. IL4's do the same thing, they're just 2 180° twins mounted together - again so that the rocking motions cancel out.

The main difference with a 3 cylinder engine is that is also has better secondary balance. That is, vibrations at twice engine speed are less than in a twin.

Someone needs to explain to him what a 2 stroke is.

The biggest source of vibration in an engine is the pistons going up and down. Explain how being 2 or 4 strokes effects that

Balance is not just about reciprocating mass and how it's distributed, but also about firing cycles.

It is - hence the point about why you see 180° cranks in 2 strokes more than 4s.

Technically it's "smoothness" in the biz. Balance refers to vibration due to things moving about, smoothness to the 'lumpiness' of tge torque output of the engine.

Mr. Dazzle wrote: Sun Jan 10, 2021 3:48 pm The biggest source of vibration in an engine is the pistons going up and down. Explain how being 2 or 4 strokes effects that

The biggest source of vibration is the firing sequence. And how often.

There is naff all vibration if the engine is just spinning, without load.

The biggest source of vibration is the firing sequence. And how often.

There is naff all vibration if the engine is just spinning, without load.

That is defintely not true.

When 'they' test engines for this sort of thing they frequently drive them around 'dry' with an electric motor.

FFS, I give up.

Mate, this is literally my day job don't forget!

Mr. Dazzle wrote: Sun Jan 10, 2021 3:58 pm Mate, this is literally my day job don't forget!

Yeah, have you ever seen a 2 stroke triple ?


Go annoy somebody else.

Yorick wrote: Sun Jan 10, 2021 4:01 pm
Yeah, have you ever seen a 2 stroke triple ?

No I haven't.

Have you ever worked for an engine design consultancy?

Dunno what your argument is really. I don't deny 2T engines or 3 cylinder engines or anything else can have low vibrations, I've even told you how it's done.