Why a [360°] parallel twin?

Hi guys, with all the inherent issues with a parallel twin engine design over a 180 degree twin, why did Norton & indeed the majority of the British motorcycle industry go down the parallel twin road??
I cant believe the technology & tooling wasn't available at the time so was it down to cost or just dog headed management?

Do you mean a 360 crank versus a 180 crank?

They are both parallel twins.

If the question is why 360 then possibly ability to use a single carb plus the magneto.

olympus said:

Hi guys, with all the inherent issues with a parallel twin engine design over a 180 degree twin, why did Norton & indeed the majority of the British motorcycle industry go down the parallel twin road??
I cant believe the technology & tooling wasn't available at the time so was it down to cost or just dog headed management?

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Not quite sure of the question but basically a parallel twin engine will fit where a single engine was
As Edward turner did with the first speed twin
No need for a new frame/gearbox or anything really
The bike was so successful every manufacturer copied

What: 'inherent issues' Malcolm?
Given most if not all early examples were low compression 500cc jobbies, did these 'issues' only appear as they were eventually (over?) stretched??

The 360 degree twin gave smooth, even firing intervals. It fitted in the cycle parts of a 500. It was truly a game changer.

A pre war 500 Triumph is a real eye opener. By the standards of the day it was so easy to start, so smooth, and so fast. And no heavier or wider than the single.

The ‘inherent’ problems only became inherent when twins grew bigger, had higher CR, and revved higher than their original designers ever intended or thought wise.

The 360 twin, I believe, started with an Edward Turner pre-war design. I think it may have been a stationary engine - generator?
Fuel was scarce so the 360 crank allowed the use of a single carb, for economy reasons, as @johnm alluded to above.
You'll never see a 180 crank Jap bike or 270 "modern" twin with a single carb/injector.
The lack of a central crank bearing was a weakness - Matchless/AJS had a half-arsed go at putting one there in the 60's.
So... the 360 agree twin has its weaknesses but it also has its charm - which is why we love them so much.
Ever noticed that the BMW boxer twin sounds the same? The sound we all love!
Cheers

gforce said:

It strikes me that it would not suffer from leaks in the same way as a 360, because one piston would be up while the other is down, taking the air pump effect away to a large extent.

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Wrong! - the Jap 180 twin has one up while the other is down.
The 270 is halfway between - sort of sounding like a fake Ducati (961??)

@robs ss sorry I spoke - I'll get back in my box and leave you to it.

The small cb175 Honda's were 180° and single carb as far as I remember

baz said:

The small cb175 Honda's were 180° and single carb as far as I remember

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CB175 had twin carbs:
"The CB175 was a high revving engine producing 20 bhp (15 kW), this was a beefier version of the CD175 but with twin carburetors and wilder cams."

"The Honda CD175 was a 174 cc bike introduced in USA, Canada, UK, Africa, Asia, Australia and New Zealand in 1967. Main features of this bike included 360 degree cranks, a single ignition, single carburetor to cut down maintenance, odd styling, a parallel twin high revving engine and a speedometer that calibrated to 100 mph (160 km/h)."

L.A.B. said:

CB175 had twin carbs:

Honda CB175K: review, history, specs - CycleChaos

www.cyclechaos.com

"The CB175 was a high revving engine producing 20 bhp (15 kW), this was a beefier version of the CD175 but with twin carburetors and wilder cams."

69-73 CB175 69-73 CL175 69 CA175 70-71 SL175 CRANKSHAFT RODS ASSY 13000-302-000 | eBay

Find many great new & used options and get the best deals for 69-73 CB175 69-73 CL175 69 CA175 70-71 SL175 CRANKSHAFT RODS ASSY 13000-302-000 at the best online prices at eBay! Free shipping for many products!

www.ebay.com

Honda CD175: history, specs, pictures - CycleChaos

www.cyclechaos.com

"The Honda CD175 was a 174 cc bike introduced in USA, Canada, UK, Africa, Asia, Australia and New Zealand in 1967. Main features of this bike included 360 degree cranks, a single ignition, single carburetor to cut down maintenance, odd styling, a parallel twin high revving engine and a speedometer that calibrated to 100 mph (160 km/h)."

NOS HONDA CD175 CRANKSHAFT CRANK | eBay

CD175 CD 175. CRANKSHAFT.

www.ebay.co.uk

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Sorry I meant the CD not CB
The literature I have read states it's a 180° ?
I have never had dealings with one so I wouldn't know
Until now

https://www.motorcyclespecs.co.za/model/Honda/honda_cd175_73.htm
This states it's a 180° crank
But the eBay photos prove otherwise!

baz said:

This states it's a 180° crank

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Not "CDI" ignition either.

Yamaha TX750, XS1/XS650 were 360°.

While both pistons up and down together is a balance nightmare, one up and one down gains balance in the up-down direction at the expense of balance in the transverse direction.

Did not stop BMW however.

Slick

Torque!

Hi all,
As has generally been pointed out, a 180’ twin is not a nicely balanced engine, the two piston working as a rocking couple. Not a nice vibration with bigger engines. In my last year of school I had a Yamaha TX 500 with a two into one hooker exhaust that went like stink. It was very complex with 2 counter rotating balance shafts and twin cam 4 valve head. I just read the review in the previous post of the Yamaha XS 750 and my bike had all the same problems including balance shafts coming out of phase and trouble with the starter sprag clutch. However it was beautifully smooth to ride. It would eat my mate’s 500/4 and wasn’t far behind the big two strokes of the day.
So I guess that the 360’ twins the British used served them quite well up to a point. When built down to a price, of smaller capacity and with lower compression, they were ideal, especially when compared to hammering around on a big single. Obviously by the sixties the writing was on the wall but none are as blind as those who will not see, so british management refused to capitalise into new tooling to make their bikes competitive into the future.
I believe that so good was the skill of the famous engineers (i won’t bother naming them) that they were able to keep tweeking the outdated design and kept them competitive long after their use by date. I wonder if this actually worked to the companys’ disadvantage, as by the time it was clear to everyone that the bikes were outdated it was too bigger leap to catch up with the predominately Japanese opposition.
I am an unashamedly big Trident fan (and Norton of course) but in quiet moments of reflection I have to admit they were too little, too late. Done right with new tooling, more capital and done a few years earlier……but that’s another story
The previous post also and a write up on the little CB175. I also had one. What a little gem! One wonders what the leaders of Triumph Norton BSA etc must have secretly thought when they rode one. Surely they must have known the writing was on the wall.
So, in brief, I think I’m saying a well designed 360’ vertical twin is a hell of a good package with all sorts of advantages when strapped into a good frame. However, it ultimately is not possible to make strawberry jam out of horse excrement. Ultimately they were not capable of keeping British bikes at the forefront of world biking.
As a final aside, am right in thinking that Bonneville, in their last gasps in the eighties, made a version with harmonic balance shafts? A mate of mine had a well sorted T140V with an eight valve Norish head. It went like crazy and had acceptable vibration. Such a shame the TSS wasn’t done earlier and better.
regards
alan

texasSlick said:

While both pistons up and down together is a balance nightmare, one up and one down gains balance in the up-down direction at the expense of balance in the transverse direction.

Did not stop BMW however.

Slick

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I read somewhere that with the one up one down design , it makes a lazy motor , because you have a piston slowing down toward tdc and the other slowing for bdc .

One curious thing I don't know why. My -72 CB250K4 has a 180 crank. The -78 CB250 has a 360 crank. Why did Honda change?

Years ago I used to race my short-stroke 500cc Triton against guys who rode 450cc Honda twins - they did not seem to be much good. The 850 Commando engine is really great. The only thing which has stuffed it is the hole in the bob-weight. It depends on what your intended purpose is for the bike - if you want to commute - smooth is better. If you really want to go, moving the smooth portion of the rev-range upwards is better. I use a 2 into 1 exhaust on my bike - it pulls like a train right up through the entire usable rev range. I do not think a similar exhaust would work well on a 180 degree twin. If you think about it - my tail pipe is the same length as one of the header pipes, and it resonates at twice the frequency of one of the header pipes. With a 180 degree twin, the firings are irregular.
The only things which make a Japanese bike faster is the number of valves , their engine management, and their gearbox.

There is a motor which is used in Japanese speedway racing - The Suzukl Engine Auto Race. They cannot be bought. It is a 180 degree double over head cam vertical twin. They seem to be very fast. I think the 500cc Paton is similar. Most of the British 500cc twins were 360 degree and pushrod valve operation. The Triumph 500 which Percy Tait raced was probably the fastest.