Norton Atlas with an offset crank

Interesting.

How do you do the ignition timing on one of these things? I was hoping he had a video on how to, but I watched 5 videos and ran out of time to waste.
 
Yeah! I read "Tuning For Speed" and have been intrigued by rephased cranks ever since. Enough to even go a little ways down the road of building a rephased Yamaha XS650 (not finished yet). Is there an off-the-shelf ignition system available for Nortons? What about the cam? (I admit I did not watch the video).
 
I beleive Pazon do offset ignition units. I'm sure knowing Andy he will be able to make something to suit.

Camshafts will be more of a challenge I expect.

But the big one I worry about with Nortons and offset cranks is harmonics in the cam chain drive. I'm interested in trying a 270 crank but anecdotally I understand they can be unreliable due to cam drive issues. . Very interesting in knowing some real experiences from anyone who has actually tried it.
 
The term "offset" is ambiguous. We have a staggered or rephased crankshaft, and an engine design providing a desaxe layout.
I read somewhere the Atlas engine is of small desaxe design, the figure is -0.093" (yes, negative), a result of some development shortcuts, and too small to be of any significance.
An intenional desaxe layout is found in the AMC lightweights, G2 and G5, which were good performers in their day.

These staggered crankshaft show-offs are tiresome to look at. Let's have a review of the real thing!

- Knut
 
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I think that for a FACTORY to produce a 270 twin makes sense. They will put in the necessary R&D into balancing etc. They will also have the equipment to make the cams without issue etc.

But when a DIYer does it, I tend to think that the the amount of opportunity to get such things wrong, or at least sub optimal, must go a long way to undoing the whole point.

And also, I’m just biased, I just prefer the nice crisp sound of a 360 degree Brit twin. When people say ‘it’s great, it’s like a v twin’ I always think, well why not just buy a v twin then ?!?
 
Fast Eddie said:
I think that for a FACTORY to produce a 270 twin makes sense. They will put in the necessary R&D into balancing etc. They will also have the equipment to make the cams without issue etc.

But when a DIYer does it, I tend to think that the the amount of opportunity to get such things wrong, or at least sub optimal, must go a long way to undoing the whole point.

And also, I’m just biased, I just prefer the nice crisp sound of a 360 degree Brit twin. When people say ‘it’s great, it’s like a v twin’ I always think, well why not just buy a v twin then ?!?
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I could not agree more, as the sound of a Commando running fluted peashooters is pure music. I did think about a 961 Norton a few years back, but the fact that it didn't have a 360 deg. crank put me off even though I love the sound of vee twins, & my Buell especially. There is no way I would change any of my Brit twins in this way. Other opinions are of course available.
 
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74ok said:
Prob see more of it down that road, certainly, the yam xs650 folks have already.
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What about the rationale? I miss an explanation in the presentations. It can't be a power surge - no one has yet demonstrated a rephased engine produces more power.
A little less vibration at high rpm's? Probably, if done right, but then, why are they constructing bobber bikes for around town riding? The original 180 or 360 degree engines were low on vibration already.
Constructing a rephased engine is no big feat in the case of an XS650. i wonder if the true motivation is a strange fixation for the V-twin sound only.

- Knut
 
Not sure if you are asking me specifically about presentation, rational etc, but for me, ended up seeing vids & stuff, new to me that i found interesting, so posted it

Then was aware that xs650 folks went there quite a bit, for whatever reasons, including various re-phasing angles, combined with power increases etc, my motivation is fwiw

then imo, it's inevitable that over time, others will go there with nortons
 
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Why climb Mt. Everest? If you want to be 35,000 feet up in the air, you could go in an airplane, in fact you could get to that altitude (and higher!) much faster and safer with an airplane. And then you could go lots of other places too, and not just to the summit of Mt. Everest...

Can't speak for anyone else, but my own motivation for doing things like offset cranks and Sportster based dirt bikes and restomod P11's is just because I want to. I amuse myself by going my own way when I can.
 
I'm sure that the "desaxe" bit was just a cheaper way for Norton to fit a bigger bore into the existing Dominator tooling.
Why shift the crank centre backwards? To avoid the barrels fouling the existing camshaft position.
They obviously worked out it was cheaper to do that than move the camshaft forward.
You'll note the Atlas also does not have the timed breather port on the rear of the crankcase. Why did they put it on the end of the camshaft? Because the larger bore gave no room for the passage rearwards - again to allow further use of existing tooling.
 
Didn't Phil Irving write an article on this back in the 50's. A friend who isn't on this forum built one using a Commando engine with Hinze Keglers help.
I have seen it run and he wasn't afraid to take it on long rides either. I remember him riding it up to the rally in Torry Utah from Albuquerque. They also built a 2 finger clutch for an Atlas. It almost didn't need clutch springs. I know I have pictures of the clutch, just not sure where.
 
Phil Irving 270 crank

Phil Irving

www.cycleworld.com

Engine Architecture: Twins

Harley, Indian, Triumph, Guzzi, Ducati, BMW; so many famous motorcycle companies made their mark with twin-cylinder engines.
www.cycleworld.com www.cycleworld.com

Crossplane Cranks​

Today the parallel twin is back, and for some of the same reasons that motivated Edward Turner. Compactness was important to Turner because his Speed Twin had to replace a single in an essentially unchanged frame. Compactness is important now, when the twins’ leanness, light weight, and reduced parts count are so often replacing the “beaminess” of the inline-fours that have been with us since Honda’s 1969 CB750.

These new-generation twins offer something else: 270-degree crankpin spacing, something which Vincent engineer Phil Irving (author of the classic text Motorcycle Engineering) recommended back in the 1950s. Why? With crankpins at either 360 or 180, both pistons are stopped at top and bottom center, imposing an “inertia torque” on the crankshaft. Having to start and stop both pistons simultaneously imposes a fluctuating load on the crank. This motivated Yamaha engineer Masao Furusawa, now retired, to give Valentino Rossi an inline-four MotoGP engine whose crankpins were spaced at 90 degrees to each other rather than the traditional 180. That fluctuating load from piston starting/stopping interfered with traction on the track; with Furusawa’s and Irving’s 90-degree crankpin spacing half the pistons are stopped while the other half are near their top speed. Thus they exchange energy with each other, and crankshaft instantaneous speed varies but little.

But why a crossplane crank for production parallel twins? The real reason is probably engine sound—the 90-degree crankpin spacing transforms the normally droning even firing interval into a syncopated sound more like…more like a Ducati V-twin.
 
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acotrel

Thread '270 degree crankshaft'

've been having a discussion with a friend about fitting a 270 degree crank to a Commando engine and the matter of secondary balance has come up. I'm wondering if any of you guys have built a motor with this type of crank ? Secondary balance is something of which I know very little. Found this article interesting :

https://en.wikipedia.org/wiki/Engine_ba ... 29_balance

jseng1

Thread '270 degree crank'

Just got this beauty in today. Shortstroke 270 deg nitrided crank going into a fully kitted racer down under.

270 degree crank

J

Thread '270 degree crank'

As any of you lads ever built a 270 crank for a Commando? Phil Erving suggested this revised throw to Edward Turner in 1947, But they {triumph} dismissed it. Proberly due to Re-tooling costs.plus the little 500 twins where "fairly "smooth any way.
I have read plenty of advantage's using this design [Yam TRX 750], one being a 47% reduction in vibes,plus improved torque compaired to 360 throw.. Exhaust beat is "V" twinish....your views on this please.
 
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