Interesting 650 Norton prototype

hybridracer said:
Norton were a small company making an insignificant number of bikes compared with Triumph & BSA. They did not have the capacity properly to develop more than one design at a time and with AMC dominated by Matchless/AJS, too much attention was focused on trying to keep those going rather than throwing everything at Norton.
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They sure cranked out a lot of bikes during WWII though.
 
acotrel said:
Norton were primarily interested in racing, and their bikes reflect that. They sold road bikes to finance their racing. A Norton was usually a real motorcycle. The 1960s was the space age. Unit construction engines leak less oil. However a unit construction Bonneville handles better than a preunit Bonneville. I think it was mainly due to steering geometry. A Commando 750 is a detuned Atlas 750. It is a different mindset - the Honda CB750 gave Norton a fright. I rode a CB750 when they first arrived - to me, it felt like riding a brick. But I was used to bikes which handled. There are two mentalities involved - fast in a straight line - or fast in corners. Some people die in the bum when they are blown to the weeds in a straight line. Two-strokes were faster, but could often not accelerate when in a corner - they usually had to be upright and pointed. Reed valves changed that. Most race circuits are half corners and half straights, but public roads go on forever. So you need a different motorcycle. It is all about having fun.
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Al,

Please tell us SPECIFICALLY how Norton de-tuned the engine from Atlas to Commando…
 
Reading behind the headline Norton in WW2 were in the middle of Birmingham with a lot of auto parts makers with nothing to do. A lot would have retooled for aircraft parts production but they would also been available to take on making motorcycle parts leaving Norton to concentrate on final assembly. Pure speculation but the Norton site in Aston is very small even when you add on the spanner factory next door would had bought Norton.
 
The Commando crank has a hole drilled into the wrong side of the flywheel to make the motor run smooth. The balance factor affects the way the motor spins up, regardless of how the motor is suspended. A racing 650 Triumph Bonneville usually has a balance factor of nearly 80%. My friend has an Atlas 750 in which the crank is balanced to 79%. It won many All-powers A Grade races in the early 1970s. Crank balance in a race bike is not about rider comfort - my 850 motor is balanced at 72% -at 7000 RPM, it is dead smooth and its throttle response is excellent. If the crank moves the motor about, that is energy which is not going down the chain.
 
mdt-son said:
Very noisy engine (valve clatter). I wonder if some chains were eating metal?

Compare this to a well-fettled G15:


- Knut
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Paul Henshaw made a comment (sometime ago) about engine 'sound' in his videos. I think it was something that was accentuated by his camera's microphone, I presume the camera is mounted on a harness on his chest? There was a chain induced noise on the bike, there are a number of videos of him investigating the noise. No spoilers, you will have to watch the series of videos...
 
daveforty said:
Paul Henshaw made a comment (sometime ago) about engine 'sound' in his videos. I think it was something that was accentuated by his camera's microphone, I presume the camera is mounted on a harness on his chest? There was a chain induced noise on the bike, there are a number of videos of him investigating the noise. No spoilers, you will have to watch the series of videos...
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Yep he's not a sound engineer, neither does he claim to be
The mike on his camera makes most bikes sound awful
As does my phone if I record a bike running it sounds absolutely nothing like the actual bike!
 
acotrel said:
The Commando crank has a hole drilled into the wrong side of the flywheel to make the motor run smooth. The balance factor affects the way the motor spins up, regardless of how the motor is suspended. A racing 650 Triumph Bonneville usually has a balance factor of nearly 80%. My friend has an Atlas 750 in which the crank is balanced to 79%. It won many All-powers A Grade races in the early 1970s. Crank balance in a race bike is not about rider comfort - my 850 motor is balanced at 72% -at 7000 RPM, it is dead smooth and its throttle response is excellent. If the crank moves the motor about, that is energy which is not going down the chain.
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Complete nonsense. You can't compare balance factors in radically different frames as though they are identical. The whole assembly acts as a very heavy crank counterweight in the Isolastic design. The balance factor chosen for the Commando was optimized for this condition. 70%-80% is the optimum range in many solid mount frames, but far too high in a Commando frame. This isn't just about comfort, but once you've chosen the Isolastic frame, which is about comfort, you need a balance factor to match.
 
mdt-son said:
Very noisy engine (valve clatter). I wonder if some chains were eating metal?

Compare this to a well-fettled G15:

- Knut
Click to expand...
The prototype 650 was quiet under power, noisy putting around or off throttle like all old Nortons. Nothing special about the engine noise from the G15 as far as I could tell. I could still hear the typical clatter. I'm wearing earphones though, so the sound reproduction on my computer might be higher quality.
 
hhh said:
Complete nonsense. You can't compare balance factors in radically different frames as though they are identical. The whole assembly acts as a very heavy crank counterweight in the Isolastic design. The balance factor chosen for the Commando was optimized for this condition. 70%-80% is the optimum range in many solid mount frames, but far too high in a Commando frame. This isn't just about comfort, but once you've chosen the Isolastic frame, which is about comfort, you need a balance factor to match.
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the near 50% balance factor for Commando/Iso is presumably beacause the idea is for the engine to shake in small circles ‘around’ the crank, which gives the Iso rubbers best chance of minimum transmission to rest of bike, above the low revs resonance.
 
jan nelder said:
the near 50% balance factor for Commando/Iso is presumably beacause the idea is for the engine to shake in small circles ‘around’ the crank, which gives the Iso rubbers best chance of minimum transmission to rest of bike, above the low revs resonance.
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That's exactly what I was trying to convey. The whole assembly (engine, gearbox, primary, and swing arm) replaces about a third of the crankshaft counter weight that would be needed in a rigid mount frame because it has the freedom to oscillate opposite to the pistons. Of course it's not immediately obvious because the far higher mass of this "counterweight" means that it achieves the same force with very little movement. I wonder if they realized this at the time? Does anybody have a whitepaper or know of discussions available anywhere on the Isolastic concept?
 
hhh said:
That's exactly what I was trying to convey. The whole assembly (engine, gearbox, primary, and swing arm) replaces about a third of the crankshaft counter weight that would be needed in a rigid mount frame because it has the freedom to oscillate opposite to the pistons. Of course it's not immediately obvious because the far higher mass of this "counterweight" means that it achieves the same force with very little movement. I wonder if they realized this at the time? Does anybody have a whitepaper or know of discussions available anywhere on the Isolastic concept?
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they did: they aimed for something like 1/64 inch vibration radius or diameter(?) at design stage. The concept was done by the two engineers pretty quickly, and it worked well. ( this is remembered from their own description , which I read some time ago)
 
jan nelder said:
they did: they aimed for something like 1/64 inch vibration radius or diameter(?) at design stage. The concept was done by the two engineers pretty quickly, and it worked well. ( this is remembered from their own description , which I read some time ago)
Click to expand...
Which is why the isolastic rubber hardness is so crucial, and why Norman White was so scathing of those who made harder ones when I spoke with him about it once. He said that a lot of effort was put into getting them right at the factory, which, based on how well they work, I think is quite true.
 
I just read the patent and it doesn't mention crankshaft balance factor, but that's not terribly surprising since it's not essential to the patent.
 

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