true or false

jseng1 said:

Oh Yeah - I just remembered another repeated saying that slipped my mind - bringing down the compression will reduce vibration. I've never found anything to reduce vibration but lower reciprocating weight.

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I wonder why they used concaved pistons on Atlas and P11's ?
My P11 came with concaved or dished pistons. I thought the reason for the dished pistons was to reduce vibration on a solid frame like the featherbed and the P11 frame.
All that being said the P11 had an Atlas engine dropped into the frame.

So they could run them on cheap gas with 7:1 compression. Probably not the right reason, that is just what came to mind.

By the way, I wish I'd kept those dished pistons. Phil Radford was more than happy to take them off my hands for free when I went +.020 with flat top Commando pistons chasing a delusional dream.

My bone stock P11 engine was a serious bone rattling paint shaker. It isn't anymore, but it is still a Norton and you can't stop a Norton from vibrating.

CanukNortonNut said:

I wonder why they used concaved pistons on Atlas and P11's ?
My P11 came with concaved or dished pistons. I thought the reason for the dished pistons was to reduce vibration on a solid frame like the featherbed and the P11 frame.
All that being said the P11 had an Atlas engine dropped into the frame.

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My 850 motor does not vibrate, however it rocks the whole motorcycle backwards and forwards when it is idling. It smooths out at about 4,000 RPM, and is dead smooth between 5000 RPM and 7000 RPM - such a motor should be. The Commando was designed to compete with the Honda CB750. When the CB750 is compared with an Atlas, the comparison is ridiculous if you do not know anything about motorcycles. One of my friends has an Atlas which he raced in the 1960s. I rode a new Honda CB750 in 1973. It was like riding a brick. I was very young when I had my first adrenalin rush - some people just do not get it. To me, a turbine-smooth motorcycle is a piece of shit - how do you know when to change gear ? On my bike, when the note becomes an 'E', the motor has exploded.
The long and short of it is, a race developed bike, is often useless as a road bike but it can feed a fantasy.

acotrel said:

My 850 motor does not vibrate, however it rocks the whole motorcycle backwards and forwards

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Yep - I'm sure your 850 doesn't vibrate - that is, until you start it!

acotrel said:

My 850 motor does not vibrate, however it rocks the whole motorcycle backwards and forwards when it is idling. It smooths out at about 4,000 RPM, and is dead smooth between 5000 RPM and 7000 RPM - such a motor should be. The Commando was designed to compete with the Honda CB750. When the CB750 is compared with an Atlas, the comparison is ridiculous if you do not know anything about motorcycles. One of my friends has an Atlas which he raced in the 1960s. I rode a new Honda CB750 in 1973. It was like riding a brick. I was very young when I had my first adrenalin rush - some people just do not get it. To me, a turbine-smooth motorcycle is a piece of shit - how do you know when to change gear ? On my bike, when the note becomes an 'E', the motor has exploded.
The long and short of it is, a race developed bike, is often useless as a road bike but it can feed a fantasy.

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acotrel said:

To me, a turbine-smooth motorcycle is a piece of shit - how do you know when to change gear ? On my bike, when the note becomes an 'E', the motor has exploded.
The long and short of it is, a race developed bike, is often useless as a road bike but it can feed a fantasy.

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Seriously? You'd know when to change gears, unless your brain stopped working.

I liked all my pieces of shit motorcycles except for a Kawasaki ZX-10. Did not fit me at all. Too stretched out.

I never believed my 850 motor would ever be any good for racing, however I was pleasantly surprised when it responded so well to tuning. It would be interesting to know the thinking behind the Domiracer. I used to think Triumph 650s were good, however the Norton motor is so much better. The fastest Triumph 650 motors I had, had flat top pistons - one had a squish band inside the combustion chamber.

H2 Kawasakis are a laugh a minute. I watched a guy get chucked up the road off one as he crossed the start/finish at full noise during a race. It was on full noise and just suddenly swapped ends and launched him. 1970s H2 and Z1 Kawasakis are not as fast as you might think, most guys cannot use them properly. I could sometimes beat them around most parts of a moderately fast circuit, but never in a drag race down a long strait.

Taking a wander into the off topic weeds.

Funny thing for me is the only bike I let go that I actually wish I had today is the first one I ever bought. It was a used 60's
something Triumph with high CR pistons in it and it was all there tin wise. Not sure which model. Maybe a T100T 500cc. I didn't take pictures back then. I rode the heck out of it.

So when was the last time you rode any bike Al and all 360 swing twin motorcycles will vibrate to some point, and in your own words you say your Norton don't vibrate but then you say it smooths out once it gets over 4k so that tells me it does vibrate, anyway there are a few different ways you can make them run smoother, some by throwing a lot of money at the motor or you can just do minor improvement without spending a bucket full of money, like crank balance, open the ports and open exhaust system, better tuning, bigger spark and jetting all makes a difference to a smoother motor and everything connected to it.
Back in the late 70s early 80s when I built my 850 for the Featherbed frame, there weren't the good gear around what Jim sells now, but there were minor upgrade around if you had the money, I wasn't working when I built my motor but there were some knowledgeable old timers around with very long experience in building race engines both British bikes and cars and how to make them get up and go, I always listened to good advice at the time (no internet in them days) and things were cheap to do at the time in machining etc. and getting things done, lots of smart people around then.
Most of the work done to my motor back then was using the high-performance section at the back of my Norton workshop manual, it had a stage one section for street bikes and a stage 2 section for race/track builds, it even had a small bit about the supercharge kit and bigger carbs as well how to make open megaphones and tuned length for the exhaust system as well a Boyar ignition system and jet sizes if you kept the old Amals.
I went the stage one way for my motor somethings I did by the book and a few things I went another way and balancing the crank to 72% was one way I went after talking to Ivan Tighe my cam builder, he also put me onto the crank balancer.
Well that was back in 1980/82 when I went down this road and my motor is still the same as when I first built it and for a 360-swing twin it's not too bad on the vibes, it has 2 sections in the rev range it gets some bad vibes but it always smooths out once past.
I love to build a motor with all of Jims bits and pieces but it's way past my budget, but what I done with my motor at the time has worked well for me and has been a very reliable motor with a lot more get up and go than when it was just a stock 850.
It all boils down in what you want out of your motor, how much money you want to throw at it and all, Norton decided to go down the isolastic way but there was other things they could have done as well but of course cost and time was running out for them at the time, I am glad I went down the road I took with my new Norton at the time and the conversion to the Featherbed frame.

Ashley

concours said:

View attachment 116180

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have the same year and color H2, but mine is unmodified. Nice heads.

It’s a good topic this, does increasing CR cause worse vibration?

Can’t say I’ve ever noticed a clear link either.

Maybe the idea comes from the fitting of HC pistons, that have high does (like Triumohs) or are taller (like period Norton offerings) all of which make for significantly heavier pistons ?

acotrel said:

My 850 motor does not vibrate, however it rocks the whole motorcycle backwards and forwards when it is idling. It smooths out at about 4,000 RPM, and is dead smooth between 5000 RPM and 7000 RPM - such a motor should be. The Commando was designed to compete with the Honda CB750. When the CB750 is compared with an Atlas, the comparison is ridiculous if you do not know anything about motorcycles. One of my friends has an Atlas which he raced in the 1960s. I rode a new Honda CB750 in 1973. It was like riding a brick. I was very young when I had my first adrenalin rush - some people just do not get it. To me, a turbine-smooth motorcycle is a piece of shit - how do you know when to change gear ? On my bike, when the note becomes an 'E', the motor has exploded.
The long and short of it is, a race developed bike, is often useless as a road bike but it can feed a fantasy.

Click to expand...

There is a big difference between race bike vibration and road bike vibration…

On a racing Commando between 4-7k rpm the rider has significant other things to focus his mind on ! Vibration would therefore have to be quite bad to be noticed.

Conversely, on the road, if a bike vibrates at a particular rpm that the rider likes to cruise at for long periods, then even relatively low vibration can become annoying to that rider.

On a race bike (well, at least a short circuit race bike), provided the vibration isn’t going to cause any mechanical issues with the bike, it is more than likely considered smooth. Or looked at the opposite way, if the vibration is bad enough to upset the rider it’s likely to be bad enough to break things, cause fuel frothing, and hold back the motors revs, etc.

I recently had a track Norton that made my hands tingle for a week after an event ! I stripped it, fitted JS rods and pistons and rebalanced the crank. My hands no longer tingle and it pulls more revs and got 7rwhp more on the dyno …

ilikevelos said:

have the same year and color H2, but mine is unmodified. Nice heads.

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TY.
Fork legs & seat to be refreshed this winter. The clock ran out on me in May.
Any idea where a good speedo for H2B/C could be found?

Schwany and SteveA
Did either of you try loosening the head bolts a few turns and using the kickstarter to loosen the head? I haven't needed to do that with Pliobond but I'm curious.

If not then a couple taps on the ex nut as show below should do it.

jseng1 said:

Schwany and SteveA
Did either of you try loosening the head bolts a few turns and using the kickstarter to loosen the head? I haven't needed to do that with Pliobond but I'm curious.

If not then a couple taps on the ex nut as show below should do it.

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I removed the head bolts, the headers and exhaust, the carburetors, and the intake manifolds. I did not try using the kickstart to pop the head up. When I couldn't get it off with my hands, I went straight for the hammer and wood block. I can't remember where I placed the wood block exactly. Probably under the exhaust ports and intake ports working my way around. Light tapping did not work. The final blows with the hammer and block were on the exhaust ports. I definitely should have tried using the kick start.

No big deal what I think. Others should try Pliobond sealer with copper head gaskets and see how they feel about using it. It is sticky stuff. I was using it on iron barrels at the time. I have not tried it on aluminum barrels.

Fast Eddie said:

There is a big difference between race bike vibration and road bike vibration…

On a racing Commando between 4-7k rpm the rider has significant other things to focus his mind on ! Vibration would therefore have to be quite bad to be noticed.

Conversely, on the road, if a bike vibrates at a particular rpm that the rider likes to cruise at for long periods, then even relatively low vibration can become annoying to that rider.

On a race bike (well, at least a short circuit race bike), provided the vibration isn’t going to cause any mechanical issues with the bike, it is more than likely considered smooth. Or looked at the opposite way, if the vibration is bad enough to upset the rider it’s likely to be bad enough to break things, cause fuel frothing, and hold back the motors revs, etc.

I recently had a track Norton that made my hands tingle for a week after an event ! I stripped it, fitted JS rods and pistons and rebalanced the crank. My hands no longer tingle and it pulls more revs and got 7rwhp more on the dyno …

Click to expand...

When a motor vibrates at high revs that is energy going into the frame of the motorcycle, and not down the chain. The fastest 650 Triumph I had, was fitted with 7 to 1 flat top pistons, and the counterweights on the crank had been built up with weld. It was an old motor which I think had been raced in a sidecar. I sold it for about 30 pounds in 1965, when I bought the short stroke Triton. I was an idiot - I should have raced that motor - not the short stroke one. On 7 to 1 comp. on methanol, it was extremely rapid. The guy who bought won a sidecar race with it, then blew it up. it's balance factor must have been up around 90%. With a low balance factor, the motor always seems to run with distress, and does not spin-up so easily.

acotrel said:

When a motor vibrates at high revs that is energy going into the frame of the motorcycle, and not down the chain. The fastest 650 Triumph I had, was fitted with 7 to 1 flat top pistons, and the counterweights on the crank had been built up with weld. It was an old motor which I think had been raced in a sidecar. I sold it for about 30 pounds in 1965, when I bought the short stroke Triton. I was an idiot - I should have raced that motor - not the short stroke one. On 7 to 1 comp. on methanol, it was extremely rapid. The guy who bought won a sidecar race with it, then blew it up. it's balance factor must have been up around 90%. With a low balance factor, the motor always seems to run with distress, and does not spin-up so easily.

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It is not easy to raise the balance factor of the Commando crank above about 72%. I would not drill or machine the cast iron flywheel.

acotrel said:

It is not easy to raise the balance factor of the Commando crank above about 72%. I would not drill or machine the cast iron flywheel.

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I must be misunderstanding something. Anyone that has had a stock iron Norton crank rebalanced got a crank back with holes drilled in it. Particularly if they wanted it lightened a little.

This 750 crank lightened and rebalanced to 62% dry went to 8000RPM a few times with Jims rods and pistons attached and the bottom end never let go. I think 8000 RPM is the same RPM on the street as it is on a race track.



At 8000 RPM vibration is the last thing I am paying attention to. Mostly I'm looking in my rearview for the PoPo and enjoying the exhaust note.

If you ride your motorcycle using about 4000 RPM as the max, the low Commando balance factor might be better, than a much higher one. The balance factor is raised to suit higher revs, but going much over 7000 RPM with a Commando crank is probably not smart. I think my motor might rev easily to 8000 RPM and stay together, but proving that might lead to disappointment. In any case the Commando 850 motor works well enough, that 8000 RPM is not needed.
I am accustomed to using motors which rev high to get power. With my Commando 850 motor, it is about torque. My overall gearing is insanely high, but with the close ratio gears, less revs are lost on each gear up-change. And a down-change is only ever a quick blip. I use the crank inertia better. I suggest many guys do not know how to race-change. The guy who used to be my major competition is one of them, so I do not discuss it with him. He has mentioned that when he has watched me racing - if I change gear, he never hears a pause, only a change of exhaust note. All I do is back-off enough to take the load off the gears, and press the gear change. It is like playing a piano. With wide ratios, life becomes more difficult. The standard Norton box is useless.
When you build a race bike, the most important things are the frame, gearbox and the front brake. It is easy to get the motor to be fast enough.
Most road motors do not need much tweaking. My 850 motor cost $1300 in 1975, the carbs cost $600. The Manx gear cluster cost $730. You do not need to know about the Mk3 Seeley frame - I kept track of it for a long time, before I was able to buy it.