Crankshaft balance

When Commandos were first made, the CB750 Honda had set the benchmark for smooth running. Smooth running is not what vertical twin four-strokes are about. If you want to ride a brick, buy a CB750. I rode a new one when they first came out - I was appalled by how bad it was. My old Triumph 650 road bikes would eat it alive. But I never had a road bike to go touring and look beautiful. Honda CB750s won races which were push-start, when the riders pushed the button during the start. It is the reason most historic races are now started with the motors running.

Al you say your motor is at 72% balance factor, how do you know that, just plugging a hole and you come up with that figure, maybe you know something more than the experts who do know about MC crank balancing, maybe you were lucky and you say your motor is stock, if it was that simple why didn't the factory get it right.
My stock 850 Commando when new only rev to 6500 RPM and valve bounced after that but building my motor for the Featherbed frame, crank balanced by an expert motor engineer who knew all about Featherbed frames (Ivan Tighe put me onto him when he was building my cam) and Norton motors at the time, with the crank balanced, 2S cam profile, port work and an open exhaust system with very little restriction, my motor runs way past 6,500 RPM, runs very freely and will keep revving, but we all know what happens if you let them rev too freely like that, but I do know the sweet rev range with my motor and gets to the ton pretty quick, I no need to prove how fast my Norton goes or if its quicker than my mates, its built for my riding style and built for fast, tight twisty roads, but is also good in the suburbs and around town, but up in the ranges is where it built for, using the torque of my motor where it performs the best, been riding it for near 50 years and 45+ years in the Featherbed frame, I think I got it right first up and seems to get better as it ages and even better with well tuned Amals and that huge spark from the Joe Hunt, it just gets better all the time.
The best thing I can take it out when ever I want and away from all the fun police, cameras and idiots on the road, yes I am not scared to take it out on the road, just got to keep your eyes open what's going on around you at all times.

ashman said:

My 850 crank balanced at 72%, my cam was built up to a 2S profile/grind, running 40thu Hepilite flat top pistons, the head been shaved and still running original valve but the ports have been ported back in 1981, I run with 5mm steel engine plates and is the same set up today, my motor is a very smooth running motor, has never shaken anything off in the 44+ years of running with this set up, my motor will run very freely right through the revs, just got to watch it don't over rev.
In all the time I built this motor for the Featherbed I have only lost one muffler mount nut and the top gear box mount nut, I am still running all my original engine bolts and the bolts that mount the motor to the engine plates, I also have hollow round spacers in-between the engine mounts to frame lugs as well one big through bolt with spacers at the mid bottom rail lugs and engine plates.
I had some vibration in the ends of the steel handle bars at some revs but fix that by a set of renthal alloy bars with 6mm wall thickness, steel bars are only 1.6mm wall thickness I think, I can ride my Norton all day without any problems at all, its a great fun bike to ride.
When I first built this bike back in the early 80s there wasn't stuff like what Jim sells for smooth running engine bits, mine was built on the cheap when I had the money took me 2 years to build as money was tight back then (in between jobs) but it has never let me down motor wise and is very reliable, was an everyday ride right up to 2013 (when I retired early at 55) and has clocked up a lot of miles, it still pulls strong after all these years with a few newer upgrade added in the last 15+ years.
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This is encouraging.

Years ago I had an 850 crank balanced and the vibration was awfull...and this was mounted in a Commando frame. I sent the crank back and he welded it together and no improvement. I have a suspicion that he might have bolted the flywheel on backwards. Any body dealt with this problem?

Onder said:

Is balance factor related to engine speed? Go fast you want to balance for high rpm smoothness if you are more
the road going type then you may want to have it best at your normal cruise speed what ever that may be?

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You are correct. It is not possible to have a 360 degree vertical twin will run fast and smooth under all circumstances. For racing, most 650cc Triumphs has balance factors of 80% and could rev safely to 8000 RPM. A Commando can probably also do that - I just do not go there. My frame has no bars over the top of the motor. The heavy crank stores a lot of energy, but it has a cast iron flywheel. If I needed a faster motor, I would make a billet crank.

I suggest that a lot of this stuff about cams and compression ratios is bullshit. Whatever you create, you need to work with. There is a movie about John Surtees titled 'Built to win'. Somebody rode his Manx Norton. Their comment was that the way it was tuned made it fast. In the UK, the Manx would have been running on petrol. In Australia, I use methanol - getting the jetting right is about one half as difficult, but still not easy. In the 1950s when Australian riders raced in Europe, they found the British Manx Nortons using petrol were as fast as our Manx Nortons in Australia which used methanol. Australian riders were accustomed to the speeds in the UK. Our guys also had Symco conrods which allowed higher revs.

When I began road racing, I knew absolutely nothing about jetting, gearing or handling. Many people probably never get beyond that.

acotrel said:

When Commandos were first made, the CB750 Honda had set the benchmark for smooth running. Smooth running is not what vertical twin four-strokes are about. If you want to ride a brick, buy a CB750. I rode a new one when they first came out - I was appalled by how bad it was. My old Triumph 650 road bikes would eat it alive. But I never had a road bike to go touring and look beautiful. Honda CB750s won races which were push-start, when the riders pushed the button during the start. It is the reason most historic races are now started with the motors running.

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No, the Honda CB750 hadn't set any benchmark in 1967 when Commandos were first made because the CB750 wasn't to be made for another two years. The Commando is all about smooth running and is the smoothest running parallel twin powered motorcycle ever made.

360 degree parallel twins are brutal for vibration! 180 degree twins less so but higher frequency vibration due to the rocking couple. Lighter reciprocating components helps
My single cylinder 500cc single cylinder drag bike has a modern GM speedway engine, the piston is not much more than a crown with gudgeon pin bosses. It is very smooth right up to the 13,500rpm rev limit. I change up at around 12,000rpm

Andy Higham said:

360 degree parallel twins are brutal for vibration! 180 degree twins less so but higher frequency vibration due to the rocking couple. Lighter reciprocating components helps
My single cylinder 500cc single cylinder drag bike has a modern GM speedway engine, the piston is not much more than a crown with gudgeon pin bosses. It is very smooth right up to the 13,500rpm rev limit. I change up at around 12,000rpm

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13.500 rpm with a 500 single ?

Strewth, that’s awesome !

Any videos of any runs, would love to hear it ?

When quoting balance factors its very important to state whether its dry or wet. Please requote them.

hhh said:

No, the Honda CB750 hadn't set any benchmark in 1967 when Commandos were first made because the CB750 wasn't to be made for another two years. The Commando is all about smooth running and is the smoothest running parallel twin powered motorcycle ever made.

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Yes and a properly set up Commando is much smoother than any CB750. Those bikes don't have the big shaking vibes of solidly mounted parallel twins, but they produce a tingling high frequency vibration that is not at all fun on a long tour.
There is none of that with a Commando.

Glen

When I rode a CB750, it felt like a brick. Perhaps the Japanese and Norton were competing for the same market - the one which wore desert boots and duffle-coats ? One of my brothers was one of those - he rode a motor-scooter. Are you blokes who ride Commandos 'Rockers' - or are you 'MODS' ? Real men do not worry about vibration - that is for women.

When I was a kid, I loved an adrenalin rush. I still get it when a large capacity motor comes on song at 4000 RPM. I love my Seeley 850. Below 5000 RPM, it is a piece of shit - but when it gets going, it is bloody beautiful. In races,it never drops below 5,500 RPM. The slowest corner on Winton is a tight hairpin bend, and a bit bumpy - the Seeley 850 goes around it almost upright and losing traction at about 60 MPH. It is pretty funny.

We all need to learn how to have fun. I never stick my neck out, but I can outride most other guys. It is not me doing it - it is the bike. Never blame yourself for being too slow, because you never know what the other guys are riding. If you have ego and race, you are in danger. I have always raced shit - I might be slightly more adaptable. Learn to race on a good motorcycle - it will shape you better.

Some of the things you come out with Al, most on here are road/street bike owner/riders not racers and just because we ride when ever we feel like it don't mean we all ride slow and under 4000 RPMs mine only sit under that when in the suburbs but once out on the open roads and up in the tight twisties is where all the fun happen and racing a group of mates for the next corner, my HOTROD Norton is built for handling and high revving, 1st gear is a bit low but who rides in first gear, maybe for pulling a few wheelies.
Riding with trusted mates who we been riding together since we left school at 15 and most ride mono shock big bore bikes so on the Norton I got to ride harder to get in front, but that is where the Featherbed comes into play, they are a lot quicker so I got to be more smarter, same when on my 1200 Thruxton with twin rear shocks I have the HP but the handling is so much more work than a mono shock sports bike, so I got to put in a bit more effort in my riding skills and knowing how my mates think, got to be one step ahead of them at all time, that where I get my fun.
With my Norton the old gentleman who balanced my crank knew what he was doing as he got it right and with the crank, cam and head work has made my motor work so well in the rev line, it will rev very freely so got to watch how far you let it run freely and that big fat spark from the modern Joe Hunt magneto, well tuned Amals and open exhaust system everything work so well together but it also need the skills to ride the bike to get the best out of it.
I had my stock cam built up and grind to a 2S profile but me thinks it might be a bit better than that and riding my HOTROD Norton for over 44+ years with this set up I think I have the skills, the torque and handling to out smart my mates when up in the tight twisty ranges we love so much.
So most of us road/street riders do get out more riding our bikes when ever we feel like it instead of the talk and how good your bike is but its so long since you been on your bike Al, you talk like a lot of my Harley mates they always talk about how good their Harley's are, but riding with me and my other bike riding mates they say we ride to long without breaks or go to fast and go too far when we are riding, so now they stick to their own little group and sit around and tell each other how good their bikes are.
My riding mates use to put a lot of sh it on me for buying a Norton, but after I built my Featherbed Hotrod they know best not to as its well set up for handling the tight twisties and having a well balanced motor has proven that they give my bike the respect it deserves and my riding skills, bike and rider they all work together.
Even my stock Commando use to run cycles around all my mates CB750 4, but not one of them still own those old heavy Honda 4s.

Ashley

Jseng messaged me asking about the piston in my drag bike. I will put the photo here as it may interest others

00501-GM-piston by Sigma Projects, on Flickr

defol rear view by Sigma Projects, on Flickr

Andy Higham said:

Jseng messaged me asking about the piston in my drag bike. I will put the photo here as it may interest others

00501-GM-piston by Sigma Projects, on Flickr

defol rear view by Sigma Projects, on Flickr

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Nice pics, but we need some SOUND dude…!

Meanwhile in the low tech world of 3000rpm max lawn tractor engines.




It had other issues LOL

Right. Issues like no skirt on the piston, so the piston could do somersaults in the bore.......