Opinions Sought: Commando 850 in Featherbed

A balance factor of 78% is best for a stock crank in a solidly mounted engine. It is best to balance the crank rather then hoping that lighter pistons will do it for you. Lighter pistons and connecting rods will also make a difference. With the computerized balancing that is available these days you can balance your crank in two planes. If you can balance the crank with engine sprocket, alternator and all gears, nuts and keys in place you'll have an even better result. Get the clutch balanced too. Any big engine in a solidly mounted frame is going to vibrate; its up to you to decide what is acceptable or not. 78% will give you the smoothest ride between 4500 and 5500 but the bike will be flinging parts like crazy when ridden at higher RPMs for any distance. Rubber mount your fenders too.

The bloke in Montreal bought four 80-degree offset cranks from me; Geoff Collins of Ed G Cranks (http://www.offsetcrank.com). So far, I know of only one running bike from this order; a 750 Commando that is isolastic mounted. Reports are that this bike is smoother then an air-head 750 BMW and accelerates a lot faster. A 90 degree offset, with perfect primary balance, will be even smoother. Some of my customers have mounted Commando engines in pre-unit BSA and featherbed frames and report that their bikes are similarly smooth. A 50% balance factor is used. I always recommend using MAP connecting rods as they are lighter and stronger then stock as well as light-weight forged pistons (250 grams or less compared to stock at 400 or more).

An offset crank will always have more torque then a stock or a 180 degree crank. This was proven in 1915 by Cadillac engineers on their big eights and today by Yamaha with their R1 racers. The crank can also be lighter (3-5 pounds) so it can accelerate faster. As for the rocking couple, anything you do to reduce vibration and reciporating weight far outweighs any rocking couple issues. Look for science and experience before opinion.

offsetcrank said:

The bloke in Montreal bought four 80-degree offset cranks from me; Geoff Collins of Ed G Cranks (http://www.offsetcrank.com). So far, I know of only one running bike from this order; a 750 Commando that is isolastic mounted. Reports are that this bike is smoother then an air-head 750 BMW and accelerates a lot faster.

Click to expand...

The isos are specifically designed for engines with no rocking couple - if there is one the vibration will be fed into the frame. Even if it doesn't reach disturbing levels how is that an improvement? It's just mixing two theories of how to counteract vibes which don't go together.

And a comparison to a flat twin? What's that supposed to say here? Compare it to a Commando which is as close as possible in spec, everything else is meaningless.

A 90 degree offset, with perfect primary balance, will be even smoother.

Click to expand...

90deg means secondary balance. Not primary - that is on a 180deg crank and only in 2D. How about science vs. opinion?

An offset crank will always have more torque then a stock or a 180 degree crank. This was proven in 1915 by Cadillac engineers on their big eights and today by Yamaha with their R1 racers.

Click to expand...

The current big bang generation is not for absolute torque or performance - it is for how the torque is fed into the tyre and was actually demanded by the tyre manufacturers. If you need more info I can ask my former boss - he was responsible for the project when he was working for said tyre maker. And it certainly does nothing to prove your (or Irving's) theory. And I've personally worked on far more modern and far more sophisticated race engines were actually a lot of comparison work concerning crank throws, flat crankshafts vs. 90deg etc was done. The more even you go with torsional vibration the better, that's about the only general statement one can make about most engines.

The crank can also be lighter (3-5 pounds) so it can accelerate faster.

Click to expand...

3 to 5 lb because of the crank offset? You're kidding me, right? Does that include the extra flanges and hardware as the bolt's don't align e.g.?

As for the rocking couple, anything you do to reduce vibration and reciporating weight far outweighs any rocking couple issues.

Click to expand...

Not if you have a design which specifically outlaws a rocking couple's presence.

Look for science and experience before opinion.

Click to expand...

I can't tell you how ridiculous I consider such a comment to be. There is exactly one guy all this 90deg theory relies on and a lot of people who built something and claim a difference. I have yet to see any kind of scientific data on a 270deg without balance shafts compared to a 360 or 180. And really, Irving was not much more of a genius as a lot of other engineers who did not agree back then. Sorry, I can't see the science in your arguments - just wishful thinking.


Tim

I just wanted to take this opportunity to thank everyone for their opinions experiences and knowledge. I enjoy the back and forth too, since one could glean much, especially from corrected erroneous engineering assumptions

Foxy said:

Click to expand...

That one was also one of the bikes that inspired me, very clean lines, looks the business.

Jean

marinatlas said:

Hi , I had allready fit a 750 Commando engine into my Atlas , but have the crank/rods statically balanced at 84%, then, the whole had been dynamically balanced , and it's far better than my previous Atlas, with a belt at 2/1 ratio and a 22 teeth , i could cruise at 4500/5000 rpm , easily.......now I had bought the Jim Schmidt set up , and will see the improvment it will bring , not cheap but nice items and well designed, it should according to Jim test and trials reduce the remaining vibes, and help the mill to rev more happily.........but take it easy as I am slow and lazy (old!!) that will take me some time to build up the power plant..........the Frog.

Click to expand...

Are you going to rebalance the crank for the lighter set up?

marinatlas said:

i could cruise at 4500/5000 rpm , easily.......now I had bought the Jim Schmidt set up , and will see the improvment it will bring , ...

Click to expand...

My Atlas also has an acceptable level of vibes at 4500rpm but is annoying from there on so I'm really interested to see how you like the JS setup. In contrast to this weird crankshaft angles at least the math Jim uses makes a lot of sense.


Tim

This is a collection of recommendations for balance factors for Norton twins from several sources. Sorry about the format. It is a cut and paste from a larger document on Norton weights and balance info I've been collecting for many years, and was in a table there. I couldn't find a way to insert the table here. Balance factors are, of course, percentages.

Stock Commando 750 & 850 - 52
Dunstall recommendation for 750 in Atlas frame - 84
Dunstall recommendation for 650 in Atlas frame - 70
Dunstall recommendation for 750 in Commando frame - 52
Mick Hemmings recommendation for 750 with isolastics - 74
Mick Hemmings recommendation for 750 with rigid frame - 84
Norton 750 Production Racer, factory specification - 62
Bob Milliken (SoCal flat track balancing guru in the '70s) recommendation for 750 flat track frames - 62
Mick O’field recommendation for 750 & 850 in isolastic frames - 52
Mick O’field recommendation for 750 & 850 in rigid frames - 85
Steve Maney recommendation for 750 & 850 long stroke in rigid frames - 75
Steve Maney recommendation for 750 short stroke in rigid frames - 78
Steve Maney recommendation for 920 in rigid frames - 78

I can add a few bits of info from my personal experience.

1. A 920 balanced at 62% works great in a Commando as a street bike and as a race bike.

2. A 750 balanced at 62% and installed in a featherbed, tilted forward a la Commando, works great as a race bike.

3. A 920 balanced at 62% is unrideable in a Caffrey Seeley replica framed race bike. An 872 engine balanced at 80+% works fine in the same frame.

Ken

RennieK said:

marinatlas said:

Hi , I had allready fit a 750 Commando engine into my Atlas , but have the crank/rods statically balanced at 84%, then, the whole had been dynamically balanced , and it's far better than my previous Atlas, with a belt at 2/1 ratio and a 22 teeth , i could cruise at 4500/5000 rpm , easily.......now I had bought the Jim Schmidt set up , and will see the improvment it will bring , not cheap but nice items and well designed, it should according to Jim test and trials reduce the remaining vibes, and help the mill to rev more happily.........but take it easy as I am slow and lazy (old!!) that will take me some time to build up the power plant..........the Frog.

Click to expand...

Are you going to rebalance the crank for the lighter set up?

Click to expand...

Another option is to get your top end reciprocating weight for your crank from the balancer and try to match that with Jim's long rods and pistons. You might find his 850 or 920 pistons and rods are in the ball park weight wise for your crank. Of coarse that would require a set of 850 barrels for starters and an 850 head.

You could also keep your balanced crank with the rods and pistons as a package and just get a replacement crank for Jim's 750 piston's and rods.

Gday SNS, well youve recieved some pretty strong and various opinions as to which way to go. Im happy with the path Im traveling with the 90 degree crank, and sounding like a Duke can only be a bonus!
Cant help notice that TINTIN on the bottom of his replies has "Parts falling of these vechiles are of the finest British craftmanship". Mmmmmm.... Perhaps he needs an offsetcrank ! :wink: LOL
Rgds and good luck FOXY

Seen that:

http://cgi.ebay.com/ebaymotors/Featherl ... cmdZViewIt

Jeandr said:

Foxy said:

Click to expand...

That one was also one of the bikes that inspired me, very clean lines, looks the business.

Jean

Click to expand...

I likey..............alot!! 8)

Not sure about the Commando into a featherbird but I have solidly mounted a commando into an A-10 frame. The crank pistons, and rods were dynamically balanced. The engine used a stock cam (low lift) and stock compression. The resultant street bike was and is a joy to ride. The solidly mounted engine does not create much vibration since there is good mass damping with hefty A-10 Frame. The featherbed project will be the same. I recommend this project

Please give us pics of your project!
How did you manage it, since when is it on the road, tell the whole story and......pictures please!!! :mrgreen:

Cheers,
Andy

Are you ok with seeing pics of these hybrid projects? No bikes were harmed during the construction, all parts were orphans. One of the hybrid bikes has thirty thousand miles on it . I will put the project on the board when I figure how to do the picfure loading. Cheers

No problems with hybrids, post away.

Jean

norsa1 said:

Are you ok with seeing pics of these hybrid projects? No bikes were harmed during the construction, all parts were orphans. One of the hybrid bikes has thirty thousand miles on it . I will put the project on the board when I figure how to do the picfure loading. Cheers

Click to expand...

There are instructions. :mrgreen:

GDay. Here are the Norsas, an A10 with a Commando 850 powertrain. Why? I have always liked the A 10 and have ridden them for years, however whenever you asked for horsepower from the A 10 engine it would blow up in a spectacular fashion. The Commando 850 has 60 HP(on a good day) and the A 10 has 40HP. Bert Hopwood (God rest his soul) designed both engines and there was enough room to shoehorn the rebalanced Commando engine into the A 10 engine bay. The frame is unaltered and the engine plates are continuous from the front to the back. As for vibration, the hefty A 10 frame soaks up a good part of it and the bike is smoother than the 650 engine. In Central Canada the empty rural country roads are ideal for this type of bike, most of the cruising is between 3 and 5 grand which is effortless for the Commando. Guys that take if for a test ride come back with compliments all around. Again, these bikes were put together from orpan parts, no Commando was hurt in this project. The bikes have offended some purists but so what if they can't take a joke.
If you want specifics I will be glad to answer
Coming to completion is a 270 cafe racer version.
Anyone up for a test ride in Lumby 2010?

Cheers
Alistair

Sweet looking bikes. If you can post more detailed pictures of the way it was put together, that would be nice. Can you elaborate more on the engine plates? Where are you located? (Calgary?)

Again wondeful integration, I don't see how anyone could be offended.

Jean

Yeah, that are really nice looking bikes!
I like the A10. I like their easy and precise handling. And I do like the Commando's engine performance and look. And both together, it just fits! Wow, very well done!
Yes, and more pics of the built and the engine plates were great!
How does it ride how does it feel, this "upgraded" (well at least "slightly tuned") Beeza?

Cheers,
Andy

Very smart looking bitza's indeed :!:
That Benelli brake doesn't look too bad either 8)