Balance factor for 850 Iso rubber mount

Ok I know I am opening a can of worms! But Norton has printed 52% balance for a 850 in isorubber mount and I heard 73% for solid mount. My engine will be on the street but probally living above 3000 to 7200 rpm What is the best your experiances say to balance?

Isolastics càñnot stop the damage a poorly balanced crànk cañ do,

acotrel said:

Isolastics càñnot stop the damage a poorly balanced crànk cañ do,

Click to expand...

If you are talking about damage to the rolling chassis and the level of fatigue to the rider yes isolastics can and do
If you are talking damage to the engine then yes I'd agree

acotrel said:

Isolastics càñnot stop the damage a poorly balanced crànk cañ do,

Click to expand...

What is a "poorly balanced crank"? Surely if you want to minimize loading of the main bearings, close to 50% would be the thing but if that results in punishing vibration, as felt by the rider, in a solid mount frame, something else may be better. If Isolastics work best with a 50(ish)% factor and if a 50(ish)% balance factor really is kindest on the main bearings then the isolastics would seem to provide benefits.

Norton's figure of 52% without oil, dry. The reason I mentioned isolastic motor mount is the fact your balance is different from solid mount. I'm not worried about frame or me
, vibration robs horsepower. I'm hoping someone with experience will tell me their experience's with varying from standard %.

Running Jim's rods and pistons and everything else claims to bring the motor up to 62% balance factor around abouts and for smoother running, so that's 10% better than factory.
My Featherbed/Commando is balanced at 72% and runs pretty smoothly.

Ashley

My MK 111 runs with too much vibration for long trips ( which I do ) so I've prepared another motor to drop in ( J .S . Carrillo rods and short pistons ) . Soon. Just have to find a place in T.O. to do it instead of the street.

mean gene said:

Norton's figure of 52% without oil, dry. The reason I mentioned isolastic motor mount is the fact your balance is different from solid mount. I'm not worried about frame or me
, vibration robs horsepower. I'm hoping someone with experience will tell me their experience's with varying from standard %.

Click to expand...

I race a short stroke 750 Commando. It lives at 5krpm to 8krpm on the track. It has a one piece Nourish crank that was supposed to be balanced at 50%, but it may have turned out a little bit higher. In terms of vibration it's very civilized, if not exactly silky smooth. If somebody offered me an extra 10 horsepower, at the cost of some significant vibration, I'd turn it down; even as a race bike my first priority is to enjoy riding it.
If your bike's a Commando I'd stick with 50(ish)%, best for motor, frame and you.

I've attached a table of Norton balance factor recommendations that I've collected over the years. Milliken's recommendation of 62% was wet, but other than that, at the moment I can't recall for sure which were dry and which were wet. The usual assumption for wet balancing is that the oil in the cranks is 50 grams.

I used 62% wet for most of the Commando engines I ran in both my Production Racer and MK3 racer Commandos (750, 850, and 920 cc), and had no issues with excessive vibration at race speeds, typically between 4500 and 7200 rpm.

I have succesfully used higher BFs for Commando engines in other, non-isolastic frames, but that's not what you asked about.



Ken

Torontonian said:

My MK 111 runs with too much vibration for long trips ( which I do ) so I've prepared another motor to drop in ( J .S . Carrillo rods and short pistons ) . Soon. Just have to find a place in T.O. to do it instead of the street.

Click to expand...

I'm surprised you are getting too much vibration from a commando!
My mk2a was incredibly smooth once you were above 2500 rpm
My 750 is not as smooth as it could be because I have 3 isolastics
Plus a norvil type headsteady

Torontonian said:

My MK 111 runs with too much vibration for long trips ( which I do ) so I've prepared another motor to drop in ( J .S . Carrillo rods and short pistons ) . Soon. Just have to find a place in T.O. to do it instead of the street.

Click to expand...

D garland in Scarborough

mean gene said:

Ok I know I am opening a can of worms! But Norton has printed 52% balance for a 850 in isorubber mount and I heard 73% for solid mount. My engine will be on the street but probally living above 3000 to 7200 rpm What is the best your experiances say to balance?

Click to expand...

My motor has 9.5:1 +40 pistons, stock cranks, Webcam 312a, 4" radius lifters, Dynamically balanced to 63%.

I live in the 4-7000 rpm range mostly.

I am very happy with how smooth it is.

Torontonian said:

My MK 111 runs with too much vibration for long trips ( which I do ) so I've prepared another motor to drop in ( J .S . Carrillo rods and short pistons ) . Soon. Just have to find a place in T.O. to do it instead of the street.

Click to expand...

I only say this because my right hand went numb after a ride to Belleville and back the next day. I'll back off the front vernier iso tomorrow and free to go around T.O. for the next 4 days of great weather. High Park with the wife, yah.

The Norton specs called for 52% as a balance factor, but I think that number is for a crank filled with oil. The MKIII factory manual I have calls for a 63% balance factor. I think 63% dry is a good number, as that equates to around 53-55% wet. The last 2 motors I built I had balanced dynamically to 63% dry and they are very smooth running bikes.

mean gene said:

Norton's figure of 52% without oil, dry. The reason I mentioned isolastic motor mount is the fact your balance is different from solid mount. I'm not worried about frame or me
, vibration robs horsepower. I'm hoping someone with experience will tell me their experience's with varying from standard %.

Click to expand...

The objective was to compete with the, CB750 for the commuters

acotrel said:

The objective was to compete with the, CB750 for the commuters

Click to expand...

The Atlas did not sell.

acotrel said:

The objective was to compete with the, CB750 for the commuters

Click to expand...

Norton commandos are very good for commuting
Much better than hustling a wide heavy gutless cb750
I've done it many times on both bikes

acotrel said:

The objective was to compete with the, CB750 for the commuters

Click to expand...

I don't think so. The Commando preceeded the CB750.

"The Norton Commando was introduced in 1967 at the Earls Court Show and the first production bikes were available in March 1968."

"CB750 appeared at the Tokyo Show in November 1968 and was publicly launched in UK at the Brighton, England motorcycle show held at the Metropole Hotel exhibition centre during April 1969"

Commandos and CB750s were in no way conceived as commuters, balance factor be damned. I commuted on both CB750 and MKIII Norton. CB750 was wide and top heavy but hardly gutless, at least speaking for my '69 model. It was, however a serious handful in snow, resulting in a sprained ankle. The MKIII was/is tractable on slick pavement and the lower CG much easier. Neither was as good in winter as the Avon-faired R60/2 BMW with K70 tires parked up next to me in the office-building steam room. I test rode /2s but have no idea what the balance factor might have been. I will say, that my R100 vibrates more at highway speed and the MKIII, whose balance factor varies from factory only by virtue of 0.020 over pistons.

I may be belaboring the point to say that BF is largely, no mostly, dependent on frame design. I set the BF on my G15 at 76% and it was reasonably smooth across the RPM band. My Atlas, according to notes I recently ran across, I set to 72% and as long as you keep the revs above 4500 it's tolerable but that's 65MPH and above and I'd like to ride it slower these days, the better to dodge the deer.

mean gene said:

Ok I know I am opening a can of worms! But Norton has printed 52% balance for a 850 in isorubber mount and I heard 73% for solid mount. My engine will be on the street but probally living above 3000 to 7200 rpm What is the best your experiances say to balance?

Click to expand...

So in fact the 73% figure is completely irrelevant to the discussion, because you have an isolastic mount frame.

Discussion of the isolastic frame balance factor shows that the 52% wasn't a bad place to be. But, but variation in calculation and parts choice, you might end up with 63% without intending to.

That doesn't seem to be a bad place to be either!

We do need to ignore a tendency to think more is better when it comes to balance factor, closer to the ideal figure for the specific configuration is better, and with isolastics, that would seem to be 52 to 62 in most cases.

We can't know until you tell us, what pistons and rods you are going to use. What we do know is that many of the standard height pistons available are heavier than the original fit Hepolites, so simple installation with no checking is likely to take the BF away from the ideal, as well as adding reciprocating weight, so not a good choice.

We also know that JSM long rods and short pistons will raise the balance factor toward the recommended BF without having to do anything at all!

And they would be a good choice for the engine you intend to build for a number of reasons, including reduced reciprocating weight and suitable BF. It could then be your choice to simply install and see how it runs, or spend money on balancing anyway! In general, people seem to have had acceptable results just through installation.