Crank Balancing

Status
Not open for further replies.
If you have a Commando with the isolastics properly adjusted and you are using it for moderately low speed work, raising the balance factor might make it unpleasant to ride most of the time. That 72% balance factor makes the motor smooth between 4000 RPM and 7000 RPM - necessary for rigidly mounted motors. The 79% balance factor was used in rigidly mounted Triumph 650 motors which revved to 8000 RPM and still used to pull the back out of the crankcases. The thing about Commandos and Triumph 650s is that revs kill them regardless of the balance factor. A lot of guys look for more power by raising the revs - with a Commando, it is better to make it pull harder in the mid-range. A race cam usually increases torque right through the rev range, but introduces a 'cam spot' at low revs, where the motor comes on song with a rush, particularly if you have separate exhaust pipes.
 
Aren’t we in danger of mixing issues here guys?

Are we questioning Basset Downs quality? Ie their ability to do the job as expected?

Or was the wrong balance factor used? And if so, why?

They could have balanced it perfectly... to the ‘wrong’ balance factor!

Even Reggie’s crank would be deemed perfect by anyone who never went above 5,500rpm, which if we’re honest, is actually a lot of classic riders.

I think these riders might be ‘polluting the body of knowledge’. When they feedback to the balancer that their motor is as smooth as silk, the balancer thinks he’s found the ideal factor, then a customer who revs higher has problems.

It seems to me that knowing, and achieving, the ideal balance factor for your bike is actually more important than getting the dynamic balance perfect.

The last crank I had dynamically balanced was my Cdo crank with JS rods & pistons. I used the balance factor advised by Jim and got a local engineering shop (in Witney, Oxfordshire) to balance it, and I’m very happy with it.

If anyone wants the details of the engineering shop, PM me.
 
Eddie, I was thinking similarly. A 52% balance factor would probably be pretty good at 5000 RPM. When I used to ride 650 Triumphs on public roads, it was in the era when the police did not have speed cameras. What I would do to a Commando motor for a road bike is probably considerably different to what SHOULD be done. These days I would never ride a big British twin on public roads, because in Australia if you lose one licence, you lose both. I don't think I would bother dynamic balancing the crank in a 360 degree firing, twin cylinder motor - the rocking couple is almost nothing. The dynamic balance is important only if it is wrong for the rev-range in which the crank is being used. A Commando crank is probably 'fit for purpose' as long as all the dimensions and clearances are correct.
 
OK...
So working on that much of what Fast Eddie applies to me.. And that the OE balance of
Factor of 52%.... how accurate was this performed in the early 70's with fairly rudimentary balance machines??
And what is best done to check confirm / adjust this.... or just leave it??
 
Which I suppose gets us to the point: what balance factor to use? Depends on the speed regime you wish to use, the frame the motor is going in.
After that it seems like a case of try it and see.
The answer to my original question is more questions.
 
Fast Eddie wrote;
Are we questioning Basset Downs quality? Ie their ability to do the job as expected?

Or was the wrong balance factor used? And if so, why?

They could have balanced it perfectly... to the ‘wrong’ balance factor!

Even Reggie’s crank would be deemed perfect by anyone who never went above 5,500rpm, which if we’re honest, is actually a lot of classic riders.
Click to expand...
Some good points there, but the original poster Onder, asked for feedback about Basset Down, and for me to not pass on my own perceived experience would have been in my opinion quite selfish, as I was disappointed with the outcome. Possibly I was expecting too much from a 360* parallel twin without isolastics, but abouve 5,500 rpm the vibration on the T140 is very unpleasant and comes up through the frame / seat.

I have to say that before I wrote the above, I took some time thinking about how I could be fair and objective and not to just try and be negative about the company as I realise that is their livelihood. I also pointed out that I didn't contact them to tell them of my findings and disappointment with my T140 crankshaft, and so my actions have not improved B.Downs knowledge base about the later L. Harris cranks (although I believe the dealer I referred to did) and I have not given them the chance to "improve" anything. In the main, this is because I didn't want to strip the engine down again, even if they had offered to take it back and adjust the factor. But I have been disappointed, the dealer was disappointed and in 2016 Kommando wrote; "
A couple of years ago there were complaints about balancing from Basset, one was on the A10 forum and another on the b50.org forum. Seems the expert balancer left and set up shop himself, after 2 years you would hope they are back up to speed.
Click to expand...
 
olympus said:
My crank is due to be dropped at Basset Down next week.... and reading the reports here is making me think twice...
I see here balance factors being quoted at 71, 72 & 79%... reading the manual I note a balance factor of 52%
Is balancing going to cause me more issues than it cures.
Click to expand...

Beware.....these figures are to be used with rigidly mounted engines. Commando standard is as you note around 50%, only because of the isolastics.

Assuming you have a road bike for normal Sunday use, why are you balancing it? Is there an actual problem you are trying to solve?

If so, use a factor suitable for your application, probably in the 60% range but check with others with similar needs.
 
Balancing the crank was to be carried out as I considered with boring & grinding it was the best way to guarantee a state of best balance
Reading all the points & after carefull consideration I will now not be having this done

Regards
 
Am somewhat puzzled as to why you would want to balance a commando crank that is going into an isolastic frame ? Are you experiancing vibration , if so have you checked the obvious , are the iso s properly adjusted ,do you have a loose rotor are the main bearings on the way out ?
And the not so obvious ,depending on what year your bike is , your money would be better spent ensuring the conrods are balanced .
 
Reggie said:
Fast Eddie wrote;
Some good points there, but the original poster Onder, asked for feedback about Basset Down, and for me to not pass on my own perceived experience would have been in my opinion quite selfish, as I was disappointed with the outcome. Possibly I was expecting too much from a 360* parallel twin without isolastics, but abouve 5,500 rpm the vibration on the T140 is very unpleasant and comes up through the frame / seat.

I have to say that before I wrote the above, I took some time thinking about how I could be fair and objective and not to just try and be negative about the company as I realise that is their livelihood. I also pointed out that I didn't contact them to tell them of my findings and disappointment with my T140 crankshaft, and so my actions have not improved B.Downs knowledge base about the later L. Harris cranks (although I believe the dealer I referred to did) and I have not given them the chance to "improve" anything. In the main, this is because I didn't want to strip the engine down again, even if they had offered to take it back and adjust the factor. But I have been disappointed, the dealer was disappointed and in 2016 Kommando wrote; "
Click to expand...

I wasn’t defending ‘em Martin! And certainly not countering your post in any way. Just wondering what the issue is (in general not just with yours) ie, shoddy work, or good work but wrong factor?

When looking at you crank in the pic, it just looks wrong, how could it possibly have been so far out to need that many holes to ‘correct’ it?! I reckon they must have been working to a different factor.

I think (and I only think) that if having a crank balanced, and in the absence of any other hard info, best bet would be to find the original balance factor for the machine concerned and demand that that is what it is Balanced to.

This topic piques my interest as I’ll be stripping my T120 motor at some stage, previously I would have had it dynamically balanced as a matter of course, but I’m not so sure now!
 
No offence taken Nigel. ;) You really did make some valid points.
On reflection, I wish that I had made contact with Basset Down, just to see what they had to say, and to make them aware of my findings for their future reference. Then I could have included their view, giving both sides of the argument in my posting.
 
oldmikew said:
Am somewhat puzzled as to why you would want to balance a commando crank that is going into an isolastic frame ? Are you experiancing vibration , if so have you checked the obvious , are the iso s properly adjusted ,do you have a loose rotor are the main bearings on the way out ?
And the not so obvious ,depending on what year your bike is , your money would be better spent ensuring the conrods are balanced .
Click to expand...

If you were going to use a Commando for very high speed work, raising the balance factor might be a good idea. But for a road bike, when you raise the balance factor to 72%, the bike rocks backwards and forwards when it is idling. Slow traffic might become a nightmare - would you want that ?
 
There is an advantage to dynamic balancing a Commando crank. It eliminates the possibility of a rocking couple which the isolastic mounts can not isolate from the chassis. A rocking couple makes the motor shake sideways which you do feel. Of course some cranks are pretty good from the factory and some are not. It was all just chance as none were ever dynamically balanced from the factory.

Have you ever noticed that some Commandos are really smooth and some are not....

There is nothing to be gained by changing the balance factor in a Commando. Isolastic mounts work best between 50 and 60% regardless of the rpm.
 
When a crank is being statically balanced, it is usually done by rolling it on level straight-edges with appropriate weights attached to the journals - NOT in a 'rudimentary balancing machine'. When you dynamically balance it, it is usually spun up to speed in a machine with sensors attached to detect the imbalance. It would be sensible to dynamically balance the crank, then adjust the balance factor to suit the operating rev range. Dynamic balancing is about the differences in mass at each end of the crank. In a 360 degree firing motor, it is less important than in a 180 degree firing motor. In the 180 degree motor you get a more pronounced rocking couple. And usually there is very little inbuilt imbalance to adjust for the operating rev range.
 
comnoz said:
There is an advantage to dynamic balancing a Commando crank. It eliminates the possibility of a rocking couple which the isolastic mounts can not isolate from the chassis. A rocking couple makes the motor shake sideways which you do feel. Of course some cranks are pretty good from the factory and some are not. It was all just chance as none were ever dynamically balanced from the factory.

Have you ever noticed that some Commandos are really smooth and some are not....

There is nothing to be gained by changing the balance factor in a Commando. Isolastic mounts work best between 50 and 60% regardless of the rpm.
Click to expand...

'There is nothing to be gained' might be supposition. Have you tried raising the balance factor in a racing Commando ? If the isolastics reach their limit of elasiticity, the crank might end up hammering the cases at high revs, if the balance factor is low.
 
acotrel said:
'There is nothing to be gained' might be supposition. Have you tried raising the balance factor in a racing Commando ? If the isolastics reach their limit of elasiticity, the crank might end up hammering the cases at high revs, if the balance factor is low.
Click to expand...
Least flexural work done on the crank (overall bending moment) with about a 50% BF. In other words, it is easier on the crankshaft at around 50%. Never, ever heard of a Commando isolastic reaching the limits of their elasticity. Talk about a bucket load of steaming supposition.
 
With a single or 360 degree twin shaking forces are at their minimum somewhere between 50 and 65% balance factor -depending on rod angles and other geometry. It has nothing to do with RPM.

Shaking forces are at there maximum at 0 or 100 percent balance factors.
[maximum amplitude in one direction]

Determining the best balance factor with iso mounts has nothing in common to finding the best factor for a solid mount motor.

Least shaking forces are always going to be the desire with a isolastic mount. So with a Commando that is going to be 53 to 56%.
 
Last edited:
comnoz said:
With a single or 360 degree twin shaking forces are at their minimum somewhere between 50 and 65% balance factor -depending on rod angles and other geometry. It has nothing to do with RPM.

Shaking forces are at there maximum at 0 or 100 percent balance factors.

Determining the best balance factor with iso mounts has nothing in common to finding the best factor for a solid mount motor.

Least shaking forces are always going to be the desire with a isolastic mount. So with a Commando that is going to be 53 to 56%.
Click to expand...

Duly noted for when I send my crank out for balancing... after my pistons arrive anyway :)
 
Status
Not open for further replies.
Back
Top