Crank Balancing

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Several cranks sent to Basset down, quite happy with the work and delivery time .....otherwise I had sent some in Luxemburg (Dynaquil), good job too, but for Uk it was more easy for me for shipping the cranks , as a english buddy of mine was coming quite often at home and pick them up and back ........!
 
Basset down was recommended to me by Minnovation, but as i wasn't comfortable with posting a Maney crank, I took it down to Andrews. A nice personable guy and he seemed to be very knowledgeable, he explained that the crank was 42 grams out of dynamic balance and explained how he had corrected it.

Now the problem is, I haven't ridden a 920 solidly mounted before so I couldn't say if he has done a good job or not, I have done about a hundred miles on it so far and the vibes are acceptable. My gut feeling is, he has done a good job.
 
When you dynamically balance the crank, don't you still have to balance it statically to suit the operating revs, especially if the motor is to be rigidly mounted ? My belief is that the reason to dynamically balance is to correct the secondary balance - the rocking couple across the length of the crank. The static balancing (primary balance) affects the counterweight for the rods and pistons. Please correct me if I have got it wrong. I have never bothered about secondary balance in any motor I have ever built, however there might be value in getting it right.
Honda twins usually have 180 degree cranks, so the rocking couple is pretty big. Norton cranks are 360 degree, so the rocking couple is minimal.
 
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acotrel said:
When you dynamically balance the crank, don't you still have to balance it statically to suit the operating revs, especially if the motor is to be rigidly mounted ? My belief is that the reason to dynamically balance is to correct the secondary balance - the rocking couple across the length of the crank. The static balancing (primary balance) affects the counterweight for the rods and pistons. Please correct me if I have got it wrong. I have never bothered about secondary balance in any motor I have ever built, however there might be value in getting it right.
Honda twins usually have 180 degree cranks, so the rocking couple is pretty big. Norton cranks are 360 degree, so the rocking couple is minimal.
Click to expand...

That is what they told me at Andrews, mine was set to 71% balance factor. I think someone told me that no matter what the balance factor, there will be a relatively smooth rev range, changing the balance factor moves that range up or down.

It is all black magic to me, I was talking to a guy at Cadwell and he told me that he had a Triumph motor that was the smoothest race motor he had ever had, he then fitted it in a different frame and it was unbearable.
 
When you statically balance you make like parts the same weight, in the case of the rods you make them the same weight in the same places (big ends and small ends), and this can work well when the weight differences (right vs. left) are small. When the differences are larger you need to add in dynamic balancing to make up the differences (different bob weights). A proper balance service will use a complimentary mix of static and dynamic to get the job done right.

Dynamic balancing of the crank alone, can be complimentary, but it is difficult and potentially unsafe to remove weight from a Norton's rods/pistons, so if your reciprocating weights are spot equal you may be good to go. Spinning the crank with proper bob weights is the best way to check your work and gives you a visual picture of what the secondary's are doing when done with a strobe light.

The service I have used for years marks and installs the engine sprocket, the rotor and the pinion gear are keyed, but installed. These and bob weights according to the balance factor chosen then spun and strobed. No guess work, no judgment, go worry about something else
 
RoadScholar said:
When you statically balance you make like parts the same weight, in the case of the rods you make them the same weight in the same places (big ends and small ends), and this can work well when the weight differences (right vs. left) are small. When the differences are larger you need to add in dynamic balancing to make up the differences (different bob weights). A proper balance service will use a complimentary mix of static and dynamic to get the job done right.

Dynamic balancing of the crank alone, can be complimentary, but it is difficult and potentially unsafe to remove weight from a Norton's rods/pistons, so if your reciprocating weights are spot equal you may be good to go. Spinning the crank with proper bob weights is the best way to check your work and gives you a visual picture of what the secondary's are doing when done with a strobe light.

The service I have used for years marks and installs the engine sprocket, the rotor and the pinion gear are keyed, but installed. These and bob weights according to the balance factor chosen then spun and strobed. No guess work, no judgment, go worry about something else
Click to expand...
Yeah, but Zindt retired....
 
If you weigh the rods and pistons then bolt steel weights to the big ends when dynamically balancing the crank, that does not duplicate the situation you have when the pistons ascend the bores when the motor is running. My crank is balanced to 72% because that is the factor you get when you fill the hole in the Commando crank with a threaded steel plug. My motor is rigidly mounted and runs extremely smoothly from 4000 RPM to the redline at 7000 RPM, and it would go straight through the top if I let it. I don't think there is a way to calculate the optimum balance factor at any particular rev range. What we have seems to have been derived from what works in practice. To my mind , Mr Norton drilling a hole in the Atlas crank to get a smooth running Commando at low revs is a bodge job. My friend has a road race Atlas which he used to race in the 60s. The crank is balanced to 79% (the factor we used to use in Triumph 650s). I think he used to rev it to 8000 RPM, because he was extremely fast in the old days. If I was ever going to race a near-standard Commando or use it for high speed road work, I'd raise the balance factor to 72 %.
 
I don't think it is a good idea to drill the cast iron flywheel to get a higher balance factor. Although that is what has been done to my friend's Atlas.
 
acotrel:

I asked a source, that I trust, in the trade about balance factors, this was an inquiry I made about a Triumph lower end I was about to send out. he told me that Triumph made up 3 engines with different factors and sent out 3 factory riders and had them swap off several times, not knowing what "factor" they were riding. When they came back in they were debriefed about what the felt was the smoothest in rank order, for street riding in England, the one that came closest to consensus ruled production for that model.

Any thing that rotates can be balanced, but not at all rotational velocities; balance factors are a compromise. The Commando's Isolastics extend the range of comfort, if you ride at an RPM that lines up with the engine's balance factor, so much the better.


"Yeah, but Zindt retired...."


Zindt is in Hawaii with his high school sweetheart. Mike does the balancing at LIndskogg now he is much better at turn-around time, more of a technician than a specialist
 
It depends on what you are looking for. If you want performance, a low factor is not as good as a high one. The Triumph Saint had the light crank with the low factor - had poor performance. There days, you can buy the Saint type one-piece crank cheaper than the heavy one. The factor that Triumph used in their street bikes could never be as high as what was used in racing. Otherwise the bikes would have been horrors to ride in slow traffic. The Norton Atlas was an excellent bike, but as far as vibration was concerned the CB750 Honda makes it look antiquated. Personally I would much rather own the Atlas any day - it is a much more purposeful bike. The Commando design was a matter of pandering to the people who thought CB750 Hondas were good. When the CB750 first arrived in Australia, I rode one and thought it's handling was horrible - the steering felt like a Harley's. But how many potential customers in the 60's, would know what good handling actually is ? - Probably one of the reasons the numbers of Commandos sold were never very high.
 
What surprised me was one of my friends used to do a dealer's pre-delivery work on Kawasakis and Commandos. The guys used to test ride them on the winding Richmond Boulevard in Melbourne and really get stuck into it. There was nothing in it, between the Commandos and the big Kawasaki two-strokes. A CB750 Honda would get nowhere near them.
 
A friend had a 750 Kawi triple back in the day. Wheelies on throttle and beat everything in sight, except a Commando which stayed right with him thru the gears.

Glen
 
I was road racing when Commandos came on the scene. Common opinion was that the young guys bought them because their fathers said 'Nortons are good bikes'. I never believed in them, even though an A-grader friend of mine won an open class road race with one. It was a dealer's bike, so winning was about selling. My feeling was that you could put that guy on a kiddie's trike and he would win. When I built my Seeley, I never believed in the motor, so it sat unraced for many years. I am now amazed at just how good it actually is. It loves methanol fuel and it pulls like a train - very easy to ride very fast when coupled to a close ratio gearbox. I'm just sad that I did not race it earlier when I had cash.
 
I have had two crankshafts balanced by Basset Down Balancing and have read many reports between 2007 and 2012/13 recommending Basset Down.

The first was in about 2011 and was a Moto Guzzi Le Mans crank. This bike engineis so smooth, you hardly know the engine is running apart from hard acceleration at low revs when you can feel the engine pulses.

The second was in 2016, and was on my T140 Bonneville. I was advised by a long time Triumph trader in the UK (who will remain nameless) that Basset Down had changed hands ?? in 2014/15 ( I can't confirm this) and that they had had a Triumph crank returned to them by B. Down resembling swiss cheese where they felt the need to break the fillets off between the many holes, they were so thin, and advised me to not use them. As B.Down had done such a good job on the Le Mans (different engine configuration...I know) I sent the T140 crank there. It came back looking like a swiss cheese and vibrates badly at +5500 rpm.

I have to confess that on both engines I had not ridden them before the balancing, so do not have a before and after impression. Also, regarding the Triumph, I didn't ring B.Down up to discuss my disappointment. Maybe they would have asked for it back and tried again. Pictures are still on the thread here.

https://www.accessnorton.com/NortonCommando/les-harris-t140-crank-balance.20439/

So what I am saying is that due to this, I personally wouldn't be happy sending my cranks there without more recent endorsements.
 
<I personally wouldn't be happy sending my cranks there without more recent endorsements.>

...which is precisely why I posted the question to begin with. I have heard muttering from others but no hard experience.
Old firms can change hands and often it doesnt work out. But then not always.
Since I had heard good things about Andrews Precision I mentioned them.
Guess it is a crapshoot and Ill have to decide this week.
Thanks for all the input.
 
I think , I had one made by Chris Applebee, once .........so far so good, my Basset down one were done prior 2006( for the seeley)!
 
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.
 
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