Crack Balance - Model 20 twin - Balance Factor

[johnnyb]

Hello Guys,

I had to get the crank in my model 20 twin machined and I would like to have the crank balanced while I'm at it. The shop doing the work has asked what the "Balace Factor" is for this crank?

Do anyone know the Balance factor for a Model 20 twin...55%????

cheers

[Eric]

The factor depends upon what you want the bike to be used for. By this I mean a factor say 55% will achieve it best balance at a particular speed/revs lets say its 55 mph in fourth gear it may or may not be by the way. So if you want to be able to cruise the bike at this speed that's what you would use.

If you want a lower or higher cruising speed the balance would be altered to suit. I will let others make the suggestions, but I am surprised the the people doing the balancing don't know chapter and verse what the factors are.

Will they be dynamically balancing it ? Or statically balancing it? Static was already done when the crank was made and doing it again will not change much.

If they are going to dynamically balance it they would have to have the conrods, pistons with rings fitted, gudgeon pin & circlips, the conrods would have to be a matched pair in weight but I cant remember the variation allowed its not much just a few grams.

If they don't want these bits then they are only proposing static balance and I would not bother.Edited by - Eric on 12 Aug 2010 11:32:33 AM

[GSAX1]

500 twins had a 50% balance factor. They were dynamically balanced at the factory. I had mine ('55 model 20) rebalanced to 60% some years ago, but did not notice any big difference.

I wouldn't bother unless you have the stone-heavy "modern" pistons. These will lower the balance factor to around 40%, worsening the vibrations.

Michael

[Eric]

Michael when I had my twin dynamically balance it had correct Hepolite or similar replacements pistons not the modern heavy ones, (this was confirmed by the balancers) and the difference was dramatic.

Perhaps the original factory static balance (they were not dynamically balanced) was never very good? I guess it depends upon what you expect.

I will try to find the balance factor used by Basset Down, I don't think it was 50%.

[GSAX1]

Acccording to F.W.Neill they were dynamically balanced. On page 60 of the "red book" it says "The two-throw crankshaft is balanced dynamically by the makers, who also weigh each piston and connecting-rod, which are paired to ensure that the balance factor is correct."

Further on it says that the balance factor is 50, and shows the calculations.

Michael

[Don Madden]

As Michael says, the F. Neil books clearly state the twin crankshaft is dynamically balanced. As proof, the balance holes drilled in each side are not at the same radial location showing the two sides are individually balanced, which can only be done dymamically. Cheers, Don. Edited by - Don Madden on 12 Aug 2010 4:53:54 PM

[Eric]

Many others on here have said they were not dynamically balanced including I think someone that worked there and why when re-balancing with pistions of the same weight size shape except for 20 thou oversize do the balancers have to remove so much more metal or put it back in the form of heavey slugs?

Also why in my case and others who have had it done does it make so much difference?

Perhaps in reality it was only competion engines that were so balanced even today with better technolodgy it takes quite a long time or was it just longditudinal balance that was done dynamically.

For me it makes a big difference, and Mr Niel was not always right.

[zwarts]

They most definitely were originally dynamically balanced at the factory!
As Don has pointed out- there is no other explanation for the difference in the size/location of the balance holes. Some years back, the 'Jampot' carried, on the front and rear covers, a series of reproductions of the original AMC adverts as they appeared in the press of the time. One of these concerned the dynamic balancing of the twin crankshaft.
Bob.

[GSAX1]

Usually other changes are done to an engine when the crankshaft is balanced, like rebore, new pistons, bearings etc. Theses changes in themselves could make the engine feel less vibratory, and it could easily be attributed to the balancing of the crank.

As I said, I had my crank rebalanced dynamically, and can't say that it vibrates less at any speed. I wouldn't bother with it. Better check conrod and piston weight, make sure they are as equal as possible. Ligthen pistons if they are too heavy, as compared to the original wire-wound ones.

Michael