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Allan G, gavin eisler, Nick H, triton thrasher
Total Likes: 4
Original Post (Thread Starter)
#834991 12/31/2020 1:16 PM
by Phil in Germany
Phil in Germany
Hi guys,

sorry for pestering you with the zillionth question around the balance factor for an A65.

I am about to resurrect my early A65 Star puzzle. As the conrods are really old and do also seem to have been mis-treaten significantly in the past new steel conrods will find their way into the engine.

[Linked Image from]

That said, dynamic balancing of the crank is mandatory.
In the past I have done this with the standard balance factor of 70% according to the workshop manual. The bikes are quite okay vibration-wise up to 4k rpm or a tad above until vibration sets in.

I have done a forum search and found recommended figures of around 70% or slightly higher.

What puzzles me a bit is the statement in Pete Crawford's wonderful book the BSA guys would have recommended 55% to 60% to smoothen out the engines at about 6k rpm.

Being well aware that my intended rev-range for a smooth engine will be more in the region of 4.5k to 5k rpm what do you recommend?
Move towards more than 70% or go for less?

Any of your input is highly appreciated.


Liked Replies
#838983 Feb 3rd a 11:01 PM
by DMadigan
To give a graphical understanding of the effect of the balance factor (BF) here are some diagrams:
[Linked Image from]
Staring at TDC the crank rotates clockwise alpha and the rod rotates counterclockwise beta. The BF is opposite the crankpin.
The mass centres move like this:
[Linked Image from]
The piston only moves vertically, the rod centre moves nearly elliptical and the BF moves in a circle.
At 72% BF the radial forces on the crank are these:
[Linked Image from]
The piston stopping at TDC and BDC makes a high radial load. It goes to zero where the rod angle and crank angle are 90 degrees (approximately 72 degrees ATDC).. The BF is a constant radial load away from the crankpin. Note the total radial load is biased on the negative radial side, the BF load overcomes the rod and piston load for most of the revolution.
The forces in the X (horizontal) and Y (vertical) look like this:
[Linked Image from]
Notice the total forces are greater in the horizontal. A rider notices the horizontal vibration less than the vertical.
At 50% BF the radial forces on the crank are these:
[Linked Image from]
The peak total forces have become more equal.
The forces in the X (horizontal) and Y (vertical) look like this:
[Linked Image from]
At 50% BF the peak loads on the cases are lower.
An additional term, not included, is the rotational inertia of the rod as it swings.
Balance weight used here is Rod(BE) + BF * (Rod(LE) + Piston)
However, as John stated, the amount of perceived vibration depends upon the resonance to the frame and other hardware..
2 members like this
#835136 Jan 1st a 09:09 PM
by Adam M.
Adam M.
My Polish friend weighted steel MAP rods small ends and find out they are nearly the same in weight as originals.
So most of a difference in weight will be included in a big end of the rods.
1 member likes this
#838720 Feb 1st a 01:55 AM
by John Healy
John Healy
Balance factors use a percentage of the reciprocating (recip) weight. The recip weight is a total of the piston, pin, circlip, and upper end weight of the rod. The percentage of the calculated recip weight Is added to the total rotating weight.

The sheet you supplied he has taken the total weight of the recip, and rotating, weight. He took the total weight and then applied the factor.

Recip weight would be: 400 + 27 + 92 x balance percentage.
519 x .60% = 311.4g

The percentage total of the recip weight would be added to the rotating weight
311.4 + 357 = 668.4 g

The figures given are from one rod so the there would be two bob weights of 668.4 g each.

The paper he gave you he took 50% of the total weight of the recip and rotating weight. It is not how this is done!
1 member likes this
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