JS Motorsport Balance Specs

Installation of the JS Motorsport lightweight pistons and rods will automatically change a 750/850 stock Commando wet balance factor from 52% to around 66% with no modifications to the crank. This means that your Commando will shake less vertically but will shake the same horizontally as explained below. Note that the Norton factory produced their Commando Production Racers with a 62% balance factor. Solid mounted motors run smoothest at approx 66% wet BF for street and racing. Detailed balance specs and recommendations are provided below (Note: 50 grams of crank oil is included in these “wet” specs).

WET BALANCE FACTOR FOR LIGHTWEIGHT 750 PISTONS/RODS IF YOU DO NOT RE-BALANCE

Isolastic mounted Commando racers & street bikes and all solid framed bikes at 66% (stock 750 crank counter weight approx 1,197)

Example: 750cc with 66% balance factor
Lightweight piston 175 grams per cylinder
Rings 18
Pin & retainers 51
Ultralightweight Carrillo rod – small end 95
339 X 2 cylinders = 678 total reciprocating weight
Reciprocating weight X balance factor: 678 X 66% = 447

Big end of one rod including cap, nuts, bolts and shell bearings =370
Example: Ultralight Carrillo rod big end = 350 grams X 2 cylinders = 700 big end weight plus 50 grams (wet) for oil in crank = 750
Rotating weight X 100% = 750
Add the total rotating weight (750) to the 66% reciprocating balance factor (447) for a total crank counter weight of 1,197 grams for a 750cc.

WET BALANCE FACTOR FOR LIGHTWEIGHT 850 PISTONS/RODS IF YOU DO NOT REBALANCE

Isolastic mounted Commando racers & street bikes and all solid framed bikes at 66% (stock 850 crank counter weight approx 1,273)

Example: 850cc with 66% balance factor
Lightweight piston 185 grams per cylinder
Rings 18
Pin & retainers 58
Lightweight Carrillo rod – small end 105
366 X 2 cylinders = 732 total reciprocating weight
Reciprocating weight X balance factor: 732 X 66% = 483

Big end of one rod including cap, nuts, bolts and shell bearings = 370
Example: Carrillo rod big end = 370 grams X 2 cylinders = 740 big end weight plus 50 grams (wet) for oil in crank = 790
Rotating weight X 100% = 790

Add the total rotating weight (790) to the 66% reciprocating balance factor (483) for a total crank counter weight of 1,273 grams for a 850cc.

WET BALANCE FACTOR FOR LIGHTWEIGHT 920cc PISTONS/RODS IF YOU DO NOT REBALANCE

Isolastic mounted Commando racers & street bikes and all solid framed bikes at 65% (stock 850 crank counter weight approx 1,307)

Example: 920cc with 65% balance factor
Lightweight 920 210 grams per cylinder
Rings 20
Pin & retainers 58
Lightweight Carrillo rod – small end 105
423 X 2 cylinders = 796 total reciprocating weight
Reciprocating weight X balance factor: 796 X 65% = 517

Add the total rotating weight (790) to the 65% reciprocating balance factor (517) for a total crank counter weight of 1,307 grams for a 920cc.

NOTES:

– All Atlas cranks should be checked for proper balance.
– It is not so important to achieve an exact balance factor. Increasing the balance factor 1% or 2% (e.g.: 67% instead of 66%) simply makes the motor shake front to back a little more than up and down.

SHOULD I RE-BALANCE MY COMMANDO CRANK FOR THE LIGHTER PISTONS?

Not necessary – and here is why:

If you just lighten the pistons and don’t change the flywheel counter weight of a Commando crank, the balance factor goes up and the motor runs smoother because – the higher balance factor reduces the up & down shaking of the pistons (reciprocating weight), but the fore & aft shaking (rotating weight) stays the same because the lighter pistons do not affect the fore/aft counterweight shaking forces when the flywheel counterweight is at 90 and 270 degrees (half way through the stroke). So the fore & aft shaking stays the same but the up & down shaking is significantly reduced and this is why a Commando crank will run smoother and with less stress with lighter pistons. As noted earlier, the Norton production racers used a 62% balance factor.

REBALANCING THE COMMANDO CRANK BACK TO FACTORY SPECS WITH LIGHTWEIGHT PISTONS

You can re-balance your Commando crank at 52% for the lighter pistons. To do so you will need to remove approx 90 grams from the counterweight – that’s roughly two 1/2″ holes drilled 1″ deep in the counterweight exactly opposite the journals. See specs below. Note that a wet balance factor of 52% equals a dry balance factor of 63%.

FACTORY SPECS

FACTORY WET BALANCE FACTOR FOR LIGHTWEIGHT 750 PISTONS/RODS FOR ISOLASTICS AT 52%

Isolastic mounted Commando racers & street bikes and all solid framed bikes at 52% (stock 750 crank counter weight approx 1,103)

Example: 750cc with 52% balance factor
Lightweight piston 175 grams per cylinder
Rings 18
Pin & retainers 51
Ultra Lightweight Carrillo rod – small end 95
339 X 2 cylinders = 678 total reciprocating weight
Reciprocating weight X balance factor: 678 X 52% = 353

Big end of one rod including cap, nuts, bolts and shell bearings =350
Example: Carrillo rod big end = 350 grams X 2 cylinders = 700 big end weight plus 50 grams (wet) for oil in crank = 750
Rotating weight X 100% = 750
Add the total rotating weight (750) to the 52% reciprocating balance factor (353) for a total crank counter weight of 1,103 grams for a 750cc.

FACTORY WET BALANCE FACTOR FOR LIGHTWEIGHT 850 PISTONS/RODS FOR ISOLASTICS AT 52%

Isolastic mounted Commando racers & street bikes and all solid framed bikes at 52% (stock 850 crank counter weight approx 1,170)

Example: 850cc with 52% balance factor
Lightweight piston 185 grams per cylinder
Rings 18
Pin & retainers 58
Lightweight Carrillo rod – small end 105
366 X 2 cylinders = 732 total reciprocating weight
Reciprocating weight X balance factor: 732 X 52% = 380

Add the total rotating weight (790) to the 52% reciprocating balance factor (380) for a total crank counter weight of 1,170 grams for a 850cc.

FACTORY WET BALANCE FACTOR FOR LIGHTWEIGHT 920cc PISTONS/RODS FOR ISOLASTICS AT 52%

Isolastic mounted Commando racers & street bikes and all solid framed bikes at 52% (stock 850 crank counter weight approx 1,310)

Example: 920cc with 52% balance factor
Lightweight 920 210 grams per cylinder
Rings 20
Pin & retainers 58
Lightweight Carrillo rod – small end 105
423 X 2 cylinders = 796 total reciprocating weight
Reciprocating weight X balance factor: 796 X 52% = 414

Add the total rotating weight (790) to the 52% reciprocating balance factor (414) for a total crank counter weight of 1,307 grams for a 920cc.

BALANCE FACTOR THEORY AND HOW IT WORKS

The Balance factors below are simplified with numbers rounded off for convenience.

Total weight of Norton twin reciprotating weight is rounded off to 750 grams (pistons, pins, rings and small end of rod)

Total weight of Norton rotating weight is rounded off to 750 grams (rod big ends, shell bearings and oil)

combined total is 1500 grms

For perfect balance at TDC you want 100% of both the reciprocating weight and rotating weight = 1500 grams

For perfect balance half way through the stroke with the rod and journal sticking out at right angles to the side you want just the rotating weight = 750 grams

So now you have to compromise the balance between TDC and half way through stroke. 1500 + 750 = 2250 divided by 2 = 1125 grams But this is not the calculation we use (see below).

For a solid frame you want 100% of the rotating weight (750 gms) and 66% of the reciprocating weight (495 grms) for a total bob weight of 1245 grams This is called a 66% balance factor.

The 1245 grams is a bit higher than the 1125 grams and this is OK because you get less vertical vibration and a little more horizontal vibration. This works because you don’t feel the vertical vibration as much as the horizontal vibration. If you put in a lower balance factor with a solid frame it feels worse.

With a vertical twin or a single there is no way to get perfect balance – its always a compromise between vertical and horizontal vibration. The heavier your pistons are the worse its going to shake throughout the RPM range. The lighter the pistons the smoother it will run. Assuming you adjust the balance factor.

Higher balance factors do not make the motor smoother at higher RPM. I used to believe this but testing showed otherwise.

The only way to prove these theories is to mount a sharp needle to a separate fixed object so it rubs against the crankshaft area of a side case. Touch the needle for a split second and then look at the orbital scratch mark on the side cover with a microscope. You want the orbital scratch to be slightly elongated horizontally for the same reason mentioned above – you feel vertical vibrations more than horizontal vibrations.

Some people quote 70% and up to 85% balance factors but there is a lot of confusion between dry balance factor and wet balance factor (oil included). Note that a 66% wet balance factor equals a 73% dry balance factor and the type of rod can change things.

The only way to get (near) perfect balance is to have a 90 deg crank like a Ducati or to use counter rotating counter balancers like you see on the modern Triumphs.