The importance of rod stroke ratio

When Norton went to their longer 89mm stroke they used a 5.875 length rod and ended up with a 1:68 rod length to stroke ratio. This was tolerable in the Norton 650SS but with the heavier 750 Atlas pistons it was a bone shaker and isolastics were necessary for the Commando. The Triumph 750 had a better 1:86 rod stroke ratio and they didn't have to use Isolastics. A 6.4" length Norton rod gives a 1:83 ratio to mellow out the vibes and stress.

I’ve done it in 3 engines now Jim and will say that the difference is pronounced.

I certainly wouldn’t have done it a second and third time if it didn’t obviously work !!

Must be nice on a Commando.

The long rods and high CR pistons worked on my solid mounted 750 engine in the P11 frame. So did the JS2 cam. That build had a stock crank that was lightened and balanced to 62% in the late 1980's. I did not have the engine rebalanced for the new rods and pistons, and the results were still good. The ride would more than likely have been better if the crank was rebalanced to 72% for the new part weights.

When I built the Seeley 850, I did not believe in the Commando 850 motor. However, I most certainly believe in it now . Norton were mostly into racing, not mass production of motorcycles. When a motorcycle is raced, there is more development which goes into it. I wondered about the squish band in the Command head - it is only at the rear of the combustion chamber - not right around as in the Manx. The twin motors were a later development.
With longer rods, the motor should be better at higher revs, but that might not be the best way to go. There is angularity to consider, which might adversely affect torque.

Jim, If I was going to develop my 850 motor further, I would buy your long rods and light 12 to 1 comp. pistons. However because of the way I use my 850 motor, it is actually fast enough. The gearbox and the bike's over-steering effect, means the 850 motor is perfectly 'fit for purpose'. I never believed that motor would be good enough to be competitive.

acotrel said:

Norton were mostly into racing, not mass production of motorcycles. When a motorcycle is raced, there is more development which goes into it. I wondered about the squish band in the Command head - it is only at the rear of the combustion chamber - not right around as in the Manx. The twin motors were a later development.

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The twins began with a 500 and as the capacity increased, through 600, 650, 750 to 830, design changes were driven by necessity only.
For the 600 the bore was increased by 2 mm because that (plus an allowance for overboring ) was all the 500 barrels could take. The barrels were made taller because of the increased stroke (hence an extra fin).
For the 650 the stroke increased to 89mm and big ends went up in size and con rods improved as a strength requirement. Con rods were shortened to fit the 600 barrel height. Crankcase was modified at the rear to clear the new crankshaft. Later on the barrel spigots and head counterbores were removed to cut costs.
For the 750 the bore increased. In order to keep changes to a minimum that necessitated moving the cylinder bore centre rearward, which also included repositioning some of the rear barrel to crankcase and head fasteners. This is the reason for the rearward offset of the pistons to the combustion chamber. The inlet valves went up a size (because someone thought it a good idea?). Crankcases went through strengthening processes after it was discovered that 500 cases were not good enough for a 750.
For the 830 (850) the bore increased again and the barrels got the through bolts for strength reasons.
So there we have it, maximum results for minimum cost and effort.

Back when I was racing at Willow Springs in the mid 1980s a couple of racers claimed that longer rods in Nortons made more power. They wouldn't give me any details so I couldn't find out what rods or pistons they were using. Then I saw a Brian Slark article where he mentioned that one of the big problems with the Norton vibration was the short rods and the harsh rod stroke ratio. Things got really interesting when DLC pins came out and Nascar racers eliminated the bronze bushing to save weight and increase efficiency/reliability. Suddenly there was an opportunity to use a lighter longer rod with a lighter piston to reduce reciprocating weight - increasing power and efficiency throughout the entire RPM range.