Balance factor for 850 Iso rubber mount

Pistons are 10.5:1 MAP and MAP long rods. Mega Cycle 560-00 cam! Thanks All. With a couple of you agreeing I have the information along with manufacture spec for their rods and piston my machinist can make a decision on %

lcrken said:

I've attached a table of Norton balance factor recommendations that I've collected over the years. Milliken's recommendation of 62% was wet, but other than that, at the moment I can't recall for sure which were dry and which were wet. The usual assumption for wet balancing is that the oil in the cranks is 50 grams.

I used 62% wet for most of the Commando engines I ran in both my Production Racer and MK3 racer Commandos (750, 850, and 920 cc), and had no issues with excessive vibration at race speeds, typically between 4500 and 7200 rpm.

I have succesfully used higher BFs for Commando engines in other, non-isolastic frames, but that's not what you asked about.

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Ken

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Ken - did the vibration characteristics change in any way when going to 62% with isolastics? Did the smooth and roughest part move up or down in the RPM range?

jseng1 said:

Ken - did the vibration characteristics change in any way when going to 62% with isolastics? Did the smooth and roughest part move up or down in the RPM range?

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Jim, the Production Racer came from the factory with 62% balance factor, and I just continued to balance the engines I used in it at that figure, so there was never a chance to compare it versus the standard Commando 52% BF. I should note that the PR also came with stiffer isolastics (differenct combination of rubber donuts inside), as well as an isolastic top mount, so the comparison to a standard Commando might not be appropriate. On the MK3 race bike, I don't think I ever rode it with a 52% balance factor. I wanted a Commando racer with an electric starter, so I bought the bike and immediately built it to 920, using a 62% BF. I did ride it briefly on the street, but don't recall much about how it felt. That was back in 1978 and 1979, so my memory of how it felt is pretty suspect. I only raced it a few times, and had handling and ground clearance problems, as well as blowing up the gearbox. I decided it was just too heavy, and didn't handle nearly as well as the PR, so I sold it.

This is a picture of it in street trim, but still with the 920 race engine 36 mm carbs, and exhaust. Unfortunately, that was back before the digital camera era, and I don't have any race track pictures.



Ken