bwolfie said:
I have a few questions for the balance factor informed. How is it calculated. What is affected by increasing or decreasing the factor. I have a 1973 Commando 850. The bottom end went out and I am putting polished stock rods, Stock cast pistons, std or .020over depending on which barrel I decide to use. I am going with a new 2S camshaft. the head has black diamond valves and I am going to do some light porting, mostly to clean up the openings. I have a 1975 MKII crankshaft that I am machining to MKII spec's. I like the larger bolts and addition of a center bolt. And it was cheap, $100.00, hand delivered from St. Louis to milwaukee. Any thoughts. do I remove weight from the center mass, and if so how much. I would like it to rev a litttle quicker, but not japanese quick. I will only use the bike for around town crusing and bike nights. No racing in it's future. Thanks, Brent
I have a 1975 MKII crankshaft that I am machining to MKII spec's.
You're machining a 75 MKII crank to MKII specs? Just wanted to make sure you were aware the 75 electric start model crankshafts are slightly wider than other 850's. Is this the machining you're doing?
How is it calculated.
Ken, Jim, Steve and others are more qualified to answer this this but it's an interesting topic so I'll try to encapsulate some info.
Balance factor is calculated from the difference in weight of the rotating mass (crank and big end of rods) and the reciprocating mass (small end of rods, wrist pins + circlips and pistons + rings). It effects vibrations.
What is affected by increasing or decreasing the factor.
The piston travels in a linear motion. It takes off, moves very fast and stops. ...then changes direction and repeats this sequence. The rod converts this linear motion to a circular motion. Once the crank is rotating it helps redirect the forces generated by the piston as it stops, into a circular motion, the full impact being spread onto areas of the big end bearings and journals as the process changes from the piston pushing the crank to the crank pulling the piston through the different strokes. These forces create vibrations and these vibrations are effected by the balance factor. Balancing mainly reduces vibrations that are energy that rob HP, limit rpm's and wear things out. Vibrations are energy in the wrong places.
I would like it to rev a litttle quicker
To make it rev quicker you can lighten pistons, rods and crank. X amount of power comes from combustion so the less weight that fixed amount of power has to push the the faster it will move. Lighter components will also slow down faster which isn't a bad thing (that's why we close the throttle). That's the main concept of lightening plus lighter objects produce smaller vibrations. Pistons and rods should be balanced in pairs.
Before you do the pistons you should check the combustion chambers to see they are equal volume as you may need to turn down the top of one piston to even them out or cut deeper valve recesses to accommodate a higher lift cam. After that start lightening them equally.
Rods are balanced in halves, that is the big ends of the 2 rods weigh the same and the 2 small ends weigh the same as each other. The bottom half is considered rotating mass and the top reciprocating mass. You require some special scales to weigh the top half separately while supporting the bottom half and vice versa. Keep this in mind when lightening the rods. The complete rods will weigh the same if you do this right.
Cranks can be balanced statically or dynamically. The timing side shaft is different from the drive side shaft and in reality the timing chain and cam become part of this equation as does the clutch, drive sprocket and alt. on the other side but most people draw the line at just balancing the crank, rods and pistons (with pins and rings). Bob weights are clamped onto the crank big end journals that represent the weight of the rods and pistons etc. This weight is determined by the actual reciprocating weight minus what you need to achieve a given balance factor.
-Static balancing is done on 2 parallel knife edged straight tracks to reduce friction so the crank can roll on the main bearing journals. With proper bob weights you should be able to place the crank in any position and it shouldn't roll.
-Dynamic balancing is the best as it takes into account particular rpm targets and accounts for velocities of moving masses etc. Material can be removed right on the balancing machine or heavy metal added.
There is a common belief that lightening a crank reduces torque. Lightening does not effect torque. The engine has less inertia due to the reduced mass so getting going from a stop takes a bit more gas but the torque is still there. The blue bike in my avatar has a lightened (1 1/2 lbs.) and balanced crank etc. It hasn't been running for 30 years and I'll be tearing it down once I get properly tooled up again. It used to idle at 500 rpm with stock amals. I had to rev it to 1200 to take off. It was my 6th Norton at the time and it was like no other I had before it. Very smooth and very fast. I've lost the stats on the engine so I don't even remember my balance factor was. I'll try to sort that when I finally get to tear into it.
Here's a harley guy trying to explain torque on a lightened crank:
http://www.youtube.com/user/RevolutionP ... AfRe23wdZs
Check out Jim's crank here:
pictures-your-norton-commandos-t6210-180.html
This guy in Germany has some interesting cranks with adjustable/interchangeable weights:
http://www.britishclassicbikes.de/HPVer ... Itemid=114
Steve Maney cranks are 5lbs lighter than stock (but the only time his engines likely turn less than 5000 rpm's is after you turn the key off)
