Jump to content
Sign in to follow this  

crankshaft balancing for a crf250r

Recommended Posts

Does anyone know the reciprocating weight percentage used to balance a crf250r crank to stock specs? I have a stroker-shaker on my hands. I'm guessing 70% to 80%., assuming the balance shaft doesnt affect the crank specs. Also I would think all similar engines are alike.

Share this post


Link to post
Share on other sites

Unless you get in tight with an engine guy, they usually keep these numbers pretty close to their chest. From my experience, a single cylinder crank will fall between 25%-35% for the amount of reciprocating mass added to the crank. Some Nortons like 74-84%, and most Vee engines like 50%. Sometimes, the geometry of the crank lends itself to balancing without adding heavy metal slugs. If you have time and a stock crank and rod, you can measure the amount of reciprocating and rotating mass and calculate the balance factor of the system.

Typically, the last 35% of the rod length on the small end is weighed to find the rod's contribution to reciprocating mass. Some books and rules of thumbuse 50% of the rod length for balancing. Then the piston, pin, rings, small end rod bearing and clips make up the rest of the recip mass. Weigh all this stuff up and record the number somewhere.

Removing the rod from the crank will let you measure the mass that rotates. Set the crank in some good Vee blocks with the bearings installed. Use a string or fishing line and make a loop around the rod journal to allow adding things like washers and other weight with a hole in it. Add enough weight to balance the rod journal level with the heavy side of the crank. Take all the stuff you added on the fishing line and go to a good gram scale and weigh it up. Subtract the mass of the big end of the rod and its bearing. Now, divide this number by the reciprocating mass you measured from the rod and piston and you have the mass fraction of all the reciprocating stuff, known as the balance factor. The other method is to sum up all the rotating mass like the crank wheels, bottom 65% of the rod, journal bearing, crank journal and then divide that into the total reciprocating mass - the balance factor number should comeout the same.

Strokers add reciprocating mass in a hurry coupled with the added piston acceleration rate, you get a first order vibrating (shaking forces) son of a gun. Adding mass to "balance" a single cylinder is counter productive as it adds to the total energy in the system as you spin it up. Playing with the counter-balancer will help a bit but it should be well understood before one starts adding tungsten to rotating stuff. A guy is best off reducing rod mass, piston mass, wrist pin mass - anything that makes the recip mass smaller.

If all else fails, adding weights to the handlebars will help at a small range of rpm, hopefully where your hands are getting numb.:thumbsup:

For more info, spend a few minutes here: http://modelenginenews.org/etw/etw_bal/p2.html

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:

Sign in to follow this  

×