Lightened Cranks and HP

[DarkFalcon]

I have been thinking about sending a crank off to Falicon to have it lightened, balanced and polished. Among the reasons to do this is to get more power to the rear wheel by reducing parasitic losses through reducing reciprocating mass and the accompanying moment of inertia. Has anybody tested the benefits of lightened cranks?

[SJJBusa]

Unless you are drag racing exclusively, you would be wasting your money in my opinion. The work you are referring to if performed by Falicon, is they're "supercrank" treatment, and it costs $475.00. For about $440.00 less, you could put a aluminum rear sprocket on in place of the stocker, and accelerate quicker than a supercrank treatment. Polishing the journals will only enlarge the oil clearance, thereby increasing the likelyhood that the oil film will be breached under heavy load, and scoured/spun bearings will be the result. Also, a lightened crank will make it a lot harder to smoothly apply power when exiting corners, and will tend to make the bike vibrate more. Best thing you can do with the crank, is leave it alone. Many supersport teams use titanium rods, but actually  add weight the end of the crank, not take it off. Some teams even have different crank end weights for different tracks, and once they started doing this, the riders benefitted from lowered lap times. Smooth power delivery is the key to improved performance. If you want to lighten the  reciprocating weight, then you would be talking about different pistons or rods, as the crank is  rotating weight.

Just my .02,

SJJBusa

[DarkFalcon]

quote:Originally posted by SJJBusa:
Unless you are drag racing exclusively, you would be wasting your money in my opinion. For about $440.00 less, you could put a aluminum rear sprocket on in place of the stocker, and accelerate quicker than a supercrank treatment. Polishing the journals will only enlarge the oil clearance, thereby increasing the likelyhood that the oil film will be breached under heavy load, and scoured/spun bearings will be the result. If you want to lighten the  reciprocating weight, then you would be talking about different pistons or rods, as the crank is  rotating weight.

Thanks for making the distinction and bring up the oiling issue. I think they remove around two pounds of weight and I am surprised it would make so little difference in drag applications.

[SJJBusa]

quote: I am surprised it would make so little difference in drag applications

It  will make a difference if drag racing, and I said as much in my first sentence. However if you do other things with the bike in addition to drag racing, I feel that you will be ahead of the game by doing other things instead. Unless of course, the object is to spend as much money as possible...!

SJJBusa

[AssAbuser]

I lightend crank will accelerate quicker so for a drag racing application it would help...

As for street us it would be a waste of money also for road racing use the bike might accelerate quicker causing the bike to step out when exiting the corners causing loss of control..  I have heard that when you lighten the crank you loose torque due to the loss of weight...

[THEMOTORHEAD]

quote:Originally posted by AssAbuser:
I lightend crank will accelerate quicker so for a drag racing application it would help...

As for street us it would be a waste of money also for road racing use the bike might accelerate quicker causing the bike to step out when exiting the corners causing loss of control..  I have heard that when you lighten the crank you loose torque due to the loss of weight...

i disagree
in corners it will have less flyweel effect thus be less aggressive out of curners
dragracing it will have less flywheel will tend to have its inertia slowed easier when trying to launch
of course how far out or how close in the weight is removed from will dictate its inertia
use to try this with a early z1 crank vs the 78 1000 which were about 4# heavier
small porkchop shape vs round throws

[DaveO]

Dennis is correct. I tested diff weight cranks back with the FZR/YZF 1000 motors. The YZF crank was about 3-4lbs lighter. The entire engine was the same except for the crank swap.

Results...They dyno'd within 2hp, with the heavier crank making more. This was probably due to diff weather conditions. At the dragstrip the heavier crank was easier to launch, due to the heavier rotating mass. The lighter crank required more clutch/higher rpm's to launch.

The bottom line is I went faster with the heavy crank due to better short times.

Dave

[Landspeed Larry]

A heavier crank revs slower so the bike will generally accelerate slower. The manufacturer does a lot of testing to find the optimum crank weight for the bike's intended use. Kawasaki made the ZX-12R crank heavier a few years ago because they wanted the increased inertia to smooth out the power pulses for smoother street riding. I guess they realized they couldn't compete with the Busa for performance so they would make the bike better for normal riding. As it turned out, the newer 12s are quicker at the drags because the heavier crank turns the tire easier for better launches. Many drag cars change flywheel weights according to track conditions. Honda's 60's GP bikes had almost no counterweights so they would rev instantaneously. Sometimes they would stall between gears changes. The laws of physics apply to all.