Lightened crank

[NAspirated]

mike, while i dont totally agree with you, im not saying your wrong or im right. I dont have the experience with broken cranks that i would need to be absolutely positively right. i guess i chose my words wrongly when i said they wont play up under ANY circumstance..


under a race application, lightened crank or not, main bearings will take a beating. Factory engines are put under the same stresses and will last a long time (i could show you ye olde truck engines that have done 3million km's on the original bearings...:wink: Harmonics, Compressive and Expansive loads on the crank itself CAN cause huge problems, and yes this is why we have balancers and the like. however, the strength of a crank cannot be measured solely by its weight from main bearing to main bearing, or by how much weight has been removed. You said you hadnt weighed your race item?

Conrods have the same problems, in fact, they are under 10 times the stress that a crank is under and are the weakest link in ANY rotating assembly, then theres hondas disgusting excuse for rod bolts, but thats a different story.

The counterweights were not solely designed to offset the weight of the journals (im assuming you meant rod journal...). they were designed the way they were for continual motion to be applied to each cylinder equally, by offsetting the weight of the complete rotating assembly for each cylinder. the counterweights mass will continue the engines revolution. if we removed the counterweights, and replaced the crank with only the rod journals in place and ran the engine, what would happen?

Its important that everyone knows, Hondas are NOT match weighted engines.
Some engines, like Yamaha and Suzuki, are match weighted. Meaning, each cylinder is identical to the next in weight (and im talking down to 0.02g). if anything is changed in these engines, they run like shit. popping, farting, detonating and eventually exploding. Hondas are a far more basic design in that small deviations will not adversely affect the operation of said engine. If you get a chance, weigh a set of standard Honda pistons and see the maximum deviation, usually it will be 5-10grams. On a match weighted engine, you should see No deviation whatsoever. Can anyone Fathom a guess as to why a match weighted engine NEEDS to be 99.9% perfect? it relates to the crank, so its on topic.....

If you lighten the crank, its mass from main bearing to main bearing MUST be equal, or like you said the crank will rip itself in half. if you remove 10grams from one counterweight, you need to remove 10grams from the opposing counterweight (assuming the weights were perfect to begin with).

Under a strobe image, from what ive seen, a crank will turn into a whip. basically taking a shape similar to a acute bell-shaped paraboloid's under certain rpms.
however, crank balancers will never, ever, remove this. cranks are of course balanced from the factory, but it is never a complete Dynamic balance compensating for the engines full operating rpm, which is party the reason the use a balancer. They work exactly the same way as Damper springs in a clutch, to remove vibratory stresses imparted on/from the crank. Cranks are statically balanced, to ensure that the weight on either side of the centre journal matches.
If you have ever seen a dynamic weight balance machine balance a crank at 20,000rpms, youll see how pointless static balancing is. absolutely amazing to see the computerized balancer tell you exactly where material needs to be removed (and it can work these points out down to .0001"). you can watch cranks being balanced with a strobe, its extremely scary... With cranks like these, harmonic balancers become somewhat unnecessary.

however, every crank will have a critical rpm. meaning, at or under a certain rpm, the crank will create excessive vibratory stresses beyond the engines average operating wavelength. in a honda engine this is approximately 7000rpm (depending on engine). using a dynamic balancer and balancing the crank 99% centrally (margin of error is usually below 0.3% depending on the machine being used) per each journal, you can move that critical rpm down as low as 3,000rpm. what this causes is another parabolic effect. after 3,000rpm, the vibration on the crank itself will actually start to decrease. however, it will never reach point zero.

Lightening a crank, as well as a flywheel, has its limitations and problems yes. But if done properly, by an experienced person with the correct tooling and equipment, they can be made far better than factory. remember every crank is heat treated Forged Steel, inherently a high strength material. Personally, i dont like the idea of shot-peening a crank. all youre doing is case-hardening the outside which can create hairline imperfections in the substructure, often causing cracks or weakspots. I dont really think that crank, or MOST lightened cranks, are shaved within an inch of their lives. as i said in an earlier post, removing any more than 15% of the overall weight becomes dangerous and in the realm of diminishing returns. until you actually weigh it and find out how much has been removed (and i am generally interested to know), i can only go off the picture, and ive seen cranks much worse than that one.


If you know your engines critical rpm, operating rpm, properly balanced per cylinder weights, you should have few problems with a crank creating major engine damage.

You are right though, if youre going to the extenet of lightening the crank to fine extra (or lost) hp, then you do have to be prepared for the consequences. I dont feel this is a certainty however, which from your wording i gather you feel removing weight from a crank is more harmful than helpful.

there are arguments for yay or nay, but Honda cranks are extremely strong pieces (some of the best in world really) and while some people say modification is unnecessary, there can be substantial benefits if you dont go overboard.

Good luck with yours dude, if you get a chance weigh them up and let me know how much has been removed...

[Green Machine]

Sorry to bump and old thread but any know now an approx price to get a crank knife edged? Just trying to get a rough budget for how much I need to save to build new motor. Cheers

[Sleepy]

Wouldnt it be fun shifting 6 times in 6 seconds while getting your organs pulled out your ass

[Horny_Devil]

y dont u ask speed factor? arent they close to you?