270 2 stroke

tecbigdog

Premium User
Chuck, I personally think the type of racing and what the engine was originally built for dictates the weight of the flywheel or wheels. The last thing you would want to do with a race engine is lose the required rotating mass for the particular type of racing. Now, having said that, yes, I would lighten the Jawa/GM flywheels for karting and add a little weight for Speedway racing. I would also add crankshaft weight (bolting on a steel plate to the flywheel) to most of the current Kart race engines. All you are doing when you modify the crank weight is adjusting your ROA (Rate Of Acceleration). The Drag Racing people have played in this area more than any other type of racing that I know of and they have very specific weights that they hang on the engines for different grips, tracks, elevations, and atmospheric conditions. I love it when I hear a person tell me that they lightened their crank and everything on it and now they just can seem to hook it up in the corners. Duhh, gee, I wonder why???? By lightening a kart engine crank, all you do is usually make it a very peaky engine to drive.
 

Ted Hamilton

helmet painter and racer
A lightened crank will spin up more quickly, but if it's not balanced with a lighter piston, can produce bad vibrational results.
 

tecbigdog

Premium User
A lightened crank will spin up more quickly, but if it's not balanced with a lighter piston, can produce bad vibrational results.
That depends on the load that you apply and the power that's available. The piston is reciprocating weight, what about the rotating weight that you remove?
 

RDub3

Member
Considering hooking up on smaller tracks is usually the issue with big power i would imagine that a lighter crank and the resulting quicker response might just blow the tires off easier, but that's just a theory. That's really where some of the 4 cycles can have an advantage at times since they can just lug the engine a bit and still have the torque to pull out of the corner while not breaking the tires loose.
 

95 shaw

Premium User
A recent trend in the high horsepower dirt latemodel world is to add a weighted spacer to the left rear wheel when the track gets slick to help traction and slow wheel spinup.
 

Ted Hamilton

helmet painter and racer
In the MXer world, flywheel weights are added to the woods bikes to help prevent stalling...14oz or so. On the MX track, the lighter crank helps accelleration to hit the triples... You picks your horses for the courses... This is part of why light reciprocating engines have upset the chassis / tires on corner entry with too much engine braking where the heavier flywheel engine don't as much (but go through rings, etc. more quickly, or rather, are more sensitive to ring leakage.) My solution would be a lighter reciprocating weight with a Hegar sprag hub to keep engine braking from upsetting the chassis....best of both worlds.
 

Phiddy

Member
I think it is all traction related. If you cant get the power you have down, might not be a good idea to accelerate it quicker.
I got a tip from a person who is an accomplished flat track motorcycle racer once. They put lead sheets between the tire and the tube in the rear wheel to slow the "Hit" of a race motor when traction doesn't allow. I do believe in flywheel weights or lightening flywheels, and all those things to tune your racing machine (Bike, Kart, Quad, ect.) to the particular application.
 
A recent trend in the high horsepower dirt latemodel world is to add a weighted spacer to the left rear wheel when the track gets slick to help traction and slow wheel spinup.
Don't you think some of that is to keep the CoG down while raising the the spoiler way up in the air when the car hikes? Basically they are using it as a lever to keep the car from flipping over when the car hikes up is how I looked at it.
 

95 shaw

Premium User
Don't you think some of that is to keep the CoG down while raising the the spoiler way up in the air when the car hikes? Basically they are using it as a lever to keep the car from flipping over when the car hikes up is how I looked at it.
Part of it is to keep weight on the LR even without the chassis hiking up, like would be the case on a very slick track.
Then lessening the hit of a big engine.
If you looked at the dynamic weights of a hiked up late model, numbers would look very much like a high cross setup on a kart.

As far a CoG is concerned, much of the added weight is high in the chassis, vs putting a big chunk bolted to the floorpan directly under the driver. Body roll is much of getting dynamic RR weight.
 
Part of it is to keep weight on the LR even without the chassis hiking up, like would be the case on a very slick track.
Then lessening the hit of a big engine.
If you looked at the dynamic weights of a hiked up late model, numbers would look very much like a high cross setup on a kart.

As far a CoG is concerned, much of the added weight is high in the chassis, vs putting a big chunk bolted to the floorpan directly under the driver. Body roll is much of getting dynamic RR weight.
My understanding is they are bolting lead to the axle tube, using unobtanium axle tubes that are heavier than lead on the LS as well as birdcages an inch thick.
 

95 shaw

Premium User
My understanding is they are bolting lead to the axle tube, using unobtanium axle tubes that are heavier than lead on the LS as well as birdcages an inch thick.
Yes, that also is, or was happening. All in the name of getting enough LR traction to get on the bars, while still getting enough RR traction to drive around the corners. For years, unsprung weight was bad, bad, BAD.
Amazing how things change.

Most sanctions have made the heavy components and bolted weight to axle tubes illegal. The newer bump stop/ light RF spring/ light LR spring setups have helped get the LR on the bars.
 
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Adding tungsten to a crank wheel would change the balance factor , where the rotor weights will not. I think the balance shaft would add some rotational weight like both other methods but very different as it's counter balancing the worst vibration. The only reason to add tungsten to a crank is if you change to a heavier rod and or piston and you want to get back to the same balance factor.
 
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