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(OT) Effects of weight on flywheel

drom68

Cast Iron
Joined
Jan 13, 2013
Location
VA, USA
Background, I got into a discussion about the effects of additional weight on a flywheel for a 2-stroke dirt bike.

It comes down to the placement of the weight.

- Take the crankshaft, horizontally and the rotation of it and the flywheel added to the end of the crankshaft.
- Add weight to the flywheel, do not change the circumference location just add say 8.6oz.
- One company is stating that the longitudinal location of the weight will increase or decrease the effect of the weight. So, take the crankshaft center (lengthwise) and take that same 8.6oz weight and move from center (lengthwise) out towards the end of the crankshaft. It is stated that the same 8.6oz weight will have the effect of an 11oz weight when moved closer to the center (lengthwise).

My question is, how much of an effect does this really have. The weight spins with the crankshaft and the crankshaft is supported by two (2) bearings. The crankshaft is not straight, it has two lobes and a wrist pin with piston that experiences impulses every stroke, intake and exhaust.

- Are "they" correct in their claims or are the variables enough to cancel out any effects?

- How much change or effect will an 8oz weight have at say 5.5" from the center (lengthwise) moved in to 5" from the center (Lengthwise) have?

- Do the bearings supporting the crankshaft change the effect of the weight and the "center" of the crankshaft? In other words, does the bearing support act as a new center location for rotating mass?

Thanks
 
It is in the "moment"...

Basic stuff but an engineer can define it better but...

Grab a gallon of milk and hold it close to body.

Now extend arm and hold it under finger.

Way harder to do.

Less weight further distance nets same flywheel effect due to the leverage part but ability to change rpm falls on Newton math so there are likely more than simple math involved.

Given for 2 stroke engine this could be interesting discussion as many here are into that and likely can explain where the optimum spot is and why.

Sent from my SAMSUNG-SGH-I337Z using Tapatalk
 
Moving the weight farther out on the crankshaft will only overload the crankshaft and the bearings. It will have no effect on it polar moment of inertia, but gyroscopic effects could break the crank.

I can't imagine anyone wanting to add weight to a dirt bike flywheel, as that would make it rev up slower. There are some cases where you want a lot of rotational inertia; low speed highly loaded engines such as forklifts and gas compressors.
 
At 1st order, the longitudinal location has no effect on moment of inertia. In real-world however there can be all manner of 2nd order issues that can cause vibrations, but still no substantive change in moment of inertia. I'm assuming that your added weight would be arranged radially symmetrically, and not all in one spot. All in one spot would be bad.

Regards.


Mike
 
Your question , as asked, cannot be answered because several factors that are critical have not been stated. What do you mean "Add weight" where? Is the crankshaft in balance both before and after? Is the crank in balance internally or is it in balance externally (torsionally)? Actually, it depends on the reason you would want to add weight. Are you simply attempting to add inertia? Does your question pertain to the effects of adding rotational inertia? Lastly, motor bikes don't have flywheels.
 
I can't imagine anyone wanting to add weight to a dirt bike flywheel, as that would make it rev up slower.

motocross - high revs, no obstacles correct
off road with logs, rocks, boulders, etc then you want more weight for more low end torque and it won't stall as easy
 
The weight is a common modification for 2 stroke off road bikes. The 2 stroke is a great motocross bike due to its quick revving and responsive characteristics. Plus it is light weight and can be rebuilt in half the time of a 4 stroke.

Common practice for woods, trail or enduro riders is to add weight to the flywheel to reduce the effects of a 2 stroke resulting in a more manageable bike. It aids in hill climbs and other technical situations where it is slower and not an open track at high rpm.

The crankshaft of the engine is balance and the OEM flywheel is balanced to that particular engine just as any other engine. The addition of 8, 9, 10, or 11 up to 13oz is the balanced for that particular engine as well.

The weight is added to increase inertia. Everything is balanced. Dirt bikes do have flywheels.

Here is an example and the note explains my questioning of the effects of where the weight is placed.

AOMC.mx: Yamaha GYTR Flywheel YZ25 9-18
NOTE: Flywheels are measured by inertia weights because the affect of the weight differs depending on where it is in relation to the center of the crank. A 4 oz. and 6 oz. weight might have the same affect depending on where they are positioned in relation to the center of the crank.

Keep in mind the center of the crank they refer to is length wise and not radially or based on the radius. If it is placed further from the center of the radius, I can completely understand.
 
to the original question about weight location

when i am balancing a tire statically on my truer i have magnetic weights to help dial in the weight needed. if i put a 1/2oz on the rim it makes a very noticeable difference. if i take a 1 or 2 oz weight and put it near the center it is not noticeable
 
When I was riding for team-fatass, I added an aftermarket weighted flywheel to my ATK to give it better characteristics in hare scrambles. Definitely changes the way it rides.
 
It aslo depends on final usage. A flywheel for a vehicle has to limit weight while a stationary engine can have tons of extra weight in a flywheel with no real drawback except the cost of the extra metal. Also the bigger/heavier the flywheel the longer it takes to speed up or slow down the engine. This may or may not be an advantage.
Bill D
 
The older Yamaha 2 strokes had inner rotor or outer rotor "flywheels".The street,enduro's mostly used outer rotor & YZ's used inner rotor .Quite a difference in size & weight.I've had quite a few of them,one being an SC500 2 stroke(inner rotor).Due to the running charistics I swapped the inner out for an outer,made quite a difference in manageability and acceleration.Florida is all sand and by slowing the acceleration of the motor it actually increased the acceleration of the bike ,due to less wheel spin.If you have unlimited traction lighter is mostly better.

On some SK boats we used heaver flywheels to slow the engine acceleration
rate for when the boat came out of the water to keep from over revving.

Lower class stock drag cars juggle flywheel weight to help the launch where you have excelent traction.

All the power in the world is useless if you cant transfer it to mother earth to propel your vehicle of choice and recip motors with their funky power delivery require some what of a balancing act to accomplish that.Look at flywheel inertion as a tuning aid.
 
Something that I found very interesting, and somewhat related: when I used to fly charters, the company I flew for had a couple of Beech King Airs. They were available with two different tire/wheel sizes. It made such a difference in takeoff performance that they had different performance charts based on tire size. The larger diameter tires definitely used more runway for takeoff.
 
The weight is a common modification for 2 stroke off road bikes. The 2 stroke is a great motocross bike due to its quick revving and responsive characteristics. Plus it is light weight and can be rebuilt in half the time of a 4 stroke.

Common practice for woods, trail or enduro riders is to add weight to the flywheel to reduce the effects of a 2 stroke resulting in a more manageable bike. It aids in hill climbs and other technical situations where it is slower and not an open track at high rpm.

The crankshaft of the engine is balance and the OEM flywheel is balanced to that particular engine just as any other engine. The addition of 8, 9, 10, or 11 up to 13oz is the balanced for that particular engine as well.

The weight is added to increase inertia. Everything is balanced. Dirt bikes do have flywheels.

Here is an example and the note explains my questioning of the effects of where the weight is placed.

AOMC.mx: Yamaha GYTR Flywheel YZ25 9-18
NOTE: Flywheels are measured by inertia weights because the affect of the weight differs depending on where it is in relation to the center of the crank. A 4 oz. and 6 oz. weight might have the same affect depending on where they are positioned in relation to the center of the crank.

Keep in mind the center of the crank they refer to is length wise and not radially or based on the radius. If it is placed further from the center of the radius, I can completely understand.

The description in the ad is trying to say the effect of the weight is relative to the radial distance to the crank centerline. They just don't say it well.
 








 
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