066 Poly vs Metal

[spacemule]

Now if you have a heavier flywheel and we know it takes more resistance to slow down a heavier object than a lighter one; then we also know that we will have a wider RPM range in which the saw will cut with a heavier flywheel before it falls on it's face.

This is what everyone is disagreeing with you on, Dean. Can you please explain why a saw with a heavier flywheel will cut with power at a lower rpm? If we're saying the same thing you are, then you're not expressing yourself in a language understandable to us. I think we've established that flywheel weight differences affect nothing aside from acceleration and deceleration characteristics, yet you maintain that a heavier fly-wheeled saw will maintain power at a lower rpm. These two items are not congruent.

 

[stihlborne]

I had both flywheels handy...the aluminum weighs 1lb 3.8 oz (26 fins) the poly was 1 lb. .02 oz (21 fins)....for scientific purposes,both had equal amounts of grime:deadhorse:

 

[Crofter]

Lighter flywheel plus stiffer crank = higher torsional resonance frequency. Torsional resonance breaks crankshafts. Anyone want to bet that the poly material of the flywheel also has some dampening properties parallell with the rubber elements of harmonic balancers in the front pulleys of car engines?

 

[Lakeside53]

I had both flywheels handy...the aluminum weighs 1lb 3.8 oz (26 fins) the poly was 1 lb. .02 oz (21 fins)....for scientific purposes,both had equal amounts of grime:deadhorse:

I also had some weighed. two different 066 aluminum flywheels (there are several versions) - 17.4 OZ and 18.3oz. The poly on the same scale weights 16.4oz. My earlier weight on the poly was off anyhow - it's missing a about 2/3 of it's fins!

 

[Lakeside53]

Lighter flywheel plus stiffer crank = higher torsional resonance frequency. Torsional resonance breaks crankshafts. Anyone want to bet that the poly material of the flywheel also has some dampening properties parallell with the rubber elements of harmonic balancers in the front pulleys of car engines?

Adding to the above...

I believe the continuous steels band acts as a dampener. It's mass is distributed around the entire wheel instead of unevenly as with the aluminum construction and two primary lobes. I have some better measurements on the laminated steel (picked some of the poly away..) - it's 5.5mm deep and 9mm wide making the center of mass further out from the crank than I first thought.

Even though the poly is slightly lighter, I'm still betting it has a higher moment of inertia - most of it's mass is concentrated at the most effective location.

I'll cut the flywheels in half when we thaw out later this weekend (metal band saw is outside..) and post some pics.

 

[Kneejerk Bombas]

Even though the poly is slightly lighter, I'm still betting it has a higher moment of inertia - most of it's mass is concentrated at the most effective location.

I'll cut the flywheels in half when we thaw out later this weekend (metal band saw is outside..) and post some pics.

That would be interesting to see.
Is there some way to measure the difference of where the mass is located on the two flywheels?

 

[Lakeside53]

That would be interesting to see.
Is there some way to measure the difference of where the mass is located on the two flywheels?

It's "relatively" easy with the Poly due to it's regularity of design and mass being in one primary location (plus a smaller second location around the rewind dogs), but really difficult with the Aluminum. Basically it consists of a bunch of math adding up all the vectors of the components... and figuring them out without a cad drawing means taking a lot of slices (both radially and "horizontally") of the flywheel and calculation for all parts...

I think I did this as a college exercise once...

The crank would be very difficult, and it is part of the inertial moment.

 

[Crofter]

Only roughly 10 % difference in total weight but because the difference in location of the weight in the poly it is not so obvious which will have the greater flywheel effect. Doesnt sound like enough to make a huge difference in a saws operation. Maybe it was for the dampening effect of the poly or maybe it was just cheaper than aluminum..

 

[bwalker]

I think we put this sack of BS to bed.

 

[Jacob J.]

So, since the 064 was designed first (with a much lighter flywheel), why did Stihl even set the 066 up initially with that big chunk of aluminum?

 

[spacemule]

So, since the 064 was designed first (with a much lighter flywheel), why did Stihl even set the 066 up initially with that big chunk of aluminum?

Not exactly a "big chunk." As was pointed out above, it's only a couple of ounces heavier.

 

[Jacob J.]

The differences described above involve the weight between the early 066 aluminum flywheel and the later 066/660 "poly" flywheel. My question is- The 064, slightly smaller in displacement than the 066, uses a smaller and lighter aluminum flywheel as well as a narrower recoil housing. The 064 also came out years before the 066, the design of the 066 being based on the 064. So, after having read all of the speculation in this thread, I'm wondering why Stihl's engineers at the time thought it necessary to outfit the 066 with the bigger flywheel and recoil housing.

 

[spacemule]

The differences described above involve the weight between the early 066 aluminum flywheel and the later 066/660 "poly" flywheel. My question is- The 064, slightly smaller in displacement than the 066, uses a smaller and lighter aluminum flywheel as well as a narrower recoil housing. The 064 also came out years before the 066, the design of the 066 being based on the 064. So, after having read all of the speculation in this thread, I'm wondering why Stihl's engineers at the time thought it necessary to outfit the 066 with the bigger flywheel and recoil housing.

Any answer would be speculation, in part because the flywheel serves various purposes--cooling, balancing, smoothing, etc. Furthermore, as Lake pointed out, changes in flywheel design correspond with changes in crank design.

 

[Jacob J.]

I'm well aware of all that, and that being said, the 064 and early 066 used the same crank. There was a later design change after 1991 when the "red light" 066 came and went where the connecting rod was beefed up and the crank throws were heavier.

 

[Lakeside53]

The differences described above involve the weight between the early 066 aluminum flywheel and the later 066/660 "poly" flywheel. My question is- The 064, slightly smaller in displacement than the 066, uses a smaller and lighter aluminum flywheel as well as a narrower recoil housing. The 064 also came out years before the 066, the design of the 066 being based on the 064. So, after having read all of the speculation in this thread, I'm wondering why Stihl's engineers at the time thought it necessary to outfit the 066 with the bigger flywheel and recoil housing.

I know about the "thinner" flywheel on the 064 flywheel but is it much lighter... maybe 1 ounce?... But I haven't weighed one on the scale as was used above though. Do you have a weight for the flywheel?

The flywheel functions to assist and smooth out the reciprocation of the engine. Larger bores need a flywheel with a greater moment of inertia. This is not the same as weight, but on the earlier Aluminum design it likely follows, so the 066 got a larger flywheel.Additionally, more HP means more heat, so likely more cooling air from the flywheel.

Then the small % of crank failures followed. Beefing up the crank/bearings, reducing the weight while maintaining the requried moment of inertia (assumed) was the solution.

I'm sure the engineers had analyzed a lot of information from the 064 and tweaked it for the 066, and has to keep tweaking it as other issues arose. Computer modeling likely helped a lot in the 90's to refine the design. It's been remarkably stable for over 10 years now.

 

[Lakeside53]

I'm well aware of all that, and that being said, the 064 and early 066 used the same crank. There was a later design change after 1991 when the "red light" 066 came and went where the connecting rod was beefed up and the crank throws were heavier.

..and at was followed by the even beefier crank when the poly flywheel was introduced in 1996.

I agree with Crofter in his post about resonance etc..

 

[Crofter]

Kickback on Cranking

Ever have an 066 snatch back the recoil handle from you? You need flywheel weight to help carry you past compression and I believe, before top dead centre ignition. A bit more flywheel is a plus for starting. Never had a 064 in my hands but with an 066 you gotta show it authority or it will laugh at you!

 

[Cut4fun]

Being I have followed this thread from the beginning and still wondering of the outcome from you pros. Because I also have a 066 that had the aluminum fly and went to a poly fly for quicker rpm pick up.
Maybe someone could take the same (one) 066-660 and run it with both flys (aluminum and poly)and put it on a dyno or a way to measure exact rpms and torque figures at given rpm ranges for a chart comparison for use.

 

[B_Turner]

Ever have an 066 snatch back the recoil handle from you? You need flywheel weight to help carry you past compression and I believe, before top dead centre ignition. A bit more flywheel is a plus for starting. Never had a 064 in my hands but with an 066 you gotta show it authority or it will laugh at you!

I've never permitted my 066 to give my half frozen fingers an unexpected and painful yank, and if I had I wouldn't admit it.

My 395 never does that.

 

[Cut4fun]

Being mine has no de-comp valve like the newer ones. I am glad that mine has that sissy fide newer handle which takes up that snap of the recoil.