Concept2 Forum

My team just got a Model C as a donation. It is near mint condition - no damage anywhere, just dust. After replacing the shock cord, putting a new handle on it, cleaning and applying oil, and replacing the PM with a spare PM3, I thought it would be just like a new one. However ...
The erg vibrates as the flywheel spins faster, but the flywheel wobbles the same regardless of the speed.
I took the flywheel apart and put it on a granite counter, it is perfectly flat. However, when it spins, it moves to the left and right, so that if I hold a screwdriver close to the side of the flywheel it hits at one point of the rotation and doesn't hit 180º off from that point.
The flywheel has no play when stationary, the bearings seem tight. At 5 rpm, it wobbles about 1/8" total per revolution, the same it wobbles at 120 rpm. The point of contact with screwdriver is the same every rotation.

Normally, I would assume the flywheel is unbalanced, but if that were the case, the motion should get worse at high speed and be unnoticeable at low speed. Something seems bent. But if the axle was bent, it would only wobble on the drive and be off consistently on the recovery. That is not the case. Anything with the bearings would be a different period. The sleeve the flywheel bolts to doesn't seem damaged nor does it appear to move.

Thoughts?

Is too losely attached to the axel, allowing it to have too much movement? This would be rectified when speed goes up.

Welcome to the forum.

I think I know what you mean, but to be sure, when you say the flywheel "moves left to right" do you mean the whole thing shifts in the axial direction, so the whole flywheel is translating laterally by some amount, or do you mean it's behaving like a bicycle wheel that isn't true due to say, a loose spoke, so at some point the "rim" is no longer in the same plane? (see the gif image of the moving wheel below)

Also, have you checked for any radial motion of the flywheel? (what we call a "hop" on a bicycle wheel)

https://bicyclewheel.info/articles/wheel-truing/

Here's the motion of the flywheel:
https://youtube.com/shorts/Yxry3PX9iKQ

As the video should show, it is the same motion as if a spoke on a bicycle is loose.

(Thanks for the welcome to the forum: long-time forum reader, first time forum poster)

JaapvanE wrote: ↑

January 29th, 2023, 8:19 am

Is too losely attached to the axel, allowing it to have too much movement? This would be rectified when speed goes up.

No, it has no wiggle on the axle when stationary. It does not slide left or right on the axle, the nuts on both sides were properly tightened, and I've reassembled twice to make sure I saw it correctly the first time, then make sure I didn't mess it up when testing.

Naetimus wrote: ↑

January 28th, 2023, 10:41 pm

...Thoughts?

As long as the wobble and vibrations don't worry you, I would just take them for granted. As far as I can see, they don't affect the proper functioning of the erg nor the performance results on de PM.

https://www.c2forum.com/viewtopic.php? ... on#p393249

Thread above from January 2017 addresses one source of axial runout in C2 flywheel. I believe there is an additional thread in which Carl Watts describes shimming the fit of the shaft bearings in the rubber bushes with half circles of bubble pack from household lead/acid batteries. I had and have the same arrangement of rubber-bushed axle bearings in my YOM2012 mod. D. I noticed the vibration or judder on my machine only during decay of flywheel momentum after final stroke. I used Jon Wilson’s reversible solution of couple turns of plastic tape on the outer races—basic Tom Joad fix—and never looked back.

In this scenario, heavy side/light side flywheel (mass imbalance bounce) was not a major source of vibration. Dynamic eccentricity (lateral runout) of flywheel-axle unit appeared to be the major contributor according to Carl. The engineering choice of loose-fit rubber bushes described by C2Jon was most likely responsible imho. I have no knowledge of what models, model versions, or production runs used the rubber-bushed bearing hangar arrangement for the flywheel axle bearings. If your machine does, info above may put paid to the head-scratching.

Based on your YouTube video, it looks to me like the flywheel is mounted to the axle housing crookedly. You stated the wheel sits flat on a granite counter, not warped itself. Lay the flywheel flat on the counter and check the perpendicularness of the axle housing to the flat part of the flywheel with a square.

How you would get the two parts of the flywheel perpendicular might be a challenge, I don't know how they are attached (and I'm not going to disassemble my model C to check, sorry ). Friction fit? Tack welded?

Addendum

Replace ‘bubble pack’ by ‘blister pack’ in my previous comment 5:58PM.

The flywheel is out of balance as evidenced by the vibration when rowing. The runout or wobble shown in the video is not too unusual even in a well balanced flywheel. However, if the flywheel was originally straight and was then somehow bent to the degree shown it would certainly cause a vibration. Did you check the three screws that hold the flywheel plate to the center hub? If one or two of those were to loosen over time it could result in what you are seeing. Also check the fan blades for evidence of missing balancing clips. An engineer might be able to balance the flywheel using modeling clay but it's a tedious process- it might be best to just order a new flywheel...

The flywheel is definitely mounted tightly and squarely on the axle housing. I've taken it off three times and it is an absolute tight fit with no wiggle room. The clearance is less than 1/1000", it is an excellent fit.
It sounds like it is the balance, and that means a new flywheel (with no absolute measure of whether a change makes it worse or better, it will take a really long time to hand balance it. There is no clear tab missing off the plastic air fan). After I get it installed, I'll report back on if that fixes it.