Not sure you can take much from this test. A couple of reasons:

1. The majority of the friction is not coming from the bearing balls/races. It is the drag of the balls through the grease, the drag of the bearing seals, and the freehub ratchet. Differences in any/all of these will greatly exceed the differences in the condition of the bearings.

2. The test does not place the bearings under significant load (just the weight of the wheel). On the road the wheel will be supporting ~100lbs of bike+rider.

3. How much energy you can place into the wheel by spinning it hard is a function of the wheels inertial moment (the rotational equivalent of mass). Fill the innertube with water and I bet you can get the wheel to spin a lot longer (much the same way you can throw a baseball farther than a styrofoam ball).

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Additionally there is going to be a substantial difference in rotation drag between the two especially as 1 is a disk wheel. The difference should be somewhere in the ball park of 10 watts at 30 mph.

Where did you get that number? Are you referring to aero drag? In a truing stand?

Not to mention that the disk will have more intertia that will make it spin longer than if it was lighter. (Assuming you put the same initial speed into each wheel...)

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Kevin

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I'm referring to rotation drag from the spokes rotating through the air. Though at 30 mph there is significant interaction between the translational wind and the rotational aspect of the spokes so likely less than 10 watts

Here the data https://docs.google.com/...UliYVotdVc2d2c#gid=0

New bearings could have seals that will have greater friction when new. New or different types/qty of grease could also play a part.

Preload can vary when the skewer is tightened. Make sure your preload is set for when the skewer is done up, which means it may have to be a smidge loose when the skewer is open.

Like others have said, the weight will also play a big part. And how well the wheel is balanced, a heavy spot will slow the wheel faster at low speeds.

So if I remove tires, tubes etc so its just each wheel, you'd expect the disc to rotate for longer, provided I spun it hard enough to begin with?

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Different weight, different moment of inertia. You are comparing apples to oranges.

Well yes I would expect the disc to spin longer. I don't thin the difference in the time to spin to a stop should be as different as what you report. Here is another test you could try. Let the wheels spin down so the heaviest spot is at the bottom. Then try to offset the heavy spot with a weight on the opposite side so you have a neutral balance. Basically any starting position doesn't make the wheel spin. Then add a new weight And set it at a known position , say 3 or 9 o'clock. Then let it go and count how many oscillations it takes before the weight stops moving.

I swapped out my standard bearings for some fancy ceramics and the difference in the number of oscillations was significant. Does this reduction in drag mean huge wattage savings? Probably not, but a watt is a watt. You should check out the Friction Facts site for some ideas on the potential savings. There are probably 3-5 watts of drag you can eliminate with careful parts selection and lubricants.

I'd agree with Titanflexr. The bearings have to be under load to make it a significant test.

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AndyF
bike geek

I'm going to give you a suggesting that could turn into a test that works if you mind the details. This is just a rough sketch. You need a fork that will fit both wheels (steel will bend, old frames can be cut apart), a good deal of weight, a way to attach the first two items to each other, a very solid stand, and a data acquisition system of some sort (camera + ruler?). You'll need a good deal of ingenuity too.

Basically you're going to swing these weights like a pendulum. The hub is the pivot for your pendulum. The fork is the pendulum arm, and the weight is the bob. The weight also loads the bearings appropriately. The wheel should be held stationary, and firmly stationary by a very heavy rigid stand. You don't want recoil affecting your results. You're going to measure the rate at which the amplitude of oscillation decreases, or the Q factor. Since you've used the same fork for both wheels, the only difference in Q factor should come from bearing drag. It wouldn't require much imagination to do the same measurement with a torsion spring and a mass as well.

This is how you can do it, but it's probably not worth the effort. If you disagree and want to set it up, I can analyze data for you.

ok this was quite a telling experiment. With the 808 I get 171 oscillations, and with the disc I get 5! Something is not right here... maybe I'll just put my wheel cover on the 808 and race with that...

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I had the same situation with a set of Hed wheels. After not getting any improvement in spin time by changing bearings and adjusting I pulled the metal/rubber seal out of the bearings and voila. The spin time went from around one minute to over 6 minutes.
I raced those wheels without the seals for a couple of years, just maintaining the bearings like old school loose bearings.

ha ok - thanks I didn't know that was an option - I'll play around with that before cashing out my 401K on CeramicSpeed bearings

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It is probably possible to change a hub such that it spins longer when unloaded and has worse friction when loaded.

For instance, might a lubricant that is too light potentially cause this scenario?

In general the kinds of differences you see with unloaded spin tests all but disappear when loaded, and I'm talking not even a watt, less than a watt!


Still, spin tests like this might be a way to keep track of whether bearings have gone bad. Keep in mind thought that a heavier wheel, or heavier tire, is going to make it spin longer.

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There are so many variables here, but what you are most likely seeing is that the Zipp disc hub uses a larger drive side bearing that has larger balls, than what you have in your 808. Zipp bearings are high grease fill, like 80%, so you have larger balls traveling through grease at a larger diameter. As pointed out here, the difference is on the order of a tiny fraction of a N/m, but with so little mass/inertia in the system is seems like a big difference. Also of note is that the disc and 808 are similar in total weight, but the disc has less rotational inertia as there is more weight in and around the hub, so that is affecting the experiment.

On the road, you are talking about maybe a hundredth of a watt or less.. When we first did ceramic bearings in 2001, we did spin testing of unloaded wheels and the standard disc would spin for like 4-5 minutes with initial impulse and the ceramic with non-contact seals and lighter grease would spin for 4-5x as long, but when you measure it out in a loaded spin test, the total savings is on the order of 0.15-0.20w per bearing at 30mph assuming ~160lb rider..so for Zipp hubs this is 0.8-1.0 watts saved (6 bearings in a hubset).

Anymore, it is easy to find bearing suppliers and hubs that feel as if they have zero friction, I've felt some really impressive ones lately, but remember, what you feel in your fingers and on the truing stand is almost entirely seals and grease, so what feels like no friction in your hands is really more like no seals, no grease (or very little..20% grease fill is common)

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Thanks Josh. I'm really just trying to verify that the spin time I'm seeing for the disc is way off what should be expected. It also feels a bit "sluggish" when riding but maybe that's just in my head now! With a regular spin of the disc when the wheel is mounted on the bike, I'm only getting about 10 sec spin which seems eay less than what I'd expect from the super 9 cc

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