Bill, do the same thing on a mag trainer instead of a workstand. On Thursday, January 2, 2014 9:43:06 PM UTC-6, Bill Lindsay wrote: > > We're talking about two components of momentum that are orders of > magnitude different from one another. Imagine a cyclist starting from a > dead stop and spinning up to 30kph. How much effort does it take to do > that? Let's call it "a lot". He did two things: > > 1. He got his whole mass moving to the velocity of 30kph > 2. He got his wheels spinning to the right speed > > Whatever "a lot" is, it is the sum of 1 and 2. With me so far? > > OK, now here's the thought experiment. Put his bike in the stand. Grab a > pedal and spin up to 30kph. How much effort did that take? A small child > could do it with one hand. You just did #2 above (to the rear wheel) and > reduced #1 above to zero. Whatever force it took, It's not "a lot". It's > not even 1/10th of a lot. It's tiny. Put on the brakes. Does the wheel > gradually slow down? Or does it stop almost instantly? Why is that? > Because it doesn't weigh anything. Comparing 200g of tire weight > difference is comparing two miniscule forces. > > Anybody with a powertap rear hub can do that thought experiment in real > life. Measure the power it takes to spin up to 30kph. Then do it again > with a tire that's 200g heavier. How much difference is it? I don't even > know if powertap hubs can measure forces that small. Does the lighter > wheel spin up faster and easier? Of course! Could you feel it? Maybe. > But both were ridiculously easy in comparison to getting that 100kg mass > moving up to speed. > > Math can't tell you the whole story, but it can get you into the > ballpark. The rotational momentum of bicycle wheels is tiny in comparison > to the linear momentum of a cyclist in motion. Orders of magnitude. Tell > me you've worked up a sweat pedalling a race bike on the workstand. > > On Thursday, January 2, 2014 6:38:41 PM UTC-8, Benz, Sunnyvale, CA wrote: >> >> I don't know. Let's do a thought experiment. Let's assume that the wheels >> have a very high rotational inertia. Wouldn't that smooth out the sine wave >> you're talking about? The slowing down part is when rotational >> potential+kinetic energy gets converted to potential energy against >> gravity. Using a high rotational inertia will actually help in maintaining >> speed (to whatever extent it does) and thus create lower amplitude sine >> waves. >> >> >>
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