Ron, Great idea, let's do that. Get on the mag trainer and pedal up from 0 to 30kph over some appropriate number of seconds. Say, 10. Capture the power from your power tap hub and plot it. So far so good? That's power trace A
Next put 200grams of lead weights on your rear rim, a little 25 gram blob between every 4th spoke hole. Repeat the test. Spin up to 30kph over the same period of time. Grab that power trace. That's power trace B The difference between A and B will be the extra work the rider had to do to "spin up" the extra 200g of rolling weight. I'm saying that difference will be small. The heavier wheel is harder to spin up, but the magnitude of the difference is small. If the total power output of the rider is ~100 Watts, then the difference between the two will be 1 or 2 Watts. Less than the difference we suffer by running a dynamo. On Friday, January 3, 2014 6:51:19 AM UTC-8, Ron Mc wrote: > > 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. >>> >>> >>> -- You received this message because you are subscribed to the Google Groups "RBW Owners Bunch" group. To unsubscribe from this group and stop receiving emails from it, send an email to rbw-owners-bunch+unsubscr...@googlegroups.com. To post to this group, send email to rbw-owners-bunch@googlegroups.com. Visit this group at http://groups.google.com/group/rbw-owners-bunch. For more options, visit https://groups.google.com/groups/opt_out.
