Re: Re:
compete_clean said:
jyl said:
If the forks are 1-2 mm from the front rim, the front fork tubes have a very long front-to-back dimension, and you have a powerful battery, maybe . . . But it won't look anything like a WT racebike.
Here is a video of someone's prototype eddy current brake. You can see he needs a large conductive disk, and the magnets are very large and mounted very close to that disc.
http://youtu.be/FRepnCIa01k Even with that, the wheel takes a while to stop - the device is not creating much torque.
Come to think of it, if we wanted to build something like this, best to wait until disc brakes get adopted. Embed rare earth magnets in the disc, replace the brake pads with electromagnets, now we have a <1 mm air gap and some chance of actually getting a working stealth motor. The problem is that the brake won't work anymore as a brake, so best to use the rear brake . . .
I imagine a completely different design where the rotor magnet is the spoke and rim, with stator windings along the entire stays and seat tube. A 2.5lb brushless servo can generate 500W+. I can imagine a wheel structure design that creates 75W.
I wonder if the reason sky are so secretive about Froomes numbers is that they are too low to produce his times. Most think the reason Froomes doesn't want his numbers put out for analysis is that they are equal power numbers to the EPO era. But what if this wrong? What if his numbers are too low to account for his climbing speed?
Maybe that is why he is not worried about having his urine and blood frozen for ten years. They don't freeze bikes and keep them for retrospective testing.
I think that's not going to work.
- The spokes can't be part of the rotor magnet, they have to be steel to support the load of being spokes, rare earth does not have that kind of tensile strength and elasticity - it is brittle and weak. Also, the spokes are quite far from the stays.
- Magnets could be concealed in the interior of the rim at the brake track, but they'd still be about 2 cm separated from the seat stay and about that much separated from the chainstay. Magnets placed further along the rim, toward the spoke bed, are even further away from the stays. Exact distances depend on the frame.
- The whole stay can't be the stator, only the four sections, of a centimeter or so length, that are closest to the rim brake track.
- So you basically have an electric motor with a rotor that is call it 4 cm^2 area, and a stator of similar area, separated by around 2 cm air gap.
- That doesn't describe any electric motor you or I have ever seen, because it would be a terrible one, if it worked at all.
To illustrate the importance of the air gap, go to this site
https://www.kjmagnetics.com/calculator.asp?calcType=block
Input a block shape magnet 10 mm high x 10 mm wide x 5 mm deep (that's something you could attach to the inside of the rim brake track area). Use the highest grade magnet (N52). Now imagine the same magnet is hidden in the stay. Have the applet calculate the attraction between these two magnets if the stay is 1 mm from the rim (air gap 1 mm). 5.12 lb. Now change the distance to 10 mm. 0.39 lb. Now change to 20 mm. 0.05 lb.
Air gap is critical. If the stays of the bike are 1 mm from the rim, then maybe something could work. Wouldn't be a powerful motor, might not be worth all the extra weight from lining the rim with magnets, but we could maybe get the bike to roll on flat ground. But if the stays are 10 mm from the rim, this rim drive "motor" is now 10X weaker. At 20 mm, which is about what the real bike would be, the motor is 100x weaker. With a reasonable battery, I'd guess we be doing well to get the wheel to turn even on a workstand.
Now, if we could build the frame and wheels custom, then it might work. We'd use a disc rear wheel, and shape the stays so that they were 1-3 mm separated from the disc wheel surface from hub to tire. We'd shape the stays like blades. I think we'd embed magnets in the inner surface of the stays, and embed windings on the outer surface of the wheel disc. Place battery and electronics inside the wheel. Use a wireless control. It would be cool. But hardly stealth . . .